ccd xray detector overview

REVIEW OF SCIENTIFIC INSTRUMENTS VOLUME 73, NUMBER 8 AUGUST 2002

REVIEW ARTICLE

Charge-coupled device area x-ray detectorsSol M. Grunera)

Department of Physics, Laboratory of Atomic and Solid State Physics, Cornell University, Ithaca,New York 14853 and Cornell High Energy Synchrotron Source (CHESS), Cornell University, Ithaca,New York 14853

Mark W. TateDepartment of Physics, Laboratory of Atomic and Solid State Physics, Cornell University, Ithaca,New York 14853

Eric F. EikenberrySLS-Project, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland

~Received 3 January 2002; accepted for publication 21 April 2002!

Charge-coupled device~CCD! area x-ray detector technology is reviewed. CCD detectors consist ofa serial chain of signal components, such as phosphors, fiber optics or lenses, image intensifiers andthe CCD which serve to convert the x-ray energy to light or electron-hole pairs and to record thespatially resolved image. The various combinations of components that have been used to makeCCD detectors are described and the properties of each of the critical components are discussed.Calibration and correction procedures required for accurate data collection are described. Thereview closes with a brief description of future directions for solid-state area x-raydetectors. ©2002 American Institute of Physics.@DOI: 10.1063/1.1488674#

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I. INTRODUCTION

Charge-coupled device~CCD!-based area x-ray detectors ~hereafter simply called simply ‘‘CCD detectors’’! haveenormously improved the quality and speed of x-ray dacquisition for many scattering and imaging applicatioCCD detectors are the outgrowth of several decades ofvelopment of area x-ray detectors based on electro-opimagers~e.g., vidicons, CCDs, and diode arrays! and sharethe general characteristics of modularity, being composea sequential cascade of components that were usually doped for other applications. On the one hand, this modulaprovides the detector designer with a wealth of choices;the other hand it provides a bewildering number of possibties. The primary purpose of this article is to review tconsiderations involved in optimizing a detector for a givapplication, and thereby serve as a guide to the detectorsigner or user.

A secondary goal of this review is to provide a contefor related x-ray detector developments. Much of the tenology of CCD-based detectors, such as procedures fordetector calibration, turn out to also be useful for other typof x-ray detectors. CCD detectors also provide the basisthe next generation of pixel array detectors~PADs! now un-der development in laboratories across the world. Thugood understanding of CCD-detector principles is also usin a wider context. This review complements many excell

a!Author to whom correspondence should be addressed; [email protected]

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papers on CCD detectors already in the literature. For sogeneral references, see Refs. 1–7.

II. ELECTRO-OPTICAL AREA X-RAY DETECTORCOMPONENTS

A. Overview

CCD detectors most generally consist of a relayelectro-optical elements~Fig. 1! that function to stop the xrays and generate a primary signal that is, perhaps, ampland eventually coupled to a CCD. CCD detectors are disguished one from the other by the components that perfthese functions. An understanding of the signal relay requan analysis of the five functions performed by the sigrelay components:

~1! X-ray conversioninvolves stopping the x ray in, for example, a phosphor or semiconductor layer and the cversion of the x-ray energy into more readily maniplated visible light or electron quanta.

~2! Intensificationof the resultant signal may be requiredthe signal is weak.

~3! Various coupling methods~e.g., lenses, fiber optics! areused to couple the signal relay components.

~4! An imaging array, e.g., the CCD, position encodes thresultant signal.

~5! The operating modeof the imaging array~e.g., whetherit is read out at video rates or in a cooled, slow-sclow-noise rate! strongly determines the selection of thcomponents and the signal-to-noise ratio of the detec

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2816 Rev. Sci. Instrum., Vol. 73, No. 8, August 2002 Gruner, Tate, and Eikenberry

The design of the signal relay is dictated by the x-rapplication and the practical constraints of available comnents. As examples, CCDs are directly sensitive to x raysmake excellent soft x-ray detectors without additioncomponents.7 However, the available CCDs may be tosmall, too susceptible to radiation damage, or have tooan x-ray stopping power for a given application. An alterntive is to absorb the x rays in a larger area, radiation-hahigher stopping power phosphor screen and couple thesultant light onto a CCD. The drawback here is that whilex-ray stopped in a CCD may create thousands of signal etrons, only a few tens of electrons may result from a retively inefficient combination of a phosphor-CCD optical rlay, thereby necessitating the imposition of a light amplifibetween the phosphor and the CCD. Thus, it is necessa

FIG. 1. Electro-optical x-ray detectors consist of a relay of signal elemeThe energy converter stops the x ray and producesN visible light photons orelectrical charges. These are amplified by a gain element, such an iintensifier, to produceN3M quanta that are relayed to a readout devisuch as a CCD.

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understand the properties and efficiencies of the varicomponents in the optical relay in order to winnow down tnumber of possible configurations.

Figure 2 shows examples of CCD detector systemshelp set the context for the discussion that follows.

B. X-ray converters

The function of the x-ray converter is to stop the x raand produce more readily manipulated quanta, such asible photons or electrons. We discuss phosphors and sconductors, which are by far the most important convertin current use.

1. Phosphors

The use of x-ray phosphors dates back to the first dcovery of x rays when, in 1895, Roentgen noticed the glof a barium platino-cyanide screen next to his discharge tuAlthough Roentgen soon turned to photographic emulsito permanently record his findings, it was quickly realizthat photographic emulsions had very low x-ray stopppower and that sensitivity could be gained by pressing a thigher stopping power phosphor sheet against the emulsPupin had already proposed CaWO4 for this purpose in1896.8

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FIG. 2. CCD detector configurations consist of variocombinations of luminescent screens, fiber-optic plaand tapers, lenses, image intensifiers, and CCDs. Soexamples:~A! Phosphor screen fiber-optically coupleto a reducing image intensifier, that is fiber-opticalcoupled to a CCD~Ref. 150!; ~B! phosphor screenfiber-optically coupled to an image intensifier thatlens coupled to a CCD~Ref. 1!; ~C! phosphor or scin-tillating fiber-optics screen coupled to a fiber-opttaper, image intensifier, a second taper, and the C~Ref. 87!; ~D! phosphor or scintillating fiber-opticsscreen coupled to a fiber-optic taper and the CCD~Ref.87!; ~E! a matrix of closely fitting phosphor screentaper-CCD modules~Ref. 2!; and ~F! direct conversionof x rays into electrical signals within the CCD.

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2817Rev. Sci. Instrum., Vol. 73, No. 8, August 2002 CCD area x-ray detectors

TABLE I. Some characteristics of phosphors commonly used for analytical x-ray CCD detectors. The initial decay is fast if it is less than a milliseco, slowif it is greater than 1 millisecond. The persistence is very high if it is very visible to the eye.

CsI:Tl NaI:Tb ZnS:Ag Gd2O2S:Tb ~Zn,Cd!Se

Robustness hydroscopic very hydroscopic stable stableEfficiency ~%! 10 13 20 15 19

Initial lightdecay

fast slow fast slow fast

Persistence low low very high low lowColor green green blue green red

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Not surprisingly, a very large number of phosphors habeen developed in the century since Roentgen’s discovPhosphors have very complex chemistry and physics andused for a diverse variety of applications, ranging fromedical and analytical x-ray detection to fluorescent lighticathode ray tubes and television projection systems, resuin a literature that is large and spread out over many yeand in many journals. An unfortunate consequence ofeconomic importance of phosphor-based devices ismuch industrial phosphor research has been proprietaryis not in the open literature. Industrial work on phosphowas especially intense during the twenty years followWorld War II due to the rapidly evolving needs of the telvision and lighting industries. Tragically, much of the unpulished phosphor expertise developed during that time is nbeing lost as old company records are discarded andoriginal researchers pass away. Even though the use of pphors is larger than ever, there is relatively little new phphor research being performed today. Hopefully this sittion will change with the advent of combinatoriaapproaches9,10 and new sol-gel processes.11

A superb review of modern phosphors, with chaptdevoted to x-ray applications, is Blasse and Grabmaie12

Garlick,13 Leverenz,14 and Curie15 are classic books on phosphor physics. Birks16 covers scintillators. Other reviews include Ouweltjes,17 D’Silva and Fassel,18 and Blasse.19–21

TEPAC22 is an invaluable reference for cathode ray tuphosphors. Phosphor characteristics and valuable advicealso listed in the information available from phosphor manfacturers~see, e.g., Refs. 23–26!.

Our concern in this section is with solid-state materiwhich luminesce~emit light! when irradiated by x rays—weshall generally call all such materials phosphors. We preto call materials ‘‘luminescent,’’ rather than ‘‘fluorescent’’ o‘‘phosphorescent’’ because the distinction between fluorcence as a spin-allowed transition (DS50) and phosphorescence as a spin-forbidden transition (DS51) is not univer-sally applied~e.g., for discussion see Ref. 27; Appendix 3Blasse and Grabmaier12!. Similarly, there are no universallused definitions which distinguish scintillator and phosphmaterials. We will follow the common practice of callinmaterials scintillators if they are usually used in photcounting applications and phosphors if they are usually uin photon integrating applications. The reader shouldaware, however, that the term ‘‘scintillators’’ and ‘‘phophors’’ often refer to the same materials. To be unambiguothey are both luminescent materials.

A phosphor screen is a thin layer of phosphor that

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used to convert an x-ray image into a light image. Phospscreen characteristics of importance for CCD detector x-imaging include:

~i! robustness and stability;~ii ! x-ray stopping power;~iii ! spectral matching of the light output to the ne

optical relay element;~iv! energy efficiency for conversion of x rays to light~v! luminescent decay time and afterglow;~vi! linearity of light output with incident x-ray dose an

intensity;~vii ! noise; and~viii ! spatial resolution across the screen.Some important x-ray luminescent screen materials co

monly used for analytical x-ray CCD detectors are listedTable I and in Fig. 3. The selection of an x-ray phosphora given application invariably involves compromises amovaried, and often conflicting characteristics of availabphosphor materials. For example, the use of thin screenoptimize the spatial resolution compromises the x-ray stping power. As another example, NaI:Tl, which is very eergy efficient, is made of low-Z atoms and has a relativellow stopping power per unit thickness. Its spectral emissis also not well matched to the red sensitivity of many CCDThe effective use of a phosphor requires knowledge of alrelevant characteristics and the ways in which these chateristics affect the use of the phosphor and its couplingother detector components.

Relatively little recent research has focused on develing new x-ray phosphors,20,28,29mostly because phosphor development is difficult and potential markets are small~see,however, Refs. 30–35!. A promising new ceramic phosphois Gd2O2S:Pr,Ce,F.9,10,33,36

In order to be useful for two-dimensional imaging, thphosphor must be fashioned into a thin screen. Phospscreens may consist of a single crystal,37 or may be a thinpolycrystalline sheet which is formed by vacuum evapotion or sublimation38–42 or solution deposition.42 Most com-monly, however, screens consist of carefully milled powdthat are settled, pressed, or otherwise deposited onto astrate in the presence of a low concentration of a bindmaterial.43–49An important reason to use powder screensthat many of the important x-ray phosphors are only avable as fine powders.

1. Robustness and stability.Robustness and stability refer both to chemical and crystal structure changes that maffect the luminosity of the screen and to bulk physicchanges, such as resistance to cracking or peeling. The

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FIG. 3. The attenuation length is defined as the thicness of a material required to reduce the transmitx-ray intensity to 1/e of its incident value. Attenuationlengths are shown for four of the phosphors of TableThese figures were produced with the online calcator at http://www-cxro.lbl.gov/optical–constants/atten2.html.

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terials listed in Table I are mostly radiation-hard solid-stmaterials, such as doped salts~e.g., CsI:Tl! or ceramics~e.g.,Gd2O2S:Tb!. Organic plastic scintillators16 are available andeasily made into thin screens, but they are rarely usedx-ray imaging applications because they have low x-stopping power and low energy efficiency~;3% or less!.Although inorganic phosphors tend to be intrinsically radtion hard, modern synchrotron sources are capable of deering x-ray doses that cause even glasses to rapidly devcolor centers, and quickly degrade plastic binders and phphor window materials.

CsI and NaI are very energy efficient phosphors. Bare hydroscopic and readily poisoned by water, which necsitates hermetic sealing behind water-vapor tight windowsis desirable to make the windows as x-ray transparenpossible. Beryllium is totally impervious to water vapor aworks well, but is expensive and highly toxic. Unfortunateberyllium corrodes readily when exposed to liquid watand, therefore, must be kept completely dry. Since manytectors involve cooled parts and suffer from water condention on the window, it may be necessary to use heaaround the edge of the window to keep the beryllium aboroom temperature, a flow of dry gas across the windowpolymer or thin inorganic films to protect the beryllium. Flow energy x rays, attention must also be given to berylliupurity. Heavy metals, such as iron, are common contanants in beryllium at concentrations that substantiallycrease the transparency for low energy x rays. Another plem with contaminants is that they introduce unexpecabsorption edges that complicate the window transparencpolychromatic x-ray applications.

Aluminum foil windows are excellent light and watevapor barriers, as long as the aluminum is pinhole free.though aluminum is less x-ray transparent than berylliumis more readily available in very thin sheets. Since thin aminum windows are delicate and easily ruptured, polym

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metal composite films are an attractive window alternatiThin polymer films always have some permeability to air awater, and do not, in general, provide the long term prottion against water entry afforded by metals; furthermothey are rarely light tight. The food packaging industry alfaces this problem because water, light, and oxygen degthe quality and shelf-life of many packaged goods. Insponse, the food industry has developed a variety of tpolymer/aluminum film composites which work very well along-term vapor and light barriers~e.g., a manufacturer oappropriate composite films is Fres-co System, Inc.50!. Manyof these packaging materials are also reasonably x-ray trparent and make excellent windows.

The x-ray window is ideally placed as close as possito the phosphor to avoid difficulties of x-ray scatter from twindow material. If the experiment requires a gap betwethe window and the phosphor, then careful consideratmust be given to both the small angle and wide angle scafrom the window,51 especially if the window precedesbeam stop for the primary x-ray beam.

2. X-ray stopping power.The minimum useful thicknessof a luminescent screen is largely a compromise betweenx-ray stopping power and it’s spatial resolution. In generthin screens are desirable in order to improve the sparesolution across the screen, especially in cases in whichincident x rays strike the screen at a substantial angle toscreen normal. Phosphor screens are typically specifieterms of the areal density in mg/cm2, which, for a givenatomic composition allows direct computation of the x-rstopping power at a given x-ray wavelength. The thicknof the screen is given by the areal density divided bymass density. Since many screens are made of settled gof phosphor powder, the mass density may be considerless than the phosphor bulk crystal density. A typical settscreen is about 50% void volume. Surprisingly, as discus

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below, optimum resolution is not always achieved by elimnating the void volume.

Structured phosphor screens and scintillating fiber opare alternative ways of increasing stopping power while pserving resolution—see Sec. II B 1 8 on spatial resolution.

3. Spectral matching.The detector efficiency dependon the spectral matching of the luminescence to succeselements of the light relay~Fig. 1!. In general, photocathodeof image intensifiers have peak responses toward the bCCDs are available in front-illuminated and bacilluminated52 forms. The most common front-illuminateCCDs are relatively insensitive in the blue end of the sptrum. Back-illuminated CCDs are much more expensive ththeir front-illuminated counterparts, with the consequenthat their use is limited. Figure 4 illustrates the largecreases in efficiency when coupling to back-illuminatCCDs. Recently, Kodak has introduced front-illuminatCCDs with electrodes that are more transparent in the bthereby enhancing blue sensitivity.53

A less important, but not insignificant additional spectmatching concern has to do with transmission throughoptics which couple the phosphor screen to successive relements. Many phosphors have part of their emission innear ultraviolet~UV!, where glass transmission starts to faAnother coupling consideration is that coupling fiber opthave a higher numerical aperture in the red, due to ancrease of the critical angle of reflection at longer wavlengths. This can result in a significant enhancement inlight captured and conveyed through the fiber optics~see Fig.11!.54

Spectral matching considerations may be subtle. A gexample is given by Gd2O2S:Tb, one of the most importanx-ray phosphors.33 Many of the major phosphor manufactuers list several Gd2O2S:Tb phosphors under different produnumbers, depending on whether the phosphor is optimfor a cathode ray tube or x-ray applications. The specmanufacturing differences between the phosphors~e.g., dop-

FIG. 4. The efficiency of the phosphor is greatly improved by couplingback-illuminated CCDs~open bars!, rather than front-illuminated CCDs~filled bars! ~Ref. 33!.

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ant concentrations, particle sizes, and particle finishes! areoften proprietary and not fully described in the product dscription. The luminescence arises from the Tb13 center andis due to transitions between5D4–7FJ levels, mainly in thegreen, and5D3–7FJ levels, mostly in the blue. The relativexcitations between these two level systems are a functiothe Tb dopant concentration, with lower concentratioyielding more luminescence in the blue. ‘‘X-ray phosphorare typically used in radiological applications to either ehance the sensitivity of x-ray film or to couple to x-ray imaintensifier photocathodes, both of which are blue sensidetectors. Hence, as far as most phosphor manufacturerconcerned, an ‘‘x-ray phosphor’’ should be optimized fmaximum blue emission. On the other hand, the cathodetube and lamp variants tend to be enhanced in the greebetter match the peak spectral sensitivity of the eGd2O2S:Tb is now often used in detectors in which the phophor is directly coupled via fiber optics to a CCD. In thcase, both the fiber optics and the CCD are more effectivcoupled in the green than the blue, so using the matelisted as an x-ray phosphor is exactly the wrong thing to

4. Energy efficiency.Energy efficiency refers to the fraction of the stopped x-ray energy that is emitted as ligRobbins55 has shown that the maximum energy efficiencynis given by

n5hne /E, ~2.1!

whereh is Planck’s constant,ve is the averaged frequency oluminescent radiation, andE is the average energy requireto create an electron-hole pair.E varies from about threetimes the band gap for NaI and CsI to about sevenCaWO4, and is dependent on the band gap, the highquency and static dielectric constants, and the frequencthe longitudinal optical vibration mode. High efficiencies rsult if the optical vibrational modes are of low frequency athe emission energy is close to the band gap energy. Enefficiencies~i.e., the fraction of the stopped x-ray energwhich is emitted as luminescence! range down from abou20% for ZnS:Ag. Gd2O2S:Tb is about 13% efficient andCaWO4 is about 6.5% efficient.56,57 A consequence of Eq~2.1! is that x-ray phosphors, which are dramatically mumore energy efficient than ZnS:Ag, are unlikely. It shouldnoted that it is very difficult to measure the absolute eneefficiency of a phosphor. Further, the efficiency is a functiof the exact phosphor composition and synthesis procedthe grain size, the grain surface treatment, etc., with thesult that energy efficiency values in the literature55–60 varyconsiderably.

Note that the detailed mechanism of electronic exction, and, hence the energy efficiency, may be differentUV and the very energetic photoelectrons producedhighly ionizing radiation.

5. Luminescence decay time and afterglow.Luminescentdecay refers to the way the intensity of luminescent emissdecreases with time after excitation. Measurement of phphor screen decay times in the context of x-ray detectordifficult, but important.32,33,40,60,61Ideally, the phosphor lu-minescence decays exponentially with a short time constUnfortunately, in real phosphors the decay curve~Fig. 5!

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generally divides into two regimes: prompt decay and afglow, the latter also being known as persistence. The prodecay is an initial, typically exponential decrease of intenswith time and is due to the primary radiative process. Cain which the exponential decay dominates for three or fdecades of luminescent intensity are rare. More commoafter one or two decades of decay, the luminescence is cacterized by an algebraic decrease of the form

I ~ t !5t2m. ~2.2!

I is the intensity of luminescence,t is the time, andm isgenerally less than 2. The afterglow is due to trapping pnomena which is complex and often not understood. Fquently, a complicated temperature dependenceobserved.14 Figure 5 illustrates that the interplay betwetemperature and persistence can be very complex.

Persistence limits the rate at which a rapidly changx-ray image may be recorded. The problem increases wthe desired dynamic range of the data. Consider, forample, the acquisition of diffraction patterns of a crystal bing rotated in an x-ray beam. Diffraction spots appebrighten, and then disappear, all in less than a few tentha degree of crystal rotation; furthermore, nearby spots indiffraction pattern may differ in intensity by several ordersmagnitude. Imagine a weak spot that appears in nearlysame position on the detector as where a very bright spotjust appeared. It is desirable for the afterglow of the brigspot to have decayed by at least four orders of magnitudthe time the weak spot appears. At an intense synchroradiation source, the rate of crystal rotation may be vrapid and the time between appearance of the spots mayfew tenths of a second. Most phosphors will not decayfour orders of magnitude in this short time. The algebr

FIG. 5. The luminescent emission of a phosphor ideally decreases expotially with time. At some time, the luminescent decay from most phosphswitches from exponential to a slower algebraic form. In this example,gebraic decay is observed at intermediate temperatures for rbZn2SiO4 :Mn(0.3) ~Ref. 14!.

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nature of persistence also may lead to a slow buildupbackground intensity due to the ever-present low leveldiffuse background between the Bragg spots. In conquence, as time progresses, it becomes increasingly diffito accurately measure very weak spots because of ribackground levels.

Some materials have traps which are too deep to be tmally excited in short amounts of time. These traps mbecome excited and luminesce upon x-ray exposure. Thesult is a ghost image of the last x-ray image taken superposed on the diffuse background of a new image. In socases, the trapped ghost images may be stored indefinonly to reappear upon further x-ray exposure. In other cashallower traps may be populated which may be opticaphotoexcited into luminescence. Such ‘‘storage phosphohave recently found important applications as x-ray detec~also called ‘‘image plate detectors’’!.62–68 The phosphor isexposed to acquire the x-ray image and then later read ouscanning the phosphor with a laser to excite the trapsluminescent emission, which is detected by a photomuplier. The most important example of such a storage phphor is BaFBr:Eu21, which may be photoexcited with reHeNe laser light.

6. Linearity of light output.The output light intensity ofa phosphor will vary nonlinearly with the input x-ray intensity if there is significant ground state depletion of the acvator and certain trapping phenomena. Excited state abstion and Auger processes may also result in nonlineaeffects.12 Phosphor saturation effects have mainly beenvestigated in cathode ray tube applications where the speenergy doses are very high, both because of large eleccurrents incident over very small phosphor screen areasbecause the electrons do not penetrate deeply into the pphor, thereby yielding most of their energy in a thin phophor layer. By contrast, typical x-ray energy doses have bvery much smaller than electron energy doses, especiallcases where one requires a large dynamic range of intenlinearity in the x-ray signal, with the consequence that fintensity linearity studies have been performed. Howevvery high specific x-ray intensities are now obtainable wsynchrotron radiation sources. This makes detailed investtion of the linearity of x-ray phosphors over a very widrange of x-ray intensities feasible; indeed, synchrotron radtion applications are likely to necessitate such studies.

Whereas intensity linearity refers to light output asfunction of the rate of incident x-ray energy, dose linearrefers to the light output per unit x-ray input as a functionthe total integrated incident x-ray exposure. Nonlinearwith respect to dose means the phosphor is changingmost typically, is suffering radiation damage. The inorgaphosphors in Table I are generally very radiation hard acan sustain enormous doses without significant damagemay be seen by considering the doses on cathode ray tua modest cathode ray tube electron beam of, say 1mA and 30kV potential delivers the same energy dose as about 1013 15keV x rays into a very small volume of phosphor. Thiscomparable to the most intense monochromatic beams aable at modern synchrotron x-ray sources. Although cathray tubes do eventually suffer from burn-in, this is typica

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only after very long periods of exposure~months or years!.Many phosphor screens consist of micron-sized ph

phor grains held together by a binding material, such asorganic polymer. Organic materials are much more susctible to bond scission and degradation, with the result thatfirst sign of radiation damage may be a breakdown ofbinding matrix, leading to flaking or discoloration of thphosphor screen. Inorganic binders, such as silicates,44,45 aresomewhat more robust. However, even with typical orgabinders, one needs enormous doses before damage isdent.

7. Noise.Phosphor noise arises from statistical vartions in the light output per incident x ray. In high-gain phton counting detectors, it is frequently possible to set a dcrimination threshold such that the counting statisticsdominated by the shot noise of the incident signal andstopping power of the phosphor. The statistics are more cplex in integrating detectors, where the light from a givpatch of phosphor screen is integrated before being readAssuming a Poisson x-ray source, the output signal-to-nratio ~SNR! of the phosphor is given by

SNR5~N0AQI !1/25~N0AN!1/2, ~2.3!

where N0 is the number of incident x-ray photons in thintegration time and area,AQ is the quantum absorption othe phosphor screen,I is a factor between 0 and 1, andAN

5AQI is called the noise-equivalent absorption.69–71 If eachstopped x ray yielded the same light output, thenI 51 andthe SNR is simply dominated by the incident quantum stistics and, importantly, the fraction of x rays stoppeSwank69 has shown that

I 5M12/M0M2 , ~2.4!

where Mi are the i th moments of the scintillation pulseheight distribution.I >1 if the pulse-height distribution isvery sharply peaked.

Factors that lead to light output fluctuationsper stoppedx-ray result in values ofI less than unity, e.g.,AN,AQ .Swank69 identifies three such factors: the distribution in icident x-ray energies, intrinsic variations in the numberphotons produced per x ray at a given energy, and light oput variations resulting from light collection efficiency cosiderations. The first factor simply results from the fact ththe lower energy x rays of a polychromatic x-ray souryield less light. Intrinsic variations occur because of a schastic distribution of the stopped x-ray energy into channthat do not result in light output. For example, at x-ray eergies just above aK absorption edge of the phosphor, x-rafluorescence can cause much of the stopped energy to esthe phosphor in the form of low energy x rays. Fluctuatioin the light collection efficiency result, e.g., when the phophor screen is not totally transparent to the emitted ligmore light will be collected from x rays which are stoppcloser to the side of the phosphor screen facing the opdetector than those stopped further from this side. Swa69

has calculatedAN for several x-ray phosphors.The phosphor limits the performance of the detector

fundamental ways. As shown in Sec. IV F, the detectquantum efficiency~DQE!, a measure of the overall detect

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sensitivity, has the stopping power of the phosphor asupper limit. In other words, the performance of even an oerwise ideal, noiseless detector will not be optimal unlessphosphor stops all the incident x rays—this is intuitiveobvious. What is not so obvious, is that the accurate msurement of an integrated signal is also limited by the sysresolution. In so far as the phosphor is frequently tresolution-limiting element of the detector, it can degradeaccuracy of the desired measurement, even if the signal rafter the phosphor adds no additional noise. Thus, improment of the phosphor stopping power, by increasing its thiness, usually degrades the spatial resolution, which alsocreases the measurement accuracy of the system. Forreason, the compromise between phosphor stopping poand resolution must be carefully selected. Methodschoosing compromise values will be detailed in later stions.

8. Spatial resolution.The spatial resolution of a luminescent screen is determined by its ability to stop x rthrough the thickness of the screen while minimizing tlateral spreading of the resultant light. Three kinds of sctillating screens are in use: screens of settled phosphor pders, single crystal screens, and microstructured screensspatial resolution of each of these screens is determineddifferent light propagation physics, as discussed below,this section, and in the next section.

The resolution of a phosphor screen is characterizedthe transfer function which maps the x-ray image into toutput light image. Because the incident and output imaare incoherent, it is sufficient to specify a real transfer funtion. The normalized point spread function~PSF!,

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specifies the distribution in output signal for a point inpsignal, where the right-hand integral applies in the usual cof a uniform phosphor screen which is rotationally invariaabout the screen normal.72,73 Insofar as the phosphor may bapproximated as linear in intensity response and station~e.g., the PSF is translationally invariant across the scre!,then the output response,h(x,y), to an input image,g(x,y)is simply the convolution~represented by ‘‘* ’’ ! of the PSFand the incident image

h~x,y!5PSF~x,y!* g~x,y!. ~2.6!

Other commonly used resolution measures are the modtion transfer function~MTF! which is the modulus of theFourier transform of the line spread function~LSF!, definedby

LSF~x!5E2`

1`

PSF~x,y! dy. ~2.7!

As detailed in Sec. IV, detector noise, sensitivity, aresolution are related such that degradation of the resoluusually reduces the detector sensitivity. Phosphor screentimization inevitably involves complex compromises btween resolution and efficiency.46,47,58,74,75Since the x-rayphosphor is frequently the resolution-limiting element of t

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detector, optimization of a detector involves full charactization of the phosphor screen resolution, i.e., measuremof the PSF out to at least several decades of intensity dfrom the peak response. Commonly used single-valued cacterizations, such as the full width at half maximu~FWHM! of the PSF or the spatial frequency at which tMTF55%, are insufficient.

This may be illustrated with an example of two screeof settled phosphor powder. The two screens differ onlythat the void volume between the phosphor particles inscreen is filled with an index matching oil. In the absencethe oil, the very high index of refraction of the phosphparticles causes the path of the luminescent photons to bsharply as the photons emerge from the phosphor intoadjacent void between the particles. The result is thatscreen is strongly scattering and light photons executnearly diffusive, random walk in the screen. By contrast,oil filled screen is more transparent. The oil filled screena sharper short-range PSF, as measured by the FWHM.is a simple consequence of the fact that photons in a trparent screen have a mean free path that is long comparthe screen thickness, so the short-range PSF has a wcomparable to the screen thickness. However, the long-raPSF of the transparent screen is wider than for the diffusscreen; in the transparent screen, photons directed nearlyallel to the surfaces travel a long distance before being stered. In the diffusive screen the photons are constantly bredirected with a step size comparable to the phosphor gwidth and so to diffuse a distancex must travel a path lengththat scales approximately asx2. If the phosphor is not totallytransparent to its own luminescence, as is typically the cabsorption limits the long-distance diffusion of [email protected]~B!#.47 Thus, a properly settled powder screen often hasurprisingly good PSF, which accounts for its popular~Fig. 7!.

The overall light collection efficiency of a properlsettled powder screen also often exceeds that of a screena transparent phosphor. The reason, again, has to do witdiffusive propagation of light in a settled screen: By Snelaw, light cannot enter the screen substrate if it is incidentoo glancing an angle to the surface. In transparent phoslayers, this reflected light can travel a long distance andlost. In a diffusive screen, however, the nearly random-wtrajectory of the light means that the photons have a vhigh probability of returning to the substrate at a differeangle and within a short distance from the initial pointrefection. Thus, these photons have multiple opportunitieenter the substrate.

The efficiency of a phosphor screen can be enhanceovercoating it with a reflective surface, such as an evaporlayer of aluminum or a thin glued-on layer of aluminizemylar, or depositing it directly onto the aluminized side omylar film.47,58This directs light that would otherwise be loback into the phosphor. However, reflective layers also casome loss of resolution, as the reflected light has a lonpath length in the phosphor before hitting the substrateorder to be effective, the reflective screen must be a greflector, a situation which does not always apply torough surface of a settled screen.

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Long-range tails on the PSF have important conquences on the detector sensitivity for certain kinds of msurements, e.g., the measurement of resolution limited ssuperimposed on a uniform x-ray background.

Measurement of the resolution of a phosphor scretypically involves the imposition of an x-ray shadow maskcreate a known incident x-ray image, the luminescent imof which is then recorded and analyzed to determinetransfer function.76 A mask of an x-ray opaque material containing a thin slit or a set of fine holes may be used,conjunction with Eq.~2.5! or ~2.7! to determine the shortrange PSF. The analysis is most straightforward if the wiof the slit or the diameter of the holes is small comparedthe FWHM of the PSF. Otherwise it is necessary to decvolute the hole via Eq.~2.6!, a procedure that is difficult andsusceptible to error if the holes are much larger than the PDeconvolution procedures also require very high quality dand may be limited by Poisson noise in the signal, whigiven the small size of the mask holes, necessitates veryx-ray exposures. Parallax artifacts may be eliminated byof a small x-ray source at a distance and minimizingdistance between the mask and the phosphor. Since x

FIG. 6. Luminescent photons follow ballistic trajectories in clear, sincrystal phosphor screens~A!, but take an almost random walk in screeconsisting of settled high index of refraction phosphor powders~B!. Theconsequences are discussed in the text.@From Gruneret al. ~Ref. 47!.#

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are penetrating, the masks are preferably of a dense m~e.g., lead, gold, or tungsten! and the holes must be fabrcated without a taper to the walls.

As an example, the mask we have used to determinePSF of CCD detectors is a 10 cm310 cm, 50-mm-thicktungsten sheet with an array of lithographically fabrica75-mm-diam holes on a 0.5-mm-square lattice.76 In order tolimit parallax, the x-ray source was a microfocus~0.2 mmspot size! x-ray tube at the end of a 1 mevacuated flight tubeand the mask was placed within a few mm of the phosphThe Gd2O2S:Tb phosphor screens being analyzed weresigned for good stopping power in the 8–15 keV ranwhich translated to areal densities of about 10 mgphosphor/cm2. These screens were settled at roughly 4bulk crystal density, corresponding to a thickness of aboumm. This sets a lower limit on the short-range PSF and sgested that a mask of 25-mm-diam holes would require nodeconvolution. On the other hand, standard lithographic frication of precise holes in metals is most readily accoplished if the holes are at least as wide as the thickness ometal. But 25-mm-thick sheet tungsten is not only fragile,is slightly transparent to the high energy Bremsstrahlux-ray background from the x-ray tube. The compromise wto use 75mm holes in 50-mm-thick tungsten material, whichnecessitated some slight deconvolution. In addition, the xtube was operated at a relatively low dc voltage of 10 kwhich was enough to excite theKa line of the copper anodebut limited the production of higher energy Bremsstrahlunrays.

Whereas the diffusive nature of light propagation domnates the light transmission through settled powder screthe resolution of luminsescent screens made of single~or afew! crystals of scintillator is dominated by long-range sctering @Fig. 6~A!#. Arndt pointed out many years ago77 that

FIG. 7. A trace through a Laue diffraction pattern recorded with a setphosphor screen fiber-optically coupled to a CCD with unity magnificatshows the excellent resolution possible with a settled screen. A 20mmdiameter glass capillary was used to create a micro x-ray beam incupon a gallium arsenide/gallium aluminum arsenide multiplayer. TFWHM values of the peaks are on the order of a single 27mm CCD pixel.Reproduced from Eikenberryet al. ~Ref. 164!.

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lens coupling of a luminescent image through a transpasingle crystal screen yields the highest short range PSingle crystal screens have recently been put to excellentfor microtomography applications,37 in which a thinY3Al5O12:Ce crystal was lens coupled to a CCD cameraachieve 0.8mm ~FWHM! resolution.

Microfabricated screens utilize specifically microstrutured luminescent materials in order to simultaneouslyhance stopping power and spatial resolution, typicallyfabricating the screen out of long, thin, ideally optically islated columns of luminescent material. An approach thatbeen successfully used in x-ray image intensifier tubes igrow CsI phosphor crystals in needlelike crystal habit.38–41

The long phosphor needles grown perpendicularly tosubstrate act similarly to optical fibers and guide the lightthe substrate. More recently, textured substrates madetched glass fiber-optic plates have been used to growtinct columns of phosphor via evaporation78,79 ~Fig. 8!. An-other approach is to fabricate screens of an array of scilating optical fibers. Scintillating fiber optics were firssuggested by Reynolds80 many years ago for high energphysics applications. More recently, scintillating fiber-opglass plates have been developed.81–87As will be discussed

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FIG. 8. CsI~Na! phosphor grown in columns on an etched fiber-optic pla~top panel!. A micrograph of the plate before phosphor deposition, but afiber matrix etching, is shown in the middle panel and after phosphor desition in the bottom panel~Ref. 78!.

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in the section on fiber optics, the technology of high sparesolution fused fiber-optic glass plates is sophisticated.comparison, relatively little work has been done on fusscintillating glass fiber optics, so there is certainly muroom for improvements in the overall efficiency and resotion of scintillating fiber optics. A final alternative is to fabricate a plate consisting of an array of parallel hollow hoby etching out the cores of fused glass fiber-optic platesby other lithographic methods. These may then be filled wphosphor grains88 or by evaporation or by solution depostion of phosphor.42 Ideally, the inner walls of the plate arcoated with reflective gold to optically isolate the columnalthough, in practice, it is very difficult to provide an aequate coating along the full length of long aspect raholes. Another difficulty with this approach is that the waof the matrix separating the phosphor columns do not lunesce and, therefore, the fill factor of the screen is less100%.

The discussion, above, has been oriented toward lemitting phosphors. However, phosphors can also be usephotocathodes. Indeed, porous, mossy CsI photocathodeder high electrical potentials can have significant electgain89 and have been used in experimental x-ray imaintensifiers.90

2. Semiconductors

The semiconductors out of which CCDs are made malso be used to directly convert x rays to electron-hole pathereby avoiding the many problems associated with phphors and light collection. CCDs made of standard, lowsistivity silicon have active charge-collecting depletiongions that are only a few microns thick, which severelimits the efficiency of direct x-ray conversion. The alterntive is to make deep depletion CCDs out of high resistivsilicon or, even better, higher stopping power semicondtors. Since efficient direct-conversion CCDs generally neto be custom fabricated, it is often as practical to also csider other custom fabricated direct conversion architectusuch as diode arrays and pixel array detectors. An unstanding of the physics of x-ray conversion in semicondtors is required in order to evaluate whether a CCD or alnative semiconductor architecture best meets a given nFor this reason, the sections on semiconductors summathe general physics of x-ray conversion.

Studies in the early part of the 20th century establishthat certain crystals conduct electricity when exposed todiation. Early work was plagued by the lack of well-undestood, well-characterized materials. The situation beganchange in the 1940’s with better understanding and availaity of good quality semiconductor materials~e.g., see Cocheand Bertolini91 for a brief history!. Throughout the middlethird of the 20th century, semiconductor detectors wereveloped primarily as single, or a few, element energy resoing detectors for radiation spectroscopy. The developmensemiconductors for multielement detector arrays had to athe development of lithographic fabrication technology dveloped during the last third of the 20th century. Recentvelopments have also been aided by the expertise and

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nology developed for the semiconductor electronics indusSemiconductor x-ray converters may be expected to incringly supplant phosphors.

Semiconductor converters have significant advantaover phosphors: Semiconductors directly convert the x-energy into electrical charge, which often simplifies the dsign of the detector. The physics of x-ray absorption aelectron-hole production in a semiconductor is generasimpler and, therefore, better understood than light prodtion in phosphors. Compared to phosphor converters, seconductors are also usually much more efficient, resultingmore charge carriers, are more linear and less noisy, andcharges are more rapidly and more efficiently collected. Tdisadvantages of semiconductors that have limited theiras x-ray converters is that large area semiconductor scrare difficult to fabricate, thick detectors are hard to maand the semiconductor of choice, namely silicon, has retively low stopping power.

Excellent descriptions of the physics of semiconducdetectors may be found in Bertolini and Coche91 and Dear-naley and Northrop92 and in course notes posted by Spieler93

The process begins with the photoabsorption or Compscattering of the x ray in the material. Within about a nansecond the emitted electrons cascade through an energyradation process resulting in many electron-hole pairs. Toccurs in a very localized region since the range of lowergy electrons is quite short in most materials, and mayapproximated by

R50.090r20.8E1.3 for E,10 keV,~2.8!

R50.045r20.9E1.7 for E.10 keV,

whereR ~microns! is the thickness of material to reduce thelectron transmission to 1%,E is the electron kinetic energy~keV!, andr is the material density (g cm23).94 For silicon,this translates into a range of about a micron for 10 keVrays, which affords excellent intrinsic spatial resolution. Tmean ionization energy to produce an electron-hole pa«determines the average number of electron-hole pairsduced,Neh, as

Neh5E/«, ~2.9!

whereE may be taken to refer to the x-ray energy. Since«typically is a few eV, many electron-hole pairs are producFurthermore, the fractional variation,s(Neh)/Neh, in thenumber produced is quite small because the energy degrtion process offers few channels which do not leadelectron-hole pairs:

s~Neh!/Neh5s~E!/E5A~F«/E!5A~F/Neh!, ~2.10!

whereF is an empirical factor known as the Fano factor91

For silicon, F'0.1. The fact that the Fano factor is smaultimately accounts for why semiconductor detectors haexcellent energy resolution.

The ionization energyEion is related to the energy bangap of the materialEgap and is approximately given in eV byKlein’s empirical formula95

Eion52.67Egap10.87. ~2.11!

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The electron-hole pairs rapidly recombine unless thare swept apart by the presence of an electric field, suchnormally present in the depletion zone of a reverse-biasemiconductor diode. The behavior of electrons and holethe depletion zone are the primary determinants of the xdetecting characteristics of the semiconductor. The deplezone is either~1! of the surface barrier type, which formsthe junction of a semiconductor and a suitable metal, or~2!of n-p type, which forms at the junction ofn- andp-dopedsemiconductor materials. What follows is a simplified dscription of a p-n detector. More detail on both types ojunctions may be found in Bertolini and Coche.91

Imagine a junction betweenn- and p-doped material.Initially each material is charge neutral, the main differenbetween them being that then-type material effectively hasonly electrons as mobile charge carriers with the charcompensating holes fixed in the lattice, while thep-type ma-terial is the other way around. The mobile carriers mayconsidered to be a kind of gas of charged particles diffusabout fixed counterparts of the opposite electrical sign. Wthe two types of material are brought into contact at rotemperature, the electrons diffuse across the junction intop material and the holes diffuse across into then material. Indoing so, however, charge neutrality in each material is vlated. Thep material now has a surplus of electrons andn material a surplus of holes, thereby creating an elecfield which points fromn to thep material. At equilibrium,the electric field strength is sufficient to prevent further nseparation of holes and electrons. Call the resultant nettential across the junctionV0 ; it is a function of the temperature and the properties of the materials. A consequence ofield is that any mobile charges introduced into the fieldgion are swept, depending on their sign, toward one endanother of the region; hence, the region is depleted ofcharge carriers and is known as the depletion zone. Sincedepletion zone has no free charge carriers, it effectively isinsulating gap. It is readily shown that the width of thdepletion zone is given by

x5F2«V0

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where « is the semiconductor dielectric constant,e is themagnitude of charge of the electron, andNa andNd are thebulk concentrations of acceptor and donor dopants in thep-andn-type materials, respectively.

If the n andp sides of the junction are now connectedthe positive and negative terminals, respectively, of a batthen there is an additional component of electric field acrthe junction that adds to the contact field, displaces the elibrium separation of charge, and expands the width ofdepletion zone. The junction is now reverse biased. Tycally, one material, say thep type, is much more heavilydoped than the other, in which caseNa@Nd , and the im-posed potentialV@Vo . In this case, it can be shown that

x'A2«mnrnV, ~2.13!

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If an x ray stops in the depletion zone the electron hpairs that are created are swept to the respective ends odiode and appear as a current pulse. Since the numbeelectron-hole pairs is ideally proportional to the x-ray enerthe charge of the pulse is proportional to the x-ray energ

In order to be useful as a quantitative x-ray convertthe semiconductor detector should ideally meet many cstraints:

~1! The width of the material between the depletion zoand the incident x rays should be thin, since x rays tare stopped in these regions do not contribute tosignal.

~2! The depletion zone should be thick enough to stop prtically all the x rays. Because of stopping power conserations, high atomic weight semiconductors becomecreasingly attractive at higher x-ray energies. In viewEq. ~2.13!, this suggests that the semiconductor resisity should be high. The imposed voltage is limited bbreakdown considerations.

~3! The rate of loss of the dominant signal carriers~i.e.,electrons in the above example! should be low. Carriersmay be lost through recombination or trapping.

~4! Spontaneous generation of electron-hole pairs shouldlow, since these constitute a dark current in the abseof x rays. Electron-hole pairs may be created thermaor at various defects, especially at the interfaces ofdevice.

~5! The junction capacitance should be as low as possisince, in general, the performance of the charge senspreamplifier connected to the detector degrades withcreasing input capacitance. This argues for a thick detion layer.

~6! The Fano factor of the semiconductor should be asas possible for low noise performance.

~7! The detector should be robust, stable, of adequate setc. Most importantly, good quality semiconductor marial, and the techniques to fabricate it, must be availab

Not surprisingly, the availability of a high quality semconductor is directly linked to its use by the electronicsdustry. Silicon is nearly ideal in all respects except for stoping power considerations~Fig. 9 and Table II!. Theattenuation length of Si rises from 130mm at 10 keV to 1038mm at 20 keV. Germanium and gallium arsenide are both seffective at 20 keV, but large area, high quality, high restivity materials are more difficult to obtain than with siliconAlthough CCDs have been fabricated out of other seconductors,96 practically all commercially available CCDare made of silicon. This situation is different for other sesor architectures, such as diode arrays, in which the dilayer fabrication is simpler than for CCDs. Alternative sesors arrays are discussed at the end of this review.

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FIG. 9. Attenuation lengths are shown for the semicoductors of Table II. These figures were produced wthe on-line calculator at http://www-cxro.lbl.govoptical–constants/atten2.html

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C. Optical coupling methods

Optical couplings are often required to connect elemeof the signal relay of Fig. 1. Optical couplings may be dovia proximity coupling, lenses, and fused fiber optics.

1. Proximity coupling

Proximity coupling simply involves keeping the distanbetween successive optical elements as small as possibleexample, in the large area x-ray intensifiers used for radogy, the phosphor is deposited on the internal~vacuum! sideof the x-ray entrance window and the photocathode isrectly deposited onto the phosphor. CCD detectors mayuse phosphors directly deposited onto the CCD.79,97,98Prox-imity coupling may be efficient, but it limits the use of com

TABLE II. Values of Egap for some semiconductors of interest.

Semiconductor Egap ~eV! at 300 K

C ~diamond! 5.5Si 1.12Ge 0.67

GaAs 1.45Gray Se 1.8

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mercially available modular components and does not ala change of magnification of the coupled image, such apossible with lenses and fused fiber optics.

2. Lens coupling

Lens coupling is simple and convenient, but often unceptably inefficient. The efficiencyC of lens coupling animage, purely from solid angle considerations, is given b

C5@M /$2 f ~11M !%#2, ~2.14!

where the magnificationM is the ratio of the sizes of theimage to the object, andf is the ‘‘f number’’ of the lens, i.e.,the ratio of the focal length to the lens diameter. The eciency given by Eq.~2.15! is listed in Table III for af /1.0

TABLE III. Efficiency of f /1.0 lens and fiber optic tapers at various manifications.

Magnification~image size/object size!

f /1.0 lensefficiency ~%!

Fiber optic taperefficiency ~%!

1 6 750.5 3 200.3 1.3 130.25 1 9

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lens; it is seen that the efficiency falls rapidly if the objehas to be demagnified. Unfortunately, it is frequentlyquired to couple a large area phosphor to smaller intensior CCDs. Moreover, since the number of visible photonray produced in the phosphor is very limited, this couplihas to be done as efficiently as possible.

The situation is very different if the x-ray image canmagnified, in which case lens coupling is often the simpland most effective means of coupling. Magnification of timage can be done efficiently because, in this case, thecan subtend a very large solid angle relative to the lumincent screen. This is, of course, the basis of microscopy.example of an application where magnified lens couplserves well is microtomography, in which the x-ray imamay be only a small fraction of a millimeter across.37,99,100

3. Fiber optics coupling

In demagnifying geometries fused fiber optics are genally more efficient than lenses~Table III!. A fused fiber-opticbundle, henceforth to be simply called fiber optics, consof a coherent bundle of glass-fiber light guides, each fibeing, say, 10mm or so in diameter. The bundle may bheated until the glass softens and is stretched so as toduce a fiber-optic taper to magnify or demagnify an imaModern fiber optics approach the theoretical limit of demanification efficiency imposed by solid angle consideratioand Snell’s law~see Fig. 11 and Coleman54!.

A numerical example will illustrate why fiber optics aroften required. In general, the response of the signal chaiFig. 1 to a detected x ray is given by

NS5NPCPIGPPCISQS , ~2.15!

whereNS is the number of quanta in the image sensor,electrons in the CCD,NP is the net number of visible lighphotons/x-ray emitted from the phosphor towards the gelement, say an intensifier,CPI and CIS are the efficiencieswith which quanta are conveyed from the phosphor tointensifier and from the intensifier to the sensor,GPP is thephoton gain of the intensifier, andQS is the quantum effi-ciency of the sensor to the intensifier output.

Some best case numbers are useful in evaluating~2.15!: efficient x-ray phosphors, such as Gd2O2S:Tb, con-vert about 15% of the x-ray energy to light~see Table I!.Given that many of these photons are emitted in the wrdirection, even with reflective coatings over the phosphone is doing very well if 10% of the energy can be directtoward the intensifier. For an 8 keV CuKa x-ray and lightemission of 2.5 eV photons, this corresponds to aboutlight photons. The efficiency of coupling an image depenon the magnification ratio and whether the coupling is dovia lenses or fused fiber-optic bundles~see Table III!. Front-illuminated CCDs have quantum efficiencies of about 30Finally, as a benchmark we want the integrated signaexceed the CCD noise. The signal-to-noise ratio will depeon the area being integrated and specifics about the CCDfor our example, assume that the detector PSF is extremsharp and single pixel performance is needed. This assution is, in fact, overly stringent for most systems, but serv

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For simplicity, let’s first examine the case of directcoupling a phosphor to a front-illuminated CCD without itervening intensification. The most efficient way of opticacoupling a phosphor to a CCD is to directly deposit it onthe CCD, i.e., a 1:1 area coupling. This corresponds(CPIGPPCIS)51. Hence, the signal per pixel, is 3203130.3596 electrons, comfortably above the 10 electronoise per pixel for a S/N ratio of almost 10.

Next, consider the case of demagnifying the phospimage onto a CCD via a magnification of 0.3 and withoimage intensification. This is not unusual, since CCDs larthan 2 or 3 cm across are difficult to obtain and one ofwishes to have x-ray sensitive areas at least 6–9 cm acReferring to Table III, we see that the efficiency of couplinwith a f /1.0 lens, is only about 1.3%. The average signanow 3203130.01330.351.2 electrons/x ray, yielding S/N'0.1. On the other hand, if a fiber-optic taper is used,coupling efficiency is 13%, which is about seven times bethan lens coupling. The signal is now 3203130.1330.3512.4, or S/N'1, which is much more acceptable.

Lens coupling inefficiencies might be acceptable aftegain element, such as an intensifier, where there are mmore signal quanta, or, as noted in the previous sectwhen the luminescent image must be magnified. Howelenses, more so than fiber optics, limit the ultimate controf small features due to reflections and imperfections atair–lens interface.101 To limit these effects, the lens surfacemust be coated with antireflection films and kept meticlously clean. This is often more difficult than it might seesince volatile oils are ever present in laboratories and haway of forming foggy films on glass surfaces, necessitathermetic enclosures. By contrast, fiber-optic surfacesusually coupled with thin layers of optical coupling gelsepoxy. Although these have to be applied in clean envirments, once applied, they are effectively self-sealing agadust and dirt.

1. Fiber optics characteristics.Fused fiber-optics technology is reviewed by Siegmund.102 Additional useful infor-mation may be found in product guides from Schott FibOptics and Incom fiber optics, two major U.S. fused fiboptics manufacturers.103,104Attention to detail is essential ifiber optics are to perform well, and x-ray detection neepush the state of the art. Key characteristics that have toconsidered include

~a! numerical aperture;~b! fiber size and bundle size;~c! extra mural absorption~EMA!;~d! core to cladding ratio;~e! shears, defects, and gross distortion; and~f! actinide contamination.

We first summarize these characteristics and thenscribe tests to characterize fiber-optics parts in Sec. II C 3 2.

A single fiber in a fused fiber optic bundle consists ocore glass of high index of refractionNC , surrounded by a

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FIG. 10. An optical fiber of index of refraction,NC , and cladding of indexNS , will accept incident light from a material of indexNA according toSnell’s Law:NA sinuA5NC sinuC . Light will propagate down the fiber provided there is total internal reflection at the cladding-core interface.

FIG. 11. The measured light transmitting efficiency of fiber-optic tapcompared to the theoretically expected values for red light versus theratio. A taper ratio of 4 corresponds to a demagnification of a factor oThe difference in transmission between red and blue light is a consequof the wavelength dependence of the indices of refraction in the glass cponents. Reproduced from Coleman~Ref. 54!.

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losses as the incident angle approaches 90°. Still, thispsteradian acceptance cone is why NA51 fiber optics are soefficient relative to lenses. Note that the indices of refractiand hence the efficiency of transmission, are dependent uthe wavelength of the light.54 The fiber transmits red lighmore efficiently~Fig. 11!.

For tapered fibers,

dsmallsinumax,small5dlargesinumax,large, ~2.17!

whered refers to the fiber diameter and ‘‘small’’ and ‘‘largerefer to the respective ends of the fiber. Equation~2.17! de-scribes the divergence of light propagating down a reducfiber. Since the NA will still be governed by Eq.~2.16!, someof the uncollimated light entering the large end of the fibwill exceed the maximum allowed angle and be lost enroto the small end.

Typical fiber sizes range from about 3 to 25mm. Thefiber size directly determines the resolution of the bundHowever, transmission losses rise as the fiber size decretowards a few microns; this is a major consideration limitithe size of fibers on the small end of the fiber-optic taperthe usual process, fibers are drawn and then cut into lenwhich are stacked into a cane of square or hexagonal csection, which is itself then fused, drawn, cut into lengtstacked, and fused into a large boule or bundle. The sizthe boule determines the ultimate size of the fiber-optic pParts greater than 15 cm across are available.

Some light always leaks out of the fibers due to scating and imperfections. Large light losses also are expectereducing tapers. Stray light can lead to ‘‘cross-talk’’ betwefibers unless it is removed. For this reason, fiber opticsavailable with interspersed black absorbing glass, caEMA, which serves to absorb stray light. EMA can consisttiny fibers stacked into the interstices of the regular fibersblack cladding around the fibers, or a sparse substitutionoccasional black fibers for transmitting fibers. Fiber optwithout effective EMA is of very little use in high-resolutiox-ray detectors.

The core to cladding ratio is the ratio of the crossectional areas of the core and cladding. Since light incidupon the cladding of a fiber-optic plate is not propagated~it,in fact, contributes to stray light!, the core to cladding ratiosets an upper limit on the fraction of incident light that wbe transmitted. The natural mismatch between core and cding of two butted fiber optic plates accounts for most of tinefficiency of fiber optic to fiber-optic couplings.

The fusing and drawing involved in producing fibeoptic boules and parts introduces distortions and defectsbers at cane edges get flattened and distorted, leadingvisible hexagonal or square mesh known as ‘‘chicken wirThis comes from the reduced transmission of the flattefibers. Fibers also break and small inclusions of gas orlead to point blemishes where light transmission is reduor blocked. Plastic distortion when the canes are fused leto discontinuous breaks, called shears, between the caOther plastic distortions when, for example, tapersdrawn, cause gross geometrical distortions of an image trmitted through the fiber-optic part. Careful specificationlimits to all these defects and distortions are needed for g

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Actinide contamination refers to the inclusion of radiactive elements in the glass formulation. Most fiber-opglass formulations contain rare earths and most rare eoxide feed stocks are naturally contaminated with actinidsuch as thorium.105,106Another source of actinide contamination is the polishing of parts with actinide-contaminatrouges, such as cerium rouges. Fiber optics in x-ray detecare usually butted against CCDs or overlaid with phosphboth of which are very sensitive to excitation by radioactemissions stemming from decay of the actinides and tdaughters. This results in bright spots, or so-cal‘‘zingers,’’ which accumulate in the detector images in prportion to the length of the exposure. The level of contamnation may be very small in absolute terms (,1 ppm), butthis still results in a significant cumulative signal in lonintegrations. X rays resulting from stopped radioactivitythe bulk of the fiber optics may also excite the phosphorthe CCD. The end result is that zingers are typically distruted in intensity from very bright to vanishingly dim and cabe numerous. Some ‘‘zinger’’ removal is possible via digimanipulation of the x-ray image~see Sec. IV F!, but this istroublesome. The best solution, of course, is to avoid theof actinide contaminated fiber optics. In our experienthere is significant batch-to-batch variation in the levelactinide contamination in fiber optics from most vendoThis is really inexcusable, since the cause of the problemunderstood. The best advice is to set a specification onpart and test the result to see if it is acceptable. Hopefufiber-optic vendors will eventually respond to customer dmands for actinide-free parts.

One could potentially identify the sources of actinicontamination by examining decay spectra using varienergy-resolving detectors. These detectors have very dient responses to alpha, beta, and gamma decays, and italways possible to relate these specific activities to the plem that they will introduce in a detector. A simpler measument that has direct bearing on detector performance is mby placing the fiber optic in direct contact with a phosphon transparent mylar optically greased to the surface ofin. photomultiplier tube. A very radioactive sample woustill only result in a count rate of say 1 Bq for a 2-in.-diaactive area. More typically, one observes count rates frseveral per minute to several per hour. With these corates, we have found it practical to simply record tube crent on a chart recorder and manually count events ovspecified period of time.

2. Characterization of fiber optics.Several simple testscan be performed to assess the quality of a particular fioptic before the unit is affixed to the CCD in the detectGross defects are easily seen by casual inspection. Msubtle performance characteristics should be examined qtitatively.

Inspection of a large fiber optic for shear dislocatiocan be facilitated with the use of a pair of crossed Ronrulings.101 The parallel lines in the rulings will normally cre

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ate a regular Moire´ pattern. When a fiber-optic part is placebetween the rulings and one of the rulings is rotatedspread the Moire´ fringes to infinity, distortions become quitevident. Even small shears will be shown as discontinuiin the Moirepattern. Note that for fiber-optic tapers, the twrulings should have spacings in the same ratio as the tapprovide best results.

Light transmission will depend on physical parameteof the glass, such as the core/clad ratio, numerical apertand the taper ratio, as well as the particular fusing conditiused in making the fiber-optic bundle. Transmission will adepend on the angular distribution of the incident lightwell as its wavelength. Light from a phosphor emanates wa Lambertian distribution. We have found good comparisobetween various fiber-optic samples can be made usinstandard light box with fluorescent tubes and a diffusplate, such as opal glass. A photodiode is used to measurlight/unit area at the surface of the light box, and then agat the end of the fiber optic. Adjustment is made for tdemagnification ratio by multiplying by a factor of 1/M2.Transmission is measured at several positions as transsion is usually expected to vary from center to edge. Oshould consider the changes in magnification that often ocfrom center to edge in this measurement as well. It shouldnoted that this measurement is affected by the quality ofEMA absorbers. For example, a fiber optic with no EMA wtransmit much more light, but much of this light is scatterincoherently and only adds to the background noise andduction in resolution.

Since a fiber optic is designed to transmit light alongfiber, as opposed to between fibers, a rough qualitative teresolution ~and the effectiveness of the EMA fibers! is toshine a strong light into the side of the fiber optic and lofor light coming out the ends. Poor resolution bundles weasily show light up to 1 cm or more from the edge.

More quantitative testing can be made by shadowinflood illumination with a knife edge mask and measuring tlight intensity scattered into the region shadowed byknife. Detector parameters are sensitive to light scatterethe 1024 level or lower and as such, this test should be ato measure over this dynamic range. Measuring directlylight level in the unshadowed portion concurrently with tscattered signal is difficult. One can, however, attenuateunshadowed portion at the fiber-optic output with a neudensity filter of a given value~see Fig. 12!. Overlap of theneutral density filter onto the output of shadowed regioninevitable, but it becomes desirable in this case, sincecan adjust the overlap region such that the intensity offlood region can match the intensity just beyond the overregion quite accurately with visual comparison alone. Othen measures the distance the filter extends into the sowed region. Measurements are usually taken for filters wneutral density values of 1.0 to 4.0.

The spread of light in the fiber optic depends criticaon the angular distribution of the incident light, as well aswavelength. Typically, light impinging at normal incidencspreads very little. We have also observed much more lspreading in the red wavelengths as compared to the blugreen, especially for some types of EMA. These fact

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should obviously be considered when making tests fogiven detector. Semiquantitative comparisons between fioptics can be made if one uses a consistent light sourcethin fiber-optic blanks, we have found it convenient to mameasurements on a microscope using the microscopesdensing illuminator and 1003magnification with eyepiecereticle. The knife edge consists of a piece of black vinelectrical tape107 affixed to one side. For large fiber-opttapers, measurements can be made on a light box with mof the area masked off. Again, the edge is formed with blatape, and the source is the opal glass diffuser of the light bagainst which the face of the taper is placed. Measuremcan be made with a hand held magnifier with a reticle.

3. Coupling to fiber optics.Reliable, mechanicallystable couplings between fiber optics and the CCD are nessary for a robust, stable detector system. The simpleslution, butting the fiber optic next to the CCD, suffers fromlower coupling efficiency and reduced resolution owingthe larger mismatch in the index of refraction betweenfiber optic, intervening air, and the CCD.

Optical epoxies and clear silicone rubbers are possibties for coupling, but these bonds must be able to withststresses of repeated thermal cycling between room tempture and a typical operation point of220 to 260 °C or be-low. Not only must the bonding material be designedthese low temperatures, but the fiber-optic glass, the Cand its mounting carrier should be matched as closelypossible in their thermal coefficients of expansion. We hahad good success with two-part silicone coupling gels108

Needless to say, CCDs with small areas are much easibond reliably than larger ones since the stress from edgedge is much less for the same temperature change.

Reliable joints require uniform, clean surfaces. Sogenerations of CCD chips have been notorious for their l

FIG. 12. Measurement of light spread in fiber optics. Here a fiber optic wa knife edge mask is illuminated from below using a source with diverglight such as an opal glass diffuser. A neutral density filter~O.D. 1.0 to 4.0!covering the illuminated portion of the fiber optic compresses the dynarange required in the measurement system. The filter also provides abrated reference for the scattered light intensity in the unilluminated porof the fiber optic. One can visually adjust the overlap of the filter (Dx) untilthe scattered intensity matches that transmitted through the filter. Meaments can then be repeated for various filter densities.

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of flatness (.20mm). Good couplings in this case requirthe back end of the taper be ground and polished to mathis shape.

An alternative to using a hard bond is to use a fluid filsuch as an optical coupling oil~e.g., ‘‘laser liquid,’’ CargilleLaboratories, Inc., Cedar Grove, NJ 07009!. Again, this ma-terial should remain fluid at the operation temperature. Aditionally, a reliable means for containing the fluid long teris necessary, realizing the volume of the oil changes qsubstantially between room temperature and the operatemperature. Additionally, an external mechanical couplingrequired to keep the taper and CCD aligned. Movementsless than a pixel can be quite evident. In addition, if the gbetween taper and CCD changes with time, the patterninterference fringes between the two surfaces can chanecessitating frequent recalibrations for detector sensitivAdvantages of this technique are that it allows the usefiber optic glass which has a larger thermal mismatch toCCD and the ability to bond larger area CCDs.

Bonding techniques for one CCD/fiber optic combintion might not be appropriate for other systems. For examback-illuminated CCDs are typically coated with an antirflection ~AR! coating. A bond then has the potential to fabetween the AR coating and the CCD when the thermstress is applied.

D. Image intensifiers

1. When is intensification needed?

Image intensifiers are used to introduce gain betweenx-ray converter and successive parts of the detector. Inintensifier, light falls on a photocathode that emits photoeltrons. The electrons are accelerated across a large volsay 10 keV, and impact, typically, into a phosphor. Sineach electron now has 10 keV of energy, it results inemission of many optical photons from the phosphor, iproduces optical gain.

Although intensifiers are expensive and delicate athus, to be avoided whenever possible, there are situatwhere they are invaluable. A simple example, along the liof Eq. ~2.16! and the discussion of Sec. II C 3, shows thintensification may be needed when it is necessary to eciently couple a large area phosphor to a much smaller aoptical sensor. It was desired to couple a phosphor on amountable fiber optic plate to a CCD that had a permanemounted 0.5 cm fiber optic faceplate.109 A particularly lowdistortion 5:1 fiber-optic taper was available. It had a mesured efficiency of 2.5% to green light. The phosphor hameasured output from the fiber-optics faceplate of 200 ptons for each 5.9 keV x ray. The 70% efficiency at eachthe two fiber-optics interfaces~phosphor faceplate to tapeand taper to CCD faceplate!, resulted in a net of 2.5photons/x ray incident on the CCD. Folding in a 30% CCquantum yielded an average of 0.8 photons/x ray, which wunacceptably low.

An intensifier coupled to the phosphor faceplate wused to improve the statistics. It was a custom fabricaproximity focused tube with input and output fiber optic widows and a 10% photocathode efficiency and an opt

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gain of 39. Thus each x ray produced 20030.730.1514 photoelectrons in the intensifier and 20030.733930.730.02530.730.3520 electrons in the CCD, welabove the 11 electrons read noise of the CCD. Note, incidtally, that the alternative approach of putting a smaller, lexpensive intensifierafter the taper would have resulted ithe same average signal per x ray but a much noisier detebecause there would be a spot along the signal chain, naat the intensifier photocathode, where the number of quawould have been too small. Specifically, the average numof photoelectrons/x ray in the intensifier would have be20030.730.02530.730.150.2. It is necessary to maksure that the number of quanta everywhere along the sichain is sufficiently large such that signal fluctuations doresult in lost ‘‘counts.’’

Our example shows that intensification may be bencial if the detector has a large demagnification ratio. Otsituations in which intensification is useful follow from consideration of the assumptions of the example. A noisy potion sensor at the end of the signal chain~e.g., a video-rateCCD camera! can be compensated by adequate gain bethe sensor. Finally, intensification is useful if the requirsignal from each x ray should be much larger than the noThis is the case, for example, for astronomical x-ray imagin which the x-ray flux is extremely small and the detectiof every x ray is important. So intensification should be cosidered in situations involving large demagnification ratinoisy position sensors, or the need for very high signalnoise ratios.

2. Types of intensifiers

Overviews of image intensifiers are given by Rose110

and by Johnson and Owen.111Additional information may beobtained from the major manufacturers of image intensifiewhich includes most of the large companies that make ptomultiplier tubes.112 Because the photoelectrons emittedthe photocathode are emitted in random directions, somethod must be used to focus the electron image ontointensifier output screen. Four categories of intensifiers mbe distinguished by the focusing method employed: theage can be proximity, electrostatically, or magneticallycused, or a microchannel plate may be used. The releprinciples of operation as applied to CCD x-ray detectorsgiven in the next few sections.

1. Proximity focused intensifiers.Proximity focusingsimply means that the output screen is kept sufficiently clto the photocathode that the electrons have little opportuto drift parallel to the photocathode surface before impactthe output screen. The primary disadvantage of this metis that limited accelerating potentials can be sustained othe short distance needed to maintain a high-resolutionage, thereby limiting the overall gain which is achieveResolution can be traded for gain by lengthening the acerating gap. An advantage of proximity focusing is thatintroduces no geometrical distortion into the image. Proxity focused tubes are available with clear glass or fused fiboptic input windows, the latter being necessary if an inpphosphor image is to be efficiently fiber optically coupled

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the photocathode, as is usually desired for sensitive x-detection. Optical gains of about 50 are readily obtained.is the case with all types of intensifiers, cascaded stagesbe used to achieve higher overall gains. An example oCCD detector based on a proximity tube custom built forapplication is an intensifier/fiber optic/CCD detector dscribed by Tateet al.109

2. Electrostatically focused intensifiers.Electrostaticallylensed tubes use electrostatic lens elements to focus theage. These tubes are also known as inverter tubes beceach lens element inverts the image. Electrostatic focusinthe only practical method of making reducing intensifiewith very large input areas and is most commonly usedmedical radiological imaging. Medical image tubes, mofied for use with softer x rays by the installation of berylliuwindows and thinner photocathodes, are the basis for a nber of detectors.113,114 Electrostatic lenses usually requircurved photocathodes for good focusing. For small diametubes this is usually done by making the input window outfused fiber optics with a sculpted, curved photocathodeface. The disadvantages of electrostatic tubes include gmetric image distortion~especially of the pin-cushion type!,the need for a curved input window in large area tubes,sensitivity to stray magnetic fields.

3. Magnetically focused intensifiers.In a magneticallyfocused intensifier, an accelerating potential is imposedtween parallel photocathode and output screens. Simuneously, a uniform magnetic fieldB along the acceleratingdirection causes the photoelectrons to execute helical pfrom the photocathode to the output screen. The angularquency of rotation of all the photoelectrons is the cyclotrfrequencyeB/mec, which depends only on the magnetfield and fundamental constants~i.e., the massme andchargee of the electron and the speed of lightc!. Therefore,after each orbital period the electrons refocus back tooriginal photocathode image. Since the speed profile ofelectrons along the electric field direction is essentiallysame for all photoelectrons, there exist a succession of plain space, separated temporally by the cyclotron periwhere the photoimage is refocused. It is only necessaradjust the accelerating potential and the magnetic fieldthat one of these planes is coincident with the output scre

Magnetically focused intensifiers are capable of vehigh resolution and, since the photocathode and ouscreens are well separated, can sustain high acceleratintentials of 10 kV or so. Four stage intensifiers with opticgains of several million are feasible. The disadvantagesmagnetically focused tubes are cost, susceptibility to stmagnetic fields, and some slight geometrics distortion dueto nonaxial components of magnetic field. The size ofinput area is also limited by the need for a uniform magnefield.

An example of a CCD detector based on a magneticfocused intensifier is the intensifier/lens/CCD detectorTateet al.109

4. Microchannel plate intensifiers.Microchannel plateintensifiers maintain the image by confining the electrpaths to parallel microchannels in a perforated glass plafew millimeters thick.115 The plate is made of a glass with

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resistivity of some gigaohms per millimeter so that a pottial impressed upon the two faces of the plate divides uformly along the length of the channels. Photoelectronsproximity focused onto the microchannel plate and execaccelerating, arcing collision paths into the walls of the chnels. Each impact produces secondary electrons, whichcelerate, collide, and produce more secondaries, etc. Thsult is high electron multiplication. The electron showemerging from the microchannel plate is then proximitycused onto the output screen. Gains of several millionreadily achieved in multistage tubes, which are usually ofzig–zag or chevron arrangement. Microchannel plate intsifiers are very compact, have practically no geometric dtortion, are relatively robust, and comparatively insensitto stray magnetic fields. In consequence, they have displathe other forms of intensification in many imaging applictions. Microchannel plate intensifiers are used, for examin most modern night vision goggles.

Unfortunately, microchannel plates do not match tquantitative performance of other types of image intensifieThe secondary multiplication process is very noisy; at retively low potential, the histogram of the multiplication factors for single electron events, the so-called pulse heighttribution, is exponentially distributed. This may be remedby raising the potential across the microchannel plate somost events saturate the channel, that is to say, effectidischarges the channel. This puts a well-defined peak inpulse height distribution, corresponding to a well-definelectron multiplication factor, but at a price: the channeldead until it can be recharged, which, due to the high retivity of the channel, typically takes several millisecondThe result is a local count rate limitation on the intensifiMicrochannel plates also have stability problems. The gof a channel decreases over its lifetime. The change of gmay be as large as a factor of 2 and is a function of the tcharge that has gone down that particular channel. As asequence, microchannel plates do not maintain a long-tgain calibration under use.

Rodrickset al.116 describe a multimodule CCD detectoincorporating microchannel plate intensifiers.

3. Intensifier output screens

The output screen of an intensifier need not be a phphor. The electron image can directly bombard a positsensor, such as a CCD.117 The disadvantages of not convering the image to an intermediate light image is a lossmodularity and, most importantly, the danger of electrondiation damage to the CCD. The advantages include elnation of intervening phosphors and optical couplings avery high gain with a single stage of intensification. In tsilicon intensified target~SIT! vidicon tube a single stagintensifier directly bombards a vidicon target, which is aequately radiation hard for long life. Phosphor-SIT vidicohave been the basis for single-stage x-ray detectors.118

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4. Intensifiers: General considerations and futuredevices

Many intensifiers contain phosphors on fiber-optic paOptical distortions and blemishes in the fiber optics a‘‘zingers’’ due to actinide excitation of phosphors by the fiboptics~see Sec. II C 3 1! can compromise the intensifier. Another consideration is that intensifiers typically have wdows whose internal surfaces are at potentials of manyThere is always some leakage through the window, leadto charging of the external surface. This can be fatalCCDs, which are extremely sensitive to electrostatic dcharge. For this reason, all windows supporting high voltain the vicinity of a CCD should have an external grouplane, for example, by a grounded, conductive, transpacoating of indium tin oxide. Note that this may place specrequirements on the voltage competency of the optical wdows of the intensifier. Fiber-optic windows, in particulahave to be rated for high voltage, lest they break down otime and develop luminescent fibers.

In general, image intensifiers are expensive and sufrom a number of other problems. Photocathodes share cmon features with photomultipliers in that they have loquantum efficiencies, are easily damaged by exposure totense light, and exhibit fatigue. The deposition of low dacurrent photocathodes is an art form. The most efficient,therefore, popular, image tube phosphors exhibit persistewhich may necessitate custom phosphor screens and cpromised gains. Field emission, which causes persisbright spots, can be a problem. Intensifiers are vacuum tuand, therefore, are mechanically fragile. Internal partssuspended in vacuum and may be susceptible to vibratespecially since high potential parts usually have tight toances on their mutual separation. Over time, image tutend to accumulate residual gas~e.g., He!, which causesbright scintillations known as ion spots. Intensifiers requvery stable high voltage power supplies for stable gains.

It must be noted that the introduction of low noise CCDhas considerably decreased the demand for image inteners, with the result that there are fewer manufacturers tothan a few decades ago. The manufacture of high quaimage intensifiers demands a considerable infrastructureis well beyond the capabilities of small laboratory groupUltimately, the limited availability of image intensifiers is thmost severe constraint to their continued use.

A device that may someday overcome many limitatioof vacuum tube intensifiers is a two-dimensional variantthe solid-state photomultiplier. Solid-state multipliers amonolithic, thickly depleted reverse-biased semiconducdiodes which have sufficiently high internal fields to effecarrier avalanche and multiplication. Small, nonimagivariants are now commercially available as replacementsphotomultiplier tubes. High spatial resolution twodimensional arrays of such devices are, in principle, possi

E. Position sensitive arrays

The last element in the signal chain of Fig. 1 is tsensor which position encodes the signal. The most imptant position sensors for x-ray detectors include

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complementary metal-oxide semiconductor~CMOS! im-agers.

1. Vacuum tube TV cameras

Although vacuum tube TV cameras, most notably vicons, have been almost completely supplanted by solid-ssensors, vidicon-based x-ray detectors were important fordevelopment of area x-ray detector technology. The esseaspects of modern CCD detector technology—phosphorstensifiers, fiber optics, coupling procedures—were firstveloped for vidicon-based sensors; as a consequence, knedge of this literature is important. A personal review hbeen given by Gruner.119

The remaining parts of this section will focus on solistate sensors, since these are the devices which will bein practically all cases in the future.

2. CCDs

Charge-coupled devices~CCDs! have revolutionized im-aging technology. Hall120 and Janesick and Elliott52 provideexceptionally lucid and recommended reviews of CCDEarly books on CCDs include Sequin and Tompsett121 andHowes and Morgan.122 Principles of operation are summarized by Tredwell123 and Holst.124 Volume 26, Nos. 8–10 ofOptical Engineering~1987! contains collections of articleon CCDs. The Society of Photo-Optical Instrumentation Egineers ~SPIE! publishes excellent books and video tacourses on the use of CCDs.125 Practical books on the assembly of CCD cameras have been written for the amateurtronomy community.126,127Comments here will be confineto an overview and details which especially pertain to theof CCDs in x-ray detectors.

A modern CCD typically consists of a silicon chimonolithically fabricated into distinct columns by implantepotential barriers called channel stops@Fig. 13~a!#. The sur-face of the silicon is overlaid with an insulating silicon doxide or silicon nitride layer and this, in turn, is overlaid wirows of conducting electrodes, called clocking gates, runnperpendicular to the columns. It is primarily the externvoltages impressed upon the gates that define the separof the pixels down the columns. Photocharges generatethe depletion region buried in the chip accumulate inpotential wells@Fig. 13~b!#. By clocking these potentials inperiodic way @Fig. 13~c!#, the accumulated charge packeare systematically shifted down the length of the columwhile still maintaining the separation of adjacent packetsthe column. Thus, the CCD consists of a series of paraanalog charge shift registers organized into columns.columns terminate in an analog output shift register perpdicular to the columns, which, itself terminates in the inpstructure of an on-chip preamplifier~13d!. Because the capacitance presented to the preamp is very small—arisfrom a single pixel of the final shift register—CCDs are cpable of noise figures of astonishingly few electrons.

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In normal operation, the gates are clocked to simuneously shift all the packets of charge down the column opixel toward the output shift register. The pixels of charthat were immediately adjacent to the output shift registerclocked into the buckets of the output shift register. The oput shift register is then clocked to sequentially shift tcharge packets one at a time onto the input structure ofon-chip preamp. Once all pixels in the horizontal shift regter have been so processed, the columns are again shdown to load the output shift register with the next rowpixel charges, and so on, until all the pixels in the CCD habeen clocked into the preamp. The fact that the pixels havbe processed one at a time onto the preamp sets a sbottleneck that limits the rate at which the chip can be reout. Some CCDs are segmented into halves or quadraeach with its own preamp, to improve the readout time.though pixels can be clocked out of CCDs at rates of tenMHz, the noise rises with the readout rate. For most CCoptimum noise performance occurs for cooled, slow-scrates below 50–500 kHz, depending on the CCD andanalog to digital conversion electronics. At slow-scan ratmodern scientific CCDs readily achieve noise figures ofelectrons rms/pixel. Noise figures of 5 electrons rms areuncommon.

Cooling the CCD is generally needed to reduce the dcurrent of thermally generated electron-hole pairs. Dark crents on the order of 10e2/pix/s are routine at240 °C, withmuch of the dark current coming from the Si–SiO2 interface.This may drop by an order of magnitude or more by the uof multipinned-phase~MPP! CCDs, in which dopants im-planted under certain of the gates allow biasing of the gaso as to drastically reduce Si–SiO2 dark current. The use oMPP chips is becoming the norm in x-ray detectors.

Modern CCDs are buried channel devices. The te‘‘buried channel’’ refers to an arrangement of the potentwells so the charges collect in the bulk of the silicon, wbelow the insulating overlayer. Buried channel operatturned out to be pivotal toward realizing high performanCCDs, because it keeps the charges away from the inevitinterface traps at the semiconductor/insulator junction.keeping the charges buried below this interface, chargetime could be made very long~hours! and the charges couldbe efficiently shuttled from one end of the CCD to the othwith little loss.

The potential well which confines the charges to a piis itself a function of the accumulated charge and onlymany charges will fit into a well before it spills over, oblooms, into the next pixel. Most CCDs used in x-ray detetor work have full-well capacities in the range of a few hudred thousand electrons and some have capacities of alhalf a million. In general, the larger the full-well the bettfor x-ray detector work. Some CCDs have antibloomistructures built in, but this is not the norm for scientifiCCDs. Janesick and Elliott52 describe how MPP CCDs cabe clocked in a clever way during integration so as to elimnate blooming. Unfortunately, few CCD controllers take avantage of this capability.

Fill factor refers to the fraction of the imaging areathe CCD that is photoactive. Serially readout CCDs w

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FIG. 13. ~a! A CCD has columns defined by implanted channel stopsrows defined by overlaid electrodes. The crossings of the columns anrows of electrodes define the pixels. The columns terminate in a ouanalog shift register.~b! Charges generated by light accumulate in the ptential wells beneath each pixel.~c! The charges may be ‘‘walked’’ down thecolumns by an appropriate sequence of voltages clocked onto the electrIn a hydraulic analogy, the charges pass in bucket-brigade fashion dowcolumn. The charges are transferred onto the output shift register and shonto the output gate of an on-chip preamp. Again, in a hydraulic analthe outputo the preamp is proportional to the charge shifted onto the ougate.~a! is from ~Ref. 207!; and ~b!–~d! are from~Ref. 208!.

100% fill factor are most commonly used for x-ray detecwork. These CCDs have the disadvantage that the imagstructure is also the readout structure, so image signal warrives while the CCD is being read out winds up smeaover the shifting image already accumulated. Thus, it is ually necessary to stop the x-ray signal during read othereby imposing a duty cycle. For slow-scan operationsynchrotron sources, the time spent reading out the Cmay exceed the exposure time. Alternatively, interlineframe-transfer CCDs may be used. Interline transfer CChave a photoinsensitive shift register next to each columThe image to be read out is very rapidly~microseconds!transferred to the adjacent shift registers and the photopiare then free to continue image accumulation while the sregisters are being clocked out. Interline transfer CCDs tycally have less than unity fill factors due to the area takenby the shift registers. In frame-transfer CCDs half of tlength of the columns are covered and can be clocked inpendently of the uncovered half. These CCDs take advanof the fact that the columns can be clocked down vequickly ~microseconds! without compromising performancei.e., the bottleneck starts at the preamp. So the image amulated in the uncovered half of the columns is very rapiclocked into the covered half, i.e., the frame is transferrThen the transferred frame is slowly read out while the ucovered half integrates the next exposure. Although thecovered half has 100% fill factor, the CCD must be twicelarge to accommodate the storage frame.

CCDs are available in either front- or back-side illumnated versions. In front-side illuminated chips, the ligpasses through the polysilicon gates into the photosensbulk of the chip. Silicon is not very transparent toward tblue, which limits the blue sensitivity, typically to,30% at550 nm and,10% at 450 nm. Alternatively, the CCD substrate may be thinned to about 10mm and illuminated fromthe back, thereby limiting the path length of light througphotoinsensitive absorbing material before entering the ptoactive depleted region. In combination with antireflecticoatings, this can improve the spectral response belownm by factors of 2–3. The process of thinning a large achip to 10mm entails extra processing and inevitable losschips and, consequently, back-side-illuminated devicesvery expensive. Moreover, since the demand for back-silluminated CCDs is low, there is limited availability of thesCCDs. Recently, Kodak has introduced a series of CCDwhich the electrodes consist of transparent indium tin oxithereby enhancing blue sensitivity.53

CCDs are directly responsive to x rays, as well aslight. Their use as direct conversion x-ray detectors is limiboth by stopping power and radiation damage concerns.depletion regions of most commercially available CCDsonly a few microns thick, thereby presenting a small actcross section for hard x rays. The radiation damage probis also a concern, since the cost of large CCDs is so hAlthough the mechanisms of radiation damage are compthe most severe problem at typical crystallographic x-rayergies is charging of the oxide. Hole mobility in SiO2 is verysmall, whereas electrons leak through relatively quickly. Tresults in the build up of a positive space charge in the ox

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that changes the potential felt in the silicon underneaSince the effect is dependent on the local exposure histocannot be trivially compensated for by changing the gpotentials. CCDs made of high resistivity material, whican be deeply depleted, have been built.128–133Some of thesedevices were designed to be resistant to radiation damagnumber of authors have evaluated deep-depletion CCDsx-ray applications.7,134–139

CCD fabrication is a very demanding technology wthe result that even good processes have low yieldsblemish-free devices. As a consequence, top grade largeCCDs are very expensive—prices in excess of $10 000not uncommon and, in some cases, prices appro$100 000. CCDs are usually graded according to the denof point and column defects, with lower grade devices comanding considerably less money. It is important to reathat lower grade devices may very well still have.99%unblemished area and exhibit the same noise performanctop grade devices.

The performance of a CCD is also limited by the eletronics of the CCD controller. A CCD with a full well of 43105 electrons and a noise of 5 electrons has an intrinsignal-to-noise ratio of 80 000. The off-chip analog signchain must be designed with great care to realize this permance. CCDs also have many different modes of operatinvolving various speeds and sequences of clockinggates, ranges of clocking voltages, etc., and a controllerpable of taking full advantage of this flexibility necessarhas some relatively unused features and high cost.

The CCD end of a CCD detector generally has thparts: The detector body, which includes the cooled Chead, i.e., the CCD socket, cooling arrangement and etronics immediately adjacent to the chip; the CCD; andremote CCD controller and associated computer. Typicathe CCD is fabricated by one vendor, the controller by aother, and the detector body is often built by a third vendThe market for scientific CCD controllers is dominated bysmall number of vendors and the quality of the produranges widely. Important considerations in selecting a ctroller include robustness of the hardware, software, quaof the analog to digital signal chain, flexibility and easeadjusting the clocking patterns, the difficulty of changifrom one CCD to another, cooling capabilities and, if onebuilding ones own detector body, the assistance the venprovides in designing the CCD head. The last point is ccial, since proper design of a CCD head is nontrivial.

There are significant advantages of efficiency, radiatdamage protection, and stability of calibrations affordedbonding CCDs directly to fiber optic bundles.

3. Diode arrays, CIDs, and CMOS imagers

We close this section with mention of diode arrays.diode array is simply an array of reverse-biased diodesare addressed inx–y fashion. The simplest incarnation is aarray of diodes that is addressed by an array of horizonta~xdirection! polysilicon electrodes lines on one side of a fudepleted silicon wafer and an array of vertical~y direction!electrode lines on the other side of the wafer. Any pixel mbe addressed by selecting thex andy lines which cross at the

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pixel. Diode array detectors are distinguished by the waywhich the signal is read out. An early competitor to the CCwas the CID.140–143Whereas charge is shifted out in a CCDit is thex–y matrix addressed in the CID though a clever sof electrodes onto an on-chip amplifier. Because the on-cpreamp now sees the full capacitance of the electrodes topixel, the noise is much higher than with CCDs. On the othand, the image is fully addressable, so a small desired~e.g., the expected positions of an x-ray spot! can be readout. Finally, the CID can be nondestructively read out byclever procedure that does not destroy the accumulatingage. By repeatedly nondestructively reading out the saimage and averaging, the noise can start to approach Cvalues. The practicality of this approach for full-frame reaouts is limited, since the noise falls as the square root ofnumber of frames averaged together, so hundreds of remay be required for each low noise frame. Examples of x-detector applications of CIDs are given by Hanleyet al.144

The CMOS imager is a modern competitor to tCCD.145–149 Whereas CCDs require specialized silicofoundries, CMOS imagers are made using normal CMfabrication techniques, thereby realizing important advtages in cost. A CMOS imager consists of a diode arraywhich CMOS fabrication is used to make a diode array wactive switching transistors in each pixel. Although CMOimagers have not achieved the low noise of the best scienCCDs, their cost and flexibility of architecture make thempotent future competitor to the CCD for many application

III. REALIZED DETECTORS

It is clear from the preceding sections, that many cofigurations of detector components have been construcThe literature on CCD detectors is too large to list evedetector that has been reported; therefore, with a few exctions, we reference a large sampling of CCD detectorswere designed for x-ray analytical purposes. Somewhat atrarily, we divide realized CCD detectors as to wheththey contain image intensifiers,113,114,116,150–159 usephosphors without image intensification,5,97,98,100,160–174

or ones in which the x rays are captured by the CCitself.7,129,130,135,137,139,175–179Some overviews of CCD detectors are included in Refs. 3, 180–183.

The decisions that dictate the use of a given configution depend crucially on the details of the application, athe cost and availability of components. However, a generule of thumb applicable to most applications is clear: tsmaller the number of components in the optical relay~Fig.1!, the better the quantitative performance and long-teability to maintain the calibration required for quantitativaccuracy. Thus, simple combinations of phosphors coupto CCDs are generally to be preferred over intensified detors. Direct conversion of x-ray within radiation-hardendeep-depletion CCDs is even better, assuming that the Ccan be obtained to cover the required area at an accepcost and with an acceptable lifetime.

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IV. DETECTOR CHARACTERIZATION ANDCALIBRATION

The quantitative usefulness of any detector dependsthe degree to which it has been characterized and calibraDetector characterization and calibration have beencussed by many authors.76,184–191In this section we summarize the factors most relevant to CCD detectors~see, espe-cially, Barnaet al.76!. These include

~1! spatial resolution;~2! intensity ~flat-field! calibrations;~3! energy and angular calibrations;~4! geometric distortions;~5! background subtractions;~6! detective quantum efficiency; and~7! zingers and other considerations.

A. Spatial resolution

The measurement of the spatial resolution of a CCDtector was discussed in Sec. II B 1 8 on phosphor resolutionThe assumptions which were made about the PSF, namthat it is cylindrically symmetric, independent of intensitand translationally invariant, are reasonable approximatifor most phosphor screens. The ranges over which thesegood approximations are likely to be more limited for tdetector as a whole, especially at the extremes, e.g., neaedges of the detector or near saturation. In fact, breakdof these approximations are often used as practical indicaof the usable ranges of values of the detector.

B. Intensity „flat-field … corrections

Many factors contribute to variation in sensitivity acrothe face of a typical CCD detector, including nonuniformitiin the phosphor, the optical couplings, the transmission offiber optics, and the response of the CCD pixels. These nuniformities are almost always sufficiently large as to requcalibration for accurate quantitative work. Calibration ivolves exposing the detector to a known signal and mappout the pixel-by-pixel response of the detector. This produre is complicated by many factors:

~i! It is difficult to produce a uniform, known x-ray signal broad enough~i.e., a uniform flood field! to coverthe face of large area detectors. Procedures for dothis are discussed by Moy190 and by Barnaet al.76

The alternative, namely to scan a known, small asignal across the detector, can be very tedious. Amendous number of x rays need to be accumulatedgood statistics. So, e.g., from Poisson statistics aloto flat-field correct a 100031000 pixel detector to anaccuracy of 0.5% requires at least 43104 x rays/pixelor 431010 x rays.

~ii ! The PSF complicates the use of flood fields becathe signal recorded at each pixel involves contribtions from the signal, referred to the input face,neighboring pixels. Ideally, one wishes to deconvoluthe PSF out of the recorded image, but in practice,PSF is rarely sufficiently uniform to allow this. Thalternative, namely to scan the face of the detec

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with a signal that is small compared to the FWHMthe PSF is tedious to the point of impracticality. Fthese reasons the usual procedure is to use a flfield and accept the resultant limitations on accuraThis consideration highlights the importance of a nrow PSF in order to minimize these limitations anallow more accurate flat-field calibrations.

~iii ! Subpixel granularity limits the uniformity of responsGranularity arises from the microstructure of thphosphor screen, fiber optics, and CCD, as wellfrom dust and imperfections in the optical couplingGranularity may be observed by scanning a stamicro-x-ray beam~i.e., small compared to a pixel!across a pixel in subpixel steps. One observes repducible variations in the integrated response. Tpractical effect of granularity and the width of thPSF is to limit the accuracy with which small-aresignals may be calibrated.

~iv! Geometric distortions, such as those typically encotered with fiber optic tapers and electrostatically fcused intensifiers, may not be area preserving~seeSec. IV C!. This means that the factor that maps smunit areas on the detector input onto the detector oput varies with position on the detector face. Thesult is that the flat-field calibration must account fthe way in which the geometric distortion compressand expands a signal of a given magnitude and sover the output area. This illustrates the interactibetween geometric and flat-field corrections. One wto deal with this is to first geometric distortion correthe images used to determine the flat-field correctiThis is discussed in Barnaet al.76

The practical effects of these complications are that ioften straightforward to flat-field correct a CCD detector fsignals several pixels across to an accuracy of a few perof the intensity. It is much more difficult to flat-field correcto 0.5%. The difficulty of flat-field correction rises as thsignal shrinks in size. Signals on the order of a pixel widacross can rarely be corrected to better than a percent oBecause of these complications, the flat-field correctioclaimed in the literature for many~if not most! CCD detec-tors are optimistic. In this regard, note that the most serilimitations arise from the phosphor, fiber optics, and opticouplings. Further, the range in silicon of electron-hole pduction from a typical 5–10 keV x ray is only about a mcron, so direct conversion CCDs have a near ideal sinpixel PSF. It is likely that direct conversion CCDs cancalibrated to greater accuracy than phosphor-based detec

C. Energy and angular corrections

The amount of signal from a CCD detector may varya complicated way with the x-ray energy and angle of indence. This is especially true for detectors with settled phphor powder screens, which represent a majority of commcially available CCD detectors.47 This is the result of twocompeting processes: first, the absorption efficiencygreater for obliquely incident x rays because of the lonpath length through the phosphor. Second, the luminesce

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is attenuated as it travels through the phosphor, so x raysare stopped nearer to the incident surface yield less liThus, x rays that are obliquely incident tend to be stoppnearer to the incident surface and yield less light than nmally incident x rays of the same energy. At low x-ray engies, most of the x rays are stopped in the phosphor. Buhigh x-ray energies a significant fraction of the normallycident x rays are not stopped in the phosphor at all, in whcase, the increased absorption with angle results in mstopped x rays and more light. At intermediate x-ray engies, there will be angles where the two effects cancel.angle effect is not negligible and can easily change thecorded luminescence per x ray by 10% or more~Fig. 14!.

The response of the detector as a function of energyincident angle is measured by exposing the detector tmonochromatic beam of x rays of the appropriate energyangle. A monochromator arrangement working off the brestrahlung background of a laboratory x-ray tube is suitafor making the measurements. It is of course necessarcollimate the beam to accurately define the angle and toan scintillator/phototube combination to determine the nuber of x rays/s in the beam at each angle and energy. Bet al.76 found that the measured response is well fit bytwo-dimensional surface given by

I ~u,E!5I ~0!1a~E!u2, ~4.1!

whereu is the angle of incidence with respect to the screnormal,I (0) is the response at normal incidence for a givx-ray energy, anda is an empirically determined function oenergy. The angle/energy effect is smooth and slowly vaing, so a sparse set of measurements at perhaps four enefor each of four angles is often sufficient to map out the fresponse. A least squares fit to the resultant two-dimensisurface results in a small number of coefficients that prova compact way to compute the needed correction overrange of incident angles and x-ray energies.

FIG. 14. The luminescent response of a settled powder phosphor screangle for two different x-ray energies. At high x-ray energies, the angeffects are dominated by stopping power. At lower energies, the respondominated by light loss. From Gruneret al. ~Ref. 47!.

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D. Geometric distortions

Geometric distortions of the recorded image result pmarily from image intensifiers and from the optical coplings in the system. Geometric distortions may be area pserving ~affine! or nonarea preserving~rubber-sheet!transformations. Insofar as the distortions are smoothslowly varying, they can be calibrated out. The most comon calibration procedure is to use a shadow mask witknown, fine pitch of holes on a regular lattice. It is clearnecessary for the mask to be accurately made with an apitch that is small compared to the scale of the distortiBarnaet al.76 uses a lithographically fabricated array of 7mm-diam holes on a 1 mmpitch in a 50-mm-thick tungstenmask. It is also important to place the mask as close assible to the detector face and to illuminate it with an x-rsource sufficiently far away to avoid parallax effects. Thexandy centroids of the spots in the recorded image are fitwith cubic polynomials, from which one computes an intepolated transformation from the incident to the recordedage.

A check on the distortion correction procedure isdistortion-correct an image of the shadow mask after a rdom rotation and displacement. The resultant centroids ofcorrected image should be nearly identical with a perflattice to better than a small fraction of a pixel. The procdure used by Barnaet al.76 resulted in less than 0.25 pixedeviation from a perfect lattice for all centroids on a 1031024 pixel CCD detector with 50mm pixels.

As noted in the discussion on fiber optics~Secs. II C 3!,fused fiber-optic bundles are subject to discontinuities, cashear distortions. These cannot be accounted for with athing less than a pixel-by-pixel calibration procedure, whiis usually quite impractical. A better approach is to use fiboptics with a tight specification on shears.

Fiber optic tapers usually introduce rubber sheet distions that necessitate coupled flat-field and geometric distion corrections. See the discussion on the flat-field corrtion, above, and Barnaet al.76

E. Background subtraction

CCD detectors generally have two sources of zero x-dose instrumental background which need to be subtracteobtain the true intensity of recorded images. The instrumtal background arises from~a! CCD dark-current and~b! anintentional offset ~pedestal! voltage of the electronics toavoid poor electronic behavior at near-zero voltage. At ltemperatures, and especially for MPP CCDs, the dark curmay be sufficiently small to allow integrations many houlong. Typical values are 1.0– 0.01e2/s/pix in the range of220 to 250 °C. The rate of dark current accumulation vaies from pixel to pixel, so the zero-dose backgroundI maybe characterized as

I ~x,y,t !5I 0~x,y!1a~x,y!t, ~4.2!

whereI 0(x,y) is the zero-dose, zero-time map anda(x,y) isthe pixel-by-pixel rate of accumulation. These parametmay be empirically determined and stored. However, itusually safer to simply acquire occasional zero-dose ex

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F. Detective quantum efficiency „DQE…

The DQE72,184 is a measure of the overall system efciency. It is defined as

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where,S5signal,s5noise, and the subscriptso and i referto input and output of the detector, respectively. All inpsignals have noise, e.g., for a Poisson input of a mean ofN xrays, the (SNRi)5N/AN5AN. Hence, the DQE measurethe additional noise imparted by the detection process. IfDQE51, then no additional noise is added and the SNo

5SNRi , i.e, the detector, is an ideally noiseless detecNote that a signal recorded with a noiseless detector isnecessarily noiseless because the signal itself has nNoisy detectors have DQE!1.

To model the DQE, one writes down expressions fornoise and signal of each element in the serial signal re~Fig. 1! and then combines all these into an equation spfying how the detected signal and cumulative noise progates through the detector.2,101,175,184,186,192–196Modeling theDQE can aid in detector design by identifying detector coponents which dominate the overall noise behavior. TDQE of the system can never exceed the DQE of the noislink in the serial signal relay.

As an example, consider the primary phosphor inphosphor-based CCD imager. If the phosphor stops a ftion, a, of the incident x rays, then the DQE of the phosphis given by

DQE5a/~11g21!, ~4.4!

whereg is the mean number of visible photons/x-ray emittby the phosphor. Sinceg is typically very large~several hun-dred!, the DQE of the phosphor is dominated by the stopppower, which in turn becomes the upper limit of the systDQE. For many very low noise detectors, the stopping poof the phosphor is a good approximation to the DQE. Tmakes excellent intuitive sense: suppose the detector wtruly ideal, except for the limited stopping power of the pmary x-ray converter. Then, from the definition of the DQ@Eq. ~4.3!#, setting DQE5a, and recognizing that forN in-cident photons obeying Poisson statistics, (Si /s i)

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5(N/AN)25N, we haveaN5(So /so)2. In other words,the output signal has identical statistics to an input signaaN x rays.

The DQE will be seen to be functionally dependent uppractically every aspect of the detector and the measuremincluding, the area of the integrated signal, the detector dcurrent, and the rate of signal read out. Although the DQEuseful in the design process, detailed performance clabased on numerical evaluation of the DQE from a model

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always suspect. The numbers that go into the modelrarely known to great accuracy. Further, the model assucomplete knowledge of all noise sources and signal behain a complicated piece of apparatus. Hence, theactual DQEshould always be measured.

In principle, the DQE can be measured by taking aquence of nominally identical x-ray exposures and comping the variance in the integrated intensity for various arin the exposures.72,184An accurate measurement of the DQrequires attention to many details and, in our experiencerarely done properly. Care must be taken to accuratelycount for signal-to-noise in the incident signal and for subinteractions between the DQE and the way in which the msurement is taken. For example, failure to account foreffects of signal averaging due to the PSF have led to moverestimations of detector efficiency. A method whichparticularly prone to this type of error is the computationsignal variance based on the integration of small subareaa flood field.47,76

In general, the type of signal used to measure the Dshould be as similar as possible to the signals of the intenapplication. Thus, the signals used to characterize detecintended for crystallography should ideally be spots simin area to crystallographic spots. In order to accuratelyclude noise arising from detector calibrations, images shobe calibrated, just as real data would be, before compuvariances. In order to more realistically account for thecuracy of calibrations, the detector should be displacedallel to it’s face between each measurement so that a gincident calibration spot falls on different parts of the detetor face. A general consequence of all these factors is thaDQE of a CCD detector is larger for small integrated featuthan for larger features.197 This is ultimately a consequencof the fact that granularity and signal spreading inherenthe detector PSF makes it very difficult to accurately detmine the response of the detector to a signal smaller thapixel, as discussed in the preceding section. The bestprinciple, way to measure the detector DQE~and generatethe calibrations! is to measure the detector response, pointpoint, for a signal which is small compared to the half widof the PSF. However, this approach is practically impossifor detectors with very large numbers of pixels.

G. Zingers and other considerations

As discussed in the section on fiber optics, CCD detecimages are prone to an accumulation of occasional~few/frame/s! spurious spots, called zingers, which range in intesity from barely above background to very bright. Besidspecifying CCD parts with low levels of actinide contamintion, there is little one can do but to ignore or remove tzingers. Ignoring the zingers is typically done for short eposures with sparse data, such as with protein crystagraphic images at a synchrotron source. Barnaet al.76 de-scribe two methods for zinger removal. The first is basedsimple cross comparison of two nominally identical imagSince the zingers are sparse and occur randomly, therevery low probability that two zingers will occur at the samlocation in two images. The second method is based on

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fact that that the intensity associated with zingers falls oside of the Poisson statistics of the local x-ray intensity atherefore, can be identified to an acceptable level of codence. This second procedure can be used even with simages, as long as the x-ray image has no sharp, hcontrast features. This is useful, for example, for radiograand for diffuse x-ray scattering from liquids and gasses.

Other considerations concern the stability of calibtions. In the experience of the authors, calibration datawell-built fiber optically coupled phosphor-based CCD dtectors are remarkably stable over time spans of mmonths. Even so, occasional calibration is required formost accurate data. Other types of CCD detectors may nto be calibrated more frequently. Specifically, detectors ctaining air or vacuum optical paths need to be monitobecause of the accumulation of dust or film on optical sfaces. This occurs even with hermetic sealing unless gprecautions are taken to avoid materials which can ouorganic film-making substances. Detectors containing imintensifiers are particularly susceptible to drifts in high voages and changes in ambient magnetic fields. Stray magfields are sufficiently deleterious that it is sometimes necsary to encase the detector in magnetic shielding mate~e.g., mu-metal!. Even then, routine stray magnetic fields ccause calibration drifts.

V. FUTURE DETECTORS

CCD detectors have had an enormous impact on mx-ray applications, such as protein crystallography. Whileuse of CCD detectors is certainly expected to continuegrow, emphasis within the detector development commuis shifting towards other solid-state sensor architecturespromise considerably more power and flexibility. The coponents in most CCD detectors~e.g., fiber optics, image intensifiers, CCDs, etc.! have typically been developed for visible light applications and adapted to x-ray detector uDeep-depletion CCDs are notable exceptions in that theylize the integrated circuit~IC! technology infrastructure tofabricate custom radiation sensors. The IC infrastructurpowerful and increasingly accessible, making it attractiveconsider alternative sensor architectures.

One of the most attractive alternative architectures isarray of directly radiation-detecting pixels, each with its owprocessing electronics. Several groups around the worldcurrently working on such ‘‘pixel array detectors’’~PADs!.Although many PAD variants are being explored, the mcommon theme is to assemble two-layer devices thatconnected, pixel-by-pixel, via lithographically fabricatemetal connecting ‘‘bumps’’~bump-bonding!. The x rays arestopped in a thickly depleted, high-resistivity semiconduclayer. The resultant charges are conveyed via the connecbumps to the second layer, which is fabricated in CMOThis architecture has many advantages: the radiatdetecting layer is a relatively simple matrix of diodes, whiis relatively simple to fabricate and can be made out of nstandard semiconductors, such as high-resistivity silicgermanium, or other high-stopping power materials. Ththis layer can be tailored for x-ray detection. The CMO

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layer is fabricated by standard, commercial silicon foundrusing normal IC design rules. This allows much flexibilitythe design of the functional electronics in each pixel. PAcan be analog, digital, or hybrid detectors. Although itbeyond the scope of this review to survey PAD technologyfew representative references illustrate the power ofapproach.198–206

ACKNOWLEDGMENTS

The authors have had the pleasure of working with mastudents and colleagues on detector research. We wisespecially acknowledge those for whom detectors have ba primary effort: Sandor Barna, Alper Ercan, John LowranJim Milch, Matt Renzi, George Reynolds, Giuseppe RosJohn Shepherd, Richard Templer, and Bob Wixted. We halso enjoyed detector support from the NSF, NIH, ONR, amost especially, from the Office of Biological and Enviromental Research of the Department of Energy~Grant No.DE-FG-0297ER62443!, which has supported our detector rsearch for many years.

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