x-ray science and applications - postechphome.postech.ac.kr/user/atl/note/chapter4.pdf · x-ray...

X-Ray Science and Applications

2008 Fall SemesterLecturer; Yang MO KOO

Tuesday and Thursday 14:45~16:00

X-ray & AT Laboratory, GIFT, POSTECH

4. X-ray Detectors

4.1 Characteristics of X-ray Detectors4.2 X-ray Film 4.3 Electron Multipliers4.4 Gas Detectors4.5 Scintillation Detector4.6 Phosphors4.7 Solid State Detector4.8 Electronics of Detector


4.1 Characteristics of X-ray DetectorsThere are many different types of detector suitable for use. Each of these is characterized by (i) counting efficiency, (ii) counting loss, (iii) energy resolution, and (iv) spatial resolution.

Counting Efficiency(ε): the product of the absorption efficiency and the detection efficiency . Since counters have thin window (usually Be) the fraction of theIncident radiation absorbed by the window is . The fraction absorbed by the counter itself is . The detection efficiency is simply where represents the fractional counting loss. The overall efficiency is

( ){ }( )losscabswabsabs fff −−== 11 ,,detεεε

( )absε( )detε

( )wabsf ,( )cabsf , ( )lossf−1 lossf

Detector efficiency is usually 100%, so that counting efficiency is determined by absorptionefficiency, which can be calculated from the dimensions and absorption coefficients of thewindow and counter.

Note particularly the dependence of on wavelength, due to the dependence of absorption coefficients on wavelength. The efficiency of any counter is low for very shortwavelengths, because most of these hard x-rays pass right through window and counter andare absorbed neither; at long wavelengths the absorption efficiency decrease because ofincreasing absorption of soft x-rays by the window.

( )absε


Calculated values of absorption efficiency in % of various kinds of detectors and photographic x-ray film

Calculated values of absorption efficiency for Si(Li) detector

4.1 Characteristics of X-ray Detectors


Counting loss:As counting rate increases, the time interval between pulses decreases and may become so small that adjacentpulses merge with one another andno longer resolved. At this point counting loss has begun. The quantity that determines this point is resolving time ts of thecounter-electronics system. The resolving times are depend on the kinds ofcounter-electronics of the detector system.

Correction of the counting loss:Dead time correction

This figure shows the non linearity of the counting the photon by counter at high courtingrate, however, this non-linearity can be corrected, so-called dead time correction.



Energy resolution

The size of voltage pulse produced by the counter is proportional to the energy of x-ray quantum absorbed.If the width of the curve at half its maximum heightis W and if V is the mean pulse size (voltage), then resolution R(%) of counter is

VWR ×

=100(%)

Pulse size is also varied with the types of counter.

Pulse-height distribution curves for three kinds of counterincident radiation is MnKα and MnK β



Spatial resolutionThe measure of how closely lines can be resolved in an image is called spatial resolution, and it depends on properties of the system creating the image, not just the pixel resolution in pixels per inch (ppi). For practical purposes the clarity of the image is decided by its spatial resolution, not the number of pixels in an image.

Pixel resolutionThe term resolution is often used as a pixel count in digital imaging. An image of N pixels high by M pixels wide can have any resolution less than N lines per picture height, or N TV lines. The pixel resolution is described with the set of two positive integer numbers, where the first number is the number of pixel columns (width) and the second is the number of pixel rows (height), for example as 640 by 480. The pixel resolutions are true resolutions, but they are widely referred to as such; they serve as upper bounds on image resolution.

An illustration of how the same image might appear at different pixel resolutions, if the pixels were poorly rendered as sharp squares


http://en.wikipedia.org/wiki/Pixels_per_inch

http://en.wikipedia.org/wiki/Pixel

http://en.wikipedia.org/wiki/Integer

http://en.wikipedia.org/wiki/Upper_bound


4.2 X-ray Film

Film is a position sensitive flux integrating x-ray detector and is widely used in imaging applications such as medical radiography. Film composed of 1μm silver halide(AgBr) coatedor embedded in gelatin matrix. When a photon is absorbed by a grain the absorbed energy results in the dissociation of several silver atoms. On the development these silver atoms act as nucleation points for further silver deposition so that an exposed grain is reduced to a cluster of silver with halide being dissolved away in the developer. The unexposed grains are subsequently removed by fixation. The choice of film depends on the x-ray energy.

Surface grain (100~1500eV) Thick-emulsion films (< 1500eV)


Fluorescence screen

4.2 X-ray Film


4.3 Electron Multipliers

Discrete dynode electron multiplier


Channel electron multiplier (CEM)

)~ 910510Typically ( electron initial each for channel the of end theby produced electrons of #The :Gain

oxides metal alkali of percentfew a withglasses Lead5cm~L

1mm~0.1d ≈



Curved channel electron multiplier (CEM)

For low x-ray fluxes the gain is limited to by saturation effects. As the flux increases these effects cause the gain drop.Typical gain at high counting rate;

98 10~10~

417

516

14

10G s10 rate counting at Gain10G s10 rate counting at Gain

flux low of that of 1/3 s10 rate counting at Gain

≈

≈−

−

−

:::



Micro channel plate (MCP)

Microchannel plate showing cross section (a) straight and (b) curved channel.

Microchannel plates are arrays of very fine bore channel electron multipliers, typically with channel diameters and channel separations both a few tens of micrometers.

Microchannel plate detectorfor imaging x-ray microscope



4.4 Gas Detectors

Gas counter and basic circuit connections torr-610~ pressure Gas

Effect of voltage on the gasamplification factor.


The filled gas is ionized by x-ray absorption, and produces electrons and positive gas ions. The electronsmove toward the wire anode and the gas ions move toward cathode shell.

53 10~10

Region I (0~100V): Ionized gas ions can combine with electrons

Region II (100~200V): No recombination occurs (Ionization chamber)Amplification factor: 1(no amplification)

Region III (200~1000V): New phenomenon occurs so-called electron avalanche. The electrons produced by primary ionization are rapidly accelerated toward wire anode. These energized electrons knock out electrons out of other gas atoms and this reactioncontinues until the electrons reach anode. Amplification factor (Proportional counter)

4.4 Gas Detectors


Region IV (~1500V): The applied voltage is so high that not only are some atoms ionizedbut others are raised to excited states and caused to ultraviolet emission. These photons are knocking electrons out of other gas atoms. Amplification factor(Geiger counter). However, the resolving time of Geiger counter for of the x-ray pulse is slow, , so that counting losses begin at a few hundred cps. Because it cannot count at high rates without losses, the Geiger counter is now obsolete in diffractometry. It is still used in some radiation survey meters.

Region V (corona discharge): An electrical discharge characterized by a corona and occurring when one of two electrodes in a gas has a shape causing the electric field at its surface to be significantly greater than that between the electrodes.

Higher voltage than region V (glow discharge) : A discharge of electricity through gas at relatively low pressure in an electron tube, characterized by several regions of diffuse, luminous glow and a voltage drop in the vicinity of the cathode that is much higher than the ionization voltage of the gas. Also known as cold-cathode discharge.

98 10~10

.sec10 4−

4.4 Gas Detectors


Ionization chamber

Gas pressure: Pressure of the gas can be adjust according to the photon energy

torr10 ~ atm 1 -6

Ionization chambers are excellent for measuring the strength of the x-ray source. The low limit on the dynamic range is determined only by sensitivity of electronics. The low limit on the energy range is ≈ 50eV, and the upper limit is in the hard x-ray range, depending on the chamber dimensions and the type and pressure of the gas.

4.4 Gas Detectors


Proportional counter

Cylindrical proportional chamber

Multiwire proportional chamber

Electric field lines

Can be achieved a spatial resolution

4.4 Gas Detectors


Charge-division readout of proportional chamber

Charge-division readout circuit

Choice of gas: Noble gases (He, Ne, Ar, Cr, or Xe) are used because of their high avalanche multiplication at relatively low electric fields.

Dynamic range: The upper limit of the dynamic range of proportional counters is primarilylimited by the accumulation of space charge near the anode wire. At high incidence fluxesthe positive field due to the anode wire may be masked by electron cloud (space charge)from a previous avalanche this causes the gains to be depressed and limits the maximumcounting rate per second.610≈

4.4 Gas Detectors


4.5 Scintillation Detector

The scintillant absorbs a photon giving rise to an excited electron state whichsubsequently decays by emission of visible light photons (Luminescence process). These are absorbed by the photocathode of a photomultiplier causing electrons to be ejected. The electron pulse is amplified and the electric output is measured.

Scintillators: plastics, liquids, organic crystals and gases. Alkili halides, such as NaI and CsI, are commonly used since they have good stopping powers and may be shaped for efficient optical coupling.


Luminescence process:

Pure alkali halide crystal Alkali halide crystal with an impurity added

NaI has a filled valance band and an empty conduction separated by an energy gap Eg. The absorption of an x-ray can result in electrons being raised from the valence band tothe conduction band where they are free to travel through the crystal. Alternatively anelectron can be raised from the valence band to an exciton state. Excitons are electron-hole pair which remain associated with each other through the Coulomb interaction. They canmove throughout the crystal but there is no net current since no net charge moves. Excitonscan decay to valence band with emission of photon. Since Eg≈ 5~10eV for most insulatorultraviolet radiation is produced.

ultravioletvisible light



If an impurity, such as Tl to give NaI(Tl), is introduced so that it causes inclusions in the lattice, crystals can be made to emit in the visible range. The impurity must have a smaller energy gap between the excited and the ground states, and it distorts (lowers) the exitonlevels in the vicinity of the inclusion.

Types of photomultiplier dynode;

Venetian blind Box and grid Linear focused Side-on circular focus

Activated alkali halide scintillation detectors have working ranges of a few keV to a few MeV, with reasonable energy resolution. They have good dynamic range with upper detection rates limited by the scintillator decay time, which is ≈≈ 11 μμm for NaI(Tl). Their spatial resolution is poor and they are large and not very robust. They an inexpensive and are widely used for a large number of x-ray applications.



4.6 Phosphors

Phosphors works in essentially the same way as scintillators, but they are used in thin layers of 1 ~10 μμm rather than in bulk. There are two main types of phosphor,

(i) Rare earths activated with terbium;(ii) Organic phosphors; Tetraphenyl butadiene.

etc. (Tb)OY(Tb),OLa S(Tb),OGd 222222

Phosphors, particularly the rare earths, are not very transparent to their own radiation and because they are used in thin layers they are not often used for hard x-rays.

Spatially resolving phosphor detectors using CCD and MCP


4.7 Solid State Detector

There are several different kinds of semiconductor detectors which are commonly calledsolid state detector. Discussion here is restricted to the widely used lithium-drifted silicon detector.

When Si absorbs an x-ray photon of energy hν, hν/ω electron-hole pairs produced where ω is the average energy needed to create and electron-hole pair. The collected charge is proportional to hν. For Si at room temperature, ω=3.62eV. For an x-ray energy of 10keV ≈ 2750 electron-hole pair will be created by each x-ray photon.

Bang gap energy of Si : ~ 1 eVThe density of conduction band electrons (ni) excited by thermal energy becomes

⎟⎟⎠

⎞⎜⎜⎝

⎛ −=

kTE

ATn gi 2

exp2/3

For silicon at room temperature . Si contains , and thus at room temperature only 1 in atoms are ionized. Since the number of electrons in conduction band is huge compared with those produced by x-ray photons, the current Induced by x-ray photon can not be measured.

This may be overcome by using a doped (extrinsic) semiconductor.

31610~ −mni1210

32810 −m~ atoms


p-n junctions

When an n-type and a p-type semiconductor, which are originally both electrically neutral, are placed in contact the electrons will drift towards the p-region and the holes towards the n-region and both can be captured. The junction between the two regions thus becomes charged, the p-side negatively and the n-side positively, setting up a contact potential in theso-called depletion region containing no charge carriers between the two regions.

For real case, a reverse bias VB is applied, and thus the depletion region gets wider. Thewidth of the depletion regions increases with VB, for example VB=300V, the width ≈1mm.The depletion region can act as a sensitive volume for x-ray detection.

depletion region



Li drifted junction

(a) Using p-type crystal Si(B), Li is applied to one face and diffused into the crystal at anelevated temperature

(b) It produces a gradient of Li concentration from high to low through the thickness.(c) A voltage is then applied, aslo at an elevated temperature, to opposite faces, positive

on the n side and negative on p side (called reverse bias). This cause the Li ions to “drift” toward the p side, resulting in wide central region of constant lithium concentration; this region is now intrinsic because it has equal Li and B.



Li drifted junction

• Excellent energy resolution• Must operate at the temperature of liquid nitrogen • The crystal should not allowed to warm too often to room temperature.



4.8 Electronics of Detector

Pulse-height analysis• Pulse-height discriminator• Single-channel pulse-height analyzer• Multi-channel pulse-height analyzer

All the counter produce pulses having amplitude (voltage) that is proportional to the energy of the incident x-rays. Electric circuits that can distinguish between pulses of different voltage can therefore distinguish between x-ray of different energies.

Pulse-height discriminator: A circuit thatallow only pulses larger than a certainselected voltage (V1) to pass anddiscriminate against smaller one.

Single-channel pulse-height analyzer (SCA):In addition, it rejects only pulse larger than V2. The net result is only pass having voltage between V1 and V2 are passed.


Multichannel pulse-height analyzer (MCA): It is designed to separate pulses from a counterthat is receiving incident radiation of many wavelengths, by sorting pulses according to theiramplitude. Example; If the x-ray energy range to be examined from 0 to 20keV and MCA has 1000channels, then each channel spans an energy range of 20eV. For Si(Li) counter, the base of the pulse distribution appears to be about 300eV wide; information about this pulse would be spread over 15 channel of the MCA. Full width of half maximum (FWHM) would be 150 eV.



Schematic diagram of the energy dispersive spectrometer

Scaler: counts each pulse produced by the counter. i) counting for fixed time, ii) countinga fixed number of pulse.

Ratemeter: measures average counting rate.



Homework

Due date: November-13, 2008

Solve the problems; 4.1, 4.3, 4.5, 4.6 and 4.7

x-ray science and applications - postechphome.postech.ac.kr/user/atl/note/chapter4.pdf · x-ray...

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