high resolution neutron resonance absorption imaging at a pulsed neutron beamline

3272 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 59, NO. 6, DECEMBER 2012

High Resolution Neutron Resonance AbsorptionImaging at a Pulsed Neutron Beamline

Anton S. Tremsin, Member, IEEE, Jason B. McPhate, John V. Vallerga, Oswald H. W. Siegmund,Winfried Kockelmann, Erik M. Schooneveld, Nigel J. Rhodes, and W. Bruce Feller

Abstract—The existence of resonance peaks in neutron absorp-tion spectra in the epithermal range of energies enables uniquenon-destructive testing techniques. The deep penetration of ep-ithermal neutrons provides an opportunity to perform a composi-tional analysis of a sample which is opaque to X-rays and thermalneutrons. The neutron resonances in the transmission spectra con-stitute a characteristic pattern for many isotopes, which can beused to identify the isotope and to map the distribution of the iso-tope in a sample. The neutron transmission spectra can be mea-sured with the time of flight (TOF) technique using a pulsed neu-tron source. Combining this method with a high resolution neu-tron counting detector enables substantial improvements of spatialresolution of neutron resonance transmission imaging. Such a de-tector has been developed to register neutrons with 55 spatialand 10–1000 ns temporal resolution Our proof-of-principle exper-iments at the ISIS pulsed neutron spallation source demonstratethat compositional analysis of multi-element samples can now beperformed with spatial resolution. Images of a testmask consisting of thick foils of Au, Ag, In andGdwerecollected in the 1–100 eV energy range. The experimental resultsdemonstrate the potential for compositional analysis via resonanceabsorption transmission with high spatial resolution. In-bulk tem-perature measurement through Doppler broadening analysis willalso benefit from this technique.

Index Terms—High-resolution imaging, materials testing, neu-tron radiography, nondestructive testing.

I. INTRODUCTION

N EUTRON resonance absorption spectroscopy is basedon the presence of narrow neutron absorption features

at unique energies characteristic of individual isotopes of eachelement. For many elements the energies of neutron resonantabsorption are in the epithermal range of 1 eV. The ab-sorption of a neutron results in a nucleus in an excited state

Manuscript received January 20, 2012; revised June 20, 2012 and August09, 2012; accepted August 22, 2012. Date of publication October 03, 2012;date of current version December 11, 2012. This work was supported in part bythe U.S. Department of Energy under STTR Grants DE-FG02-07ER86322 andDE-FG02-07ER86353.A. S. Tremsin, J. B. McPhate, J. V. Vallerga and O. H. W. Sieg-

mund are with the Space Sciences Laboratory, 7 Gauss Way, Universityof California, Berkeley, CA 94720 USA (e-mail: [email protected];[email protected]; [email protected]; [email protected]).W.Kockelmann, E.M. Schooneveld andN. J. Rhodes are with STFC, Ruther-

ford Appleton Laboratory, ISIS Facility, Harwell Oxford, Didcot, OX11 0QX,U.K. (e-mail: [email protected]; [email protected];[email protected]).W. B. Feller is with Nova Scientific, Inc., Sturbridge, MA 01566 USA

(e-mail: [email protected]).Color versions of one or more of the figures in this paper are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TNS.2012.2215627

that promptly returns to the ground state via the emission ofseveral gamma-ray photons. These photons can be detected byconventional gamma-ray detectors and the energy of absorbedneutron can be obtained with a neutron time of flight technique,providing the sample is interrogated by a sharp neutron pulse[1]. In this method, often referred to as neutron resonancecapture analysis (NRCA), there is no need to measure theenergy of the gamma photons. This substantially simplifies thedetection equipment used in the experiment, however that iscompensated by the requirement to use a pulsed neutron beam.NRCA is somewhat similar to prompt gamma ray activationanalysis (PGAA) [2], [3], where the sample is irradiated bythermal or cold neutrons and the spectrum of prompt gammarays is measured. In the latter case a continuous beam of neu-trons can be used, but the energy of gamma ray photons needsto be registered. However, some materials can be activated bythis method and can be radioactive for a long period of time.In contrast, the interrogation by epithermal neutrons leads to alower activation of the samples as the absorption primarily oc-curs only at the nuclear resonances and only a small fraction ofincoming flux contributes to sample activation and the excitednucleus, in many cases, returns to the ground state promptly.The thermal and cold neutrons in these experiments often canbe filtered out by a relatively thin thermal neutron absorber.Another advantage of interrogation with epithermal neutronsis their ability to penetrate thick samples which are opaque tothermal and cold neutrons as well as to X-rays.Both NRCA and PGAA techniques have limited spatial res-

olution defined by the area of the interrogation beam. A vari-ation of the NRCA technique, where the spectrum of neutronbeam transmitted through the sample is measured enables neu-tron resonance absorption imaging [4]–[9]. In this method theneutrons transmitted through the sample are measured by thedetection system and the energy of each registered neutron is re-constructed from its time of flight (TOF), providing the detectorhas sufficient spatial and temporal resolution for each detectedneutron. This non-destructive imaging technique is unique in itsability to provide a spatial distribution of isotopes within thesample. Moreover, the 2-dimensional distribution of isotopescan be obtained from a single measurement performed with aproper neutron counting detection system.The potential of neutron resonance absorption imaging

(NRAI) was demonstrated by previous measurements, wherea number of museum artifacts were studied in terms of theirconstituent materials [4], [6], [9]. The depths of neutron ab-sorption resonances can be translated into quantitative amountsof specific elements in the sample, with the spatial resolution

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TREMSIN et al.: HIGH RESOLUTION NEUTRON RESONANCE ABSORPTION IMAGING AT A PULSED NEUTRON BEAMLINE 3273

defined by the detector pixellation used in the experiments. Fora typical flight path of 15 m the time of flight of a 10 eV neutronis . For such a beamline the timing resolution of thedetector needs to be in the sub- range to well characterizethe narrow resonance features. A detector with a small numberof GS20 glass scintillator pixels coupled to multichannelphotomultipliers has demonstrated the power of the neutronresonance absorption imaging to spatially map given elementswithin samples [9].To advance these experiments to a higher spatial resolution a

neutron counting detector with microchannel plates (MCPs) anda Timepix readout was tested in initial proof-of-principle exper-iments conducted at the ISIS pulsed neutron source at Ruther-ford Appleton Laboratory, UK. The MCP detector used in ourexperiments is capable of neutron counting with simultaneous

spatial and temporal resolution, enablingan order of magnitude improvement compared to previous mea-surements performed with a 10 10 pixel detector with

pixels [6], [9].

II. EXPERIMENTAL SETUP

Microchannel plate based event counting detectors are widelyused in time of flight applications (e.g. detecting electrons inangle-resolved photoemission spectroscopy, ARPES, on syn-chrotron sources [10] as well as photons in fluorescence lifetimeimaging [11] requiring sub-ns temporal resolution). With theadvent of neutron sensitive MCPs all the advancements in thistechnology can now be applied to NRAI. The spatial resolutionofMCP detection systems is typically on the scale of 20–50 .Their intrinsic temporal resolution is sub-nanosecond, howeverfor neutron detection it is energy dependent and is defined bythe depth of the neutron interaction within 1 mm thick MCPs.For thermal neutrons the resolution is , while in theepithermal range it is much better since these neutrons travelwith higher velocities. The efficiency of our MCP detector wasshown to reach 70% for cold neutrons [12], [13], while in theepithermal range it is significantly lower due to the smaller crosssection of absorption, which provides the primary signal inthe MCP neutron detectors. However, the combination of a rela-tively low efficiency and a high flux of epithermal neutrons stillprovide sufficient statistics for NRAI, as is demonstrated by theresults of our experiments described in the next section.The active area of the detector used in the experiments was

limited to by the size of the Timepix readout [14].The detector was mounted 16 m from the source moderatorin the direct neutron beam at the ROTAX beamline [15]. Thesamples were placed 12 mm from the detector active area.For each detected neutron the position (55 precision) andtime of flight ( 40 ns precision) were provided by the detector.During data acquisition the energy of each detected neutron wascalculated from its time of flight and then the count was added toa corresponding energy slice at the position of the event XY. Aset of images, each corresponding to a 500 ns time of flight in-terval, was recorded in our experiments. From this X, Y, T “datacube” a transmission spectrum can be reconstructed for each

pixel of the detector. From transmission spectra

in the epithermal range the elemental distribution of certain ma-terials was reconstructed through the resonance absorption fea-tures which provide a high contrast in a narrow energy range.

III. RESULTS

A. NRAI Imaging of an In, Gd, Ag, Au Foil MaskTo test the sensitivity of our experimental system in the reso-

nance absorption range, a mask consisting of foils made fromseveral materials known to have resonances in the few eVrange was imaged (Fig. 1). The foils had straight edges whichwere offset so that edges of individual materials could be im-aged at the resonance energies. In order to reconstruct the trans-mission spectrum of a particular material the measured trans-mission of the material should be normalized by the spectrum ofthe unattenuated neutron beam. An area not containing any foilswas used to obtain a spectrum of the unobscured (“open”) neu-tron beam. It was discovered by post-experiment analysis thatthe spectrum of the neutron beam had some variation across thefield of view. Therefore, for an accurate normalization of mate-rial transmission in a given area, the open beam spectrum shouldbe measured for exactly the same area. The largest spectral gra-dient across the field of view was observed at the highest ener-gies (shortest time of flight) and was most likely caused by theresidual shadow of the beam chopper, which was used to elim-inate the gamma flash emitted at the time of the spallation. Thetime of flight spectrum measured for the entire active area of thedetector and the spectrum of the “open beam” area are shownin Fig. 2. Resonance absorption dips are seen at certain ener-gies in the spectrum shown in Fig. 2(a). In the present work alltransmission spectra were normalized by the open beam mea-sured over the entire area, Fig. 2(b). For accurate quantificationexperiments the open beam spectrum should be taken for thesame area as the one to be normalized. Variation of the spectrumacross the field of view can be properly corrected in the lattercase. The high flux at the TOF 0–100 range indicates thatthere are a large number of fast neutrons as well as secondarygammas in the neutron beam. The neutron counting capabilityof our detection system with accurate timing for each neutroncan be used to eliminate these neutrons from transmission ra-diography measurements as the contrast in this energy range inmost objects is low.Transmission spectra of areas corresponding to different foils

are shown in Fig. 3 as a function of neutron energy calculatedfrom the time of flight. Two resonance lines are fully saturated,absorbing all neutrons from the incident beam; the non-zero bot-toms of these lines are due to background neutrons present in theexperimental bunker that are not correlated with the interroga-tion neutron pulse or arrive at the detector from a different di-rection. Absorption spectra, produced by normalizing the trans-mission spectra by the spectrum of interrogating neutron beam,are shown in Fig. 4 together with the database spectra [16]. Thetransmission spectra of Fig. 4 were obtained by normalizationof spectra corresponding to Au, Ag, and In (Fig. 3(b), 3(c), 3(d))by the open beam spectra Fig. 3(a). The gradual increase of thetransmission at low energies seen in Fig. 4(c) and 4(e) is dueto the variation of open beam intensity across the field of view,which was not properly normalized in the present work as no


Fig. 1. Schematic diagram of a test mask containing thin foils of Au (20 ),Ag (50 ), In (400 ) and Gd (250 ). The mask was placed on the inputwindow of the detector at a distance of 12 mm from the active area.

Fig. 2. Transmission spectra measured with the foil mask as a function of neu-tron time of flight. Vertical axis represents the average number of counts in a 55

pixel per 0,521 time bin. Integration time 3 hours. (a) Transmissionspectrum integrated over the entire image. (b) Unobscured (“open beam”) areaat the left bottom corner. This spectrum is used to normalize the transmissionof the foils.

proper open beam spectrumwas acquired. The absorption peaksare also somewhat smoothed by the 0.5 binning used duringmeasurements.The amplitude of a resonance absorption peak is determined

by the strength of the resonance as well as the thickness of thefoil and some of the peaks reached the saturation value. The ab-solute value of some measured peaks does not correspond ex-actly to the values predicted by theory due to two factors: the

Fig. 3. Neutron transmission spectra measured with the foil mask. Pixels incertain areas, corresponding to particular materials as well as open beam area,were grouped together and the average number of registered neutrons per pixelper energy bin is shown as a function of neutron energy.

Fig. 4. Neutron absorption spectra measured (and calculated from tabulatednuclear data on cross sections) at the regions of the image containing foil of thematerial specified in the legend. The theoretical curves are calculated for theabsorption of 50, 400 and 20 thick foils of Ag, In and Au, respectively. Thenormalization of experimental data was performed by the same open beam areain the left bottom corner of Fig. 1 as there was no full open beam transmissionspectra acquired in the measurements.

presence of background and scattered neutrons in the experi-mental bunker and the finite time binning of our data acquisi-tion system. A proper beam characterization can be performedto account for the background neutrons and used for all consec-utive measurements at the same beamline. A set of thick foilsor sheets of different materials with saturated peaks at differentenergies can be measured, and the difference between the satu-rated unity value and the measured peaks can be obtained at dif-ferent energies. An interpolated curve of background level canbe calculated from this measurement and used for proper spectra


Fig. 5. Neutron transmission images of the test mask measured with the MCPdetector. Dark areas in each image are caused by the resonance absorption of aparticular material. Each image represents a 5 —wide range of neutron timeof flights (TOFs), centered around the TOF shown in the image legend togetherwith the corresponding neutron energy. The dashed boxes indicate the areasused to obtain the foil edge profiles in Fig. 6.

normalization. Our future measurements will use that proce-dure. Taking into account the less than ideal normalization thereis a good agreement between measured and database spectrashown in Fig. 4, enabled by the high timing resolution of ourneutron counting detector. Although the quantitative compar-ison between the measured and calculated transmission spectrais left for the future experiments, the energy of resonance peaksand their relative amplitudes explicitly identify the presence ofparticular materials in the certain areas of the sample. With aproper calibration and open beam normalization we hope to beable not only to image the presence of certain isotopes in eachpixel of the image, but also to reconstruct the effective thicknessof these materials from the measured resonance spectra.The energy resolved imaging capabilities of our detection

system in the range of epithermal energies are demonstratedin Fig. 5. The images corresponding to a narrow energy rangewere acquired simultaneously by time-tagging each neutron.Several resonance energies are shown for In, Ag and Gd. To in-crease neutron statistics the time slices were combined into 5time bins centered around the TOF (energy) valueshown in the legend of each image. The edges of different foilsare observed to be quite sharp (Fig. 6), on the order of(10 to 90% of the edge amplitude), indicating that spatial res-olution of NRAI can be as good as providing thereare sufficient neutron counts for statistical accuracy.

B. NRAI Imaging of a Belt Mount Replica

The spatial resolution of NRAI with the MCP detector wasalso tested with a replica of an ancient belt mount, made of twoiron plates with silver and brass inlays, Fig. 7. Two areas of

Fig. 6. Edge profiles of the foils obtained from the resonance absorption imagesof Fig. 5, demonstrating the spatial resolution of NRAI achievable with ourpresent system.

Fig. 7. Photograph of the belt mount replica used in the experiments. Thedashed boxes show the areas imaged at 1–100 eV energy range.

the belt mount imaged with the full beam spectrum and withonly neutrons of 1–100 eV energies indicate that the contrastof the silver and brass components placed over the iron plate ismostly generated in the epithermal range, Fig. 8. Narrow energyrange images of the same areas (Figs. 9 and 10) taken around thesilver resonance of 5.3 eV exhibit very sharp contrasts of thepieces containing silver. Capture of the fine detail in these im-ages is enabled by the high spatial resolution of our detectionsystem. Previous measurements performed with a 10 10 de-tector only allowed a crude map of the silver distribution acrossthe same belt mount [6]. The improved spatial resolution shouldenable high resolution non-destructive elemental/isotopic map-ping within objects containing elements and isotopes with rela-tively low resonance energies in the range 1–100 eV.

IV. CONCLUSIONS AND FUTURE DEVELOPMENTS

The time-tagging capabilities of the MCP neutron countingdetector with high spatial resolution can be used to resolve theenergy of each detected neutron through the time of flight tech-nique. The experiments conducted at the ROTAX beamline withnarrow initial neutron pulse demonstrated that high resolution


Fig. 8. Neutron transmission radiography of a belt mount replica. (a) Full spec-trum of the beam containing epithermal, thermal and cold neutrons. (b) Onlyneutrons in the energy range of 1–100 eV are registered in this image.

Fig. 9. Transmission images of the belt mount taken at the resonance energy ofAg and away from it, same area as in Fig. 8. (a) Transmission image obtainedwith neutrons around 1.63 eV energies. No resonances are present around thatenergy range. (b) Image obtained at 5.3 eV energy at the silver resonance. Bothimages are normalized by the open beam and represent the spatial variation oftransmission coefficient at the corresponding neutron energy.

Fig. 10. Transmission images for the same settings as in Fig. 10, except for theother area of the belt mount shown by the lower dashed rectangle in Fig. 7.

elemental/isotopic maps can be obtained for the materials withneutron resonance energies below 100 eV. The relatively goodagreement between measured and predicted absorption spectrawas observed despite non-optimal spectra normalization used inthe present experiment: variation of the beam spectrum acrossthe field of view requires the use of the same area for the trans-mission and “open beam” data. A proper normalization proce-dure shall be used in the future experiments. A spatial resolu-tion of was demonstrated for In, Ag, Au, and Gdfoils. To reconstruct the integrated thickness of a particular ma-terial/isotope along the direction of the beam propagation fromthe resonance transmission data a proper calibration should be

conducted with foils of different thicknesses. Fitting absorp-tion curves into measured data will also enable more accuratequantification and improve the image statistics compared to thesimple image binning used in the present experiments.The isotopic/material sensitivity of NRAI technique makes

it unique in some cases and in others complimentary to ex-periments with thermal and cold neutrons where variation ofneutron absorption across the sample (radiography and tomog-raphy) and/or diffraction are measured. It is very important tomention that at a pulsed neutron source NRAI can be conductedsimultaneously with thermal and cold neutron imaging/diffrac-tion providing capability to study different characteristics ofthe same sample in one experiment, e.g. its material composi-tion, strain, grain, phase and temperature distribution. It is thequality of the beam (width of the initial pulse, intensity andbeam divergence) and the characteristics of the neutron countingdetector (spatial and temporal resolution, detection efficiency,dead time, etc.) which determine the accuracy of the measuredphysical properties of the sample. Results presented in this papertogether with the previous measurements of energy resolvedimaging in thermal and cold ranges as well as neutron radiog-raphy [19]–[22] demonstrate the MCP/Timepix detection sys-tems can be very attractive for such imaging experiments. Afaster readout electronics and firmware/software will have to bedeveloped for simultaneous NRAI and thermal neutron/Braggedge imaging.One of the strongest shortcomings of the detection system de-

scribed in this paper is its limited active area of .The next generation detector hardware and electronics utilizethe fact that Timepix chips are 3-side buttable. The new 2 2MCP/Timepix system has an active area of , withreadout time of only 300 for all 4 chips. It is possible to fur-ther increase the active area in one dimension and build deviceswith , which still can be inadequate for somerelatively large samples. It is likely that theMCP/Timepix detec-tors with high spatial and timing resolution will find the nicheapplications in resonance absorption imaging where high spa-tial resolution is required and relatively small areas of interestcan be investigated in one exposure with the possibility to scanacross the sample if necessary.

ACKNOWLEDGMENT

The authors are grateful to Z. Vykydal, J. Jakubek and D.Turecek of the Institute of Experimental and Applied Physics,Czech Technical University in Prague for their prompt helpwith the FITPix readout electronics [17] and data acquisitionsoftware [18] developed within the Medipix collaboration. Thiswork was performed within the Medipix collaboration.

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high resolution neutron resonance absorption imaging at a pulsed neutron beamline

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