IMAGING OF HETEROGENEOUS MATERIALS BY. P. Staples, T. H. Prettyman, and J. Lestone

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1 Tifle: IMAGING OF HETEROGENEOUS MATERIALS BY PROMPT GAMMA-RAY NEUTRON ACTIVATION ANALYSIS R'6iCElVED OCT Author@): P. Staples, T. H. Prettyman, and J. Lestone os Submitted to: 998 Symposium on Radiation Measurements and Applications Ann Arbor., MI USA May 2~4,998 (EXTENDED SYNOPSIS) ASTER Los Alamos NATIONAL LABORATORY Los Alamos National Laboratory. 8n affirmativeactionlclgual opponunity employer. is operated by the University of C8ldomie for the U.S. Department of Energy under contract W NO-36. By acceptance of thu anide. the publtsher recogrues that the U.S. Government retains a nonexclusive. royaltyfree license to publish or reproduce the published form of thls COntnbutlon. or to allow others to do eo. for U.S. Government purposes. Loo Alamos NItiorId Laboratory quests that the publisher identity this 8rticle as work performed under me rurpices of me U.S. Department of Energy. Loo Alimos National 'Laboratory strongly supports 8cademic freedom and 8 researchefs nght to publish: as an institution. however. the Laboratory does not endorse the viewpoint of publication or guarantee ne technical correctness. FWmNO.ll3BR6 st2629mq

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4 Imaging of Heterogeneous Materials by Prompt Gamma-Ray Neutron Activation Analysis Panish StaDles, Tom Prettyman, and John Lestone Los Alamos National Laboratory, P.O. Box 663, MS E540, Los Alamos, New Mexico, (505) , fax (505) ~~ Abstract We have used a tomographic gamma scanner (TGS) to produce tomographic prompt gamma-ray neutron activation analysis imaging (PGNAA) of heterogeneous matrices. The TGS was modified by the addition of graphite reflectors that contain isotopic neutron sources for sample interrogation. We are in the process of developing the analysis methodology necessary for a quantitative assay of large containers of heterogeneous material. This nondestructive analysis (NDA) technique can be used for material characterization and the determination of neutron assay correction factors. The most difficult question to be answered is the determination of the source-to-sample coupling term. To assist in the determination of the coupling term, we have obtained images for a range of samples that are very well characterized, such as, homogenous pseudo onedimensional samples to three-dimensional heterogeneoussamples. We then compare the measurements to MCNP calculations. For an accurate quantitative measurement, it is also necessary to determine the sample gamma-ray self attenuation at higher gamma-ray energies, namely pair production should be incorporated into the analysis codes. modified such that a large graphite reflector is positioned at the top and bottom of the modular test stand. These graphite reflectors are constructed to hold up to five isotopic neutron sources each. Their purpose is to provide a uniform neutron flux over the measurement region with the maximum intensity possible. They also provide significant shielding for the experimental area and help to minimize room return contributions. Lastly, The graphite reflectors and sources are fixed to the sample container where a constant neutron flux is maintained within the sample during scanning. This last fact is important when attempting to determine the source to sample coupling term. Introduction The tomographic gamma scanner (TGS) enables imaging with variable resolution per voxel by adjusting tungsten plates in front of the HPGe detector. The resolution used for these measurements was intentionally degraded to about 4 cm. The purpose was to decrease the scanning time in order to produce an PGNAA image of a container that is approximately three-quartersthe height of a 208-L drum with the same diameter as a 208-L drum (- 60 cm high by -60 cm in diameter). To obtain the information for an accurate characterization of the instrument for use in PGNAA tomographic imaging, it is necessary to have very well characterized samples and geometry. Thus, we have built a six-layer modular design stand with a diameter equal to that of a 208-L drum. Each layer of the stand encompasses between two and three TGS scan layers. We have at this time a large number of 0.2-cm per side blocks of polyethylene, borated polyethylene, aluminum, graphite, and iron. We also use a number of similar sized aluminum containers to hold other samples such as cadmium pieces, polyethylene shavings or liquid samples. The modular design allows us to interchange a sample at any one position with minimal disturbance to the rest of the matrix and allows for a previous arrangement to be reproduced. This design also allows for simpler MCNP modeling. The TGS has been Discussion For this paper we will detail two measurement series that we have performed. The first is a single measurement of a four-pillar arrangement, one pillar is located 90 degrees apart from the others at the edge of the drum or sample holder. The pillars were placed on the edges of the sample holder to minimize cross talk between each of them. We used the following arrangement of pillars: a polyethylene of density 0.98 gm/cm3, a pillar of polyethylene shavings of density 0.6 gm/cm3, a pillar of borated polyethylene of density 0.95 gm/cm3, and a pillar of iron blocks of density The purpose of this measurement was to characterize Prettyman,T.H., Estep, R.J., Sheppard, G.A., Development of a Tomographic Instrument for Gamma-Ray Nondestructive Assay, Trans. Am. Nucl. Soc. 69, (993).

5 L * the neutron flux reduction as a function of distance from a source that was placed in the top center graphite reflector. This group of materials was chosen such that we could measure a range of neutron interactions, such as neutron moderation in the polyethylene, capture in the boron, and inelastic scattering in the iron. Figure shows the total time-integrated gammaray spectrum that was obtained for both the transmission and emission parts of the tomographic scans. MCNP and TGS results for the four pillar measurement -WNP Fwnbl e kl".n'l Gamma-Ray Spectrum from a TomographicPGNAA Measurement Layer in Drum Figure 2. MCNP and TGS results for the four-pillar measurement. Each pillar was separatedby 90 degrees from the others. A single normalization factor to account,for the source. strength was used to correlate the TGS measurement to the MCNP calculation. "he source is located at approximately layer in terms of the TGS scanning protocol. The solid line is the corresponding MCNP calculation for each of the data measurements. A single normalization factor to account for the source-strength term was used for all of the curves. The magnitude of the '% capture reaction appears to be biased high in the measurement, this could be attributed to the actual boron loading in the material being different from that used in the calculation. The only line with a different shape is the Fe(n,n'y) line. That calculation relies upon the less measured inelastic scattering cross section for the first excited state in 56Fe. The flattening of the measurement at low layer numbers also appears to demonstrate a room return effect on the neutron capture reactions for those regions close to the floor. The MCNP model did not incorporate the experimental room geometry in the calculation. The second result to be presented is comprised of several independent measurements of a 4-cube arrangement in each layer centered directly below the source. For these measurements, a single source was also located just in the top graphite reflector. Polyethylene blocks were then placed in each layer with the exception of layer three of the stand (TGS scanning layers 6 and 7). Measurements with polyethylene, borated polyethylene, iron, aluminum, and graphite were measured. Figure 3 shows the results of these measurements. Figure. A time-integratedgamma ray obtained for the measurement results shown in Figure 2. The ROIs highlighted in yellow are the peaks used for the tomographic analysis. The peaks highlighted are those used for the measurements, in addition to the Cd(n,y) line at 558 kev. Selenium-75 and %o are used for transmission measurements to determine the sample gamma-ray attenuation. The figure displays an inset spectrum with the entire energy range, while the enlarged region displays some peaks of interest. Normal TGS operations simply use the 7 5 ~source e for gamma-ray attenuation measurements. Pair production is estimated as contributing less than 20% of the gamma-ray attenuation at 2223 kev and has not been factored in. The measured gamma-ray production rate for each of the materials as a function of layer or depth into the sample is shown in Fig. 2. 2

6 energies. The detector also views a cone through the sample. The layered geometry for these measurements enables the germanium detector to view parts neighboring layers on the far side of the stand during scans. Thus the 2223-keV gamma-ray is measured for positions 6 and 7. The difference in neutron capture in hydrogen seen for these two measurements is calculated in MCNP and represents a method of characterizing material in different regions as scattering/moderating neutrons or as capturing neutrons. This is a method we are hoping to use to provide information to help determine the source-tosample coupling term. Interrogation with neutron sources of different energy spectra could also aid in this decoupling, as MCNP modeling has indicated. Measurementof Nsumn hlsnctbn Rats for Mafir Clung08 0 byen 6 and Law in h u m Figure 3. The measured gamma-rayproductionrate for each layer. Comparison for different matrix materials in layers 6 and 7. The aluminum and graphite data are not shown for clarity, they are nearly identical to one another, and mimic the polyethylene result within measurement error. The solid blue line represents the H(n,y) reaction for each layer in the measurement with polyethylene in layers 6 and 7. The maroon line comprised of small dashes represents the H(n,y) reaction for each layer with the borated polyethylene in layers 6 and 7. The red line made of large dashes represents the H(n,y) reaction for each layer in the measurement with iron in layers 6 and 7. The solid maroon and red lines represent the?b(n,ay) and Fe(n,n y) production rates respectively. The TGS instrument does not have perfect collimation, especially for the higher gamma-ray Conclusion We are beginning a series of measurements to quantitatively measure heterogeneous matrices via prompt neutron activation tomography. This paper presented some of our results for simple systems in which we are trying to validate MCNP for use in modeling the PGNAA measurements and also a possible technique for determining the source-tosampling coupling term. Acknowledgment This work is supported by the US Department of Energy. 3