A 7{9 MeV isotopic gamma ray source for detector testing. Abstract. An isotopic source of high energy gamma rays has been constructed and tested.

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1 A 7{9 MeV isotopic gamma ray source for detector testing Joel G. Rogers a, Mark S. Andreaco b, and Christian Moisan a a TRIUMF, 4004 Wesbrook Mall, Vancouver, B.C., Canada V6T 2A3 b CTI, 810 Innovation Drive, Knoxville, TN 37932, U.S.A. TRI{PP{96{7 Apr 1996 Abstract An isotopic source of high energy gamma rays has been constructed and tested. The source uses a long-lived americium-beryllium neutron source to produce fast neutrons, which are moderated in paran and then absorbed in a 76 mm diameter cylinder of metallic nickel. The radiative capture of thermal neutrons in natural metallic Ni produces 5 prominent gamma ray energies in the range 7.5 to 9 MeV, of which the MeV line is the strongest by more than a factor of two. The source was optimized by measuring gamma ray energy spectra with a 76 mm diam NaI detector, and then used to test an imaging BGO detector with gamma rays in the energy range 7.5 to 9 MeV. (submitted to Nuclear Instruments and Methods) Corresponding author. rogers@rhythm.triumf.ca.

2 1. Introduction An imaging gamma ray detector has been developed as part of a contraband detection system [1]. To test the detector, we needed a source of gamma rays at or near the system design energy of 9.17 MeV. Available isotopic sources at TRIUMF included a 1 Ci americium beryllium source, which produces neutrons in the range 0{10 MeV [2]. Starting with this 1 Ci Am-Be source, we constructed a compound source to produce gamma rays in the 7-9 MeV range by a two-step nuclear process: thermalization of the neutrons in paran followed by radiative thermal neutron capture on nickel. Table 1 shows the expected strengths of the strongest gamma ray lines from that process on Ni, taken from the compilation by Troubetzkoy and Goldstein [3]. The table shows that the MeV gamma ray production rate is stronger than that of the next lower energy gamma ray by more than a factor of two. The next most promising metal considered for photon production was chromium. However, the inclusive production rate for gamma rays with energies above 7.5 MeV would be almost a factor of two lower than for nickel [3]. For detector testing, the potential advantages of this isotopic source compared to the contraband detection system's accelerator source of gamma rays [1] are that it is relatively inexpensive, portable, and absolutely stable in ux. Its disadvantage is that the desired 9 MeV gamma rays are accompanied by several others from Ni in the range 7.5 to 9.0 MeV, as well as larger numbers of lower energy gamma rays from (n,) reactions in surrounding passive materials and from the scintillation crystal material itself. However, at the low rates needed for detector testing, the lower energy sources of gamma rays can be rejected using the energy discrimination of the detector being tested. Only signals in the energy range above 7.5 MeV were accepted by the detector for imaging. The imaging performance of the detector is known to vary only slightly with energy, so that the expected distribution of energies among the ve strongest gamma lines shown in Table 1 would be acceptable for our purposes. The remainder of this paper describes the construction of the source and its characterization in terms of the measured ux of gamma rays in the range above 7.5 MeV. 2. Experimental method The source was constructed as shown in g. 1. A plywood box measuring cm was lled with paran except for a cm void which was centered on the 6060 cm horizontal section of the paran. At the bottom of the void, the 2.24 cm diam Am-Be source was placed in a clearance hole drilled in the paran. The cylindrical source capsule, was closely surrounded on all sides by paran except for its top face, which was ush with the bottom of the void. Various congurations of nickel and paran were arranged in the void above the source capsule. The rate of high energy gamma rays was measured for each conguration, to determine the conguration which maximized the gamma ray ux. Gamma ray energy spectra were measured using a 76 mm diam 76 mm long Harshaw Matched Window NaI scintillation detector, which viewed the source capsule at a distance of 50 cm from its top. The detector was connected to an Ortec 4890 preamp-amplier-sca NIM module. The single-channel-analyzer(sca) output of the Ortec module fed a 2

3 scaler and also provided a gate signal into a multichannel analyzer which acquired an energy-gated energy spectrum. The contents of the multichannel analyzer's memory was displayed on an oscilloscope screen and used to accurately set the SCA levels to count only gamma rays which had energies in a selected energy band. The lower SCA level of 7.5 MeV served two purposes: it selected gamma rays of approximately the energy of interest in the contraband detection system [1] and also helped eliminate gamma rays from other materials than the Ni. Suppressing as much as possible the detection of gamma rays from other sources than the Ni was important because only gamma rays from the Ni arrive at the detector with the correct geometrical relationship to the detector's position for imaging. Although the imaging detector can measure the position of gamma rays of other energies, the formation of an image requires that the gamma rays originate in a compact source, on-axis and at a large distance from the detector. Uncontrolled sources of room background, even with the correct energy, would not be properly imaged. Common materials, such as iron and nitrogen, also have substantial cross sections for producing gamma rays with energies above 9 MeV. To reject possible room background from any such unknown sources, an upper level discriminator was also employed in the electronics. By setting the SCA window to cuto above 9.5 MeV, only gamma rays with energies in the desired range were counted. The most important parameter varied was the thickness of Ni above the source. With too little Ni, not enough thermal neutrons will interact. With too much Ni, most of the gamma rays emitted in the Ni will be absorbed and won't reach the detector. A second parameter varied was the thickness of paran on top of the Ni, between the Ni and the detector. This paran thermalizes and reects epithermal neutrons which would otherwise pass through the Ni and be lost into the void. The thickness of Ni and the thickness of paran above the Ni were separately varied and the rate of gamma rays detected in the range 7.5 to 9.5 MeV measured for each thickness. A 76 mm diam 99% pure metallic \Nickel-200" rod was bandsaw cut into \hockey puck" sized wafers, each measuring 76 mm diam by 25 mm long. These pucks were added one at a time to form a cylindrical stack, centred on-axis between the Am- Be source capsule and the NaI detector. The rate of gamma rays in the selected energy range was measured after each addition of Ni. The gated energy spectrum was continuously monitored in the multichannel analyzer to verify that the SCA level and electronic gain of the detector/amplier combination remained stable during the course of the measurements. 3. Results Fig. 2 shows the gamma energy spectrum, measured with the 76 mm NaI, for the case of 25 mm of Ni plus 76 mm of paran above the 1 Ci Am-Be source. Of all the congurations tested, this combination gave the highest rate of gamma rays in the selected energy window. The spectrum shows three prominent gamma rays, as indicated by the three arrows: 2.22 MeV from p(n,)d in paran, 4.44 MeV directly from the Am-Be source, and 8.99 MeV from thermal neutron capture in Ni. Just below the indicated 4.44 and 8.99 MeV photopeaks are other peaks from single- and double-escape of the secondary MeV gamma rays produced in the NaI. The singleescape peak from the 8.99 MeV gamma ray is partially obscured by the photopeak of 3

4 the 8.53 MeV line from Ni. The spectrum in g. 2 was used to determine the energy scale of the multichannel analyzer's pulse-height conversion, with a view to setting the SCA's lower and upper levels. Fig. 3 shows the channel numbers of the three photopeaks mentioned above, as a function of the energy of the gamma rays which caused them. The straight line veries that the energy/calibration system is linear and that the zero-oset of the multichannel analyzer is small. The vertical lines in g. 3 indicates the positions which were chosen for the 7.5 MeV lower and 9.5 MeV upper level discriminators. Fig. 4 shows a gated energy spectrum which includes only gamma rays in the selected energy band. The rate of gamma rays detected in the selected range was measured using a scaler to count the output pulses from the SCA for measured periods of time. The times were chosen to be long enough to accumulate more than 10,000 counts from each conguration, so that the statistical standard deviation of each measurement was typically smaller than 1% of the measured value. Fig. 5 shows the variation of the measured count-rate caused by varying the thickness of Ni. The maximum counting rate was obtained with a 25 mm thick puck of Ni, overlayed by 76 mm of paran. The uncertainty of the peak value was estimated by repeating this conguration four times, at intervals of several hours, stacking and unstacking the Ni and paran for each measurement. The error bar on the peak value in g. 5 shows the total span of the four measurements which were 16.6, 16.2, 15.6, and 16.2 counts per second (cps). Fig. 6 shows the variation of count-rate of gamma rays as a function of the thickness of paran above a 25 mm thick puck of Ni. As discussed above, non-ni gamma rays can confuse the imaging performance of the contraband detector, because they enter the detector from a direction other than from the direction of the Ni source. For this reason, an eort was made to measure the rate of such non-ni background gamma rays. As shown in g. 5, even without any Ni in the source, the rate of gamma counting is 3.2 cps, about 20% of the peak rate (16 cps), which was the value measured with the optimum 25 mm thickness of Ni in place. In a geometry similar to that of the contraband detection system [1], we measured the transmission factor for gamma rays of the selected energy through a standard mm thick lead (Pb) brick attenuator, which was positioned on-axis between the source and detector, just above the surface of the paran. Over the selected energy range of 7.5 to 9.5 MeV, the transmission factor of gamma rays through 50 mm of Pb is known to vary from.06 to.05 [4]. However, gamma rays which originate from anywhere in the room outside the shadow of the Pb, which masked only the Am-Be source, Ni, and a small portion of the oor below the paran box, may still reach the detector without passing through the small Pb brick. These gamma rays are unattenuated by the Pb and therefore cause the apparent transmission factor to be bigger than the expected.057 average transmission over the 7.5 to 9.5 energy range. The apparent transmission factor of the gamma rays in the selected band was measured to be for the conguration which maximized the counting rate. 4. Discussion and conclusions Figs. 5 and 6 show that the counting rate is a strong function of the thickness of Ni used to convert the thermal neutrons to high-energy gamma rays, and a rather weak function of the amount of paran. We selected 25 mm of Ni and 76 mm of paran as 4

5 optimum. The discrepancy between the measured transmission factor for 50 mm of Pb and the expected value of is partially due to a background of high energy gamma rays coming from directions other than from the Ni source. If this were the only cause of discrepancy, the apparent transmission factor (T a ) would be related to the true transmission factor(t t ) and the fractional rate of contamination gamma rays (F c ) by: F c = (T a -T t )/(1-T t ) = (0.130{0.056)/(1.0{0.056) = This contamination fraction of 0.08 translates to about 1.3 cps of non-ni background out of the maximum of 16 cps total rate in the selected energy band. It is also possible that some of the extra counts with the Pb degrader come from (n; ) reactions in the Pb itself, which has a strong gamma line at 7.38 MeV [3]. Although the mean energy of this line is below the SCA's lower energy discrimination level, the tail may still contribute in the selected band due to the nite energy resolving power of the NaI detector. No eort was made to estimate the size of this eect quantitatively. The compound isotopic source, as described herein, is a practical and convenient source of gamma rays in the range 7.5 to 9.0 MeV. It combines the features of low cost, portability, and excellent long-term stability, that are not present in acceleratorbased gamma ray sources. The drawbacks of low overall counting rate and the presence of many lower energy gamma rays were found not to be a problem for the intended use of this source. Low energy gamma rays were rejected using energy discrimination, implemented with a single channel analyzer. The source was used successfully to image various phantom objects with a position sensitive BGO detector. The results of these tests will be published in a future article [5]. Stronger Am-Be sources than the 1 Ci one we used, up to 25 Ci, are listed in the Amersham catalogue [2]; these could also be used to increase the counting rate. References [1] J.J. Sredniawshi, T. Debiak, E. Kamykowski, J. Rathke, P. Schmor, B. Milton, G. Stanford, J. Rogers, J. Boyd, and J. Brondo, \A proof-of-principle contraband detection system for non-intrusive inspection", SPIE Proceedings of the 5th International Conference on Applications of Nuclear Techniques, Crete, Greece, June, [2] \Radiation sources for industrial gauging and analytical instrumentation", Amersham Corporation, 2636 S. Clearbrook Drive, Arlington Heights, IL 60005, U.S.A., 1995 Catalog. [3] E. Troubetzkoy and H. Goldstein, \A compilation of information on gamma ray spectra resulting from thermal neutron capture", USAEC Report, ORNL-2904 Oak Ridge National Laboratory, [4] R.D. Evans, The Atomic Nucleus, (McGraw-Hill, New York, (1955), p [5] J.R. Rogers, C. Moisan, A. Altman, and E. Kamykowski, \A position sensitive detector for 5-10 MeV gamma rays", to be presented at the upcoming IEEE Nuclear Science Symposium in Anaheim, CA, November, 1996 and to be published in the Conference Issue of IEEE Trans. Nucl. Sci. 5

6 Table 1 - Thermal (n,) Rates from natural Ni taken from ref. [3] Gamma Energy (MeV) Rate (photons/100 captures) Figure Captions 1. Schematic drawing of the compound isotopic source construction. 2. Multichannel analyzer (MCA) gamma ray pulse height spectrum from the isotopic source. The ordinate is on a logarithmic scale. 3. Pulse height vs. energy for three prominent gamma ray photopeak positions, from the spectrum of g Pulse height spectrum of gamma rays gated by an SCA gate of 7.5 to 9.5 MeV. The ordinate is plotted on a linear scale. 5. NaI detector counting rate as a function of Ni thickness, for an energy band of 7.5 to 9.5 MeV. 6. NaI detector counting rate as a function of paran thickness. 6

7 Fig. 1 Fig. 2 7

8 Fig. 3 Fig. 4 8

9 Fig. 5 Fig. 6 9