Evaluation of a Boron-Lined Neutron Detector

Transcription

1 Evaluation of a Boron-Lined Neutron Detector Prepared by: B. M. van der Ende Atoemic Energy of Canada Limited Chalk River, ON K0J 1J0 Scientific Authority: Guy Jonkmans DRDC Centre for Security Science The scientific or technical validity of this Contract Report is entirely the responsibility of the Contractor and the contents do not necessarily have the approval or endorsement of the Department of National Defence of Canada. Defence Research and Development Canada Contract Report DRDC-RDDC 2014-C April 2014

2 IMPORTANT INFORMATIVE STATEMENTS This project Boron lined neutron detector evaluation CSSP-2013-CD-1028, was supported by the Canadian Safety and Security Program (CSSP) which is led by Defence Research and Development Canada s Centre for Security Science, in partnership with Public Safety Canada. Partners in the project include Atomic Energyof Canada Limited, Natural Resources Canada, and Canada Border Services Agency The CSSP is a federally-funded program to strengthen Canada s ability to anticipate, prevent/mitigate, prepare for, respond to, and recover from natural disasters, serious accidents, crime and terrorism through the convergence of science and technology with policy, operations and intelligence Her Majesty the Queen in Right of Canada, as represented by the Minister of National Defence, 2014 Sa Majesté la Reine (en droit du Canada), telle que représentée par le ministre de la Défense nationale, 2014

3 Technical Evaluation EVALUATION OF A BORON-LINED NEUTRON DETECTOR TE-001 Revision 0 Prepared by Rédigé par Reviewed by Vérifié par van der Ende Bryan - Reactor Physicist Atanackovic Jovica - R & D Scientist Approved by Approuvé par Bentoumi Ghaouti - G. BENTOUMI O'Kane Mike MA - Branch Manager Applied Physics 2014/03/19 AECL - OFFICIAL USE ONLY Atomic Energy of Canada Limited Chalk River, Ontario Canada K0J 1J0 2014/03/19 À L'USAGE EXCLUSIF D'EACL Énergie Atomique du Canada Limitée Chalk River (Ontario) Canada K0J 1J0

4 Technical Evaluation Evaluation of a Boron-Lined Neutron Detector Research and Development TE-001 Revision March AECL - OFFICIAL USE ONLY mars 2014 À USAGE EXCLUSIF - EACL Atomic Energy of Canada Limited Énergie atomique du Canada limitée

5 Revision History Liste de révisions AECL - OFFICIAL USE ONLY À USAGE EXCLUSIF - EACL TE 001 Document Details / Détails sur le document Evaluation of a Boron-Lined Neutron Detector 37 For Release Information, refer to the Document Transmittal Sheet accompanying this document. Pour des renseignements portant sur la diffusion, consultez la feuille de transmission de documents ci-jointe. Revision History / Liste de révisions D1 2014/03/03 Issued for Review and Comment. B. M. van der Ende G. Bentoumi J. Atanackovic /03/19 Issued as Approved for Use B. M. van der Ende G. Bentoumi J. Atanackovic M. O Kane M. O Kane 01

6 TABLE OF CONTENTS SECTION PAGE 1. INTRODUCTION Purpose and Scope Abbreviations DETECTOR PRINCIPLES, DESCRIPTION, AND PERFORMANCE PARAMETERS Neutron Detection Principle BCS Detector Configuration Detector Performance Parameters The absolute neutron efficiency The gamma intrinsic efficiency The gamma absolute rejection ratio for neutrons Evaluation Goals DESCRIPTION OF MEASUREMENT SETUPS Neutron Tests Gamma Tests Gamma-plus-neutron Tests Data Acquisition Electronics MEASUREMENT RESULTS Neutron Efficiency Measurements Pulse height spectra and choice of discriminator level Effect of moderator thickness GEANT4 simulation: determine optimal moderator thickness Experiment: replacing the front incident moderator panel with smaller thickness Influence of BCS detector orientation Shadow cone measurements Discussion Gamma Insensitivity Measurements Gamma Absolute Rejection Ratio for neutrons (GARRn) SOME NOTES ON INTERFACING WITH PROPORTIONAL TECHNOLOGIES INC. AS A CUSTOMER SUMMARY AND CONCLUSIONS REFERENCES

7 TABLE OF CONTENTS SECTION PAGE 8. ACKNOWLEDGEMENTS TABLES Table 4-1: Summary of shadow cone measurements performed with the BCS detector. Counts are for 30 s real time, and corrected for dead time. The detector orientation was vertical Table 4-2: Summary of estimated detection efficiency for configurations employing 1 aluminum tubes, each consisting of seven 7 mm diameter boroncoated straws FIGURES Figure 2-1: Schematic of a cylindrical boron-coated straw, depicting the detection of alpha particle resulting from neutron capture by 10 B. The 7 Li ion produced (not shown) recoils in the direction opposite to the alpha particle s trajectory at the point of neutron capture Figure 2-2: The BCS detector from Proportional Technologies, Inc. It is shown together with a DC ±5 V power supply for the preamplifiers and summing amplifier of the detector, a biasing high voltage power supply, and an oscilloscope for observing the detector output Figure 2-3: The 3 He detector from Natural Resources Canada Figure 3-1: The setup employed for shadow cone measurements. This shadow cone is placed 14 cm in front of the holder where the 252 Cf source appears when it is exposed. The BCS detector, encased in HDPE moderator, appears in the background in photo (b.) Figure 3-2: The setup employed for gamma insensitivity measurements in GC60. The 3 He detector appears in photo (a), while the BCS detector, encased in HDPE moderator, appears in photo (b) Figure 3-3: The setup employed for gamma insensitivity measurements in B469. The holder where the 192 Ir appears after it is exposed appears in the foreground Figure 3-4: The setup employed for GARRn measurements in HPNG. In photo (a), the 252 Cf source holder appears in the foreground, and the 192 Ir source holder appears behind the BCS detector, attached to a pylon. The glow of the pylon in photo (b) is due to camera flash Figure 3-5: Photo of NIM modules, displaying settings and connections employed for neutron counting

8 TABLE OF CONTENTS SECTION PAGE Figure 4-1: A typical pulse height spectrum for the boron-lined neutron detector. In this case, the 252 Cf source-detector distance is 2 m, and the counts are collected over 30 s Figure 4-2: Percentage change in the MCA counts when varying the lower bound of the ROI from channel 34, between channels 0 and Figure 4-3: Snapshot of a GEANT4 simulation investigating the ideal moderator thickness to thermalize neutrons emitted by a 252 Cf source Figure 4-4: GEANT4 detector simulation counts vs. moderator thickness Figure 4-5: Energy distribution of neutrons detected in the GEANT4 simulation. In this case, the source-detector distance was set to 2 m, and the moderator thickness was 5 cm Figure 4-6: Count rate of the BCS detector versus source-detector distance, when exposed to a 252 Cf source for differing moderator thickness configurations: encasing entirely in 2 thick HPDE moderator (red squares), and replacing a 2 panel with a 1 thick panel at the front incident side (black circles) Figure 4-7: Count rate of the BCS detector versus source-detector distance, when exposed to a 252 Cf source for differing detector orientation: horizontal orientation (red squares), and vertical orientation (black circles) Figure 4-8: Count rate of the BCS detector (without shadow cone) versus inverse source-detector distance squared, for distances in the range of 2 to 4 m. The BCS detector was in the vertical orientation, with 2 thick moderator on all sides Figure 4-9: Efficiency in detection of neutrons from moderated 252 Cf versus the number of 4 mm diameter BCS straws mounted in a hexagonal array surrounded by HDPE moderator, from Proportional Technologies, Inc. Data is taken from Refs. [7,12,13] Figure 4-10: Pulse height spectrum for the BCS detector when irradiated with a 20 mr/hr 137 Cs gamma field, in GC60 facility Figure 4-11: BCS detector response to 192 Ir gamma irradiation, in the vertical orientation, versus LLD Figure 4-12: BCS detector response to 192 Ir gamma exposure, in the horizontal orientation, versus LLD Figure 4-13: 3 He detector response to 192 Ir gamma irradiation versus LLD Figure 4-14: BCS detector background without 192 Ir gamma irradiation Figure 4-15: Average gamma insensitivity versus LLD, for the BCS detector in vertical (black squares) and horizontal (red circles) orientations, and for the 3 He detector Figure 4-16: Gamma absolute rejection ratio for neutrons versus gamma exposure rate, for 252 Cf BCS detector distances of 2 m (red triangles) and 4 m (black squares)

9 1. INTRODUCTION Proportional gas detectors filled with 3 He are the gold standard for detection of thermal neutrons. 3 He has a large cross-section for the capture of thermal neutrons (5330 barns) [1], while remaining insensitive to the detection of gamma rays. 3 He detectors are typically simple and robust in design, and they are useful for a wide range of applications, including border security technology for monitoring of special nuclear materials [2]. Due to an increased demand for 3 He for security applications, and a very limited supply of 3 He [2[3], it has become imperative to develop alternative technology for the detection of neutrons. A commercially available technology that can serve as a viable replacement for 3 He detectors are boron-lined proportional counters. 10 B also has a high cross section for capturing thermal neutrons (3840 barns) [1]. Boron-lined neutron counters from Proportional Technologies Inc. (Houston, TX, USA) consist of a thin layer (~1 m thick) of solid 10 B 4 C deposited on a metal substrate. When the boron layer captures a neutron, it undergoes a nuclear reaction yielding a 6 Li ion and an alpha particle. These charged particles ionize the gas inside a proportional counter, which leads to the creation of a detector signal from charge collection driven by high voltage applied in the proportional counter [4]. 1.1 Purpose and Scope This report evaluates the performance of a particular boron-lined proportional counter manufactured by Proportional Technologies, Inc. (Houston, TX, USA). The report will describe the detector principles and design, as well as performance parameters that were used to assess the efficacy of the detector for detection of special nuclear materials, for border security purposes. The results of measurements performed with the detector at facilities at AECL Chalk River Laboratories are evaluated in terms of these parameters. 1.2 Abbreviations B469 Building 469 B513 Building 513 BCS 10 B-Coated Straw GARRn Gamma Absolute Rejection Ratio for neutrons GC60 Gamma Co-60 GEANT4 GEometry ANd Tracking, version 4 (Monte Carlo package) HDPE High-Density PolyEthylene HPNG Health Physics Neutron Generator NIM Nuclear Instrument Module PNNL Pacific Northwest National Laboratory PTI Proportional Technologies, Inc. ROI Region Of Interest SCA Single Channel Analyzer

10 2. DETECTOR PRINCIPLES, DESCRIPTION, AND PERFORMANCE PARAMETERS 2.1 Neutron Detection Principle The 10 B capture process has long been suggested as an alternative to 3 He. With an absorption cross section of 3840 barn, an incident thermal neutron may be captured by 10 B producing 7 Li and an alpha particle as follows [1]: Li(1.015MeV) 2He(1.777 MeV) 6% n 5B (2-1) Li(0.840 MeV) 2He(1.47 MeV) (0.478 MeV) 94% The 7 Li and the alpha particle are emitted from the point of neutron capture in exactly opposite directions. The direction distribution for the pair of ions is isotropic. In a boron-coated detector, the boron compound layer is made only a few micrometers thick, so that one of the two charged particles will escape into a nearby proportional gas chamber. In boron-coated straws (BCS) fabricated by Proportional Technologies Inc., the gas chamber is in a cylindrical geometry, where a 10 B 4 C layer lines the inside of the cylindrical (cathode) surface, and an anode wire is set through the center of the cylindrical chamber. The gas within the straw is a mixture of CO 2 and Ar in the molar ratio of 10/90. See Figure 2-1 for a diagram, as well as Figure 1 in Ref. [4]. Figure 2-1: Schematic of a cylindrical boron-coated straw, depicting the detection of alpha particle resulting from neutron capture by 10 B. The 7 Li ion produced (not shown) recoils in the direction opposite to the alpha particle s trajectory at the point of neutron capture. 2.2 BCS Detector Configuration The BCS detector configuration evaluated in this report consists of seven sealed aluminum tubes (2.54 cm diameter, 1 m long), each containing 7 enriched 10 B 4 C coated straws (BCS) of 7.5 mm diameter, and 1 m length. Each straw is filled with Ar/CO 2 gas (90/10 ratio) at 10.5 psi. The overall BCS detector therefore has a total number of 49 sealed straws, with an active area of 1 m x 0.18 m. Figure 2-2 below displays the detector.

11 Figure 2-2: The BCS detector from Proportional Technologies, Inc. It is shown together with a DC ±5 V power supply for the preamplifiers and summing amplifier of the detector, a biasing high voltage power supply, and an oscilloscope for observing the detector output. 2.3 Detector Performance Parameters The performance evaluation centres on three key parameters [5,6]: 1) absolute neutron detection efficiency ( abs n ), 2) intrinsic efficiency of gamma rays detected as neutrons (gamma insensitivity) ( int ), and 3) Gamma Absolute Rejection Ratio in the presence of neutrons (GARRn) The absolute neutron efficiency The absolute neutron efficiency ( abs n ) represents the number of pulses recorded per number of radiation quanta emitted by the source in a given geometry. PNNL guidelines [5] state that an acceptable neutron detector should read a count rate of 2.5 cps when a 252 Cf source of 1 nanogram (ng) surrounded by 0.5 cm thick lead (for blocking gamma rays) and 2.5 cm thick polyethylene (for moderating the neutron spectrum), is placed at a distance of 2 m from the detector [5,6]. This efficiency is often reported in cps/ng, and it connects the rate of signal detection to the activity of the nuclear material being detected. Note: 10 ng of 252 Cf emits x 10 4 n/s [7,8] The gamma intrinsic efficiency The gamma intrinsic efficiency int measures the response of a neutron detector to the presence of a gamma ray field. It is the net number of registered counts divided by the number of gamma photons striking the detector. At an exposure rate of 10 mr/hr (0.1 msv/hr), a well-designed 3 He

12 detector has a ratio int, ~ It is reasonable to require int 10-6 for good gamma ray insensitivity at an exposure level of 10 mr/hr [5,6] The gamma absolute rejection ratio for neutrons The gamma absolute rejection ratio for neutrons (GARRn) measures the detector response in the presence of both a large gamma ray source and a neutron source. GARRn is defined as the absolute neutron detection efficiency in the presence of both sources ( abs n ), divided by the absolute neutron detection efficiency ( abs n ): GARRn = abs n / abs n Where the gamma ray source has no impact on neutron detection, the GARRn = 1. The proposed specification for acceptable neutron detection is 0.9 < GARRn < 1.1 at 10 mr/hr exposure [5,6]. 2.4 Evaluation Goals The overall objective of this test evaluation is to determine the real performance of the boronlined detector from Proportional Technologies, Inc., for each of the three above parameters. This evaluation will be performed in comparison with a 3 He detector on loan from Natural Resources Canada. The 3 He detector is a RSN-4 model from Radiation Solutions, Inc. (Mississauga, ON, Canada), employing a RS-705 mobile data acquisition unit. The detector consists of four model cylindrical 3 He tubes from LND, Inc. (Oceanside, NY, USA). The 3 He pressure in each tube is 2052 torr (39.68 psi). Each tube has an effective length of (81.27 cm), and an effective diameter of 1.96 (4.98 cm). Figure 2-3 displays the 3 He detector. Figure 2-3: The 3 He detector from Natural Resources Canada

13 3. DESCRIPTION OF MEASUREMENT SETUPS 3.1 Neutron Tests Measurements of the absolute neutron efficiency were performed at the Health Physics Neutron Generator (HPNG) facility in B513 at AECL. The HPNG facility is equipped with a calibrated 252 Cf source (half-life of yr). The current source was purchased from Frontier Technology Corporation (Xenia, OH, USA), which on Feb. 18, 2010 had a neutron output of 2.3 x 10 8 n/s. The facility is also equipped with calibrated Bonner sphere and rotating proton recoil spectrometers that have carefully characterized the neutron dose and fluence at various distances from the 252 Cf source in the facility room. These characterizations were carried out using solid, boron-loaded shadow cones, as prescribed in the ISO 8529 standard [9]. In particular, the shadow cone method provides a means of determining the component of detected neutron flux that is incident directly from the source, in isolation from the component of detected neutron flux that is incident from scattering in the facility room. By performing shadow cone measurements with a neutron detector at various source-detector distances, the direct component of the neutron count rate can be determined in each case. Figure 3-1 displays the setup employed for shadow cone measurements. Table 3-1 provides the decay-corrected source neutron emission rate, and the corresponding 252 Cf mass on the dates that shadow cone measurements were performed. (a) Figure 3-1: The setup employed for shadow cone measurements. This shadow cone is placed 14 cm in front of the holder where the 252 Cf source appears when it is exposed. The BCS detector, encased in HDPE moderator, appears in the background in photo (b.) (b)

14 Table 3-1: 252 Cf mass and neutron emission rate on days when shadow cone measurements were performed in HPNG. Date 252 Cf Neutron Emission Rate (n/s) 252 Cf mass ( g) Dec. 19, Jan. 15, Jan. 16, Jan. 21, Jan. 22, Gamma Tests Measurements of the gamma intrinsic efficiency (or, gamma insensitivity) were performed at the Gamma Co-60 (GC60) facility in B513 at AECL (Figure 3-2), as well as in a gamma radiography facility in B469 at AECL (Figure 3-3). Initial measurements were performed at GC60, using 60 Co and 137 Cs sources. Subsequent, and the most reliable measurements, were performed in B469, using a 192 Ir source (half-life of days). The source-detector distance was varied to produce various gamma exposure fields at the detector face, and the registered counts of the detector were recorded. (a) Figure 3-2: The setup employed for gamma insensitivity measurements in GC60. The 3 He detector appears in photo (a), while the BCS detector, encased in HDPE moderator, appears in photo (b). (b)

15 Figure 3-3: The setup employed for gamma insensitivity measurements in B469. The holder where the 192 Ir appears after it is exposed appears in the foreground. 3.3 Gamma-plus-neutron Tests Measurements of the gamma absolute rejection ratio for neutrons (GARRn) were conducted in the HPNG facilities at AECL (Figure 3-4). For these measurements, the detector was exposed to both 252 Cf and 192 Ir sources simultaneously, at various source-detector distances. The corresponding registered counts of the detector were recorded. (a) Figure 3-4: The setup employed for GARRn measurements in HPNG. In photo (a), the 252 Cf source holder appears in the foreground, and the 192 Ir source holder appears behind the BCS detector, attached to a pylon. The glow of the pylon in photo (b) is due to camera flash. (b)

16 3.4 Data Acquisition Electronics BCS neutron detector measurements were collected using an AmpTek PocketMCA 8000A, as well as with a Nuclear Instruments Module (NIM) counter. The PocketMCA records signal output from a neutron detector that is shaped through a NIM shaping amplifier, using either an Ortec 570 or Ortec 671 amplifier. Dead time in the recorded count rate was corrected by multiplying it by the ratio of the MCA s real time to live time. The NIM counter (either an Ortec 996 or Ortec 776 model) received detector signal that was relayed though a shaping amplifier (Ortec 570 or Ortec 671), followed by a single channel analyzer (Ortec 550A or Ortec 406A), and finally a gate and delay generator (Ortec 416A) see Figure 3-5. The role of the single channel analyzer was to set the lower level discriminator (LLD), and the role of the gate and delay generator was to set a fixed delay time longer the delay time of the acquisition electronics, which then became the new effective dead time of the acquisition system. Then a non-paralyzable model was used to correct for dead time in the recorded count rate [1]. For these measurements, the delay time was set to 12 s. The 3 He detector was designed to collect data through its own self-contained RS-705 data acquisition unit, so the above methods did not apply to this detector. The RS-705 unit recorded and stored counts binned in 1 second intervals, up to a 24 hour period of data acquisition time.

17 Figure 3-5: Photo of NIM modules, displaying settings and connections employed for neutron counting

18 4. MEASUREMENT RESULTS 4.1 Neutron Efficiency Measurements Pulse height spectra and choice of discriminator level A representative pulse height spectrum from the BCS neutron detector while registering neutrons from an exposed, bare 252 Cf source in AECL s HPNG facility, is shown in Figure 4-1. The spectrum shows some energy pile-up near the zeroth channel, which is due to the presence of gamma emission from the 252 Cf source. Recent literature [10] has demonstrated that the contribution of gamma dose is about 5% of the total 252 Cf neutron dose. This confirms that the gamma energy pile-up should form only a small portion of the overall pulse height spectrum. Figure 4-1: A typical pulse height spectrum for the boron-lined neutron detector. In this case, the 252 Cf source-detector distance is 2 m, and the counts are collected over 30 s. Figure 4-1 also shows a relatively low plateau region for channels 20 to 60, preceding the majority of neutron counts that appear at higher channels. The region of interest (ROI) of the pulse height spectrum was defined to be channels 34 to 511 (end of the spectrum), where channel 34 appears near the center of the plateau region. Integration of the spectrum over the ROI provides the number of neutron counts. This choice of ROI serves to effectively discriminate against counts due to gamma events, while minimally sacrificing counts due to neutron events. Based on the typical spectrum shown in Figure 4-1, Figure 4-2 below demonstrates how the counts change by at most 15% when setting the lower bound of the ROI to other channels in range of 0 to 60. Counts from the MCA defined with the ROI of channels 34 to 511 agree with NIM counter counts with the LLD of the single channel analyzer set to 320 mv, typically within 5%. The LLD of the NIM counter was therefore set to 320 mv for neutron counting. Neutron counting data for the BCS detector presented hereafter in this report will display NIM counter data.

19 Figure 4-2: Percentage change in the MCA counts when varying the lower bound of the ROI from channel 34, between channels 0 and Effect of moderator thickness GEANT4 simulation: determine optimal moderator thickness The optimal thickness of high density polyethylene (HDPE) moderator material with which to encase the BCS detector, was determined with preliminary calculations carried out with GEANT4 (GEometry ANd Tracking) Monte Carlo package [11]. The simulations consisted of a bare 252 Cf source positioned at a specified distance from a 1 m x 0.18 m detector encased with HDPE of varying thickness. The source emitted neutrons uniformly over a 4 solid angle in the simulation, and the number of neutrons incident in the detector plane was tallied. This was done for source-detector distances of 2 m and 4 m. A snapshot of the simulation is shown in Figure 4-3. The counts are plotted versus moderator thickness in Figure 4-4, which indicates that 5 cm ( 2 ) moderator thickness appears to admit the most neutrons to the detector. The energy distribution of the neutrons arriving at the detector for this moderator thickness is shown in Figure 4-5; the distribution resembles a typical Maxwellian representation of a thermal neutron spectrum, with a peak position value close to ev. Based on these simulation results, 2 thick moderator material was designed and fabricated with which to surround and moderate the BCS detector.

20 Figure 4-3: Snapshot of a GEANT4 simulation investigating the ideal moderator thickness to thermalize neutrons emitted by a 252 Cf source Figure 4-4: GEANT4 detector simulation counts vs. moderator thickness

21 counts neutron energy/ev Figure 4-5: Energy distribution of neutrons detected in the GEANT4 simulation. In this case, the source-detector distance was set to 2 m, and the moderator thickness was 5 cm Experiment: replacing the front incident moderator panel with smaller thickness Further discussion also suggested that neutrons might be better thermalized and detected if the front moderator thickness was made thinner than the other sides of the detector. Measurements in HPNG were therefore conducted with boron-lined detector surrounded by 2 inch thick moderator on all sides, as well as with the front panel replaced with 1 inch thick moderator. Figure 4-6 below shows the BCS detector counts versus distance for the two moderator thickness configurations. The graph shows that the two configurations give comparable detector count rates in when exposed to an unmoderated 252 Cf neutron energy spectrum.

22 Figure 4-6: Count rate of the BCS detector versus source-detector distance, when exposed to a 252 Cf source for differing moderator thickness configurations: encasing entirely in 2 thick HPDE moderator (red squares), and replacing a 2 panel with a 1 thick panel at the front incident side (black circles) Influence of BCS detector orientation The influence of BCS detector orientation was also investigated. For a moderator thickness of 2 on all sides, the count rate of the detector was measured when placing its broader side parallel to the floor (horizontal configuration), as opposed to perpendicular to the floor (vertical configuration). Figure 4-7 displays the measurements versus source-detector distance for these two orientations. The horizontal orientation presents a front incident area that is a factor of 7 lower than for the vertical orientation, yet the count rate is reduced by a factor of less than 2. This suggests that the horizontal orientation detects more neutrons per unit incident area, than does the vertical orientation. This is due to each incident neutron having greater likelihood of being detected in the horizontal configuration, than in the vertical configuration, since there are more BCS straws behind the incident area in the horizontal configuration, than in the vertical configuration.

23 Figure 4-7: Count rate of the BCS detector versus source-detector distance, when exposed to a 252 Cf source for differing detector orientation: horizontal orientation (red squares), and vertical orientation (black circles) Shadow cone measurements The measurements in Figures 4-6 and 4-7 include both direct and scattering components in the neutron count measurements. To measure only the direct component of the neutron counts, the shadow cone method described in the previous section was employed. Shadow cone measurements were performed at a source-detector distance of 2 m, as this is the distance that applies in the 2.5 cps/ng neutron detection efficiency standard mentioned in the Introduction section. However, due to the large active area of the BCS detector, and the size limitations of the shadow cones, the largest shadow cone could not provide full shadow coverage of the BCS active area at 2 m distance. As such, shadow cone measurements were also performed at 4 m, where the largest shadow cone could provide adequate shadowing, and therefore enable an accurate estimate of the direct neutron counts. Table 4-1 summarizes the results of these shadow cone measurements.

24 Table 4-1: Summary of shadow cone measurements performed with the BCS detector. Counts are for 30 s real time, and corrected for dead time. The detector orientation was vertical. Source- Detector Distance (m) Moderator Configuration 2 1 front + 2 other sides 4 1 front + 2 other sides 252 Cf mass (ng) 35,512 35, all sides 36,389 Counts without Shadow (Total) Counts with Shadow (Scattered) Difference (Direct) Detection Efficiency (cps/ng) The detection efficiency of the BCS detector at 2 m distance from the source was found to be 0.38 cps/ng. This is quite a bit lower than the 2.5 cps/ng standard. However, the number is suspect since the cone did not properly shadow the detector at 2 m. Since the detector was not properly shadowed, some of the direct component of neutron flux incident on the detector was included in the scattering component measurement (counts with shadow). The size of the direct component (the difference between counts without shadow and counts with shadow) is then underestimated, and therefore so is the detection efficiency. The efficiency of cps/ng estimated at 4 m distance is more reliable, since the detector was properly shadowed at this distance. Figure 4-8 demonstrates that the BCS detector count rate (without shadow cone) varies as 1/r 2 for distances in the range of 2 4 m. Figure 4-4 also indicates a factor of 3.9 between the counts tallied at 2 m, and the counts tallied at 4 m, in the GEANT4 simulation; this is close to what is expected from 1/r 2 intensity scaling. Using the cps/ng efficiency estimates at 4 m, simple 1/r 2 intensity scaling, where r is the sourcedetector distance, yields an estimate of efficiency at 2 m of about 0.7 cps/ng.

25 Figure 4-8: Count rate of the BCS detector (without shadow cone) versus inverse sourcedetector distance squared, for distances in the range of 2 to 4 m. The BCS detector was in the vertical orientation, with 2 thick moderator on all sides. There is an additional caveat to the results shown in Table 3-1. The 2.5 cps/ng standard stipulates that the 252 Cf source should be surrounded by 0.5 cm thick lead to reduce its gamma emission, as well as 2.5 cm thick polyethylene to moderate the emitted neutrons [2]. It was not possible to comply with these conditions in the measurements performed at HPNG. The measurements shown in this work are for an unmoderated 252 Cf source; only the detector was moderated. Based on measurements reported in literature, moderating the 252 Cf source can increase the efficiency estimate by a factor of 1.2 [7]. Even with this additional factor, the final estimated efficiency of the BCS detector (0.84 cps/ng) is well below the 2.5 cps/ng standard. Unfortunately, shadow cone measurements could not be carried out with the 3 He detector. The data acquisition unit that interfaces with the 3He detector is designed for optimal operation in low neutron fluence fields (less than ~1000 cps). The neutron fluence from the HPNG 252 Cf source was too high for the 3 He detector, such that the detector acquisition electronics could not yield meaningful counts Discussion Some comparison of the efficiency measurements in this report with published efficiency measurements of other BCS detectors should be made. Kouzes et al. [7] have tested a BCS neutron detector from Proportional Technologies, but in a different straw configuration. The detector consists of 171 straws, each of which are 4 mm in diameter. The straws are individually surrounded by moderator, in an array consisting of 9 rows of 19 tubes each. The array fit in a

26 space of m deep x m wide x 2.18 m tall, with an active surface area of m 2. For this configuration, they measured the neutron detection efficiency to be between 4.2 and 4.7 cps/ng. Lacy et al. [12] have conducted neutron efficiency measurements on BCS detectors from Proportional Technologies in a similar configuration. Using 85 straws, the measured sensitivity is between 3.34 and 3.45 cps/ng, and for 63 straws it is 2.86 cps/ng. The Proportional Technologies website [13] claims to have an efficiency of 3.55 cps/ng for 96 straws. These measurements are summarized in Figure 4-9, which demonstrates an approximately linear trend in the scaling. This trend is confirmed by Fig. 4 of Ref [12]. Figure 4-9: Efficiency in detection of neutrons from moderated 252 Cf versus the number of 4 mm diameter BCS straws mounted in a hexagonal array surrounded by HDPE moderator, from Proportional Technologies, Inc. Data is taken from Refs. [7,12,13]. The scaling of Figure 4-9 does not include our 0.84 cps/ng estimated for the 49 straws in the BCS detector evaluated in this report. This is because the straw configuration of the detector evaluated in this report is fundamentally different. Detectors employed for Figure 4-9 consist of 4 mm diameter straws individually surrounded by HDPE moderator in a hexagonal array. This appears to be more optimal than the configuration of the BCS detector evaluated in this work, which consists of 7 mm diameter straws bundled together in groups of 7, and surrounded by air in their 1 diameter aluminum tubes. Figure 4-7 suggests that to meet the 2.5 cps/ng standard with the configuration evaluated in this work, more 1 diameter tubes could be set behind each other. Figure 4-7 shows that the ratio in counts between vertical and horizontal configurations at 2 m is 1.53, even though the front incident area of the detector is reduced by a factor of 7 in the horizontal configuration. The vertical configuration has a single 1 diameter tube deep for a direct neutron incident on the front

27 face, whereas the horizontal configuration has seven 1 diameter tubes deep for a direct neutron incident on the front face. If one ignores for simplicity the scattering component in the counts shown in Figure 3-7, this demonstrates that having seven 1 diameter tubes deep improves detection efficiency over having a single 1 diameter tube deep, by a factor of 7/1.53 = Using this factor, and assuming that the efficiency in units of cps/ng will scale linearly with the size of the detector, Table 4-2 summarizes estimates of how much the neutron detection efficiency at 2 m source-detector distance could improve when using seven 1 tubes deep and five or seven 1 tubes wide. While these configurations are estimated to exceed the 2.5 cps/ng at 2 m standard, this is at the price of using more straws, more so than are employed in the configurations represented by Figure 4-9. Table 4-2: Summary of estimated detection efficiency for configurations employing 1 aluminum tubes, each consisting of seven 7 mm diameter boron-coated straws. No. of Tubes Wide No. of Tubes Deep No. of Straws Used Efficiency at 2 m (cps/ng) Although the 3 He detector was not able to yield meaningful measurements of the intense 252 Cf neutron field at HPNG, an estimate of its detection efficiency can be made, by examining its performance relative to the BCS detector. Of the moderator configurations available for the BCS detector, surrounding with a 1 thick HDPE panel on front, and 2 HDPE panels on other sides comes closest to the ¾ thick HDPE surrounding the 3 He tubes. With this configuration, the ratio of 3 He counts to BCS counts for identical neutron fields is 3.60 ± (For further details about the determination of this ratio, see Ref. [14].) Multiplying the BCS detection efficiency of unmoderated 252 Cf (0.7 cps/ng) and moderated 252 Cf (0.84 cps/ng) by this ratio yields an efficiency for the 3 He detector that meets or exceeds the 2.5 cps/ng standard in each case. 4.2 Gamma Insensitivity Measurements As described in section 2, measurements to determine gamma intrinsic efficiency were carried out in a radiography facility in B469. A Ci 192 Ir source was used as a source of gamma rays, and the source-detector distance was varied to effect different gamma exposure fields. The relationship between gamma exposure rate X and source-detector distance d is A X (4-1) 2 d where A is the activity of the source, and is the air kerma rate constant [15]. The air kerma rate constant value for 192 Ir was taken to be R m 2 /(Ci hr) [15]. The gamma ray exposure rate X in Roentgen/s (R/s) is also related to photon flux by

28 en E X 8 ( ) E (4-2) air where E is the average photon energy in MeV, ( en / ) air is the mass interaction coefficient of air in cm 2 /g, and is in s -1 cm -2 [16]. The average photon energy from the gamma emission spectrum of 192 Ir is 0.38 MeV. The mass interaction coefficient of air is interpolated to be cm 2 /g at 0.38 MeV [17]. One can solve for from Equation (4-2) to determine the photon flux for a given gamma exposure rate. (Note that estimating the exposure rate and photon flux in this way ignores any room scattering contribution.) From the photon flux and detector area, the incident photon rate can be determined and subsequently divided into the measured detector count rate response to the gamma field. This ratio estimates the gamma intrinsic efficiency. The response of the neutron detectors to gamma irradiation occurs primarily for low values of the Lower Level Discriminator (LLD) setting. This is demonstrated in the pulse height spectrum shown below in Figure Figure 4-10: Pulse height spectrum for the BCS detector when irradiated with a 20 mr/hr 137 Cs gamma field, in GC60 facility. Figures 4-11 and 4-12 show plots of the response of the BCS detector for varying values of LLD,. For the BCS detector, the LLD was varied through an Ortec 406A single channel analyzer. Figure 4-13 shows an analogous graph for the 3 He detector, wherein the LLD was varied through its RS-705 data acquisition unit. Figure 4-14 shows the background recorded by the BCS detector as a function of LLD, in the absence of gamma ray exposure.

29 Figure 4-11: BCS detector response to 192 Ir gamma irradiation, in the vertical orientation, versus LLD

30 Figure 4-12: BCS detector response to 192 Ir gamma exposure, in the horizontal orientation, versus LLD

31 Figure 4-13: 3 He detector response to 192 Ir gamma irradiation versus LLD Figure 4-14: BCS detector background without 192 Ir gamma irradiation

32 Figure 4-14 demonstrates that the background count rate of the BCS detector is 1 cps or lower, independent of the LLD setting. It should be noted that the background measurements include all sources of noise that would be present during a 192 Ir gamma irradiation, including the ticking klaxon alarm that sounds when gamma fields are present. In the case of the 3 He detector (Figure 4-13), the detector response to 10 mr/hr gamma exposure is essentially on the level of background count rates (1-2 cps). The low background count rate of the detectors for all LLD values in such an environment is testament to the quiet environment of room 130 in building 469. Figures 4-11 to 4-14 demonstrate that the heightened detector count rate at lower LLD values during gamma exposure represent a response of the detector solely to the incident gamma field. The GC60 facility does not offer the same degree of decoupling between noise and gamma-induced detector response, since the facility introduces additional equipment noise and vibrations during its irradiation operation. These additional sources of noise and vibration can be picked up by the detectors through a micro-phonics effect. The determination of the gamma intrinsic efficiency, or gamma insensitivity, is dependent upon the choice of LLD. In the case of the BCS detector, the gain setting of the Ortec 671 shaping amplifier also had some influence. These factors should be set such that the count rate is just above background levels when exposing the detector to 10 or 20 mr/hr gamma exposure fields [18]. In the case of the BCS detector, the appropriate LLD value to use was 80 mv when the coarse gain was set to 100x, and 20 or 30 mv when the coarse gain was set to 50x (the fine gain in each case was set to 1x). In the case of the 3 He detector, acquisition software and hardware limited the LLD settings between 50 mv and 400 mv. Steady count rates could only be obtained for LLD values between 100 mv and 400 mv for 10 mr/hr, 20 mr/hr, and 50 mr/hr gamma exposure. For 400 mr/hr, the LLD was further limited to 250 mv 400 mv for steady data-taking. It is noteworthy how the data in Figure 4-13 for 50 mr/hr and 400 mr/hr cases appears to plateau well above background for higher LLD values. This may be due to signal pile-up effect, occurring at voltages above the LLD values shown in Figure Figure 4-15 displays the average gamma insensitivity for the BCS detector in horizontal and vertical orientations, and for the 3 He detector. The gamma insensitivity values were determined from the ratio of the observed count rate to the expected incident photon rate. The expected incident photon rate was determined from the photon flux in Equation (4-2) and the incident detector area. In the case of the BCS detector, the values shown are averaged between the 50x and 100x coarse gain setting cases, and the error bars shown represent the spread in the values from these cases. In the case of the 3 He detector, the data shown is with taking 100 mv as the LLD in the case of gamma exposure fields 50 mr/hr and lower. For 400 mr/hr, 250 mv was taken as the LLD. The error bars for 3 He data points is the standard deviation of the mean of the count rates measured each second over a specific time period. Note that since Equations (4-1) and (4-2) neglect any scattering component, the actual photon exposure rate, and corresponding photon flux, is likely to be greater. Since the intrinsic gamma efficiency is the detector count rate divided by the incident photon rate (= incident photon flux detector area), the true intrinsic gamma efficiency is likely to be lower than what is shown in Figure 4-15.

33 Figure 4-15: Average gamma insensitivity versus LLD, for the BCS detector in vertical (black squares) and horizontal (red circles) orientations, and for the 3 He detector Figure 4-15 demonstrates that gamma insensitivity worsens with gamma exposure. The BCS detector displays the most superior gamma insensitivity in the vertical orientation. BCS gamma insensitivity worsens in the horizontal orientation, since each incident gamma ray has a higher probability of registering an event (in one of up to seven 1 diameter tubes, each behind each other) than in the vertical orientation. 3 He gamma insensitivity is systematically worse than the BCS detector in the vertical orientation, and in some cases worse than the BCS in the horizontal orientation. These gamma insensitivity values are comparable to those reported in literature [5,7]. Both detectors meet the acceptance criteria of < 10-6 at 10 mr/hr exposure. 4.3 Gamma Absolute Rejection Ratio for neutrons (GARRn) Measurements of the gamma absolute rejection ratio for neutrons (GARRn) were conducted in the HPNG facility. In these measurements, the BCS detector was exposed simultaneously to a 252 Cf neutron field, and a 192 Ir gamma field. The activity of the 192 Ir source at the time of testing was Ci. With the 252 Cf source BCS detector distance set at 2 m or 4 m, the 192 Ir field was varied between 20 and 400 mr/hr by varying the 192 Ir source BCS detector distance. The

34 GARRn was estimated by dividing counts with both sources exposed by the counts with only the 252 Cf source exposed. The results are shown in Figure 4-16 below, with the y-axis scale range set to the acceptance bounds of 0.9 to 1.1. The measured GARRn ratio is well within these bounds at all tested levels of gamma exposure. Due to the 3 He detector being unable to give reliable readings at the level of 252 Cf neutron exposure, no GARRn results are available to report for this detector. Figure 4-16: Gamma absolute rejection ratio for neutrons versus gamma exposure rate, for 252 Cf BCS detector distances of 2 m (red triangles) and 4 m (black squares)

35 5. SOME NOTES ON INTERFACING WITH PROPORTIONAL TECHNOLOGIES INC. AS A CUSTOMER As part of this technical evaluation, it is perhaps relevant to make some remarks about past experiences in interactions with Proportional Technologies Inc. (PTI) that could affect future decisions about further procurement from this company. At the time of procuring a boron-lined detector from PTI, detectors with straws that have optimal surface-area to volume ratios in configurations that optimize detection sensitivity for safeguard applications were sought from the company, but were not available. The detector evaluated here was the only configuration available. It took some months after purchasing to receive the detector from the company. When the detector arrived, it was found to be with an electronics configuration different what was advertised on their website [13]. The detector was also without the aluminum frame around the active area tubes that is presented on their website. None of these changes were communicated by Proportional Technologies before the detector arrived. In the fall of 2013, specifications for the detector were promised. As of yet, those specifications have not arrived. Overall, customer service from PTI has left much to be desired.

36 6. SUMMARY AND CONCLUSIONS This report presents a technical evaluation of a boron-lined neutron detector from Proportional Technologies Inc. This detector was evaluated as a potential replacement for 3 He neutron detection technology. Measurements of the detector s neutron detection efficiency, gamma insensitivity, and its gamma absolute rejection ratio for neutrons were conducted, in controlled indoor environments. All of these measurements were made in close comparison with a 3 He detector, wherever possible. Neutron detection efficiency measurements were conducted in the HPNG facility at AECL using an unmoderated, calibrated 252 Cf source. The direct component of the incident neutron flux was measured using a shadow cone method [9]. Reliable shadow cone measurements for the BCS detector were conducted at 4 m source detector distance, yielding 0.18 cps/ng sensitivity response. Simple 1/r 2 scaling to 2 m estimates the detector sensitivity to be 0.7 cps/ng at 2 m. Based on measurements in published literature, moderating the 252 Cf source would increase the sensitivity at 2 m by about a factor of 1.2 to 0.84 cps/ng. This is well below the minimum acceptance criteria of 2.5 cps/ng at 2 m 252 Cf source-detector distance [2,6]. In order for this configuration of boron-coated straws to meet this acceptance criteria, more straws (to a total of 245 straws or more) would need to be added in behind to capture a larger percentage of the incident neutron flux. Direct shadow cone measurements with the 3 He detector were not possible since its acquisition electronics could not keep up with the intense neutron field of the 252 Cf source. The relevant ratio of 3 He/BCS detector response, based on lower neutron fields, is 3.60 ± Based on this ratio, the 3 He detector response meets or exceeds the minimum acceptance of 2.5 cps/ng at 2 m for safeguard applications. The intrinsic gamma efficiency, defined as the neutron detector count rate divided by the rate of gamma photons incident upon the detector, measures the insensitivity of the detector to gamma radiation, in the absence of neutron radiation. This efficiency should be < 10-6 at 10 mr/hr exposure for accepted safeguard use. The measured gamma insensitivity at 10 mr/hr of the boron lined detector in the vertical orientation, at , surpasses that of the 3 He detector, at It should be noted that these estimates are overestimates, since the scattered component of the incident gamma flux has been neglected in the computations. The gamma absolute rejection ratio for neutrons (GARRn) provides a measure of the influence of a gamma field. It quantifies the ratio of efficiency of neutron counting when the detector is simultaneously exposed to neutron and gamma radiation, to the efficiency of neutron counting when the detector is exposed to the same neutron field in the absence of gamma radiation. If the gamma radiation has no influence, this ratio is unity. The acceptance criteria for safeguards is 0.9 < GARRn < 1.1. The BCS detector response was tested in the presence of neutron radiation from 252 Cf, with and without gamma radiation of varying strength (10 to 400 mr/hr) from a 192 Ir source. The resulting GARRn ratio is well within the bounds of acceptance. These tests could not be carried out for the 3 He detector, again due to the inability of its data acquisition system to count steadily in the presence of the intense neutron radiation from the 252 Cf source.

37 7. REFERENCES [1] G. F. Knoll, Radiation Detection and Measurement, Third Edition. Toronto: John Wiley and Sons, Inc [2] R. T. Kouzes, J. H. Ely, L. E. Erikson, W. J. Kernan, Z. T. Lintereur, E. R. Siciliano, D. L. Stephens, D. C. Stromswold, R. M. van Ginhoven, M. L. Woodring, Neutron detection alternatives to 3 He for national security applications, Nuclear Instruments and Methods in Physics Research A, vol. 623, pp , [3] R. T. Kouzes, The 3 He Supply Problem, Pacific Northwest National Laboratory Report PNNL-18388, April [4] J. L. Lacy, A. Athanasiades, N. N. Shehad, R. A. Austin, C. S. Martin, Novel Neutron Detection for High Rate Imaging Applications, IEEE Nuclear Science Symposium Conference Record, vol. 1, pp , [5] R. T. Kouzes, J. R. Ely, A. T. Lintereur, D. L. Stephens, Neutron Detector Gamma Insensitivity Criteria, Pacific Northwest National Laboratory Report PNNL-18903, October [6] P. Peerani, A. Tomanin, S. Pozzi, J. Dolan, E. Miller, M. Flaska, M. Battaglieri, R. De Vita, L. Ficini, G. Ottonello, G. Ricco, G. Dermody, C. Giles, Testing on novel neutron detectors as alternative to 3 He for security applications, Nuclear Instruments and Methods in Physics Research A, vol. 696, pp , [7] R. T. Kouzes, J. R. Ely, D. C. Stromswold, Boron-Lined Straw-Tube Neutron Detector Test, Pacific Northwest National Laboratory Report PNNL-19600, August 7, [8] P. Narayan, L. R. Meghwal, K. C. Songara, S. G. Baijapurkar, P. K. Bhatnagar, Radiological safety aspects in Californium-252 source transfer operation, Indian Journal of Pure and Applied Physics, vol. 48, pp , [9] International Organization for Standardization. Reference neutron radiations Part 2: Calibration fundamentals of radiation protection devices related to the basic quantities characterizing the radiation field. Geneva: ISO; ISO-8529, [10] N. J. Roberts, N. A. Horwood, and C. J. McKay, Photon Doses in NPL Standard Neutron Fields, Radiation Protection Dosimetry, doi: /rpd/nct249, [11] S. Agostinelli, et al., Nuclear Instruments and Methods in Physics Research A, vol. 506, pp , [12] J. L. Lacy, A. Athanasiades, L. Sun, C. S. Martin, T. D. Lyons, M. A. Foss, H. B Haygood, Boron-coated straws as a replacement for 3 He-based neutron detectors, Nuclear Instruments and Methods in Physics Research A, vol. 652, pp , [13] accessed 21 February [14] B. M. van der Ende, Detection of Neutrons Emanating from ZED-2, AECL document TD-001, March 2014.