Comparison of Neutron Sensitive Scintillators for use with a Solid- State Optical Detector

Transcription

1 Comparison of Neutron Sensitive Scintillators for use with a Solid- State Optical Detector Sharmistha Mukhopadhyay 1,Christopher Stapels 1, Eric B. Johnson 1, Eric C. Chapman 1, Paul S. Linsay 1, Thomas H. Prettyman 2, Michael R. Squillante 1, and James F. Christian 1 1 Radiation Monitoring Devices, Inc.; 44 Hunt Street; Watertown, MA Planetary Science Institute; 17 E. Fort Lowell, Suite 16, Tucson, AZ ABSTRACT The detection of illicit nuclear sources and SNM requires an ubiquitous network of sensors. While 3 He proportional counters are excellent neutron detectors, there is an insufficient global supply of 3 He to create the required number of detectors. Alternatives to 3 He must be efficient, insensitive to gamma radiation, easily manufactured, rugged, and inexpensive to enable the procurement of a large numbers of sensors. The use of a high sensitivity solid-state optical detector coupled to scintillation materials, loaded with a neutron absorber such as 6 Li or 1 B, can fulfill these design constraints. In this work, we compare the properties of neutron-sensitive scintillation materials utilizing Monte Carlo simulations and experiments. Cs 2 LiLaBr 6 :Ce is compared to commercially available boron-loaded plastic scintillators and 3 He tubes. The scintillators are compared for neutron detection efficiency, limitations on size, gamma-rejection ratio, neutron detection limits, manufacturing cost, and availability for mass-production. Keywords: Neutron Detector, CMOS SSPM, Scintillation Materials, Detection Efficiency. 1. INTRODUCTION We are building a compact neutron detector with new scintillation material coupled to a solid-state photo multiplier tube (SSPM), which will provide an alternative to 3 He tubes and PMT-based detectors. 3 He proportional counters are capable of detecting neutrons but there is an insufficient global supply to create the large number of detectors required for the application. Existing detectors like boron loaded plastic scintillators efficiently detect neutrons; however, the electronequivalent energy of the 1 B(n,α) reaction is sufficiently low (93 kev) that gamma rays can obscure the neutron signature. The reaction peak for the new scintillators is at high energy, enabling higher precision for neutron measurements than any previous loaded scintillator. The neutron detector is based on a scintillators coupled to the solid-state photo multipliers (SSPM). In this work, we investigated the use Cs 2 LiLaBr 6 (CLLB), an emerging scintillation material [1], which is capable of detecting both neutron and gamma radiation. Similar measurements were made with commercially available boron loaded plastic scintillator (BLP). We will discuss some of the measurements and simulations that we performed for selecting the material and size for the prototype detector. We characterized the neutron source used for our measurements to calibrate the scintillator segments. From the experiments and simulations, we can select CLLB crystals for building our first prototype neutron detector to achieve a performance comparable to that of a helium tube. The CMOS SSPM can be mass-produced with commercially available CMOS process. Reliability, reproducibility and performance are enhanced for SSPM devices fabricated using CMOS technology. The signal processing can be done on the chip because CMOS enables the monolithic integration of the SSPM with other read out elements. This advantage further reduces cost and simplifies the detector system. The scintillation light produced as a result of the capture of a thermal neutron in CLLB corresponds to a light flash from the full-energy absorption of a 3.2-MeV gamma ray. This allows for a simple discrimination method using the pulse Hard X-Ray, Gamma-Ray, and Neutron Detector Physics XI, edited by Ralph B. James, Larry A. Franks, Arnold Burger, Proc. of SPIE Vol. 7449, 74491P 29 SPIE CCC code: X/9/$18 doi: / Proc. of SPIE Vol P-1

2 height to separate neutrons from the background gamma rays, which are prominent at energies below 3 MeV. Based on Monte Carlo calculations and experiments, CLLB can compete with 3 He in terms of counting efficiency and provides adequate gamma ray suppression for the target applications. Advantages of coupling CLLB to an SSPM for detecting neutrons include the capability of integrating signal-processing electronics with the SSPM as a mass-producible detector platform. 2. MATERIALS AND METHOD 2.1 Neutron Source The neutron source used for the measurements is an AmBe source with a neutron flux ~1 6 neutrons/ s. The source was sealed in a lead pig, which is shielded with polyethylene to thermalize the source neutrons. The source assembly was inserted in a portable plastic box shown in Figure 1. Back Side 4 PlasticBox Front Polyethylene sheets Side 3 Source Front Tungsten Figure 1. Block diagram of the source covered with polyethylene sheets on all three sides. There is a layer of tungsten sheet at the front of the source to absorb gamma radiations. We measured the source gamma rays with a NaI detector. Peaks identified in the spectra are the 511-keV annihilation peak, the 2.2 MeV from the thermal neutron capture with hydrogen, and the 4.4 MeV from the source. These gamma ray signatures are consistent with expected emission given in [2]. The spectra measured did not show any gamma ray peak around 3.2 MeV. 2.2 Simulations of the detector response The neutron and gamma ray output of the source was modeled using the Monte Carlo radiation transport code MCNPX [3]. MCNPX was used to calculate the neutron flux and dose as well as the gamma ray background needed for source design and characterization. The code was also used to model the response of detectors used for source characterization ( 3 He and NaI) and to calculate the neutron detection efficiency and detection limits for different CLLB-based designs in comparison to other solid state neutron detectors and 3 He. The intrinsic efficiency of a detector as a function of neutron energy and incident direction is defined as the ratio of the number of pulses recorded to the number of neutrons incident on a detector. The counting rate is given by the integral over all incident energies and directions of the product of the intrinsic efficiency, the projected area of the detector, and the flux of incident neutrons. 2.3 Experimental Characterization of Neutron Detectors BLP and CLLB: The neutron spectra were measured with BLP and CLLB coupled to the PMT using the moderated AmBe source. The block diagram of the experimental set up is shown in Figure 2. We first measured the neutron spectrum with a 3x3x6 mm 3 BLP crystal coupled to the PMT and then with a 3x3x3 mm 3 CLLB crystal coupled to a PMT. These measurements were done to compare the performance of the scintillation material in terms of efficiency and gamma ray rejection ratio. Side-by-side measurements were made with helium tube to compare the performance of the scintillator coupled to PMT with the standard helium tube (Reuter Stokes # P ). The helium tube has an active length of 2.2 cm and radius of.64 cm. Proc. of SPIE Vol P-2

3 M o d e r a to r S O U R C E D e te c to r Figure 2. Experimental set up used to measure scintillation materials coupled to a PMT. 3x3x3 mm 3 and 3x3x6 mm 3 CLLB: We measured the neutron source with 3x3x3 mm3 and 3x3x6 mm3 CLLB coupled to a PMT, and a helium tube as shown in Figure 3. The experimental set up is similar to Figure 2. The CLLB crystal is hygroscopic and therefore we have packaged the crystal in a glass cuvette as shown in Figure 3. The measurements were to see the effect of sensitivity on CLLB crystal size. Packaging Material Teflon Tape CLLB Helium Tube in glass cuvette CLLB Glass Cuvette Figure 3. On the left the photograph shows a 3x3x3mm3 CLLB and a helium tube kept side by side for neutron flux measurement. The right hand side is the diagram of the packaged CLLB crystal. The CLLB crystal is inserted in a glass cuvette of height 3 mm, internal width of 3mm and external width of 5.5 mm. All five sides of the packaged crystal is wrapped with Teflon tape. 2.4 Experimental Characterization of the SSPM SSPMs are fabricated from an array of Geiger photodiodes readout in parallel so that the output signal is proportional to the incident light intensity [4]. A Geiger photodiode is similar to an avalanche photodiode, but these diodes are operated above the reverse breakdown. A large gain (>16) is associated with each photon-initiated avalanche in the photodiode, providing good signal-noise ratio. These SSPMs are fabricated using a commercially available CMOS (complimentary metal oxide semiconductor) process, which provide cost-effective detector-on-a-chip solutions, high sensitivity measurements, and the very fast time response needed for neutron measurements [5]. A photograph of the CMOS SSPM is shown in Figure 4. Proc. of SPIE Vol P-3

4 Figure 4. Photograph of a CMOS SSPM detector. The size is approximately 6.6 mm x 6.6 mm. The fill factor is greater than 47%, and the number of 5μm x 5 μm pixels is In the SSPM neutron characterization measurements, the scintillator is coupled to the SSPM, as shown in Figure 5. The open face of the CLLB sample is 3x3 mm2, packaged in a glass cuvette. The active area of the SSPM is 3x3 mm2, but it has wire bond connections at one side that extend above the surface of the chip. A 3x3 mm2 wave-guide was created to couple the crystal without damaging the wire bonds. Figure 5. Photographs showing the CLLB on a PMT and SSPM prototype. 3. RESULTS 3.1 Scintillation Material Simulations Intrinsic efficiency: Intrinsic efficiency for neutrons has been simulated for selected scintillation materials are shown in Figure 6. The capture cross section for neutrons is inversely proportional to the neutron velocity [2]. This trend is reflected in Figure 6. The intrinsic efficiency for thermal neutrons (up to.4 ev) is greater than neutrons with higher energy. For Li, there is resonance at 1 MeV and hence the intrinsic efficiency increases at this energy. The counting rate that we expect to measure with the source a foot away from the neutron detector is also tabulated in Figure 6. The ratio of the reaction rate (events/source neutron/ cm3) in the helium tube and 3x3x3 mm 3 CLLB for the detectors placed a foot away from the source has been simulated to be 29. We measured the neutron spectra with the helium tube and the CLLB crystal a foot away from the source and the ratio of the measured count rate is 33. Thus, we can say that the measured data is in agreement with the simulated data with in 15%. This agreement with result and simulation confirms the detector characteristic simulated in Figure 6. Proc. of SPIE Vol P-4

5 Intrinsic Efficiency E-3 1E-4 1E-5 3 He (5mm) 3 CLLB En. BLP (5mm) 3 CLLB (3mm) 3 CLLB Energy(MeV) Detector Helium tube CLLB_En CLLB CLLB BLP Area (cm 2 ) Count Rate (Hz) Count Rate (Hz) Norm to Area Figure 6. Left hand side shows the intrinsic efficiency calculated for helium tube (black curve), 5x5x5 mm 3 CLLB crystal (red curve), 5x5x5 mm 3 95% enriched CLLB crystal (green curve), 3x3x3 mm 3 CLLB crystal (blue curve), 1x1x5 mm 3 BLP crystal (dark yellow curve) and lithium-loaded glass (magenta). CLLB En. stands for enriched CLLB in the figure. The table in the right hand side shows the expected count rate for helium tube of area 1.27 cm 2, CLLB crystals of area.25 and.9 cm 2, and 1 cm 2 of a BLP crystal. It can be seen that the intrinsic efficiency of the 5x5x5 mm 3 CLLB with 95% Li enriched is comparable to the helium tube. From the simulations, we can also see that the intrinsic efficiency for BLP is comparable to the helium tube. The neutron peak corresponds to 93-keV for BLP, and at this energy, the gamma-ray background is very high. The source quantification limit for BLP scintillator is higher than enriched CLLB as discussed next. Neutron Quantification Limits for Loaded Scintillators: CLLB crystals that we are examining for building our prototype detector are capable of detecting both neutron and gamma radiations. The neutron peak corresponds to 3.2 MeV in the pulse height spectra and at this region the gamma ray background is relatively small. For our AmBe source we have simulated the gamma background from the decay of 241 Am and the production of high energy gamma rays by the Be(α,n) reaction using MCNPX. The minimum neutron production rate needed to produce a measurable signal in the presence of a background is defined as the source quantification limit, which depends on the detector composition, distance from the source, shielding materials, and gamma ray background. Here, the quantification limit is arbitrarily taken to be the source neutron production rate that results in a relative precision in the net counting rate for the neutron peak of less than 3%: σ N < 3%, N Where N is the net counting rate and σ is the associated uncertainty (1 standard deviation). The calculated source N quantification limits for selected loaded is given in Table 1. The source quantification is given by Proc. of SPIE Vol P-5

6 S Q ε + bδp(1 + f ) = 2 2 ( σ / N ) ε t N where ε is the counts/source neutron, b is the background counts/kev, f = Δ p /Δ b, σ N / N is the quantification limit, and t is the counting time. For our calculation we have used f=2.simulated neutron spectra is shown in Figure 7. (Counts/keV) N Δ l Δ p Δ u Pulse height, E (kev) N=Total neutron counts over the background Δ l = Background peak region of interest on the left side of the neutron peak Δ p = Neutron peak region of interest. Δ u = Background peak region of interest on the right side of the neutron peak Δ b = Background peak region of interest on the right side of the neutron peak Figure 7. Example of typical pulse height spectra used to simulate the source quantification limit, S Q. Table 1. Shows quantification limits determined by Monte Carlo for the measurements of the RMD neutron source with 1x1x5 mm 3 scintillators. The counting time was 6s. Scintillation Material Peak Energy (MeV) Resolution (%FWHM at peak) Efficiency (counts/source neutron) (ε) Background (counts/mev/source neutron) (b) Peak region of interest (Δ p ) CLLB(Li 3.2 Enriched) 3% 6.22E E CLLB(Li 3.2 Natural) 3% 7.41E E BLP.93 25% 5.81E E Source Quantification limit (S Q ) (neutrons/sec ) From the simulation results we see that the CLLB with enriched Li provides the best performance of the materials evaluated, as the S Q is about 3 neutrons/sec. Due to very high equivalent energy for the reaction peak, CLLB is relatively insensitive to variations in gamma ray background. Proc. of SPIE Vol P-6

7 3.2 Scintillation Material Measurements BLP and CLLB: Figure 8 shows the pulse-height spectra collected by the BLP and the CLLB crystal for 72 sec. The neutron count rates measured for the BLP crystal is 1.77 Hz and that for the CLLB crystal is.3 Hz. The neutron peak is fitted to a Gaussian shape and the background counts are calculated with in an interval of 3-channel width. Taking the ratio of the neutron counts to the total counts we get the gamma rejection ratio of about 78% for BLP and 82% for CLLB. Counts 7 Neutron Peak in the 3x3x6 mm3 BLP crystal Counts 3 Neutron Peak in the 3x3x3 mm3 CLLB crystal Energy(keV) Energy(keV) 4 5 Figure 8. Neutron spectra measure with 3x3x6 mm 3 BLP crystal and a 3x3x3 mm 3 CLLB crystal. The neutron peaks are fitted in red. The spectra were taken for 72 sec in the presence of the moderated AmBe source. The spectrum measured with the helium tube is shown in Figure 9. The helium tube is sensitive to neutrons and from the spectra we measure the neutron flux on the helium tube to be 87 neutrons/s cm 2. The neutron flux on the BLP crystal measured from Figure 8 is 119 counts/s cm 2. Since BLP can detect both neutrons and gamma rays and the natural background as well as the source gamma ray background is high at energies below 5 kev, the 93-keV peak measured in the BLP spectra does not originate only from the neutron interaction with the boron. The higher count rate in the BLP crystal is due to the presence of gamma events with in the neutron peak. There is no distinction in the BLP crystal for the natural background and other source producing 93-keV gamma-equivalent peak. From the simulation shown in Figure 6, we expect similar neutron flux measurement by the BLP and the helium tube but our measurements show that the count rates measured in the BLP crystal is higher than helium tube count rate. This also shows the gamma events have been counted with the neutron events. To improve the gamma-ray rejection ratio, therefore, one has to use a coincident measurement with bismuth germinate (BGO), or some other dense scintillation material that will efficiently detect the gamma rays associated with the capture of a thermal neutron. Due to the geometric constraints, the intrinsic efficiency will generally decrease for a coincidence measurement and also this detector system will consist of two scintillation materials, which will make the cost of the detection system, go higher. The advantage of using the CLLB crystal is that the high equivalent gamma energy allows for easy pulse height discrimination (PHD). Using only PHD discrimination we have achieved the gamma discrimination ratio to be 82% for the CLLB and 78% for the BLP. Proc. of SPIE Vol P-7

8 Counts 5 Neutron Spectra in the helium tube Channel Figure 9. Neutron spectra measured with the helium tube. The dashed green curve is the gamma signature. 3x3x3 mm 3 and 3x3x6 mm 3 CLLB: Similar measurements were made with different volume CLLB crystals and with a helium tube. From the measured data and normalizing the total neutron counts to volume, we get the intrinsic efficiency of the 3x3x3 mm 3 CLLB cube to be 3% relative to the helium tube. The neutron count rate measured by the CLLB is.3 Hz. From this measurement, and assuming the incident neutron flux to be 11 n/cm 2 s, we estimate the sensitivity, which is the ratio of the neutrons detected by the detector to the total neutron flux from the source, to be.3. Simulations indicate that the intrinsic efficiency increases with the crystal size; therefore, the use of a larger crystal can improve the intrinsic efficiency. The quantities associated with binning the data for sensitivity characterization and pulse-amplitude discrimination are tabulated in Table 2. The results show that the sensitivity of the 3x3x6 mm 3 crystal is 4% more than the 3x3x3 mm 3 crystal. Table 2. Sensitivity calculated from measurements made with 3x3x3 mm 3 and 3x3x6 mm 3. Total counts represent the counts under the neutron peak. Bgnd. Counts or the background counts are gamma ray events under the neutron peaks. Crystals Total Counts Bgnd. Counts Sensitivity (S) 3x3x3 mm x3x6 mm SSPM Characterization Figure 1 shows the spectrum measured with a LYSO crystal coupled to the CMOS SSPM, irradiated with a 22 Na source. The spectrum is compared to that measured with the same LYSO crystal coupled to a PMT, and we see that the peak resolution for SSPM is comparable. Proc. of SPIE Vol P-8

9 Counts kev FW HM Device PMT 13% 9% SSPM 13% 8% 5.5 x 5.5 x 5.5 mm 3 LYSO Source: 22 Na Room Temperature 1274 kev Energy (kev) Figure 1. Plot of the pulse height spectra from a LYSO crystal coupled to the SSPM, exposed to 511-keV and 1275-keV gamma rays from 22 Na. The optical properties of the individual pixels have been measured. The detection efficiency (DE) is maximized around nm wavelengths. The emission spectra for the CLLB crystal show two peaks at 392 nm and 419 nm [1]. The DE for 419 nm at 2V excess bias is approximately 15%. We have tested our CLLB samples on SSPM segments. The measured spectra with PMT and SSPM are shown in Figure 11. We can see that the performance of SSPM is comparable to the PMT tube; however, the different optical coupling used with the SSPM affects the measurement. 4 SSPM PMT 4 Int Gamma Equivalent Energy (kev) Figure 11. Comparison of CLLB spectra on PMT and SSPM. The SSPM is a 3 mm x 3 mm device operated at 2 V excess bias and.5 ms shaping time. The PMT is operated at 6 V bias and 5 shaping time. 4. CONCLUSION We have made measurements with CLLB and BLP scintillators. From the measured spectra and simulation we can see the disadvantage of using the BLP is that the neutron peak corresponds to 93-keV where the gamma background is relatively high and since BLP can detect both neutron and gamma ray events it is not possible to distinguish between the using a single crystal. Thus, to improve the gamma rejection ratio we need two scintillation materials to set up coincidence measurements that will eliminate the gamma events. From the simulated efficiency plots we see that enriched 5x5x5 mm 3 CLLB crystal is good choice of scintillation material. Our aim is to make a compact, portable, hand held solid-state neutron detector whose performances is comparable to helium tubes. We will use only neutron signature to detect threatening events. Thus, we propose to make a detector with a scintillators coupled to an SSPM, capable of detecting neutron events. The AmBe source that has been used for measurements was characterized, which provided us information for calibrating the detector system. Measurements with 3x3x3 mm 3 CLLB show that the efficiency measured agrees well with the calculated results. For our prototype detector we would like to use a bigger crystal. We made measurement with 3x3x3 mm 3 and 3x3x6 mm 3 crystals. Our results show that we do gain in sensitivity by using bigger crystal. We conclude that CLLB is better than BLP and comparable to helium for detecting neutrons. Proc. of SPIE Vol P-9

10 ACKNOWLEDGMENTS This work has been supported in part by DNDO, Grant Number HSHQDC-8-C-169. REFERENCES [1] J.Glodo, E. V. D. v. Loef, and K. S. Shah, "Elpasolite Crystals for Gamma and Neutron Detection." IEEE, Dresden, Germany, October, 28. [2] G. F. Knoll, "Radiation Detection and Measurements," pp. 546, 2. [3] D. B. E. Pelowitz, "MCNPX User s Manual Version 2.5.," Los Alamos Natl. Lab. Doc. LA-CP-5 369, Los Alamos Natl. Lab., Los Alamos, N. M., 25. [4] P. Buzhan, B. Dolgoshein, L. Filatov, A. Ilyin, V. Kantzerov, V. Kaplin, A. Karakash, F. Kayumov, S. Klemin, E. Popova, and S. Smirnov, "Silicon photomultiplier and its possible applications," Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment Proceedings of the 3rd International Conference on New Developments in Photodetection, vol. 54, pp , 23. [5] C. J. Stapels, W. G. Lawrence, and J. F. Christian, "CMOS Solid-State Photomultiplier for Detecting Scintillation Light in Harsh Environments," SNIC Symposium, pp. 1-8, 26. Proc. of SPIE Vol P-1