SNM detection by means of thermal neutron interrogation and a liquid scintillation detector

Transcription

1 Journal of Instrumentation OPEN ACCESS SNM detection by means of thermal neutron interrogation and a liquid scintillation detector To cite this article: A Ocherashvili et al Related content - Time dependent measurements of induced fission for SNM interrogation A Beck, I Israelashvili, U Wengrowicz et al. - The investigation of fast neutron Threshold Activation Detectors (TAD) T Gozani, M J King and J Stevenson - Liquefied Noble Gas (LNG) detectors for detection of nuclear materials J A Nikkel, T Gozani, C Brown et al. View the article online for updates and enhancements. This content was downloaded from IP address on 11/03/2019 at 05:33

2 PUBLISHED BY IOP PUBLISHING FOR SISSA MEDIALAB RECEIVED: December 23, 2011 REVISED: February 17, 2012 ACCEPTED: February 22, 2012 PUBLISHED: March 20, nd INTERNATIONAL WORKSHOP ON FAST NEUTRON DETECTORS AND APPLICATIONS, NOVEMBER , EIN GEDI, ISRAEL SNM detection by means of thermal neutron interrogation and a liquid scintillation detector A. Ocherashvili, a E. Roesgen, a A. Beck, b E.N. Caspi, b M. Mosconi, a J.-M. Crochemore a and B. Pedersen a,1 a Nuclear Security Unit, Institute for Transuranium Elements, Joint Research Centre, European Commission, Via E. Fermi 2749, Ispra (VA), Italy b Physics Department, Nuclear Research Center Negev, P.O. Box 9001, Beer-Sheva, Israel bent.pedersen@jrc.ec.europa.eu ABSTRACT: The feasibility of using a pulsed neutron generator in a graphite assembly together with a single liquid scintillation detector for the detection of special nuclear materials is investigated. Thermal source neutrons induce fission in fissile material present in the sample. By means of pulse shape discrimination the detector signals from fast fission neutrons are easily identified among the signals from gamma rays and the interrogating thermal neutrons. The method has potential in applications for detection of special nuclear materials in shielded containers. KEYWORDS: Search for radioactive and fissile materials; Inspection with neutrons; Liquid detectors 1 Corresponding author. c 2012 IOP Publishing Ltd and Sissa Medialab srl doi: / /7/03/c03037

3 Contents 1 Introduction 1 2 Experiments Experimental setup Measurements of non-fissile sources Measurements of fissile sources 8 3 Results and discussion 11 1 Introduction Many different non-destructive assay methods (NDA) are being studied for nuclear security purposes for the detection of Special Nuclear Materials (SNM) in shielded containers. A candidate among these methods concerns the detection of fission signatures following exposure to an external neutron source [1, 2]. In this method the external neutron source can be tailored to yield a high probability of inducing fission in a shielded fissionable sample without excessively perturbing the detection system. The detection system on the other hand is optimized for one or more of the characteristic fission signatures. These signatures include the prompt and delayed gamma and neutron emissions from fission. Some detection methods exploit the fact that a fission event causes the emission of multiple radiations allowing the detection system to seek out coincident emissions in often very short coincidence intervals [3, 4]. The fission prompt gamma rays and neutrons are in many techniques the most desirable signatures for detection because they are more intense than the delayed ones. One disadvantage however is that the prompt emissions may be difficult to distinguish from interactions of the external neutron source with the detector. In the present work we investigate the concept of a go/no go system based on a pulsed neutron source and a single BC-501A liquid scintillation detector [5] for the detection of SNM. In the experimental setup a pulsed 14-MeV neutron generator and the liquid scintillation detector are inserted into the sample cavity which is surrounded by a large graphite structure. In the proposed method a 10 µs burst of 14-MeV neutrons is thermalized in the graphite structure over approximately 200 µs. The thermalized source neutrons have an average life-time in the graphite/cavity structure of about 900 µs. The neutron generator is pulsed with a period of 10 ms. By proper time gating of the detector it is possible to observe the detector response during the interrogation by thermal source neutrons only. About 70 µs after the generator burst only epi-thermal and thermal source neutrons remain, and from 100 µs onwards only thermal neutrons. To overcome a potential thermal neutron absorber surrounding the SNM, the detector can be enabled already during the epi-thermal and thermal interrogation. These source neutrons have the ability to both penetrate the sample containment and to induce fission in the sample with a high probability. The photomultiplier anode output of the liquid scintillation detector is connected directly to a signal digitizer for 1

4 the online analysis known as pulse shape discrimination (PSD) [6] [9]. The PSD signal allows discrimination between gamma rays and fast neutrons. The reason is that whereas the gamma rays interact with electrons, the neutrons interact with the protons of the detector. However in case of thermal or low energy neutrons the energy of the recoil proton produced in an elastic scattering event is low and result in a low charge signal amplitude in the scintillation detector which is below a reasonable discrimination trigger level. The only way a thermal neutron can be detected in the PSD spectrum is through a capture reaction. E.g. capture in hydrogen (σ = barn) yields a MeV gamma line which may be detected and give rise to a gamma PSD event. Thus the prompt fission neutrons emitted by the sample can be detected and identified by means of their PSD value (see section 2.1 for definition of PSD) in the absence of signals from the epi-thermal and thermal source neutrons. The observation of signals with high PSD value, such as for fast neutrons, is taken as evidence of fission, and consequently the presence of fissile material, in the sample. The method presented here has principle similarities with the so-called differential die-away (DDA) method [10] [12] in the sense that both methods use hardware implementations to separate the interrogating thermal source neutrons from the sample response i.e. the prompt fission neutrons. In the DDA method the separation of the fast prompt fission neutrons and the interrogating thermal neutrons is done by implementing a screen of thermal neutron absorber (such as cadmium or boron) between the graphite liner and the neutron detector. In this case 3 He proportional counters embedded in a polyethylene moderator are often used as fast neutron detector. The DDA method is being investigated at PUNITA in a separate study (the fast neutron detector modules, or fission neutron counters, as described above are visible in figure 1a). As such the method implementing the liquid scintillation detector for the detection of fast neutrons constitutes an alternative to the DDA method which is both inexpensive (compared to 3 He) and versatile due to the capability of detecting other radioactive substances in a passive cycle. For these reasons the method may have potential in future multi-purpose integrated systems. 2 Experiments 2.1 Experimental setup The Pulsed Neutron Interrogation Test Assembly (PUNITA) at the Joint Research Centre is designed for experimental studies in NDA methods for nuclear safeguards and security. The facility is composed of a large graphite liner surrounding the central sample cavity. The (D-T) pulsed neutron generator, the sample and the scintillation detector are located inside the sample cavity. Various permanently installed neutron detectors are located in and around the instrument. The dimensions of the sample cavity are 500 mm by 500 mm by 800 mm. This design yields a relatively high neutron flux in the cavity and provides flexibility with respect to detector configurations. As such PUNITA is a versatile tool for studying detection methods although not an instrument intended for infield installation. The pulsing of the neutron generator (Model A-211 from Thermo Fisher Scientific Inc.) at 100 Hz is tailored to the exponential decay of the thermal neutron flux in the sample cavity with a decay time of about 1.0 ms. The interrogating thermal flux peaks at about 250 µs after the 14-MeV 2

5 (b) Figure 1. (a) Cross-section of PUNITA showing the permanently mounted neutron detectors and the neutron generator. (b) Sample cavity of PUNITA showing a) neutron generator, b) liquid scintillation detector and c) uranium sample. neutron burst. Due to the pulsing of both the Penning ion source and the acceleration voltage this generator model is able to produce a sharp burst of 14-MeV neutrons with absolutely no neutron emission between bursts. This fact together with the very short duty cycle of of the generator allows separation of the neutron interrogation into a fast period (from zero to 100 µs) and a thermal period (from 250 µs to 9 ms). Figure 1a shows a vertical cross section of PUNITA with the permanently installed neutron detectors visible. Figure 1b is an actual photograph of the sample cavity. The liquid scintillation detector used in this work in a BC-501A type from Saint Gobain Inc. of dimensions 3 by 3 inches. The anode output of the photomultiplier is connected directly to a signal digitizer. The distance between sample and detector surface is 150 mm. The digitizer is a CAEN N6720 with a 250 MS/s 12-bit ADC. This digitizer has a built-in proprietary firmware which analyses in real time the charge pulse according to the pulse shape discrimination method known as the integration method [13]. In this method the charge pulse of typically 20 to 30 ns length is analyzed in a long gate and a short gate. The so-called PSD value represents the ratio of the charge integral of the tail to the charge integral of the whole pulse. PSD is calculated in the following manner: QL QS PSD = (2.1) QL where: QL is the charge integrated in Long gate, QS is the charge integrated in Short gate. Figure 2 illustrates the elements of the standard integration method. The pre-trigger period from the opening of the gate to the signal trigger point (rising edge) serves to establish the baseline level for each signal. Due to the different scintillation mechanisms of gamma rays and fast neutrons the pulse shape differs. In a mixed gamma and neutron field signals with a long tail (high PSD value) are due to fast neutrons while signals with a short tail are caused by gamma rays [14]. 3 (a)

6 Figure 2. PMT anode pulse as recorded with the CAEN N6720 digitizer showing the trigger and time gates needed in the standard integration method for PSD of gamma rays and fast neutrons. The time axis is in units of 4 ns. 2.2 Measurements of non-fissile sources The ability of the method to discriminate between pulses caused by gamma rays and neutrons is demonstrated in figure 3 and figure 4. These measurements were carried out outside the PUNITA facility. In figure 3 an AmBe neutron source is positioned at 150 mm from the surface of the 3 by 3 inch liquid scintillation detector. The figure shows excellent separation between neutron and gamma ray PSD values. The figure of merit (FOM) value of 1.2 quoted in figure 3 is calculated in the standard fashion [15] as the ratio of the distance between peak centroids to the sum of the two peak standard deviations. Figure 4 shows measurements of the same type as figure 3(b) but for various standard neutron and gamma sources. Each curve is normalized to a total area of 1. Each curve shows a clear separation of PSD peaks for gamma and fast neutron except for the 60 Co source which has no neutron emission. The trigger threshold for the PSD analysis is set to 22 kevee (electron equivalent). The AmBe source is seen to have the highest neutron to gamma ratio of the sources investigated. The AmLi source has a low neutron to gamma ratio for two specific reasons: 1. Due to the low energy of the neutrons from the AmLi source only a small fraction of the neutrons will appear above the trigger threshold. 2. As the cross-section for the (α,n) reaction on lithium is low a relatively large amount of 241 Am is required for a given neutron intensity. The relatively large amount of 241 Am results in a stronger gamma emission than for the other neutron sources. The larger tail for the AmLi source at high PSD values is due to pile-up (in the Long gate of figure 2) mainly caused by the gamma emission from 241 Am. 4

7 Figure 3. Measurement of an AmBe source with the 3 3 BC-501A scintillation detector. The PSD diagram as obtained using the CAEN N6720 firmware. The insert b) is the projection of the PSD intensity of the same measurement. Figure 4. Standard neutron and gamma calibration sources as measured with the BC-501A liquid scintillation detector and the CAEN N6720 digitizer. The trigger threshold was set at 22 kevee. All the following experiments were carried out at a PSD trigger threshold of 22 kevee. Also the response of the pulsed 14-MeV neutron source in PUNITA was investigated in terms of count rate variation after the neutron burst, and of PSD value of each detection event. In these experiments the BC-501A scintillation detector was placed inside the sample cavity as shown in figure 1b. In contrast to the radioactive gamma and neutron sources, the pulsed source in PUNITA produces a detector response that varies strongly in time. 5

8 Figure 5. Schematic of pulsing regime of the neutron generator (a), and count rate observed in the BC-501A scintillation detector as function of time after the burst of neutrons from the generator (b) to (d). (c) and (d) are different time scales of the time spectrum (b). One particular advantage of the liquid scintillation detector for counting purposes is the fast signal formation on the anode output of the PMT. Whereas detectors requiring pre-amplifier and amplifier front-end electronics would typically saturate their input dynamic range at the high count rates in PUNITA, the scintillation detector anode output with pulse widths of typically 20 ns can sustain very high count rates. The neutron generator produces about MeV neutrons in a 10 µs interval. These neutrons produce multiple gamma rays through inelastic scattering and other nuclear reactions. Figure 5 shows a schematic of the generator pulsing regime and the formation of the thermal neutron flux inside PUNITA (a). The graphs (b) to (d) show the distinct intervals as observed in the scintillation detector. The graph (b) shows the response in the 8 ms following the burst of 14-MeV neutrons at time zero. The graphs (c) and (d) are different time scales and ranges of (b). The graphs show four distinct intervals of the 10 ms period: 14-MeV neutron emission in first half of (0 20 µs) interval, neutron slowing down (20 µs 300 µs), thermal neutron (300 µs 4000 µs), and finally the range with only activation and signal background (4000 µs 8000 µs). The two distinct single exponential decays visible in (b) and (d) are associated with the slowing-down time and thermal life time, respectively, of neutrons in the graphite/cavity assembly. Least-squares fitting to a single exponential function yields the following parameters for the two distinct intervals; decay time: ± 2.1 µs, χ 2 per degree of freedom: , and decay time: ± 4.2 µs, χ 2 per degree of freedom: , respectively. The decay time (reciprocal of exponential decay constant) obtained in the slowing-down region does not represent the slowing-down time 6

9 Figure 6. PSD response of pulsed neutron source in PUNITA. The total spectrum has been unfolded into four significant time intervals after the generator burst. of fast neutrons only but has also other components related to the gamma activity. In the thermal neutron region, however, the obtained decay time is in fact the thermal neutron life time, as the same value is obtained with the bare low-pressure 3 He detector in the corner of the cavity ( thermal flux monitor in figure 1a). During and immediately after the 14-MeV neutron pulse the detector sees a mixed field of intense gamma rays and fast neutrons. When the neutron energy falls below a few hundred kev, the neutrons can no longer produce a PSD value above the trigger threshold. Figure 6 shows the detector response in terms of PSD of the pulsed neutron source in PUNITA after 12,000 pulse repetitions. The top curve is the total response in the 10 ms period between generator bursts. The area of this curve has been normalized to 1. The four lower curves represent different time intervals inside the 10 ms period. The sum of the four lower curves equals the top curve (black). The time intervals in figure 6 are the same as in figure 5. The curve for the 14-MeV neutron emission shows a weak maximum at a PSD value for gamma rays, and a long tail up to the maximum PSD value of 1. This broad spectrum is due to detection of multiple events (pile-up) in the Long gate (figure 2). In the time interval where neutron slowing-down dominates the PSD gamma peak is becoming more pronounced. In the interval dominated by the decaying thermal neutron flux, the gamma peak is even more pronounced, and in the range above the single gamma events (PSD > 0.15) the intensity has fallen by an order of magnitude. In this interval single neutron events (PSD > 0.15) are not recorded, but only pile-up events. Finally after the total decay of the thermal neutron flux only the effect of neutron activation and background remains. In this interval only PSD values of gamma rays are observed. 7

10 Figure 7. Detector response of passive measurements outside (a) and inside (b) PUNITA of two different Pu sources of similar mass. The part of single neutron detections (0.15 < PSD < 0.25) is influenced both by the intrinsic fission rate and by the PUNITA environment. The behavior of the detector response discussed above allows for a selective and sensitive method of detection of fissile material. The time interval where the interrogating thermal flux (with high fission cross-section) in PUNITA is highest coincides with the PSD spectrum (blue curve) with a low intensity at the position where the fast fission neutrons will be found (0.15 < PSD < 0.25). See figure 4 for the PSD range of the 252 Cf spontaneous fission source. 2.3 Measurements of fissile sources Before submitting the combined fissile material source and scintillation detector to the pulsed neutron source in PUNITA, the PSD spectrum from a spontaneous fission neutron source is investigated with the neutron generator switched off. PSD spectra from the BC-501A scintillation detector at a distance of 150 mm are recorded for two Pu sources of roughly the same mass. The two sources are significantly different only in their isotopic composition. The source of 93.8% 239 Pu has a 3.5 times smaller spontaneous fission yield compared to the 75.7% 239 Pu source, and consequently 3.5 times smaller neutron emission rate. Also the gamma emission rate of the higher burn-up source (75.7% 239 Pu) source is considerably higher than the lower burn-up source. The two sources were measured in the same conditions both outside PUNITA and inside PUNITA but with the neutron generator switched off. Figure 7 shows the PSD measurement of the two sources. The area under each curve is normalized to 1. Some important features are noticed. As expected the higher burn-up source has a higher neutron PSD peak. However also events of higher PSD value than a single neutron detection are observed. These events are mainly due to multiple detections (pile-up in the Long gate) of separate radioactive events in the source. The higher burnup source has four times higher spontaneous fission rate, but a much higher gamma emission rate mainly due to the increased amount of 241 Am. As expected the fraction of pile-up in the PSD spectrum was reduced when the detector to source distance was increased (not shown). When comparing the PSD response inside and outside PUNITA the effect of the graphite moderator is clearly visible. For both types of Pu sources the neutron fraction is clearly increased, while the shape of the curves remains unchanged. This increase is likely due to 8

11 Figure 8. PSD distribution following 12,000 generator pulses of the 9.45 g Pu source of 75.7% 239 Pu. Both the source measurement (red) and background (black) are shown. In the time period for thermal neutron interrogation (d) the PSD peak from fast fission neutrons is clearly visible. After the decay of the thermal interrogating flux (e) a small fast fission neutron component from spontaneous fission remains. spontaneous fission neutrons being thermalized in the graphite structure and subsequently inducing fission in the source. This potential in PUNITA to induce fission in the source by means of external thermal neutrons will be exploited below. In the following measurements (figure 8, 9 and 10) fissile sources are investigated inside PUNITA when exposed to 12,000 pulses of 14-MeV neutrons (2 minutes measurement). The distance between the source and the BC-501A detector is 150 mm as shown in figure 1b. The PSD spectra are generated in real time using the CAEN digitizer. Again, the total response over the 10 ms period (100 Hz pulsing of the generator) as well as the response in the four sub-intervals described in figure 6 are shown. The graphs are normalized so that the area under curve (a) is equal to 1, and the curves (b) + (c) + (d) + (e) = (a). Each measurement is repeated both with (red) and without source (black). Figure 8 shows the detector response for the high burn-up Pu source (93.8% 239 Pu). In the graph representing thermal neutron interrogation only (d), the detection of the fast fission neutrons is clearly visible in the range 0.15 < PSD < In the graph representing the passive response (e) the neutron signal from spontaneous fission is visible. Figure 9 shows the detector response for the high burn-up Pu source (93.8% 239 Pu). Again the response of fast fission neutrons is clearly visible during thermal neutron interrogation (d). 9

12 Figure 9. PSD distribution following 12,000 generator pulses of the 9.45 g Pu source of 93.8% 239 Pu. Both the source measurement (red) and background (black) are shown. In the time period for thermal neutron interrogation (d) the PSD peak from fast fission neutrons is clearly visible. After the decay of the thermal interrogating flux (e) a small fast fission neutron component from spontaneous fission remains. Comparison of figure 8 and 9 shows that both the induced fission neutrons (d) and the spontaneous fission neutrons (e) are more abundant for the higher burn-up source, and the amount of induced fission is more than the spontaneous fission in both cases. Figure 10 shows the detector response for a 169 g U sample of 4.46% 235 U. Again the fast fission component is clearly visible in the graph representing thermal neutron interrogation (d). After the decay of the thermal interrogating flux (e) a small fast fission neutron component remains. This is either due to few remaining thermal neutron induced fission events or β-delayed fission neutrons. The figures 8, 9 and 10 demonstrate that for three different types of fissile material the thermal neutron interrogation range (d) is the most sensitive to the presence of fissile material. The reason for this is both the high fission cross-section for the thermal source neutrons, and the insensitivity of the scintillation detector in terms of PSD to the thermal source neutrons. In the following measurements only the time period of thermal neutron interrogation is considered. Figure 11 shows measurements of CBNM standard uranium samples inside PUNITA at a source to detector distance of 150 mm. The five standards are identical in all aspects (mass, density, geometry, container) except for the concentration of 235 U and 238 U. The mass of the fissile component 235 U is 0.52 g, 1,12 g, 3.28 g, 4.99 g and 7.54 g, respectively. All measurements include 12,000 neutron generator 10

13 Figure 10. PSD distribution following 12,000 generator pulses of a 169 g U source of 4.46% 235 U. Both the source measurement (red) and background (black) are shown. In the time period for thermal neutron interrogation (d) the PSD peak from fast fission neutrons is clearly visible. pulses. All curves are normalized to an area of 1. The figure shows the increase of the neutron PSD peak as function of the 235 U content of the sample. 3 Results and discussion We have investigated the possibility of using a single BC-501A liquid scintillation detector for the detection of SNM in nuclear security applications. The method has potential in applications where shielded containers are the object of investigation. For example air cargo containers (so-called ULDs) with a volume of a few cubic metres could be a feasible object size even without to need to increase the neutron generator source strength. In this case a 235 U detection limit of less than 100 g should be achievable with a detector volume a factor 10 larger than in the present work. Further simulations are required to establish the scalability of the present work to actual container size volumes. In PUNITA a substantial tungsten liner has been placed around the (D, T) generator target. The purpose is to achieve a broadening of the neutron spectrum by means of scattering and (n,2n) and (n, 3n) reactions. In this way the interrogating epi-thermal and thermal neutron flux in PUNITA is increased compared to the case of a bare generator (the optimization of the W liner is the subject of a separate publication currently under elaboration). The benefit of an increased epi-thermal flux is 11

14 Figure 11. PSD measurements of CBNM uranium standards during thermal neutron interrogation. The graphs show the sensitivity of the neutron PSD peak to the mass of the fissile component 235 U. Even for a mass of 0.52 g of 235 U (0.31% enrichment) the neutron PSD peak is still visible. improved penetrability of shielding materials including thermal neutron absorbers. A further study of the performance of the method when shielding materials are applied to the SNM is necessary. The method described here applies field-able instrumentation such as a sealed neutron generator, and a room temperature and inexpensive scintillation detector. Finally the data analysis (PSD) is carried out in real time in FPGA hardware. The final data interpretation in a go/no go system based on this method is straightforward. The elements investigated in the present work included: The behavior of a single (3 3 ) BC-501A based liquid scintillation detector inside the pulsed 14-MeV PUNITA facility. We analyzed the PMT anode charge pulse directly by means of the PSD method. We applied the standard integration PSD method for discrimination of fast neutrons and gamma rays. The PSD analysis has been implemented in FPGA hardware and carried out online. The neutron PSD peak is well separated at values 0.15 < PSD < About 100 to 200 µs after the generator neutron burst the sample is interrogated by epithermal and thermal neutrons. The source neutrons do not appear in the neutron PSD peak, but the (fast) fission neutrons do. 12

15 Different types of fissile material were detected in the present configuration. Less than 1 gram of 235 U is detected. The present work intended to investigate the feasibility of the method. However from the experimental data achieved, and some additional measurements, performance values such as minimum detectable mass, fast and thermal neutron flux, detector efficiency, and sample/detector configuration can be derived for the purpose of designing a full-scale instrument for practical applications. References [1] T. Gozani, Fission signatures for nuclear material detection, IEEE Trans. Nucl. Sci. 56 (2009) 736. [2] H. Rennhofer, J.-M. Crochemore, E. Roesgen and B. Pedersen, Detection of SNM by delayed gamma rays from induced fission, Nucl. Instrum. Meth. A 652 (2011) 140. [3] A. Enqvist, M. Flaska, J.L. Dolan, D.L. Chichester and S.A. Pozzi, A combined neutron and gamma-ray multiplicity counter based on liquid scintillation detectors, Nucl. Instrum. Meth. A 652 (2011) 48. [4] T.E. Valentine, L.G. Chiang and J.T. Mihalczo, Monte Carlo evaluation of passive NMIS for assay of plutonium in shielded containers, in Proceedings of the INMM 41st Annual Meeting, New Orleans U.S.A., July [5] Saint-Gobain Crystals, BC-501A Data Sheet (2008), [6] J.M. Adams and G. White, A versatile pulse shape discriminator for charged particle separation and its application to fast neutron time-of-flight spectroscopy, Nucl. Instrum. Meth. A 156 (1978) 459. [7] Z. Bell, Tests on a digital neutron-gamma pulse shape discriminator with NE213, Nucl. Instrum. Meth. A 188 (1981) 105. [8] M. Moszyński et al., Study of n-γ discrimination with NE213 and BC501A liquid scintillators of different size, Nucl. Instrum. Meth. A 350 (1994) 226. [9] F.T. Kuchnir and F.J. Lynch, Time dependence of scintillations and the effect on pulse-shape discrimination, IEEE Trans. Nucl. Sci. 15 (1968) 107. [10] J.T. Caldwell, W.E. Kunz and J.D.Atencio, Apparatus and method for quantitative assay of generic transuranic wastes from nuclear reactors, US Patent no issued on 20 Nov 1984, CL , G21G1/12. [11] K.A. Jordan and T. Gozani, Detection of 235 U in hydrogenous cargo with differential die-away analysis and optimized neutron detectors, Nucl. Instrum. Meth. A 579 (2007) 388. [12] W. Hage, Fissile mass determination by the active neutron correlation technique with the generalised Poisson parameters or its factorial moments, Nucl. Instrum. Meth. A 614 (2010) 72. [13] CAEN Application note AN2506, Digital gamma neutron discrimination with liquid scintillators (2011), [14] M. Flaska, S.A. Pozzi, Identification of shielded neutron sources with the liquid scintillator BC-501A using a digital pulse shape discrimination method, Nucl. Instrum. Meth. A577 (2007) 654. [15] G.F. Knoll, Radiation detection and measurement, 3 rd edition, J. Wiley & Sons (1999), pg. 680 [ISBN ]. 13