- Email: [email protected]
Directional detection of neutrons and photons using elpasolites: Computational study
Radiation Measurements 124 (2019) 127–131
Contents lists available at ScienceDirect
Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Directional detection of neutrons and photons using elpasolites: Computational study
T
A. Guckesa,∗, A. Barzilovb, P. Gussc a
Nevada National Secuirty Site, Nevada Operations, P.O. Box 98521, M/S NLV068, Las Vegas, NV, 89193-8521, USA University of Nevada, Las Vegas, 4505 S. Maryland Parkway, Las Vegas, NV, 89154, USA c Nevada National Security Site, Remote Sensing Laboratory - Nellis, P.O. Box 98521, M/S RSL-09, Las Vegas, NV, 89193-8521, USA b
A R T I C LE I N FO
A B S T R A C T
Keywords: Monte Carlo modeling Elpasolite scintillator Directional detector Neutron measurements Photon spectroscopy
A radiation detection system consisting of an array of Cs2LiYCl6:Ce3+ elpasolite cells was studied for simultaneous neutron measurements and gamma-ray spectroscopy. The system allowed directional detection with the localization of radiation sources. Computational modeling of detector array showed that the three-cell detector array is feasible to determine the direction to a neutron source and a gamma-ray source.
1. Introduction Neutron and photon measurement technologies are essential to fulfilling homeland security and nonproliferation mission areas including detection and localization of missing or smuggled radiological or nuclear materials (Chichester et al., 2007; Doyle, 2008; Menlove et al., 2013; Israelashvili et al., 2015), environmental and waste management (Greenberg et al., 2007; Stauff et al., 2015), and in-situ material analysis (Womble et al., 2002; Reber et al., 2005; Bodnarik et al., 2013). Typically, a combination of two different detectors is utilized for the detection of neutrons and gamma rays, separately; e.g., a moderated 3 He tube provides a neutron count rate and a NaI:Tl scintillator (or a cryogenically-cooled semiconductor detector) measures a gamma-ray spectrum (Moody et al., 2005; Knoll, 2010). This approach makes detection systems and associated electronics complicated and bulky for the use in field measurements. Ideally, a single detector is desired for measurements of both types of radiation simultaneously in the mixed radiation flux scenarios. Dual neutron/photon detection materials such as plastic and liquid scintillators typically exhibit reduced gamma-ray energy resolution (Klein and Neumann, 2002; Zaitseva et al., 2012; Cester et al., 2014; Qin et al., 2015). Recently, the elpasolite scintillator materials (Biswas and Du, 2012) were proposed that enable simultaneous neutron and photon detection: the Ce3+-doped elpasolite crystals Cs2LiYCl6:Ce3+ (CLYC), Cs2LiLaCl6:Ce3+ (CLLC), Cs2LiLaBr6:Ce3+ (CLLB), and Cs2LiYBr6: Ce3+ (CLYB) (Van Loef et al., 2002; Bessiere et al., 2005; Glodo et al., 2011; Guss et al., 2014). These elpasolites provide high energy resolution for
∗
gamma rays. The following full width on the half maximum (FWHM) values were determined for these scintillators for the gamma-ray peak at energy of 662 keV: 3.9%, 3.4%, 2.9%, and 8.5%, respectively. Due to the lithium content, these elpasolites are also capable to interact with thermal neutrons. Naturally-occurring lithium is composed of two stable isotopes 6Li and 7Li (7.5% and 92.5% abundance, correspondingly). The thermal neutron capture cross-section for 6Li (940 b) is acceptable for detection applications via the 6Li(n,α)3H reaction, where the alpha particle and Triton share the kinetic energy of 4.78 MeV. These energetic charged particles produce ionization tracks in the elpasolite material with the subsequent trapping of free charges in the Ce3+ scintillation centers that create a pulse of light as a result of the de-excitation process. Moreover, 133Cs, 35Cl, and 37Cl isotopes can capture thermal neutrons. To increase detection efficiency, the 6Li-enriched elpasolites were synthesized, e.g. the CLYC with the 95% lithium-6 enrichment (Glodo et al., 2013): its thermal neutron crosssection is 2.3 times that of 3He gas at 10 bar for the same volume. In the energy spectra, the full-energy thermal neutron peak typically appears above 3 MeV gamma-equivalent energy (GEE), with the energy resolution of 3%. Thus, the pulse-height discrimination (PHD) can be implemented with these materials. The CLYC emissions consist of three components: core-to-valence luminescence (Rodnyi, 2004) (CVL; 250 nm–350 nm), Ce3+ prompt emission (350–450 nm), and cerium self-trapped excitation (Ce3+-STE). The first two components to appear are due to the photon interactions with the scintillator; the third component appears due to thermal neutron interactions within the CLYC. The CVL is of particular interest
Corresponding author. E-mail address: [email protected] (A. Guckes).
https://doi.org/10.1016/j.radmeas.2019.04.003 Received 10 February 2016; Received in revised form 18 March 2019; Accepted 3 April 2019 Available online 06 April 2019 1350-4487/ © 2019 Elsevier Ltd. All rights reserved.
Radiation Measurements 124 (2019) 127–131
A. Guckes, et al.
an incident radiation) with an FT8 CAP treatment was used. The FT8 CAP turns the F8 tally into a neutron capture tally, which returns the number of neutron capture events on 6Li isotopes in the cell per source particle. This value was then multiplied by the number of source particles (1 × 107) resulting in the total number of neutron captures in each detector cell. The number of neutron captures tallied in each of the cells as a function of the angular rotation of the array provided the directional response of the elpasolite array to neutrons. For gamma rays, an isotropic 662-keV photon point source (representing the 137Cs isotope) was also modeled in locations along a path of a circle of the radius R in the angular increments of 10°, from 0° to 360°, in the XY-plane. The gamma-ray source was initially placed at a radius of 10 cm from the center of the array for near-field measurements. The gamma-ray source was then placed at a radius of 100 cm from the center of the array. To evaluate the number of gamma rays interacting within the CLYC crystal, an F4 tally (average flux over the cell) was utilized. The gamma-ray flux, in units of photons/cm2/source particle, was tallied in each of the cells as a function of the photon source position. This value was then multiplied by the volume of the cell in cm2 and the number of source particles (1 × 107) resulting in the total number of gamma-ray interactions in each detector cell. The total number of gamma-ray interactions in each detector cell as a function of angular rotation of the array provided the directional response of the detection system to gamma rays. The statistical error for each tally was also estimated.
as it appears only under the photon excitation. It is also fast and decays with less than 2-ns time constant. It is also absorbed by Ce3+ ions in a crystal. The significant difference in the decay times of the gamma-ray versus neutron-induced emission components allow for excellent pulseshape discrimination (PSD) capabilities to segregate neutron and gamma-ray pulses (D'Olympia et al., 2013). The feasibility of fast neutron spectroscopy via the 35Cl(n,p)35S reaction using CLYC was also studied (D'Olympia et al., 2012). The proton energy in this reaction depends linearly on the initial energy of the fast neutron; a proton fullenergy peak is induced in the energy spectrum. Thus, the CLYC elpasolite can be utilized to detect neutrons and photons with the energy spectroscopy option for both radiations. In the remote sensing of neutron and gamma-ray radiation, directional detection technologies that provide the location of the sources are often necessary (Guss and Mukhopadhyay, 2013). To address this need, the directional detection system consisting of an array of CLYC cells was investigated for simultaneous measurements of neutron and photon flux with the localization of radiation sources. 2. Computational model The computational study was carried out using the Monte Carlo particle transport code MCNP6 (MCNP6, 2013). A detection system consisting of an array of cylindrical 1-inch-diameter by 1-inch-height CLYC scintillator cells was modeled. Fig. 1 shows the models of 3-cell and 4-cell arrays arranged in a symmetrical closely-packed geometry. In these models, the CLYC scintillator crystals were encapsulated in thin aluminum housings. Other parts of the detector system (e.g., photomultiplier tubes) have little effect on the radiation interactions within the CLYC crystals and were not included into the model. To determine the detector responses at different radiation source positions, MCNP6 simulations were performed for the 3- and the 4-cell systems and for both a neutron source and a gamma-ray source.
2.2. Maximum likelihood estimation of direction to a radiation source The direction from the detection system to a neutron source or a gamma-ray source was known for the computational study. However, if this directional detection system would be employed in the field to find a lost, stolen or smuggled radioactive source or nuclear material, the exact position of the source will not be known. To enable estimation of the unknown source direction, the following procedure was enacted. The 360°-simulated data of each CLYC detector in the array was fitted with a smoothing spline model. The angle at which each smoothing spline function is a maximum is the observed source angle for that specific detector. This is based on the assumption that each detector will observe the maximum number of counts when the greatest amount of each detector's surface area is aligned with the source. It is also assumed that the observed source direction obtained from each detector in the array is normally distributed with the following probability density function (PDF):
2.1. Modeling of directional detection of neutrons and photons To study the directional neutron detection, an isotropic, point 0.0253-eV neutron source was placed along a path of a circle of the radius R in locations determined by the angular increments of 10°, from 0° to 360°, in the XY-plane. The neutron source was initially placed at a radius of 10 cm from the center of the array for near-field measurements. The neutron source was then placed at a radius of 100 cm from the center of the array. These conditions reflected those we plan to implement in the experiments. To calculate the number of neutron capture events on 6Li nuclei within the CLYC, an F8 tally (which calculates the energy distribution of pulse heights in a detector caused by
f(Θi | θ , σ 2) =
−(Θi − θ) 1 exp 2σ 2 2π σ
2
(1)
where Θi is the observed source angle from the measured data for the ith detector, θ is the unknown actual source angle, and σ 2 is the variance of θ . Since all detectors are independent, the PDF for the source angle considering all n detectors in the array is as follows: n
f ({Θi }in= 1 |θ , σ 2) =
∏ f (Θi | θ, σ 2) i=1
(2)
The likelihood function of Equation (2) is defined as:
L (θ , σ 2) = ln(f ({Θi }in= 1 |θ , σ 2))
(3)
The maximum likelihood estimate (MLE) for the unknown actual source angle is that θ which maximizes L (θ) :
θˆ = arg max L (θ , σ 2) θ
(4)
The variance of θˆ is defined as: Fig. 1. Three-cell (solid yellow cells) and four-cell (CLYC #1, CLYC #2, and pattern-filled cells) array models. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
σˆ 2 =
128
1 n
n
∑ (Θi − θˆ) i=1
2
(5)
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Fig. 2. Three-cell CLYC array exposed to thermal neutron source positioned at R = 10 cm.
Fig. 4. Three-cell CLYC array exposed to 137Cs gamma-ray source positioned at R = 10 cm.
The standard deviation is the square root of the variance. Based on this assumption, the maximum likelihood estimation method is applied to determine the estimate of the source angle for both the neutron source and the gamma-ray source at the distances of 10 cm and 100 cm using both the three-cell and the four-cell array.
3. Results and discussion The computational modeling results for the three- and four-cell arrays provide the information on how a prototype directional detection system would respond to radioactive sources. The models of the fourCLYC array serve as a comparison to the three-cell array in terms of quality of the response. It can be observed from Figs. 2–9 that the responses of both detection systems to neutrons and gamma rays had an angular pattern distinctive for the specific direction to the source. It should be noted that a single CLYC scintillator is able to detect both neutrons and gamma-rays simultaneously. However, the neutron and gamma-ray responses are presented separately in this study. Although both neutron and gamma-ray counts were collected in a single MCNP simulation for each rotation of the array, two different MCNP tallies were used to determine the counts for neutrons and gamma-rays thus, two separate sets of data. The maximum MCNP statistical error achieved in the computational study of the three-cell array for gamma rays was 2% and for thermal neutrons it was 9%. The maximum MCNP statistical error achieved in the computational study of the four-cell array for gamma rays was 2% and for thermal neutrons it was 10%. These statistical errors can be reduced with a higher number of particles ran or longer run times in each of the MCNP simulations. For near-field measurements (R = 10 cm), the response of all the detectors to both thermal neutrons and gamma rays is higher in magnitude and the difference between the actual source angle and the MLE source angle is decreased. Measurements in the position R = 100 cm are lower in magnitude with larger differences between the actual
Fig. 5. Three-cell CLYC array exposed to 137Cs gamma-ray source positioned at R = 100 cm.
Fig. 6. Four-cell CLYC array exposed to thermal neutron source positioned at R = 10 cm.
Fig. 7. Four-cell CLYC array exposed to thermal neutron source positioned at R = 100 cm.
source angle and the MLE source angle. This is due to the fact that the intensity of a point source decreases with respect to the inverse of the distance from the center of the detector system to the source squared.
Fig. 3. Three-cell CLYC array exposed to thermal neutron source positioned at R = 100 cm. 129
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4. Conclusion
Fig. 8. Four-cell CLYC array exposed to R = 10 cm.
137
Cs gamma-ray source positioned at
Fig. 9. Four-cell CLYC array exposed to R = 100 cm.
137
Cs gamma-ray source positioned at
A detector consisting of an array of 1-inch-diameter by 1-inchheight CLYC cells arranged in a symmetrical closely-packed geometry was studied for simultaneous directional neutron and gamma-ray measurements. Three- and a four-cell arrays were modeled using MCNP6 code. The evaluation of the elpasolite detector responses showed that the three-cell array is feasible to determine the direction to a point-like thermal neutron source or a gamma-ray source simultaneously. The MLE-enabled source localization for the three-cell array estimated the source angle with a difference of 1° and 4° for a 137Cs gammaray source at R = 10 cm and 100 cm, respectively, and 1° and 8° for a thermal neutron source at R = 10 cm and 100 cm, respectively. The source localization for the four-cell array estimated the source angle with a difference of 0° and 10° for a 137Cs gamma-ray source at R = 10 cm and 100 cm, respectively, and 0° and 9° for a thermal neutron source at R = 10 cm and 100 cm, respectively. Considering the standard deviation for each estimate, the actual source angle is predicted within the standard deviation for each MLE source angle. Altogether, the differences between the actual source angle and the MLE source angle are reasonable. The larger (> 1°) angle differences and MLE standard deviations all occur at R = 100 cm. This behavior is expected as the array is placed farther away from the source and thus, the intensity of the source decreases with respect to the inverse of the distance from the center of the detector array to the source squared. These angle differences and MLE standard deviations can be minimized computationally with a higher number of source particles ran or longer run time in each MCNP case and by decreasing the angular increment at which counts are collected (i.e., from 10° to 1°). These results indicate that the thermal neutron and/or gamma-ray measurements with either the three- or four-cell array and utilization of the mathematical procedure described in Section 2.2 to interpret the observed data is achievable and enables source localization. However, an array based on three CLYC cells will cost less, weigh less and be more compact than a four-cell array. The three-cell array with the described MLE procedure would allow an end-user to search for missing or smuggled radiological or nuclear materials, perform environmental and waste management, and in-situ material analysis with improved source direction estimates at each step while approaching the source. The next step is to perform an experimental study to confirm the results of the computational study and to develop a prototype dual neutron/gamma-ray directional detector.
This characteristic is apparent when the detectors were rotated 360° in the XY-plane. The source angle estimates for the computational models are provided in Table 1. It is apparent that the difference between the MLE source angle and the actual source angle increases with increasing distance for both thermal neutrons and gamma rays and for both the three-cell and four-cell array. So, the closer the detector is to a source, the more accurate the estimate of the direction to the source will be. A longer data collection time, and thus higher counts collected, would also afford a more accurate estimate of the source direction. It is apparent that for near-field measurements (R = 10 cm), the four-cell array provides a slightly (1°) more accurate estimate of the source direction than the three-cell array. For measurements at R = 100 cm, the three-cell array provides a slightly (< 6°) more accurate estimate of the source direction. These differences are small and can be attributed to active volume size and thus, detector efficiency in the near-field case and poorer statistics in the far-field case. From this study it can be concluded that a three-cell array is feasible to accurately estimate the source's direction. An array based on three CLYC cells will cost less, weigh less and be more compact than a fourcell array without decreasing the quality of the direction estimates.
Acknowledgments This manuscript has been authored by Mission Support and Test Services, LLC, under Contract No. DE-NA0003624 with the U.S. Department of Energy and supported by the Site-Directed Research and Development Program. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form
Table 1 Estimates of the source direction. Source Position
Actual Source Angle
MLE Source Angle ± σ
Actual vs MLE Angle Difference
3-cell 3-cell 3-cell 3-cell 4-cell 4-cell 4-cell 4-cell
90° 90° 90° 90° 45° 45° 45° 45°
89° ± 2° 82° ± 27° 89° ± 1° 94° ± 28° 45° ± 2° 54° ± 25° 45° ± 2° 55° ± 16°
1° 8° 1° 4° 0° 9° 0° 10°
array, array, array, array, array, array, array, array,
thermal neutrons, R = 10 cm thermal neutrons, R = 100 cm Cs: R = 10 cm 137 Cs: R = 100 cm thermal neutrons: R = 10 cm thermal neutrons: R = 100 cm 137 Cs: R = 10 cm 137 Cs: R = 100 cm 137
130
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of this manuscript, or allow others to do so, for United States Government purposes. The U.S. Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/ downloads/doe-public-access-plan). DOE/NV/25946–2704. The authors acknowledge the professional staff of RMD, Watertown, Massachusetts, for the production of the scintillators, for providing these detectors to the Remote Sensing Laboratory, and for their support and advice.
gamma-rays. Nucl. Instrum. Methods A 714, 121–127. Glodo, J., Van Loef, E., Hawrami, R., Higgins, W.M., Churilov, A., Shirwadkar, U., Shah, K.S., 2011. Selected properties of Cs2LiYCl6, Cs2LiLaCl6, and Cs2LiLaBr6 scintillators. IEEE Trans. Nucl. Sci. 58 (1), 333–338. Glodo, J., Hawrami, R., Shah, K.S., 2013. Development of Cs2LiYCl6 scintillator. J. Cryst. Growth 379, 73–78. Greenberg, M., Lowrie, K., Burger, J., Powers, C., Gochfeld, M., Mayer, H., 2007. Preferences for alternative risk management policies at the United States major nuclear weapons legacy sites. J. Environ. Plan. Manag. 50 (2), 187–209. Guss, P., Stampahar, T., Mukhopadhyay, S., Barzilov, A., Guckes, A., 2014. Scintillation properties of a Cs2LiLa(Br6) 90%(Cl6)10%:Ce3+ (CLLBC) crystal. SPIE Proc. 9215, 921505. Guss, P., Mukhopadhyay, S., 2013. Dual gamma neutron directional elpasolite detector. SPIE Proc. 8854, 885402. Israelashvili, I., Dubi, C., Ettedgui, H., Ocherashvili, A., Pedersen, B., Beck, A., Roesgen, E., Crochmore, J.M., Ridnik, T., Yaar, I., 2015. Fissile mass estimation by pulsed neutron source interrogation. Nucl. Instrum. Methods A 785, 14–20. Klein, H., Neumann, S., 2002. Neutron and photon spectrometry with liquid scintillation detectors in mixed fields. Nucl. Instrum. Methods A 476, 132–142. Knoll, G.F., 2010. Radiation Detection and Measurement, fourth ed. Wiley, New York. MCNP6 Development Team, 2013. MCNP6, Version 1.0. Report LA-CP-13-00634. Los Alamos National Laboratory. Menlove, H.O., Menlove, S.H., Rael, C.D., 2013. The development of a new, neutron, time correlated, interrogation method for measurement of 235U content in LWR fuel assemblies. Nucl. Instrum. Methods A 701, 72–79. Moody, K., Hutcheon, I., Grant, P., 2005. Nuclear Forensic Analysis. Taylor & Francis, New York, NY. Qin, J., Lai, C., Ye, B., Liu, R., Zhang, X., Jiang, L., 2015. Characterizations of BC501A and BC537 liquid scintillator detectors. Appl. Radiat. Isot. 104, 15–24. Reber, E.L., Blackwood, L.G., Edwards, A.J., Jewell, J.K., Rohde, K.W., Seabury, E.H., Klinger, J.B., 2005. Idaho explosives detection system. Nucl. Instrum. Methods B 241, 738–742. Rodnyi, P., 2004. Core-valence luminescence in scintillators. Radiat. Meas. 38, 343–352. Stauff, N.E., Kim, T.K., Taiwo, T.A., 2015. Variations in nuclear waste management performance of various fuel-cycle options. J. Nucl. Sci. Technol. 52, 1058–1073. Van Loef, E.V.D., Dorenbos, P., Van Eijk, C.W.E., Krämer, K.W., Güdel, H.U., 2002. Scintillation and spectroscopy of the pure and Ce3+-doped elpasolites: Cs2LiYX6 (X = Cl; Br). J. Phys. Condens. Mater. 14, 8481–8496. Womble, P.C., Vourvopoulos, G., Paschal, J., Novikov, I., Barzilov, A., 2002. Results of field trials for the PELAN system. SPIE Proc. 4786, 52–57. Zaitseva, N., Rupert, B., Pawelczak, I., Glenn, A., Martinez, H.P., Carman, L., Faust, M., Cherepy, N., Payne, S., 2012. Plastic scintillators with efficient neutron/gamma pulse shape discrimination. Nucl. Instrum. Methods A 668, 88–93.
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.radmeas.2019.04.003. References Bessiere, A., Dorenbos, P., Van Eijk, C.W.E., Krämer, K.W., Güdel, H.U., 2005. Luminescence and scintillation properties of Cs2LiYCl6:Ce for γ and neutron detection. Nucl. Instrum. Methods A 537, 242–246. Biswas, K., Du, M.H., 2012. Energy transport and scintillation of cerium-doped elpasolite Cs2LiYCl6: hybrid density functional calculations. Phys. Rev. B 86, 014102. Bodnarik, J.G., Burger, D.M., Burger, A., Evans, L.G., Parsons, A.M., Schweitzer, J.S., Starr, R.D., Stassun, K.G., 2013. Time-resolved neutron/gamma-ray data acquisition for in situ subsurface planetary geochemistry. Nucl. Instrum. Methods A 707, 135–142. Cester, D., Nebbia, G., Stevanato, L., Pino, F., Viesti, G., 2014. Experimental tests of the new plastic scintillator with pulse shape discrimination capabilities EJ-299-33. Nucl. Instrum. Methods A 735, 202–206. Chichester, D.L., Simpson, J.D., Lemchak, M., 2007. Advanced compact accelerator neutron generator technology for active neutron interrogation field work. J. Radioanal. Nucl. Chem. 271, 629–637. Doyle, J., 2008. Nuclear Safeguards, Security and Nonproliferation. Elsevier, Oxford, UK. D'Olympia, N., Chowdhury, P., Guess, C.J., Harrington, T., Jackson, E.G., Lakshmi, S., Lister, C.J., Glodo, J., Hawrami, R., Shah, K., Shirwadkar, U., 2012. Optimizing Cs2LiYCl6 for fast neutron spectroscopy. Nucl. Instrum. Methods A 694, 140–146. D'Olympia, N., Chowdhury, P., Lister, C.J., Glodo, J., Hawrami, R., Shah, K., Shirwadkar, U., 2013. Pulse-shape analysis of CLYC for thermal neutrons, fast neutrons, and
131
Contents lists available at ScienceDirect
Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Directional detection of neutrons and photons using elpasolites: Computational study
T
A. Guckesa,∗, A. Barzilovb, P. Gussc a
Nevada National Secuirty Site, Nevada Operations, P.O. Box 98521, M/S NLV068, Las Vegas, NV, 89193-8521, USA University of Nevada, Las Vegas, 4505 S. Maryland Parkway, Las Vegas, NV, 89154, USA c Nevada National Security Site, Remote Sensing Laboratory - Nellis, P.O. Box 98521, M/S RSL-09, Las Vegas, NV, 89193-8521, USA b
A R T I C LE I N FO
A B S T R A C T
Keywords: Monte Carlo modeling Elpasolite scintillator Directional detector Neutron measurements Photon spectroscopy
A radiation detection system consisting of an array of Cs2LiYCl6:Ce3+ elpasolite cells was studied for simultaneous neutron measurements and gamma-ray spectroscopy. The system allowed directional detection with the localization of radiation sources. Computational modeling of detector array showed that the three-cell detector array is feasible to determine the direction to a neutron source and a gamma-ray source.
1. Introduction Neutron and photon measurement technologies are essential to fulfilling homeland security and nonproliferation mission areas including detection and localization of missing or smuggled radiological or nuclear materials (Chichester et al., 2007; Doyle, 2008; Menlove et al., 2013; Israelashvili et al., 2015), environmental and waste management (Greenberg et al., 2007; Stauff et al., 2015), and in-situ material analysis (Womble et al., 2002; Reber et al., 2005; Bodnarik et al., 2013). Typically, a combination of two different detectors is utilized for the detection of neutrons and gamma rays, separately; e.g., a moderated 3 He tube provides a neutron count rate and a NaI:Tl scintillator (or a cryogenically-cooled semiconductor detector) measures a gamma-ray spectrum (Moody et al., 2005; Knoll, 2010). This approach makes detection systems and associated electronics complicated and bulky for the use in field measurements. Ideally, a single detector is desired for measurements of both types of radiation simultaneously in the mixed radiation flux scenarios. Dual neutron/photon detection materials such as plastic and liquid scintillators typically exhibit reduced gamma-ray energy resolution (Klein and Neumann, 2002; Zaitseva et al., 2012; Cester et al., 2014; Qin et al., 2015). Recently, the elpasolite scintillator materials (Biswas and Du, 2012) were proposed that enable simultaneous neutron and photon detection: the Ce3+-doped elpasolite crystals Cs2LiYCl6:Ce3+ (CLYC), Cs2LiLaCl6:Ce3+ (CLLC), Cs2LiLaBr6:Ce3+ (CLLB), and Cs2LiYBr6: Ce3+ (CLYB) (Van Loef et al., 2002; Bessiere et al., 2005; Glodo et al., 2011; Guss et al., 2014). These elpasolites provide high energy resolution for
∗
gamma rays. The following full width on the half maximum (FWHM) values were determined for these scintillators for the gamma-ray peak at energy of 662 keV: 3.9%, 3.4%, 2.9%, and 8.5%, respectively. Due to the lithium content, these elpasolites are also capable to interact with thermal neutrons. Naturally-occurring lithium is composed of two stable isotopes 6Li and 7Li (7.5% and 92.5% abundance, correspondingly). The thermal neutron capture cross-section for 6Li (940 b) is acceptable for detection applications via the 6Li(n,α)3H reaction, where the alpha particle and Triton share the kinetic energy of 4.78 MeV. These energetic charged particles produce ionization tracks in the elpasolite material with the subsequent trapping of free charges in the Ce3+ scintillation centers that create a pulse of light as a result of the de-excitation process. Moreover, 133Cs, 35Cl, and 37Cl isotopes can capture thermal neutrons. To increase detection efficiency, the 6Li-enriched elpasolites were synthesized, e.g. the CLYC with the 95% lithium-6 enrichment (Glodo et al., 2013): its thermal neutron crosssection is 2.3 times that of 3He gas at 10 bar for the same volume. In the energy spectra, the full-energy thermal neutron peak typically appears above 3 MeV gamma-equivalent energy (GEE), with the energy resolution of 3%. Thus, the pulse-height discrimination (PHD) can be implemented with these materials. The CLYC emissions consist of three components: core-to-valence luminescence (Rodnyi, 2004) (CVL; 250 nm–350 nm), Ce3+ prompt emission (350–450 nm), and cerium self-trapped excitation (Ce3+-STE). The first two components to appear are due to the photon interactions with the scintillator; the third component appears due to thermal neutron interactions within the CLYC. The CVL is of particular interest
Corresponding author. E-mail address: [email protected] (A. Guckes).
https://doi.org/10.1016/j.radmeas.2019.04.003 Received 10 February 2016; Received in revised form 18 March 2019; Accepted 3 April 2019 Available online 06 April 2019 1350-4487/ © 2019 Elsevier Ltd. All rights reserved.
Radiation Measurements 124 (2019) 127–131
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an incident radiation) with an FT8 CAP treatment was used. The FT8 CAP turns the F8 tally into a neutron capture tally, which returns the number of neutron capture events on 6Li isotopes in the cell per source particle. This value was then multiplied by the number of source particles (1 × 107) resulting in the total number of neutron captures in each detector cell. The number of neutron captures tallied in each of the cells as a function of the angular rotation of the array provided the directional response of the elpasolite array to neutrons. For gamma rays, an isotropic 662-keV photon point source (representing the 137Cs isotope) was also modeled in locations along a path of a circle of the radius R in the angular increments of 10°, from 0° to 360°, in the XY-plane. The gamma-ray source was initially placed at a radius of 10 cm from the center of the array for near-field measurements. The gamma-ray source was then placed at a radius of 100 cm from the center of the array. To evaluate the number of gamma rays interacting within the CLYC crystal, an F4 tally (average flux over the cell) was utilized. The gamma-ray flux, in units of photons/cm2/source particle, was tallied in each of the cells as a function of the photon source position. This value was then multiplied by the volume of the cell in cm2 and the number of source particles (1 × 107) resulting in the total number of gamma-ray interactions in each detector cell. The total number of gamma-ray interactions in each detector cell as a function of angular rotation of the array provided the directional response of the detection system to gamma rays. The statistical error for each tally was also estimated.
as it appears only under the photon excitation. It is also fast and decays with less than 2-ns time constant. It is also absorbed by Ce3+ ions in a crystal. The significant difference in the decay times of the gamma-ray versus neutron-induced emission components allow for excellent pulseshape discrimination (PSD) capabilities to segregate neutron and gamma-ray pulses (D'Olympia et al., 2013). The feasibility of fast neutron spectroscopy via the 35Cl(n,p)35S reaction using CLYC was also studied (D'Olympia et al., 2012). The proton energy in this reaction depends linearly on the initial energy of the fast neutron; a proton fullenergy peak is induced in the energy spectrum. Thus, the CLYC elpasolite can be utilized to detect neutrons and photons with the energy spectroscopy option for both radiations. In the remote sensing of neutron and gamma-ray radiation, directional detection technologies that provide the location of the sources are often necessary (Guss and Mukhopadhyay, 2013). To address this need, the directional detection system consisting of an array of CLYC cells was investigated for simultaneous measurements of neutron and photon flux with the localization of radiation sources. 2. Computational model The computational study was carried out using the Monte Carlo particle transport code MCNP6 (MCNP6, 2013). A detection system consisting of an array of cylindrical 1-inch-diameter by 1-inch-height CLYC scintillator cells was modeled. Fig. 1 shows the models of 3-cell and 4-cell arrays arranged in a symmetrical closely-packed geometry. In these models, the CLYC scintillator crystals were encapsulated in thin aluminum housings. Other parts of the detector system (e.g., photomultiplier tubes) have little effect on the radiation interactions within the CLYC crystals and were not included into the model. To determine the detector responses at different radiation source positions, MCNP6 simulations were performed for the 3- and the 4-cell systems and for both a neutron source and a gamma-ray source.
2.2. Maximum likelihood estimation of direction to a radiation source The direction from the detection system to a neutron source or a gamma-ray source was known for the computational study. However, if this directional detection system would be employed in the field to find a lost, stolen or smuggled radioactive source or nuclear material, the exact position of the source will not be known. To enable estimation of the unknown source direction, the following procedure was enacted. The 360°-simulated data of each CLYC detector in the array was fitted with a smoothing spline model. The angle at which each smoothing spline function is a maximum is the observed source angle for that specific detector. This is based on the assumption that each detector will observe the maximum number of counts when the greatest amount of each detector's surface area is aligned with the source. It is also assumed that the observed source direction obtained from each detector in the array is normally distributed with the following probability density function (PDF):
2.1. Modeling of directional detection of neutrons and photons To study the directional neutron detection, an isotropic, point 0.0253-eV neutron source was placed along a path of a circle of the radius R in locations determined by the angular increments of 10°, from 0° to 360°, in the XY-plane. The neutron source was initially placed at a radius of 10 cm from the center of the array for near-field measurements. The neutron source was then placed at a radius of 100 cm from the center of the array. These conditions reflected those we plan to implement in the experiments. To calculate the number of neutron capture events on 6Li nuclei within the CLYC, an F8 tally (which calculates the energy distribution of pulse heights in a detector caused by
f(Θi | θ , σ 2) =
−(Θi − θ) 1 exp 2σ 2 2π σ
2
(1)
where Θi is the observed source angle from the measured data for the ith detector, θ is the unknown actual source angle, and σ 2 is the variance of θ . Since all detectors are independent, the PDF for the source angle considering all n detectors in the array is as follows: n
f ({Θi }in= 1 |θ , σ 2) =
∏ f (Θi | θ, σ 2) i=1
(2)
The likelihood function of Equation (2) is defined as:
L (θ , σ 2) = ln(f ({Θi }in= 1 |θ , σ 2))
(3)
The maximum likelihood estimate (MLE) for the unknown actual source angle is that θ which maximizes L (θ) :
θˆ = arg max L (θ , σ 2) θ
(4)
The variance of θˆ is defined as: Fig. 1. Three-cell (solid yellow cells) and four-cell (CLYC #1, CLYC #2, and pattern-filled cells) array models. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
σˆ 2 =
128
1 n
n
∑ (Θi − θˆ) i=1
2
(5)
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Fig. 2. Three-cell CLYC array exposed to thermal neutron source positioned at R = 10 cm.
Fig. 4. Three-cell CLYC array exposed to 137Cs gamma-ray source positioned at R = 10 cm.
The standard deviation is the square root of the variance. Based on this assumption, the maximum likelihood estimation method is applied to determine the estimate of the source angle for both the neutron source and the gamma-ray source at the distances of 10 cm and 100 cm using both the three-cell and the four-cell array.
3. Results and discussion The computational modeling results for the three- and four-cell arrays provide the information on how a prototype directional detection system would respond to radioactive sources. The models of the fourCLYC array serve as a comparison to the three-cell array in terms of quality of the response. It can be observed from Figs. 2–9 that the responses of both detection systems to neutrons and gamma rays had an angular pattern distinctive for the specific direction to the source. It should be noted that a single CLYC scintillator is able to detect both neutrons and gamma-rays simultaneously. However, the neutron and gamma-ray responses are presented separately in this study. Although both neutron and gamma-ray counts were collected in a single MCNP simulation for each rotation of the array, two different MCNP tallies were used to determine the counts for neutrons and gamma-rays thus, two separate sets of data. The maximum MCNP statistical error achieved in the computational study of the three-cell array for gamma rays was 2% and for thermal neutrons it was 9%. The maximum MCNP statistical error achieved in the computational study of the four-cell array for gamma rays was 2% and for thermal neutrons it was 10%. These statistical errors can be reduced with a higher number of particles ran or longer run times in each of the MCNP simulations. For near-field measurements (R = 10 cm), the response of all the detectors to both thermal neutrons and gamma rays is higher in magnitude and the difference between the actual source angle and the MLE source angle is decreased. Measurements in the position R = 100 cm are lower in magnitude with larger differences between the actual
Fig. 5. Three-cell CLYC array exposed to 137Cs gamma-ray source positioned at R = 100 cm.
Fig. 6. Four-cell CLYC array exposed to thermal neutron source positioned at R = 10 cm.
Fig. 7. Four-cell CLYC array exposed to thermal neutron source positioned at R = 100 cm.
source angle and the MLE source angle. This is due to the fact that the intensity of a point source decreases with respect to the inverse of the distance from the center of the detector system to the source squared.
Fig. 3. Three-cell CLYC array exposed to thermal neutron source positioned at R = 100 cm. 129
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4. Conclusion
Fig. 8. Four-cell CLYC array exposed to R = 10 cm.
137
Cs gamma-ray source positioned at
Fig. 9. Four-cell CLYC array exposed to R = 100 cm.
137
Cs gamma-ray source positioned at
A detector consisting of an array of 1-inch-diameter by 1-inchheight CLYC cells arranged in a symmetrical closely-packed geometry was studied for simultaneous directional neutron and gamma-ray measurements. Three- and a four-cell arrays were modeled using MCNP6 code. The evaluation of the elpasolite detector responses showed that the three-cell array is feasible to determine the direction to a point-like thermal neutron source or a gamma-ray source simultaneously. The MLE-enabled source localization for the three-cell array estimated the source angle with a difference of 1° and 4° for a 137Cs gammaray source at R = 10 cm and 100 cm, respectively, and 1° and 8° for a thermal neutron source at R = 10 cm and 100 cm, respectively. The source localization for the four-cell array estimated the source angle with a difference of 0° and 10° for a 137Cs gamma-ray source at R = 10 cm and 100 cm, respectively, and 0° and 9° for a thermal neutron source at R = 10 cm and 100 cm, respectively. Considering the standard deviation for each estimate, the actual source angle is predicted within the standard deviation for each MLE source angle. Altogether, the differences between the actual source angle and the MLE source angle are reasonable. The larger (> 1°) angle differences and MLE standard deviations all occur at R = 100 cm. This behavior is expected as the array is placed farther away from the source and thus, the intensity of the source decreases with respect to the inverse of the distance from the center of the detector array to the source squared. These angle differences and MLE standard deviations can be minimized computationally with a higher number of source particles ran or longer run time in each MCNP case and by decreasing the angular increment at which counts are collected (i.e., from 10° to 1°). These results indicate that the thermal neutron and/or gamma-ray measurements with either the three- or four-cell array and utilization of the mathematical procedure described in Section 2.2 to interpret the observed data is achievable and enables source localization. However, an array based on three CLYC cells will cost less, weigh less and be more compact than a four-cell array. The three-cell array with the described MLE procedure would allow an end-user to search for missing or smuggled radiological or nuclear materials, perform environmental and waste management, and in-situ material analysis with improved source direction estimates at each step while approaching the source. The next step is to perform an experimental study to confirm the results of the computational study and to develop a prototype dual neutron/gamma-ray directional detector.
This characteristic is apparent when the detectors were rotated 360° in the XY-plane. The source angle estimates for the computational models are provided in Table 1. It is apparent that the difference between the MLE source angle and the actual source angle increases with increasing distance for both thermal neutrons and gamma rays and for both the three-cell and four-cell array. So, the closer the detector is to a source, the more accurate the estimate of the direction to the source will be. A longer data collection time, and thus higher counts collected, would also afford a more accurate estimate of the source direction. It is apparent that for near-field measurements (R = 10 cm), the four-cell array provides a slightly (1°) more accurate estimate of the source direction than the three-cell array. For measurements at R = 100 cm, the three-cell array provides a slightly (< 6°) more accurate estimate of the source direction. These differences are small and can be attributed to active volume size and thus, detector efficiency in the near-field case and poorer statistics in the far-field case. From this study it can be concluded that a three-cell array is feasible to accurately estimate the source's direction. An array based on three CLYC cells will cost less, weigh less and be more compact than a fourcell array without decreasing the quality of the direction estimates.
Acknowledgments This manuscript has been authored by Mission Support and Test Services, LLC, under Contract No. DE-NA0003624 with the U.S. Department of Energy and supported by the Site-Directed Research and Development Program. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form
Table 1 Estimates of the source direction. Source Position
Actual Source Angle
MLE Source Angle ± σ
Actual vs MLE Angle Difference
3-cell 3-cell 3-cell 3-cell 4-cell 4-cell 4-cell 4-cell
90° 90° 90° 90° 45° 45° 45° 45°
89° ± 2° 82° ± 27° 89° ± 1° 94° ± 28° 45° ± 2° 54° ± 25° 45° ± 2° 55° ± 16°
1° 8° 1° 4° 0° 9° 0° 10°
array, array, array, array, array, array, array, array,
thermal neutrons, R = 10 cm thermal neutrons, R = 100 cm Cs: R = 10 cm 137 Cs: R = 100 cm thermal neutrons: R = 10 cm thermal neutrons: R = 100 cm 137 Cs: R = 10 cm 137 Cs: R = 100 cm 137
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of this manuscript, or allow others to do so, for United States Government purposes. The U.S. Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/ downloads/doe-public-access-plan). DOE/NV/25946–2704. The authors acknowledge the professional staff of RMD, Watertown, Massachusetts, for the production of the scintillators, for providing these detectors to the Remote Sensing Laboratory, and for their support and advice.
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