Predicting the sensitivity of the beryllium/scintillator layer neutron detector using Monte Carlo and experimental response functionsa) J. D. Styron, G. W. Cooper, C. L. Ruiz, K. D. Hahn, G. A. Chandler, A. J. Nelson, J. A. Torres, B. R. McWatters, Ken Carpenter, and M. A. Bonura Citation: Review of Scientific Instruments 85, 11E617 (2014); doi: 10.1063/1.4896176 View online: http://dx.doi.org/10.1063/1.4896176 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/85/11?ver=pdfcov Published by the AIP Publishing Articles you may be interested in A technique for verifying the input response function of neutron time-of-flight scintillation detectors using cosmic raysa) Rev. Sci. Instrum. 85, 11D633 (2014); 10.1063/1.4896958 Digital discrimination of neutrons and gamma-rays in organic scintillation detectors using moment analysis Rev. Sci. Instrum. 83, 093507 (2012); 10.1063/1.4754633 Study of the response of plastic scintillation detectors in small-field 6 MV photon beams by Monte Carlo simulations Med. Phys. 38, 1596 (2011); 10.1118/1.3554644 Monte Carlo study of the energy and angular dependence of the response of plastic scintillation detectors in photon beams Med. Phys. 37, 5279 (2010); 10.1118/1.3488904 Monte Carlo investigations of megavoltage cone-beam CT using thick, segmented scintillating detectors for soft tissue visualization Med. Phys. 35, 145 (2008); 10.1118/1.2818957
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 132.203.227.62 On: Wed, 04 Mar 2015 19:02:20
REVIEW OF SCIENTIFIC INSTRUMENTS 85, 11E617 (2014)
Predicting the sensitivity of the beryllium/scintillator layer neutron detector using Monte Carlo and experimental response functionsa) J. D. Styron,1,b) G. W. Cooper,1 C. L. Ruiz,2 K. D. Hahn,2 G. A. Chandler,2 A. J. Nelson,2 J. A. Torres,2 B. R. McWatters,2 Ken Carpenter,1 and M. A. Bonura1 1 2
Department of Nuclear Engineering, University of New Mexico, Albuquerque, New Mexico 87131, USA Sandia National Laboratories, Albuquerque, New Mexico 87185, USA
(Presented 4 June 2014; received 1 June 2014; accepted 6 September 2014; published online 26 September 2014) A methodology for obtaining empirical curves relating absolute measured scintillation light output to beta energy deposited is presented. Output signals were measured from thin plastic scintillator using NIST traceable beta and gamma sources and MCNP5 was used to model the energy deposition from each source. Combining the experimental and calculated results gives the desired empirical relationships. To validate, the sensitivity of a beryllium/scintillator-layer neutron activation detector was predicted and then exposed to a known neutron fluence from a Deuterium-Deuterium fusion plasma (DD). The predicted and the measured sensitivity were in statistical agreement. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4896176] I. INTRODUCTION
The physical relationship between beta energy deposition and the resulting light output for organic scintillators is well known and described in detail by Birks.1 Birks’ formula predicts the light output in the absence of quenching, which is the case for energetic electrons, and is directly proportional to the beta energy deposition. In addition, the detector response will be characterized absolutely by a system specific proportionality constant.2 There are two types of experimental techniques commonly discussed in the literature to determine this constant: the Compton Coincidence Technique (CCT)3 and the Compton Edge Method (CEM).4, 5 In CCT, the electron energy is inferred by exposing the scintillator to a collimated gamma source and measuring the kinematic relationship of scattered photon and electron pairs. This requires a coincidence measurement between the scintillator detector of interest and a calibrated photon detector, typically a high-purity germanium detector at a known angle and location. The CEM method is used to determine the response of the detector at the maximum obtainable energy of a recoil electron from a collision with a gamma of known energy. This energy corresponds to the half-height of the Compton edge that can be determined from a differential spectrum. This method requires several known gamma sources of varying energies and some knowledge of the detector resolution. In this work, we are interested in the response function of a fusion neutron detector which is based on the 9 Be (n, α) 6 He reaction. The neutron yield is inferred by measuring the beta decay of the 6 He nucleus, which has a half-life of 807 ms and a beta end-point energy of 3.51 MeV.6 The beta particle is detected by intimately coupling a thin layer of plastic scintillator to a beryllium layer, similar to the original design a) Contributed paper, published as part of the Proceedings of the 20th Top-
ical Conference on High-Temperature Plasma Diagnostics, Atlanta, Georgia, USA, June 2014. b) Author to whom correspondence should be addressed. Electronic mail: [email protected] 0034-6748/2014/85(11)/11E617/4/$30.00
proposed by Rowland.7 Any or all of the betas’ energy is deposited within the scintillator and the resulting scintillation photons are collected using a photomultiplier tube (PMT) that views the scintillator on edge. It has been well established in the literature that the parameters that affect the magnitude of the pulse measured from the PMT are the scintillator type, the reflectivity of materials surrounding the scintillator, the location of scintillation event, the system electronics, and the solid angle subtended by the scintillator on the photocathode.8–10 This work quantifies these parameters and develops a detector response function for a simple detector system using a semiempirical formulation that combines experimental data and Monte Carlo calculations.11 II. EXPERIMENTAL SET-UP
For this work, an existing cylindrical beryllium detector was adopted. The housing is a steel tube (4.215 cm in diameter by 11.9 cm in height) that houses a Hamamatsu R5946 PMT and was retrofitted to accept, 5.5 mm below the PMT, rectangular geometries. Figure 1 shows the variations of beryllium-scintillator options investigated and the three possible source positions. The left image is the primary geometry for which the response function is desired. It consists of alternating 30 × 65 mm layers of 3.175 mm thick beryllium metal (3 total) and 1.0 mm thick Bicron BC-404 plastic scintillator (4 total). The two inner scintillator layers have beryllium on both sides of the scintillator and the outermost scintillator layers are reflected with beryllium and a 1.0 mm white polypropylene layer. These two geometries, shown with a source in place, are labeled case 1 and case 2 and are shown in the center and right images, respectively. Data were collected with several NIST traceable beta and gamma sources for cases 1 and 2 to develop the response for the primary detector geometry. The isotopes used throughout were Co-60, Cl-36, Cs-137, Sr-90/Y-90, and Na22. Each source has a known activity (∼10 nCi and
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 132.203.227.62 On: Wed, 04 Mar 2015 19:02:20
REVIEW OF SCIENTIFIC INSTRUMENTS 85, 11E617 (2014)
Predicting the sensitivity of the beryllium/scintillator layer neutron detector using Monte Carlo and experimental response functionsa) J. D. Styron,1,b) G. W. Cooper,1 C. L. Ruiz,2 K. D. Hahn,2 G. A. Chandler,2 A. J. Nelson,2 J. A. Torres,2 B. R. McWatters,2 Ken Carpenter,1 and M. A. Bonura1 1 2
Department of Nuclear Engineering, University of New Mexico, Albuquerque, New Mexico 87131, USA Sandia National Laboratories, Albuquerque, New Mexico 87185, USA
(Presented 4 June 2014; received 1 June 2014; accepted 6 September 2014; published online 26 September 2014) A methodology for obtaining empirical curves relating absolute measured scintillation light output to beta energy deposited is presented. Output signals were measured from thin plastic scintillator using NIST traceable beta and gamma sources and MCNP5 was used to model the energy deposition from each source. Combining the experimental and calculated results gives the desired empirical relationships. To validate, the sensitivity of a beryllium/scintillator-layer neutron activation detector was predicted and then exposed to a known neutron fluence from a Deuterium-Deuterium fusion plasma (DD). The predicted and the measured sensitivity were in statistical agreement. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4896176] I. INTRODUCTION
The physical relationship between beta energy deposition and the resulting light output for organic scintillators is well known and described in detail by Birks.1 Birks’ formula predicts the light output in the absence of quenching, which is the case for energetic electrons, and is directly proportional to the beta energy deposition. In addition, the detector response will be characterized absolutely by a system specific proportionality constant.2 There are two types of experimental techniques commonly discussed in the literature to determine this constant: the Compton Coincidence Technique (CCT)3 and the Compton Edge Method (CEM).4, 5 In CCT, the electron energy is inferred by exposing the scintillator to a collimated gamma source and measuring the kinematic relationship of scattered photon and electron pairs. This requires a coincidence measurement between the scintillator detector of interest and a calibrated photon detector, typically a high-purity germanium detector at a known angle and location. The CEM method is used to determine the response of the detector at the maximum obtainable energy of a recoil electron from a collision with a gamma of known energy. This energy corresponds to the half-height of the Compton edge that can be determined from a differential spectrum. This method requires several known gamma sources of varying energies and some knowledge of the detector resolution. In this work, we are interested in the response function of a fusion neutron detector which is based on the 9 Be (n, α) 6 He reaction. The neutron yield is inferred by measuring the beta decay of the 6 He nucleus, which has a half-life of 807 ms and a beta end-point energy of 3.51 MeV.6 The beta particle is detected by intimately coupling a thin layer of plastic scintillator to a beryllium layer, similar to the original design a) Contributed paper, published as part of the Proceedings of the 20th Top-
ical Conference on High-Temperature Plasma Diagnostics, Atlanta, Georgia, USA, June 2014. b) Author to whom correspondence should be addressed. Electronic mail: [email protected] 0034-6748/2014/85(11)/11E617/4/$30.00
proposed by Rowland.7 Any or all of the betas’ energy is deposited within the scintillator and the resulting scintillation photons are collected using a photomultiplier tube (PMT) that views the scintillator on edge. It has been well established in the literature that the parameters that affect the magnitude of the pulse measured from the PMT are the scintillator type, the reflectivity of materials surrounding the scintillator, the location of scintillation event, the system electronics, and the solid angle subtended by the scintillator on the photocathode.8–10 This work quantifies these parameters and develops a detector response function for a simple detector system using a semiempirical formulation that combines experimental data and Monte Carlo calculations.11 II. EXPERIMENTAL SET-UP
For this work, an existing cylindrical beryllium detector was adopted. The housing is a steel tube (4.215 cm in diameter by 11.9 cm in height) that houses a Hamamatsu R5946 PMT and was retrofitted to accept, 5.5 mm below the PMT, rectangular geometries. Figure 1 shows the variations of beryllium-scintillator options investigated and the three possible source positions. The left image is the primary geometry for which the response function is desired. It consists of alternating 30 × 65 mm layers of 3.175 mm thick beryllium metal (3 total) and 1.0 mm thick Bicron BC-404 plastic scintillator (4 total). The two inner scintillator layers have beryllium on both sides of the scintillator and the outermost scintillator layers are reflected with beryllium and a 1.0 mm white polypropylene layer. These two geometries, shown with a source in place, are labeled case 1 and case 2 and are shown in the center and right images, respectively. Data were collected with several NIST traceable beta and gamma sources for cases 1 and 2 to develop the response for the primary detector geometry. The isotopes used throughout were Co-60, Cl-36, Cs-137, Sr-90/Y-90, and Na22. Each source has a known activity (∼10 nCi and
