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3He and BF3 neutron detector pressure effect and model comparison
Nuclear Instruments and Methods in Physics Research A 652 (2011) 347–350
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
3
He and BF3 neutron detector pressure effect and model comparison
Azaree Lintereur a,n, Kenneth Conlin b, James Ely b, Luke Erikson b, Richard Kouzes b, Edward Siciliano b, David Stromswold b, Mitchell Woodring b a b
University of Florida, Nuclear and Radiological Engineering, Gainesville, FL 32611, USA Pacific Northwest National Laboratory, PO Box 999, Richland, WA 99352, USA
a r t i c l e in f o
abstract
Available online 23 October 2010
Radiation detection systems for homeland security applications must possess the capability of detecting both gamma rays and neutrons. The radiation portal monitor systems that are currently deployed use a plastic scintillator for detecting gamma rays and 3He gas-filled proportional counters for detecting neutrons. Proportional counters filled with 3He are the preferred neutron detectors for use in radiation portal monitor systems because 3He has a large neutron cross-section, is relatively insensitive to gamma-rays, is neither toxic nor corrosive, can withstand extreme environments, and can be operated at a lower voltage than some of the alternative proportional counters. The amount of 3He required for homeland security and science applications has depleted the world supply and there is no longer enough available to fill the demand. Thus, alternative neutron detectors are being explored. Two possible temporary solutions that could be utilized while a more permanent solution is being identified are reducing the 3He pressure in the proportional counters and using boron trifluoride gas-filled proportional counters. Reducing the amount of 3He required in each of the proportional counters would decrease the rate at which 3He is being used; not enough to solve the shortage, but perhaps enough to increase the amount of time available to find a working replacement. Boron trifluoride is not appropriate for all situations as these detectors are less sensitive than 3He, boron trifluoride gas is corrosive, and a much higher voltage is required than what is used with 3He detectors. Measurements of the neutron detection efficiency of 3 He and boron trifluoride as a function of tube pressure were made. The experimental results were also used to validate models of the radiation portal monitor systems. & 2010 Elsevier B.V. All rights reserved.
Keywords: 3 He Boron trifluoride Radiation detection Neutron detection
1. Introduction Radiation portal monitor (RPM) systems are used to interdict illicit radioactive sources being transported across international borders. RPMs must meet specified criteria for both gamma ray and neutron detection [1,2]. Currently, plastic scintillators are used to detect gamma rays and 3He gas-filled proportional counters are used to detect neutrons [3]. However, the supply of 3He has become limited and the present demand level can no longer be sustained [4]. Alternative technologies that can fulfill the homeland security neutron detection requirements are being explored [5]. Options to extend the available 3He until an alternative can be identified may need to be considered. One possibility for reducing the amount of 3He used in the RPMs is to decrease the partial 3He pressure in the tubes [6]. Decreasing the tube pressure will decrease the neutron detection efficiency; however, if a lower pressure 3He tube can meet the required neutron detection
n
Corresponding author. Tel.: + 1 352 256 6461. E-mail address: azu21088@ufl.edu (A. Lintereur).
0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.10.040
capability then the amount of 3He required for each RPM will be reduced [7]. Another technology that has been identified as a candidate for replacing 3He is boron trifluoride (BF3) gas-filled proportional tubes [8]. Boron trifluoride gas has the disadvantage of being corrosive, having a lower neutron cross-section and requiring a higher operating voltage than 3He. The efficiency of BF3 tubes increases with increasing tube pressure; however, the required operating voltage also increases. Higher voltages are more difficult to deploy in field conditions where high humidity can produce breakdown. Thus, using an increased number of lower pressure tubes to decrease the required voltage, while maintaining the required neutron efficiency, may be a more practical option for BF3 proportional counters. One of the constraints for any 3He replacement for use in RPMs is that it fits into the space available in the systems that are currently deployed. Therefore, the number of tubes that were tested simultaneously was limited to what would fit into the existing RPM moderating box. Models of the systems have been created and used for parametric studies to predict system response. The models were validated by comparing the theoretical results of the system efficiency generated with different 3He and BF3 pressures and various numbers of tubes with the experimental data.
348
A. Lintereur et al. / Nuclear Instruments and Methods in Physics Research A 652 (2011) 347–350
2. Equipment and experiments Pacific Northwest National Laboratory (PNNL) has made measurements to test the neutron detection efficiency of 3He and BF3 tubes as a function of tube pressure. The measurements were performed by positioning the tubes in the existing polyethylene moderating assembly in a Science Applications International Corporation (SAIC) re-locatable RPM system (Fig. 1a). External electronics were used for all the measurements to increase the range of voltages available for testing, as not all the tubes were operated at the standard operating voltage for the currently deployed 300 kPa (3 atm) 3He tube. The appropriate operating voltage for each tube pressure was determined by generating a voltage plateau curve. The same electronics were used for all the tests except for the 120 kPa BF3 test, which used a different multi-channel analyzer than subsequent measurements. Four different 3He partial pressure tubes and two different BF3 pressure tubes, manufactured by LND Inc. (height 182.88 cm (72 in.), diameter 5.08 cm (2 in.)) were tested. The tube pressures and configurations tested are shown in Tables 1 and 2. The experimental and theoretical efficiencies for each configuration are further discussed in Section 3.
The measurements were made with a 252Cf neutron source. The source activity at the time of the measurements was 21.571.23 mCi. The source was located in a polyethylene pig positioned 2 m from the front panel of the RPM with the center of the pig at the same height as the midpoint of the moderating box. All 3He and BF3 calculated results shown were obtained using the Monte-Carlo N-Particle transport code, MCNP, and by tallying on the total number of neutron capture reactions in the target gases [9]. One and two 3He tubes with pressures up to 300 kPa and one through four 107 and 120 kPa BF3 tubes were modeled in the SAIC re-locatable base (Fig. 1b). The theoretical results were compared with the data from the experimental measurements.
3. Measurements and results The detection efficiency of the 3He tubes increased with increase in pressure (Tables 1), following a logarithmic trend. The model predicted the same trend that was observed experimentally; however, the models under-predicted the values of the experimental results by approximately 7–9% (Fig. 2). The neutron
Fig. 1. (a) SAIC re-locatable RPM used for these measurements (with the front panel removed) and (b) MCNP model of the SAIC re-locatable base.
Table 1 3 He tube pressures and configurations tested and the corresponding neutron detection efficiencies. Pressure (kPa)
Number of tubes
Theoretical efficiency (cps/ng
101 101 200 253 300 300
1 2 1 1 1 2
1.75 2.88 2.37 2.57 2.73 4.14
252
Cf)
Experimental efficiency (cps/ng 1.90 3.10 2.63 2.81 3.05 4.04
252
Cf)
A. Lintereur et al. / Nuclear Instruments and Methods in Physics Research A 652 (2011) 347–350
349
Table 2 BF3 tube pressures and configurations tested and the corresponding neutron detection efficiencies. Pressure (kPa)
Number of tubes
Theoretical efficiency (cps/ng
107 107 107 107 120 120 120 120
1 2 3 4 1 2 3 4
1.49 2.27 2.86 3.26 1.58 2.38 3.00 3.41
252
Cf)
Experimental efficiency (cps/ng
252
Cf)
1.62 2.67 3.27 3.75 1.58 2.59 3.04 3.38
Fig. 2. Model and experimental results for the 3He tubes.
Fig. 3. Model and experimental results for the BF3 tubes.
detection efficiency of one tube as a function of tube pressure was predicted by Eq. (1), and the efficiency of two tubes as a function of tube pressure was predicted by Eq. (2)
detection efficiency. However, the geometric limitations of the current configuration limit the possible changes to the moderator dimensions. An exhaustive study has not been performed, nor have the results been verified experimentally.1 The uncertainty in the experimental results from the measurements made with the 3He and BF3 tubes was primarily due to the uncertainty in the source strength, which was 5.7%. The statistical uncertainties of the results from the models were all less than 2%, and thus any error bars would be smaller than the symbols plotted. The results for both measurements are shown in units of cps per nanogram of moderated 252 Cf. The efficiency in counts per emitted neutron can be obtained using the conversion factor 2.3 104 n/s¼5.4 mCi¼10 nanograms of 252Cf.
E ¼ 0:90 ln ðPÞ2:39
ð1Þ
E ¼ 1:15 ln ðPÞ2:43
ð2Þ 252
where E is the efficiency (cps/ng Cf) and P the pressure (kPa). Two tubes did not result in twice the efficiency of one tube because the tubes partially shield each other due to the space constraints of the moderating box. The BF3 tube configurations tested, like the 3He tubes, followed the trend predicted by the models, but the experimental efficiencies were approximately 11% higher than the model predictions (Fig. 3). Higher experimental neutron detection efficiency was obtained with the 107 kPa tubes than the 120 kPa, which was the opposite of what was predicted. It is possible that there were slight differences in the tubes that were tested (the 120 kPa tubes were manufactured at an earlier date than the 107 kPa tubes), but the discrepancy is more likely caused by the different multichannel analyzer that was used for 120 kPa tubes. As with the 3He tubes, the efficiency did not increase linearly with the number of tubes tested (Table 2). The size of the moderating box prevented the tubes from being positioned with sufficient spacing to prevent the tubes from shielding each other. The results presented here were obtained using the standard SAIC moderating box. A small parametric study was performed to determine if optimizing the moderating box could increase the neutron detection efficiency sufficiently to compensate for the decreased efficiency with BF3 tubes and lower pressure 3He tubes. The studies performed suggest that with the current space restraints changing the moderating box may allow for a modest improvement in neutron
4. Conclusions The experiments performed at PNNL demonstrate that for 3He tubes located within the standard SAIC moderating box the neutron detection efficiency increases with increasing tube pressure. However, the increase in efficiency is not linear, rather, the 3He neutron detection efficiency increases logarithmically with pressure. Thus, doubling the 3He pressure does not double the neutron detection efficiency. The amount of 3He used for certain applications may be decreased using an increased number of lower pressure tubes to obtain the required efficiency (i.e., two 100 kPa tubes in place of one 300 kPa tube). The BF3 neutron detection efficiency did not show the expected increase with tube pressure, likely due to differences in 1
2010.
Edward Siciliano, unpublished study and private communication, August 18,
350
A. Lintereur et al. / Nuclear Instruments and Methods in Physics Research A 652 (2011) 347–350
the electronics used for the measurements. Neither the 3He nor the BF3 tubes demonstrated a linear increase in efficiency with the number of tubes located in the moderating box. The non-linear increase in efficiency with increase in the number of tubes used is due to the tubes partially shielding each other. A larger moderating box would be required to prevent this effect. The experimental results followed the trends predicted by the models for all the tests; however, the model results under predicted the values of the measured results for both the 3He and BF3 filled tubes. Further work on the models is being pursued.
Acknowledgements This work was supported largely by the United States Department of Energy (NA-22). Additional support was provided by Pacific Northwest National Laboratory, the Department of Defense and the Department of Homeland Security. The first author is
supported by a National Science Foundation Graduate Research Fellowship. PNNL-SA-73002. References [1] D.C. Stromswold, J.H. Ely, R.T. Kouzes, J.E. Schweppe, PNNL Report PIET-43741TM-017, 2003. [2] R.T. Kouzes, J.H. Ely, A.T. Lintereur, D.L. Stephens, PNNL Report 18903, 2009. [3] R.T., Kouzes, J.H. Ely, P.E. Keller, R.J. McConn, E.R. Siciliano, NIM-A 584/2-3, 2008, pp. 383–400. [4] R.T. Kouzes, The 3He Supply Problem, PNNL Report 18388, 2009b. [5] R.M. Van Ginhoven, R.T. Kouzes, D.L. Stephens, PNNL Report—18471, 2009. [6] R.T. Kouzes, J.H. Ely, A.T. Lintereur, E.R. Siciliano, D.C. Stromswold, M.L. Woodring, PNNL Report 19110, 2009. [7] V.S. Cornelison, G. Sjoden, G. Ghita, Trans. Am. Nucl. Soc. 96 (2007) 24. [8] R.T. Kouzes, J.H. Ely, A.T. Lintereur, E.R. Siciliano, M.L. Woodring, PNNL Report 19050, 2009. [9] MCNP. 2003. MCNP X-5 Monte Carlo Team, MCNP—A General Purpose Monte Carlo N-Particle Transport Code, Version 5, LA-UR-03-1987, Los Alamos National Laboratory, April 2003. The MCNP code can be obtained from the Radiation Safety Information Computational Center (RSICC), P.O. Box 2008, Oak Ridge, TN, 37831-6362.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
3
He and BF3 neutron detector pressure effect and model comparison
Azaree Lintereur a,n, Kenneth Conlin b, James Ely b, Luke Erikson b, Richard Kouzes b, Edward Siciliano b, David Stromswold b, Mitchell Woodring b a b
University of Florida, Nuclear and Radiological Engineering, Gainesville, FL 32611, USA Pacific Northwest National Laboratory, PO Box 999, Richland, WA 99352, USA
a r t i c l e in f o
abstract
Available online 23 October 2010
Radiation detection systems for homeland security applications must possess the capability of detecting both gamma rays and neutrons. The radiation portal monitor systems that are currently deployed use a plastic scintillator for detecting gamma rays and 3He gas-filled proportional counters for detecting neutrons. Proportional counters filled with 3He are the preferred neutron detectors for use in radiation portal monitor systems because 3He has a large neutron cross-section, is relatively insensitive to gamma-rays, is neither toxic nor corrosive, can withstand extreme environments, and can be operated at a lower voltage than some of the alternative proportional counters. The amount of 3He required for homeland security and science applications has depleted the world supply and there is no longer enough available to fill the demand. Thus, alternative neutron detectors are being explored. Two possible temporary solutions that could be utilized while a more permanent solution is being identified are reducing the 3He pressure in the proportional counters and using boron trifluoride gas-filled proportional counters. Reducing the amount of 3He required in each of the proportional counters would decrease the rate at which 3He is being used; not enough to solve the shortage, but perhaps enough to increase the amount of time available to find a working replacement. Boron trifluoride is not appropriate for all situations as these detectors are less sensitive than 3He, boron trifluoride gas is corrosive, and a much higher voltage is required than what is used with 3He detectors. Measurements of the neutron detection efficiency of 3 He and boron trifluoride as a function of tube pressure were made. The experimental results were also used to validate models of the radiation portal monitor systems. & 2010 Elsevier B.V. All rights reserved.
Keywords: 3 He Boron trifluoride Radiation detection Neutron detection
1. Introduction Radiation portal monitor (RPM) systems are used to interdict illicit radioactive sources being transported across international borders. RPMs must meet specified criteria for both gamma ray and neutron detection [1,2]. Currently, plastic scintillators are used to detect gamma rays and 3He gas-filled proportional counters are used to detect neutrons [3]. However, the supply of 3He has become limited and the present demand level can no longer be sustained [4]. Alternative technologies that can fulfill the homeland security neutron detection requirements are being explored [5]. Options to extend the available 3He until an alternative can be identified may need to be considered. One possibility for reducing the amount of 3He used in the RPMs is to decrease the partial 3He pressure in the tubes [6]. Decreasing the tube pressure will decrease the neutron detection efficiency; however, if a lower pressure 3He tube can meet the required neutron detection
n
Corresponding author. Tel.: + 1 352 256 6461. E-mail address: azu21088@ufl.edu (A. Lintereur).
0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.10.040
capability then the amount of 3He required for each RPM will be reduced [7]. Another technology that has been identified as a candidate for replacing 3He is boron trifluoride (BF3) gas-filled proportional tubes [8]. Boron trifluoride gas has the disadvantage of being corrosive, having a lower neutron cross-section and requiring a higher operating voltage than 3He. The efficiency of BF3 tubes increases with increasing tube pressure; however, the required operating voltage also increases. Higher voltages are more difficult to deploy in field conditions where high humidity can produce breakdown. Thus, using an increased number of lower pressure tubes to decrease the required voltage, while maintaining the required neutron efficiency, may be a more practical option for BF3 proportional counters. One of the constraints for any 3He replacement for use in RPMs is that it fits into the space available in the systems that are currently deployed. Therefore, the number of tubes that were tested simultaneously was limited to what would fit into the existing RPM moderating box. Models of the systems have been created and used for parametric studies to predict system response. The models were validated by comparing the theoretical results of the system efficiency generated with different 3He and BF3 pressures and various numbers of tubes with the experimental data.
348
A. Lintereur et al. / Nuclear Instruments and Methods in Physics Research A 652 (2011) 347–350
2. Equipment and experiments Pacific Northwest National Laboratory (PNNL) has made measurements to test the neutron detection efficiency of 3He and BF3 tubes as a function of tube pressure. The measurements were performed by positioning the tubes in the existing polyethylene moderating assembly in a Science Applications International Corporation (SAIC) re-locatable RPM system (Fig. 1a). External electronics were used for all the measurements to increase the range of voltages available for testing, as not all the tubes were operated at the standard operating voltage for the currently deployed 300 kPa (3 atm) 3He tube. The appropriate operating voltage for each tube pressure was determined by generating a voltage plateau curve. The same electronics were used for all the tests except for the 120 kPa BF3 test, which used a different multi-channel analyzer than subsequent measurements. Four different 3He partial pressure tubes and two different BF3 pressure tubes, manufactured by LND Inc. (height 182.88 cm (72 in.), diameter 5.08 cm (2 in.)) were tested. The tube pressures and configurations tested are shown in Tables 1 and 2. The experimental and theoretical efficiencies for each configuration are further discussed in Section 3.
The measurements were made with a 252Cf neutron source. The source activity at the time of the measurements was 21.571.23 mCi. The source was located in a polyethylene pig positioned 2 m from the front panel of the RPM with the center of the pig at the same height as the midpoint of the moderating box. All 3He and BF3 calculated results shown were obtained using the Monte-Carlo N-Particle transport code, MCNP, and by tallying on the total number of neutron capture reactions in the target gases [9]. One and two 3He tubes with pressures up to 300 kPa and one through four 107 and 120 kPa BF3 tubes were modeled in the SAIC re-locatable base (Fig. 1b). The theoretical results were compared with the data from the experimental measurements.
3. Measurements and results The detection efficiency of the 3He tubes increased with increase in pressure (Tables 1), following a logarithmic trend. The model predicted the same trend that was observed experimentally; however, the models under-predicted the values of the experimental results by approximately 7–9% (Fig. 2). The neutron
Fig. 1. (a) SAIC re-locatable RPM used for these measurements (with the front panel removed) and (b) MCNP model of the SAIC re-locatable base.
Table 1 3 He tube pressures and configurations tested and the corresponding neutron detection efficiencies. Pressure (kPa)
Number of tubes
Theoretical efficiency (cps/ng
101 101 200 253 300 300
1 2 1 1 1 2
1.75 2.88 2.37 2.57 2.73 4.14
252
Cf)
Experimental efficiency (cps/ng 1.90 3.10 2.63 2.81 3.05 4.04
252
Cf)
A. Lintereur et al. / Nuclear Instruments and Methods in Physics Research A 652 (2011) 347–350
349
Table 2 BF3 tube pressures and configurations tested and the corresponding neutron detection efficiencies. Pressure (kPa)
Number of tubes
Theoretical efficiency (cps/ng
107 107 107 107 120 120 120 120
1 2 3 4 1 2 3 4
1.49 2.27 2.86 3.26 1.58 2.38 3.00 3.41
252
Cf)
Experimental efficiency (cps/ng
252
Cf)
1.62 2.67 3.27 3.75 1.58 2.59 3.04 3.38
Fig. 2. Model and experimental results for the 3He tubes.
Fig. 3. Model and experimental results for the BF3 tubes.
detection efficiency of one tube as a function of tube pressure was predicted by Eq. (1), and the efficiency of two tubes as a function of tube pressure was predicted by Eq. (2)
detection efficiency. However, the geometric limitations of the current configuration limit the possible changes to the moderator dimensions. An exhaustive study has not been performed, nor have the results been verified experimentally.1 The uncertainty in the experimental results from the measurements made with the 3He and BF3 tubes was primarily due to the uncertainty in the source strength, which was 5.7%. The statistical uncertainties of the results from the models were all less than 2%, and thus any error bars would be smaller than the symbols plotted. The results for both measurements are shown in units of cps per nanogram of moderated 252 Cf. The efficiency in counts per emitted neutron can be obtained using the conversion factor 2.3 104 n/s¼5.4 mCi¼10 nanograms of 252Cf.
E ¼ 0:90 ln ðPÞ2:39
ð1Þ
E ¼ 1:15 ln ðPÞ2:43
ð2Þ 252
where E is the efficiency (cps/ng Cf) and P the pressure (kPa). Two tubes did not result in twice the efficiency of one tube because the tubes partially shield each other due to the space constraints of the moderating box. The BF3 tube configurations tested, like the 3He tubes, followed the trend predicted by the models, but the experimental efficiencies were approximately 11% higher than the model predictions (Fig. 3). Higher experimental neutron detection efficiency was obtained with the 107 kPa tubes than the 120 kPa, which was the opposite of what was predicted. It is possible that there were slight differences in the tubes that were tested (the 120 kPa tubes were manufactured at an earlier date than the 107 kPa tubes), but the discrepancy is more likely caused by the different multichannel analyzer that was used for 120 kPa tubes. As with the 3He tubes, the efficiency did not increase linearly with the number of tubes tested (Table 2). The size of the moderating box prevented the tubes from being positioned with sufficient spacing to prevent the tubes from shielding each other. The results presented here were obtained using the standard SAIC moderating box. A small parametric study was performed to determine if optimizing the moderating box could increase the neutron detection efficiency sufficiently to compensate for the decreased efficiency with BF3 tubes and lower pressure 3He tubes. The studies performed suggest that with the current space restraints changing the moderating box may allow for a modest improvement in neutron
4. Conclusions The experiments performed at PNNL demonstrate that for 3He tubes located within the standard SAIC moderating box the neutron detection efficiency increases with increasing tube pressure. However, the increase in efficiency is not linear, rather, the 3He neutron detection efficiency increases logarithmically with pressure. Thus, doubling the 3He pressure does not double the neutron detection efficiency. The amount of 3He used for certain applications may be decreased using an increased number of lower pressure tubes to obtain the required efficiency (i.e., two 100 kPa tubes in place of one 300 kPa tube). The BF3 neutron detection efficiency did not show the expected increase with tube pressure, likely due to differences in 1
2010.
Edward Siciliano, unpublished study and private communication, August 18,
350
A. Lintereur et al. / Nuclear Instruments and Methods in Physics Research A 652 (2011) 347–350
the electronics used for the measurements. Neither the 3He nor the BF3 tubes demonstrated a linear increase in efficiency with the number of tubes located in the moderating box. The non-linear increase in efficiency with increase in the number of tubes used is due to the tubes partially shielding each other. A larger moderating box would be required to prevent this effect. The experimental results followed the trends predicted by the models for all the tests; however, the model results under predicted the values of the measured results for both the 3He and BF3 filled tubes. Further work on the models is being pursued.
Acknowledgements This work was supported largely by the United States Department of Energy (NA-22). Additional support was provided by Pacific Northwest National Laboratory, the Department of Defense and the Department of Homeland Security. The first author is
supported by a National Science Foundation Graduate Research Fellowship. PNNL-SA-73002. References [1] D.C. Stromswold, J.H. Ely, R.T. Kouzes, J.E. Schweppe, PNNL Report PIET-43741TM-017, 2003. [2] R.T. Kouzes, J.H. Ely, A.T. Lintereur, D.L. Stephens, PNNL Report 18903, 2009. [3] R.T., Kouzes, J.H. Ely, P.E. Keller, R.J. McConn, E.R. Siciliano, NIM-A 584/2-3, 2008, pp. 383–400. [4] R.T. Kouzes, The 3He Supply Problem, PNNL Report 18388, 2009b. [5] R.M. Van Ginhoven, R.T. Kouzes, D.L. Stephens, PNNL Report—18471, 2009. [6] R.T. Kouzes, J.H. Ely, A.T. Lintereur, E.R. Siciliano, D.C. Stromswold, M.L. Woodring, PNNL Report 19110, 2009. [7] V.S. Cornelison, G. Sjoden, G. Ghita, Trans. Am. Nucl. Soc. 96 (2007) 24. [8] R.T. Kouzes, J.H. Ely, A.T. Lintereur, E.R. Siciliano, M.L. Woodring, PNNL Report 19050, 2009. [9] MCNP. 2003. MCNP X-5 Monte Carlo Team, MCNP—A General Purpose Monte Carlo N-Particle Transport Code, Version 5, LA-UR-03-1987, Los Alamos National Laboratory, April 2003. The MCNP code can be obtained from the Radiation Safety Information Computational Center (RSICC), P.O. Box 2008, Oak Ridge, TN, 37831-6362.