3 He and BF 3 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 University of Florida, Nuclear and Radiological Engineering, Gainesville, FL 32611, USA b Pacific Northwest National Laboratory, PO Box 999, Richland, WA 99352, USA article info Available online 23 October 2010 Keywords: 3 He Boron trifluoride Radiation detection Neutron detection abstract 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 3 He gas-filled proportional counters for detecting neutrons. Proportional counters filled with 3 He are the preferred neutron detectors for use in radiation portal monitor systems because 3 He 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 3 He 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 3 He pressure in the proportional counters and using boron trifluoride gas-filled proportional counters. Reducing the amount of 3 He required in each of the proportional counters would decrease the rate at which 3 He 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 3 He, boron trifluoride gas is corrosive, and a much higher voltage is required than what is used with 3 He 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. 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 3 He gas-filled proportional counters are used to detect neutrons [3]. However, the supply of 3 He 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 3 He until an alternative can be identified may need to be considered. One possibility for reducing the amount of 3 He used in the RPMs is to decrease the partial 3 He pressure in the tubes [6]. Decreasing the tube pressure will decrease the neutron detection efficiency; however, if a lower pressure 3 He tube can meet the required neutron detection capability then the amount of 3 He required for each RPM will be reduced [7]. Another technology that has been identified as a candidate for replacing 3 He is boron trifluoride (BF 3 ) 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 3 He. The efficiency of BF 3 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 BF 3 proportional counters. One of the constraints for any 3 He 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 3 He and BF 3 pressures and various numbers of tubes with the experimental data. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/nima Nuclear Instruments and Methods in Physics Research A 0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.10.040 n Corresponding author. Tel.: + 1 352 256 6461. E-mail address: azu21088@ufl.edu (A. Lintereur). Nuclear Instruments and Methods in Physics Research A 652 (2011) 347–350