PNNL-SA-92075 Abstract— Multiplicity counters are one of the neutron detection systems affected by the shortage of 3 He. Efforts are underway to identify potential 3 He neutron detector replacements for use in multiplicity counters. Boron-10 and 6 Li based systems are two of the options being explored as near-term 3 He alternatives for neutron multiplicity counters. Simulations of BF 3 filled proportional counters, 10 B-lined proportional counters and 6 Li/ZnS(Ag) sheets in various configurations have been performed with the Monte Carlo particle transport code MCNPX, and compared to simulations of existing 3 He counters. The system performances are being evaluated using figures of merit that are the square, or cube, of the total system efficiency divided by the die-away time (the average lifetime of neutrons in the counter). Design considerations include developing a system with enough neutron detection material to achieve the highest possible efficiency, while simultaneously minimizing system size. Adequate moderation is required to thermalize the incident neutrons for increased counting efficiency, but as the system size increases so will the die-away time. The optimal moderator configuration is one for which the increase in neutron detection efficiency is not off-set by an increase in the die-away time. Thus, the entire system performance must be evaluated with every configuration change. The simulation results will be validated against a bench-top demonstrator unit design based on the system identified through simulations as having the highest performance potential. Presented here are the simulation results with various configurations of BF 3 filled proportional counters, 10 B-lined proportional counters and 6 Li/ZnS(Ag) sheets, and preliminary measurements with the initial bench-top system. Manuscript received November 16, 2012. This work was supported by the U.S. Department of Energy Office of Nonproliferation and Verification Research and Development (NA-22). Pacific Northwest National Laboratory is operated for the United Stated department of Energy under contract DE- AC05-76RLO-1830. A.T. Lintereur is a graduate student at the University of Florida working at Pacific Northwest National Laboratory, Richland WA 99352 (phone: 509- 371-7889; e-mail: Azaree.Lintereur@pnnl.gov). J.H. Ely is with Pacific Northwest National Laboratory, Richland, WA 99352 (e-mail: James.Ely@pnnl.gov). R.T. Kouzes is with Pacific Northwest National Laboratory, Richland, WA 99352 (e-mail: RKouzes@pnnl.gov). E.R. Siciliano is with Pacific Northwest National Laboratory, Richland, WA 99352 (e-mail: Edward.Siciliano@pnnl.gov). M.T. Swinhoe is with Los Alamos National Laboratory, Los Alamos, NM 87544 (e-mail: Swinhoe@lanl.gov). M.L. Woodring is with Pacific Northwest National Laboratory, Richland, WA 99352 (e-mail: Mitchell.Woodring@pnnl.gov). I. INTRODUCTION he 3 He shortage has provided motivation to identify alternative neutron detection options [1]. Neutron multiplicity counters (NMCs) currently rely upon 3 He filled proportional counters, and therefore will be affected by the decreased availability of 3 He. Non- 3 He based NMCs, in various configurations, have been simulated to establish the theoretical performance achievable without using 3 He. In addition to 3 He there are two other commonly used neutron- capture detection materials, 10 B and 6 Li. Boron-10 is available in a gaseous from (BF 3 ), and in a solid form, which can be used for coating devices. Lithium-6 does not exist in a gaseous form, and elemental 6 Li is chemically reactive, so it is typically bound in a compound, such as 6 LiF. There are other thermal neutron detectors available, but for various reasons (small size, low cross sections, gamma ray sensitivity, etc.) they are not as commonly used. In this work several 3 He alternative NMC configurations have been considered. The counter designs simulated in this study were limited to systems developed with three currently commercially available neutron detectors: BF 3 proportional tubes, 10 B-lined proportional tubes, and 6 LiF/ZnS(Ag) scintillating sheets. Some basic properties of the detector materials considered are listed in Table 1 [2]. Helium-3 has the highest cross-section of the detection isotopes listed in Table 1; to achieve equivalent efficiency to a 3 He based system from one designed with 10 B or 6 Li a greater amount of neutron detection material is required. The performances of the different configurations simulated were compared using, two figures of merit (FOMs). The first FOM was the standard (coincidence or doubles) FOM defined as [3]: FOM2 = ! ! ! (1) where ε is the system total neutron detection efficiency and τ is the system die-away time (the average lifetime of a neutron in the counter [4]). This FOM was established for use with coincidence counters, where the doubles rate is the highest multiplicity considered, but has also traditionally been used to compare the performance of multiplicity counters. However, as multiplicity counters also collect the triples, and this study considered NMC designs only, the additional multiplicity, or triples, FOM defined as: FOM3 = ! ! ! (2) Alternatives to Helium-3 for Neutron Multiplicity Counters Azaree T. Lintereur, Member, IEEE, James H. Ely, Member, IEEE, Richard T. Kouzes, Member, IEEE, Edward R. Siciliano, Member, IEEE, Martyn T. Swinhoe, Member, IEEE, and Mitchell L. Woodring, Member, IEEE T