Alternatives to Helium-3 for neutron multiplicity counters

Multiplicity counters are an example of a neutron detection system affected by the shortage of ³He. Efforts are underway to identify potential ³He neutron detector replacements for use in multiplicity counters. Boron-10 and ⁶Li based systems are two of the options being explored as near-term ³He alternatives for neutron multiplicity counters. Simulations of BF₃ filled proportional counters, ¹⁰B-lined proportional counters and ⁶Li/ZnS(Ag) sheets in various configurations have been performed with the Monte Carlo particle transport code MCNPX and compared to simulations of existing ³He counters. The system performances are being compared using figure of merits 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 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 BF₃ filled proportional counters, ¹⁰B lined proportional counters and ⁶Li/ZnS(Ag) sheets, and preliminary measurements with the initial bench-top system.