Technology Readiness and Device Design of the Li Foil Multi-Wire Proportional Counter Neutron Detector

Abstract

The rising cost of 3He gas has made an initiative to find alternative neutron detector technologies. The Li foil multiwire proportional counter (Li Foil MWPC) neutron detectors have shown promise as a 3He replacement technology and are currently being developed to increase the technology readiness. Large area Li Foil MWPCs with five layers of 75-µm thick 6Li foils have been built and characterized, yielding over 55% intrinsic thermalneutron detection efficiency, with the possibility of increasing the efficiency above 70% with ten 96% enriched 6Li foil layers. Most recently, a trade study was performed between three different device configurations (2 suspended foils with 2 wall foils (Gen3), 3 suspended foils (Gen4 MkI), and 3 suspended foils with 2 wall foils (Gen4 MkII)) for a backpack radiation detector (BRD). MCNP6 models were performed for each configuration. The resulting neutron detection response, estimated weight, number of Li foils, and number of anode wires were evaluated for an optimum commercial product. The simulated device intrinsic thermalneutron detection efficiency ranged from 39.2 – 44.5%, depending on the configuration. Gen3 devices were fabricated with advancements from prior prototype devices, including a new lid, a pinch off gas tube, and the addition of an SMA bulkhead connector. A custom electronics package was designed that couples to each device, thereby making it easy to array multiple devices into a large-area system. Four of the new Gen3 devices were sent to PNNL for a test campaign where the 1052.7 cm2 active neutron-sensitive area device was evaluated. Typical sensitivity to 252Cf at 1.5 meters was measured at 0.36 cps/ng and a gamma-ray rejection ratio of 6.46 x 10−14 for 60 mR/hr of 60Co was measured. A gamma-ray absolute rejection ratio in the presence neutrons (GAARn) was measured at 1.00 ± 0.0036, and the dead time of the system was measured at 18.7 ± 1.7 microseconds.