Ultrahigh-Sensitivity Fission Counter with Transmission Line Electrode Configuration

An ultrahigh-sensitivity fission counter (UHSFC) prototype was designed, fabricated, and tested. The objective of this research was development of a fission counter system for ex-vessel, source-range flux monitoring having a neutron sensitivity of >40 counts s-1 [neutrons/(cm2·s)]-1 (>40 cps/nv) and a size comparable to that of a BF3 proportional counter system. Also, the UHSFC system was required to be operable under conditions beyond the capabilities of BF3 counters: up to a temperature of 450 K and in a gamma radiation field of 7.2 × 10-4 C(kg·s)-1 (i.e., 104 R/h). The sensitivity requirement (40 cps/nv) was met by assembling two transmission line fission counters (TLFCs) within a common envelope. The size requirement (<14 cm diameter, <80 cm length) was met by using a high electrode-packing density with parallel, curved electrodes arranged to maximize the ratio of electrode area to sensitive volume. The measured neutron sensitivity for each TLFC is 23.5 cps/nv at 4.5 × 104 R/h and 450 K. The UHSFC is filled with a recently developed gas (80% Ar, 20% CF4 at 263 kPa pressure) of high electron drift velocity. The electrode area (5 m2) is coated with 88 g of enriched uranium (93.15% 235U, 0.99% 234U, balance 238U). The impedance of the lumped element LC transmission lines is 25 Ω, the bandwidth 100 4Hz, and the delay 4 ns per node (each TLFC has 58 nodes). Pulse-height and time discrimination is used to mitigate the effects of noise from the alpha pileup current of the uranium coating.