Low Voltage He3-Filled Proportional Counter for Efficient Detection of Thermal and Epithermal Neutrons

Neutron counters using He3 gas at absolute pressures up to 10 atm have been constructed and tested. The relative detection efficiency for thermal, epicadmium, and fast neutrons was measured as a function of pressure from 1 to 10 atm, and compared with similar data from a boron trifluoride counter of the same size filled to a pressure of 65 cm of B10F3 gas. The ratio He3/BF3 increases approximately linearly with pressure for epicadmium and fast neutrons, reaching values of 26.6 and 21.6, respectively, at 10 atm He3 pressure. This ratio shows a saturation effect for thermal neutrons, becoming 4.45 at 10 atm He3 pressure. The gamma-ray response has been found to be low with proper pulse shaping. High voltage plateau curves for 3- and 7-atm counters are given and indicate operating regions at least 100 V wide at a counter high voltage of about 1200 V.