Temperature Dependence of Thermal Neutron Detection Performance of 3He Proportional Counters

3He proportional counters are said to be able to operate at temperatures up to 200°C, but only one report (J. E. Myers, Jr.; IEEE Trans., NS-14(1), 416(1967)) was found which described the temperature dependence of the plateau curves. We have measured the pulse height distributions and the pulse shapes of thermal neutron pulses obtained from seven different types of commercially-available 3He proportional counters at temperatures up to 300°C. Their anode voltages were varied widely from the voltage that gave ionization chamber operation to that gave gas multiplication of more than 100. The peak pulse height increased and the FWHM degraded as the temperature increased, thus making neutron spectroscopy difficult at elevated temperatures, although the thermal neutron counts stayed constant unless spurious pulses due to breakdown appeared.