Limitation on the response of 3He counters due to intrinsic alpha emission

Alpha-emitters in the uranium and thorium series, present as trace elements in the construction materials of 3He detectors, can result in a highly significant count rate when these detectors are used for low-level neutron flux measurements. This is confirmed for a detector with a uranium content of only 5.72×10−8 g/g. It is shown that measurement of the contribution of the internal background to total count is not readily possible in an above-ground environment. However, a deep underground laboratory, where the effects of cosmic rays become insignificant, is an ideal location for such measurements. It is shown that the pulse heights due to alpha particles are suppressed because of electron ion recombination in the detector. The observed internal alpha background in 3He detectors is consistent with Monte Carlo simulation.