The energy resolution of the 3He counter for thermal-epithermal neutrons as a function of high voltage and shaping time constant is presented in Fig. 7a. These measurements were performed by exposing the detector to thermal neutrons from a 241Am–Be source, covered with paraffin moderator. In Fig. 7a higher values of FWHM with decreasing high voltage settings should be attributed to statistical fluctuations in the gas amplification, recombination and electronic noise, effects that broaden the peak in a Gaussian mode. Higher resolution values are observed when high voltage increases due to an enhanced asymmetry in the peak shape. The same phenomenon, observed by Dietz et al. , was ascribed to space charge effect since, they found a correlation, although weak, of the asymmetry of the peak with a larger number of preamplifier pulses having long rise times. Due to the negligible momentum of thermal-epithermal neutrons the reaction products, proton and triton, are emitted in opposite directions. Longer rise times are attributed to those cases where the proton and
the triton travel almost perpendicularly to the anode wire, with maximum of the ionization near the end of their track.