Non-linearity of recoil pulse height events in He-3 tubes

Regarding neutron spectrometry, energy calibration of a He-3 counter was performed for neutron energies up to about 7 MeV, for various operating voltages in the proportionality region. Full-energy peak position is linearly correlated with neutron energy, while the position of the 3He recoil peak deviates from a linear response, especially in the lower energy region. By means of the linear full-energy peak calibration, recoil peak energies are found to be significantly lower than those resulting from the kinematics of the reaction. The pulse-height loss is attributed to initial recombination. Using the Bragg curve of the recoiling nucleus in the counter gas mixture, it is determined that the pulse-height loss corresponds to a constant portion, 33.5±1.3 %, of the fraction of energy deposited by a 3He nucleus traversing the gas mixture with a stopping power higher than 450 eV/μm. The energy calibration equation for the recoil peaks is derived using the following: (i) the range-energy relation and the stopping power function for a 3He nucleus in the gas mixture where the coefficients of both functions were determined via Monte Carlo calculations, and (ii) the experimentally determined recombination coefficient.