Nonlinearity of pulse height of recoil helium in /sup 3/He proportional counters

The pulse height distribution in /sup 3/He proportional counters has been investigated to accurately quantify the response function to be used in fast neutron spectroscopy. The measurements were performed as a function of the operating voltage, the counting gas mixture, and the incident neutron energy. The special emphasis of this study has been put on the nonlinearity of the pulse height of a recoil helium induced by the /sup 3/He(n,n)/sup 3/He reaction. A distinct deviation of 14% at maximum to lower energies was found in the position of the recoil edge with the energy axis calibrated by the full energy peaks of the /sup 3/He(n,p)T reaction produced with neutrons of thermal energy and 2.413 MeV. It was confirmed that the nonlinear effects, such as an initial recombination and a space-charge effect, take place more significantly in the energy loss process of a recoil helium because of its higher linear energy transfer (LET) in the filling gas compared to that of a proton and triton pair emitted from the /sup 3/He(n,p)T reaction. The deviation becomes larger with an increase of the average LET in the gas mixture. The experimental results suggested that the nonlinear effect caused by a higher LET recoil helium is inevitable for the filling gas, even if the operating voltage is changed to an acceptable range.