Ideal response function of a 3He proportional counter to thermal neutrons determined by different length counters

The gas multiplication distributions along the anode wires of He-3 proportional counters with different sensitive lengths of 10, 15 and 20 cm were measured using a collimated thermal neutron beam at the Kyoto University Reactor facility. The ideal response function excluding the effect of electric field distortion near the boundaries of the anode wire was derived by taking into account the difference between pulse height spectra measured by two different length counters. The pulse height spectrum calculated by the NRESPG Monte Carlo code assuming the ideal gas multiplication region showed relatively good agreement with the response function derived from the measurements. Both the energy resolution of the full energy peak at 764 keV and the peak ratio to the entire pulse height spectrum were improved markedly. (C) 2003 Elsevier B.V. All rights reserved.