Neutron spectroscopy: study of a set of proportional counters response

The Rotating Spectrometer ROSPEC is made of six gaseous spherical proportional counters and was designed to measure the neutron fluence spectrum and provide related dosimetric data. A comparative survey with this spectrometer was carried out in 2003 on ISO neutron sources. It put forward the need for studying the effects of device rotation and for determining possible influences between the counters. By means of Monte Carlo calculations, we have shown that the rotation did not affect the measured neutron spectra but that the counter sensitive to the thermal neutrons was strongly disturbed in its immediate vicinity. The hydrogenated proportional counters responses have been calculated by taking into account phenomena influencing detection such as the wall effect and the effect of the variation of the gaseous multiplication in the detector medium. The wall effect was studied by validating the calculated recoil proton spectra with the analytical model of Snidow. The electric field lines were determined thanks to the finite element method. Thus, while taking into account both studied phenomena, we were able to obtain an accurate estimate for the experimental response. Measurements highlighted mathematical dysfunctions related to the unfolding (e.g. negative fluence values). To solve this problem, we proposed using the MAXED code which based on the maximum entropy principle. It was compared with the existing code SPEC4. We have concluded that MAXED provided better unfolding results if the user provides it with sufficient physical information (such as the presence of resonances) through the default spectrum. (author)