Measurement And Unfolding Study Of Fast Neutron Energy Spectra Using Recoil Proton Method

Neutron spectroscopy is of great importance in many basic research studies and applications, such as nuclear physics research, nuclear reactor control and radiation protection. The process of fast neutron energy spectrum measurement by recoil proton method with liquid scintillator detector contains four main steps:n-y discrimination, recoil proton spectrum measurement, response function calculation and neutron spectrum unfolding. The n-y discrimination and unfolding of the neutron energy spectrum are two key points among them. The accuracy of the neutron energy spectrum is subject to the quality of the n-y discrimination, but the n-y discrimination quality at low energy range is not as good as expected. As a consequence, to improve the n-y discrimination quality at low energy range is one of the important aspects of this thesis. The unfolding of the neutron energy spectrum is an ill-posed problem, which can only be solved by some specific algorithms. The domestic researches about the neutron spectrum unfolding are not as accurate as the international standards, so, the improvement in the accuracy of spectrum unfolding is another key point to be studied.The zero-crossing method and charge comparison method were used to perform the n-y discrimination in this work. One experiment system using zero-crossing method was tested by41Am-Be neutron source. Two-dimensional scatter plots of zero-crossing time vs. energy were obtained. The figure-of-merits (FOMs) at different energies were calculated and compared with the published results. It shows that the n-y discrimination quality at low energy range is improved. The neutron energy threshold of n-y discrimination of this system is0.4MeV, which is comparatively lower than the previous threshold (0.5MeV). The other system using charge comparison method was based on the VME modules and VME bus. A data acquisition program was developed through the use of the LabVIEW, which can enable us to control the programmable electronics and process data online via VME bus. This system also has a good n-y discrimination performance at low energy range compared with references. Its neutron energy threshold of n-y discrimination is0.45MeV, which is lower than the previous energy threshold (0.5MeV).NRESP7program was used for calculating the response function of the BC501A liquid scintillator detector. The unfolding of the neutron spectra were carried out with the recoil proton spectra and response matrix. A MatLab program based on the GRAVEL algorithm was developed for unfolding the neutron energy spectra, and the neutron energy spectra of Am-Be, Cf, D-T, D-D neutron sources were obtained. Theses neutron energy spectra are identical with the references, which prove that the unfolding program used in the experiments works well. The energy spectra unfolded by GRAVEL method were compared with the spectra unfolded by inverse matrix method, the results show that the GRAVEL method works better.