Measurement of neutron spectra using an extended bonner multisphere spectrometry system

Abstract

In spite of the poor resolution the Bonner multiSphere spectrometer (BS) has a several kinds of merits in use such as high sensitivity, wide range of energy response from thermal to a few hundred mega-electron volt neutron, isotropic response and easy operation. And therefore it has been used as a neutron spectrometer in the field of radiation protection dosimetry since 1960. But the conventional BS could not detect effectively a neutron over a few tens of mega electron volt due to the low sensitivity to it. From 1990s the need for neutron spectrometry around high-energy particle accelerators and in the atmospheric environment at high altitudes have increased because a human being is exposed to high-energy neutrons more often than before. If the BS is modified to make the neutron multiplication reaction possible by inserting a high atomic number material like a copper or a lead inside it, a high-energy neutron can be measured more effectively. In this study an extended BS that has a lead shell inside or outside the conventional BS was constructed to measure a high-energy neutron. New BS sets, which use a LiI(Eu) scintillator as the thermal neutron detector, with six kinds of lead shells including the conventional BS were designed and constructed. The response functions of new BS sets consisting of thirteen spheres, seven PE spheres and six kinds of lead shells, were calculated using the MCNPX code (Ver. 2.4.0) and constructed as the library for the unfolding code, MXD_FC31. This new BS was calibrated in the neutron calibration laboratory of Korea Atomic Energy Research Institute (KAERI) using a $^{252}Cf$ source. The thirteen kinds of neutron spectra, consisting of five spectra from radioisotopes, six from a proton accelerator and two from an electron LINAC, were measured using the new BS. Following one of the recommendations of ISO, five scattered neutron calibration fields (SNCF) were manufactured as the realistic neutron spectra used in the calibrat...