Characterization of the epithermal neutron field produced by p+⁷Li reaction in a tandem accelerator using a Bonner sphere spectrometer

The proton beam produced in the Nuclear Physics line of the tandem accelerator of the Centro Nacional de Aceleradores was used to generate a neutron field. In particular, 1.912 MeV protons were used to produce well-known epithermal neutrons through the p+⁷Li → n+⁷Be reaction. The aim of the work was to characterize this field while testing the performance of a Bonner sphere spectrometer in the epithermal range. Measurements were performed in four locations at different angle (0 deg., 30 deg., 60 deg. and 90 deg.) from beam incidence direction in order to study the angular dependence of the field. Both a parametric and numerical unfolding methods were tested to process the counts of the central detectors and obtain the energy distribution of the neutron fluence. In addition, a Monte Carlo simulation was carried out to complete the study and provide a guess spectrum for numerical unfolding. It was found that the fluence rate and mean energy decrease as the angle from beam direction increases. Total fluence was 2.75, 1.36, 0.366 and 0.216 cm^-2 per charge collected in the target at 0 deg., 30 deg., 60 deg. and 90 deg., respectively. Mean energy of the field ranges from 46 to 17 keV at 0 deg. and 60 deg., respectively. In all cases, given that the irradiation room is so large, the contribution of thermal neutrons to the field is small. Regarding the unfolding, the total fluences estimated by all methods were in agreement within the uncertainties. (authors)