Study of room-return neutrons

The coefficient that relates the neutron source strength and inner room surface area with the thermal neutron fluence rates has been calculated for neutrons whose energy goes from 1 eV to 20 MeV. This coefficient was calculated using Monte Carlo methods for 150, 200 300, 535.24, 832.10 and 1010 cm-radius spherical cavity with and without air. In the calculations monoenergetic neutron sources were located at the center of cavity and, along the spherical cavity radius, neutron spectra were determined at several source-to-detector distances. From the neutron spectra the thermal neutron (E⩽0.414eV) contribution was calculated and several coefficients were estimated. A weighted average of all coefficients was estimated being 5.6±0.15.6±0.1, this is the cc-value for the case of rooms without air, and 4.8±0.44.8±0.4, for rooms with air. In the aim to compare the results with monoenergetic neutrons, calculations were also performed using two isotopic neutron sources, Cf252 and AmBe241 in vacuum.