Spectrum of isotopic neutron sources inside concrete wall spherical cavities

The neutron spectra of Cf252/D2O, LaBe140, Cf252, Pu238O218, AmB241, AmBe241, RaBe226 and PuBe239 isotopic neutron sources due to room-return have been determined for various source-to-detector distances in concrete spherical cavities of different radius. Changes in the amount of thermal neutrons (E⩽0.414eV) were analyzed to estimate, for each neutron source, the coefficient that relates the neutron source strength and room surface area with the thermal neutron fluence rates. The study was carried out using Monte Carlo methods for 200, 400, 500, 800, 1000, 1200 and 1500-cm-radius spherical cavity in vacuum; cavities are 100-cm-thick concrete. Point sources were located at the center of cavity and neutron spectra were calculated at several source-to-detector distances along the cavity radius. The thermal neutron contribution was thereby evaluated. From these calculations a weighted coefficient value that relates the thermal neutron fluence with the neutron source strength and the total inner area surface of the cavity was estimated to be 3.76±0.03.