Development of a graphite pile to produce a neutron thermal field at the CIEMAT - Neutron standards laboratory

The CIEMAT Neutron Standards Laboratory (LPN) is the Spanish national reference laboratory in neutron metrology. Within the framework of the LPN expansion project, it has been planned to incorporate a thermal neutron field based on a graphite pile. A study has been carried out with MCNP 6.2 for a graphite pile with dimensions 180 x 150 x 150 cm3, with a 185 GBq Am-Be source (B = 1.1·107 s-1) and the internal cavity with a section of 40 x 40 cm2 and a depth of 50 cm. The main parameters of the graphite pile and the position and emission rate of the Am-Be neutron source have been varied to determine the fluence rates at several points in the pile, their thermal component and homogeneity. The optimal configuration produces a neutron spectrum with more than 98% of thermal component, fluence rates around 400 cm-2 s-1 inside the cavity and 55 cm-2 s-1 outside, and acceptable homogeneity in this thermal neutron field. From a Radiation Protection point of view it is necessary to incorporate borated high-density polyethylene (HDPE-B) around or at least as a shielding wall, and a Cd window to filter the thermal component.