CHARACTERISATION OF THE GRAPHITE MODERATED THERMAL NEUTRON FIELD AT CMI.

The graphite pile was set up at CMI to provide reference thermal neutron field for metrology and dosimetry purposes. It consists of three Pu-Be and three Am-Be radionuclide sources located in a 1.95 m (width) × 1.95 m (length) × 2.0 m (height) graphite moderator block. The neutron field in the volume of the experimental channel with dimensions of 40 cm × 40 cm × 135 cm (depth) was characterised experimentally and by Monte-Carlo calculations. Neutron fluence was determined by the activation of gold foils and Manganese tablets irradiated in nine different positions and by two 3He detectors of different construction irradiated in one position (bare and covered in three spherical PE moderators of 3, 3.5 and 4 inch diameter). Weighted mean value of the neutron fluence rate measured by all the abovementioned detectors in the pile centre was 2.91 × 104 cm-2 s-1 ± 1.8%. This value was used as a calibration factor for the Monte-Carlo model predictions. The neutron spectral fluence rate calculated by validated Monte-Carlo model was used to determine the conventionally true ambient and personal dose equivalent rates at different positions.