Assessment of fast and thermal neutron ambient dose equivalents around the KFUPM neutron source storage area using nuclear track detectors

A set of five 241Am-Be neutron sources are utilized in research and teaching at King Fahd University of Petroleum and Minerals (KFUPM). Three of these sources have an activity of 16 Ci each and the other two are of 5 Ci each. A well-shielded storage area was designed for these sources. The aim of the study is to check the effectiveness of shielding of the KFUPM neutron source storage area. Poly allyl diglycol carbonate (PADC) Nuclear track detectors (NTDs) based fast and thermal neutron area passive dosimeters have been utilized side by side for 33 days to assess accumulated low ambient dose equivalents of fast and thermal neutrons at 30 different locations around the source storage area and adjacent rooms. Fast neutron measurements have been carried out using bare NTDs, which register fast neutrons through recoils of protons, in the detector material. NTDs were mounted with lithium tetra borate (Li2B4O7) converters on their surfaces for thermal neutron detection via B10(n,α)Li6 and Li6(n,α)H3 nuclear reactions. The calibration factors of NTD both for fast and thermal neutron area passive dosimeters were determined using thermoluminescent dosimeters (TLD) with and without a polyethylene moderator. The calibration factors for fast and thermal neutron area passive dosimeters were found to be 1.33 proton tracks cm-2μSv-1 and 31.5 alpha tracks cm-2μSv-1, respectively. The results show variations of accumulated dose with the locations around the storage area. The fast neutron dose equivalents rates varied from as low as 182nSvh-1 up to 10.4μSvh-1 whereas those for thermal neutron ranged from as low as 7nSvh-1 up to 9.3μSvh-1. The study indicates that the area passive neutron dosimeter was able to detect dose rates as low as 7 and 182nSvh-1 from accumulated dose for thermal and fast neutrons, respectively, which were not possible to detect with the available active neutron dosimeters.