Photon dose mixed in monoenergetic neutron calibration fields using 7Li(p,n)7Be reaction

The ambient dose equivalents H*(10) of photons mixed in the 144, 250 and 565 keV monoenergetic neutron fields were evaluated using measurements from an NaI(Tl) detector and calculations done using the MCNP-ANT code. It was found that H*(10) of the photons produced in the target assembly dominates the dose, particularly near the target. The H*(10) of the photons produced in other materials in the field increases with the increase in distance from the target and could not be neglected at a large distance from the target. The ratios of the H*(10) of the mixed photons to that of the monoenergetic neutrons for 144, 250 and 565 keV neutron fields, were evaluated to be below 5.5, 6.9 and 1.5 %, respectively. The ratios were calculated at calibration points between 100 and 500 cm from the target. The ambient dose equivalent H*(10) of the photons mixed in the 144, 250 and 565 keV monoenergetic neutron fields was evaluated using measurements from an NaI(Tl) detector and calculations were done using the MCNP-ANT code. The target photons in the first and second categories decrease as the inverse square of the distance from the target to the calibration point. This is because the origin of these photons can be considered to be a point source at the target. Therefore, the ratio of the H*(10) of the target photons to the monoenergetic neutrons is constant. On the other hand, the third category photon doses, in the 144 and 565 keV fields, increased with the distance. This is because, the majority of these photons, in the 144 and 565 keV fields, originate from the concrete wall behind the calibration point and as the distance from the target increases the calibration point moves closer to the wall. The first category photon dose is dominant near the target, whereas, the third category photon dose dominates further away from the target. The ratios of the H*(10) of the photons in the 144, 250 and 565 keV fields to that of the monoenergetic neutrons were below 5.5±0.2, 6.9±0.2 and 1.5±0.1 %, respectively. The ratios were calculated at all calibration points between 100 and 500 cm. (authors)