Accuracy of neutron dose evaluation in the area monitoring for LHD experiments

The error in the evaluation of neutron dose during calculation of the neutron field around the large helical device (LHD) in D–D operation is discussed. The expected neutron dose at each monitoring point was derived from the dose conversion factor and neutron fluence data, which was calculated with the radiation transport code DOT-3.5. In contrast, the detected dose at the neutron counter was obtained from the fluence data and the detector response given by calculation with MCNP-4b. The neutron counter used in these calculations consisted of a helium-3 proportional counter with a cylindrical polyethylene moderator. According to the results of the calculations, the ratio of the detected dose to the expected dose was found to lie in the range 1.0–3.0 on the outdoor monitoring points. Since the response of a single neutron counter may lead to inconsistencies in the dose conversion factor, we attempted to minimize these inconsistencies by using a pair of counters with moderators of different thickness. The ratio of the detected dose to the expected dose ranged from 1.5 to 2.1 at the site boundary, indicating that the use of a paired counter allows a more accurate evaluation of dose than the use of a single counter.