Correcting the calibration curve of TLD-600 dosimeters after irradiation without phantom by thermal neutrons of Isfahan MNSR

It is shown recently that the thermal neutron field of Isfahan MNSR with stable flux and dose equivalent rate can be used as a calibration field in dosimetry. The fundamental limit of this field is not the feasibility of using phantom for the irradiation of personal dosimeters. This issue leads to underestimating the personal dose-equivalents from the true values. The subject of this work is to correct the calibration curve of TLD-600 dosimeters irradiated without phantom. To do this, Monte Carlo simulations using Geant4 are carried out and absorbed doses in this dosimeter for irradiation with and without phantom are calculated. Then, the ratio of these doses as a correction factor applies to the responses measured without phantom, and then the corrected calibration curve is determined. As an analytical approach, the correction factor is considered as the ratio of the mean number of thermal neutron reflections between air and water (with phantom) and in the air alone (without phantom). Results obtained show that the correction factors determined by the simulation and analytical methods are 1.57 and 1.44, respectively, which agree well with the 8% difference. Finally, the response correction has led to changing the calibration factor of TLD-600 dosimeters from 0.0012 to 0.0008.