High-dosage neutron dosimetry with TLDs : study of the thermoluminescence properties and the tritium build-up effect in 6LiF(Mg,Ti) dosimeters as a result of irradiation with high neutron dosages in the TRIGA Mark II reactor

Abstract

The aim of this study was to investigate the performance of LiF(Mg,Ti) dosimeters during high-dosage neutron measurements in the mixed radiation field within the TRIGA Mark-II research reactor at the Institute of Atomic and Subatomic Physics, Vienna. For this purpose both TLD-600 (6-LiF) and TLD-700(7-LiF) chips were irradiated within the reactor at different powers and for different irradiation times. In subsequent thermoluminescence (TL) measurements first the primary TL signal acquired directly after irradiation was studied. Next the secondary TL signal (tritium build-up) caused by selfirradiation of the chips through tritium produced within the TLD-600 dosimeters during irradiation (7-Li(n,)3-H) was measured at weekly intervals. Finally some of the TLD-600 chips were dissolved and measured in a liquid scintillation analyser. Due to a variety of correction factors (supra-linearity, saturation effects, sensitivity loss etc) needed to account for the high dosages within the reactor, it was clear that this would be a very complex task. Therefore the main aim of this thesis was to test and find the limits of thermoluminescence dosimetry in the radiation field of the reactor through the use of these three measurement methods. In addition we aimed to find a correlation between the results of all three techniques. Only inconclusive results were found for the (primary) net neutron TL-signal as the high dosages within the reactor were mostly outside of the dosimeter-s linear response range. The secondary TL-signal - the tritium build-up - was found to stay relatively stable over the course of several weeks. A good relationship between the tritium build-up and the reactor power was found between 1 and 10 kW. At higher reactor powers the tritium build-up showed a deviation from the expected results. In LSC measurements annealed dosimeters also showed a remarkable correlation with the tritium build-up. A comparison of the tritium activities within non-annealed dosimeters showed that only about 13 % of the calculated activity was actually measured. One possible reason for this could be the loss of tritium during the chemical dissolution of the dosimeters. Furthermore the neutron flux on which our calculations were based could have been too high as a result of a recent change in the reactor core configuration. At the time of this work measurements of the neutron flux at different irradiation positions within the reactor were still under way.