Analysis of thermoluminescent signal fading through the emission curve

Thermoluminescent dosimetry is based on the principle that some crystals can absorb and store energy from ionizing radiation, which is re-emitted in the form of electromagnetic radiation when these crystals are heated. The emitted light is then detected by a photomultiplier tube and correlated to the absorbed dose received by the thermoluminescent (TL) material. The loss of the thermoluminescent signal with the time elapsed between the irradiation and the reading is a well-known characteristic of thermoluminescent dosimeters and is referred to as fading. For the monitoring of individual doses, fading is a challenging issue, as measuring a reduced signal results in an underestimation of the irradiation dose. Fading is usually bypassed through a thermal treatment, which erases the signal from the lowest temperature peaks, which are the most responsible for the signal fading. Hence, the objective of this work was to study and analyze the TL emission curve, and to better understand the fading phenomenon and the thermal treatment methodology, aiming to propose an alternative methodology to optimize the reading and treatment time. A total of 100 TL pellets were used to select a working group and assess the TL fading signal with the time between irradiation and reading. The selection was given by the average reading and reproducibility of the pellets. After selection, these pellets were divided into subgroups, in which each group was irradiated with doses from 0.1 to 10 mGy. The TL signal of each subgroup was read after times from 0 to 60 days after irradiation, with evaluation of the fading of the low (P1) and high (P2) temperature peaks. Results were compared with and without heat treatment performed prior to the readings. From the results it was possible to notice that from the tenth day onwards, the P1 signal disappeared almost completely, in addition to the heat treatment improving the results obtained for P2, with a reduction of data dispersion up to 12%. However, the obtained results point to the potential of using the F ratio (ratio of the integral of the P2 signal by the integral of the total signal) as a dosimetric parameter. It was possible to see that the F ratio showed comparable results for readings with and without thermal treatment. This shows that the F ratio is a potential dosimetric parameter that is independent of both fading and dose. Thus, the use of the F ratio can represent a potential gain for individual dosimetry laboratories, in the sense of facilitating the processing of individual dosimeters without the need for pre-reading thermal treatments and maintaining the accuracy of the quantification of the operational quantities. (author)