Harmonization of calibration methods of TL-dosimeters at the center for personal dosimetry medical staff in Ukraine

Full text: Control of individual doses of Ukrainian medical staff, working with the radiation exposure in gamma-therapy, nuclear medicine and diagnostic radiology, is performed by the Center for Personal Dosimetry that was established in 1979. At present more than 5500 medical workers from 440 hospitals of 24 Ukrainian regions are covered by quarterly personal monitoring. In database, the information about doses of 11500 persons is available. From 1991, termoluminescence dosimetry became the main method. Three semi-automated TL-readers DTU-01 (Latvia) and single-crystal TL-detectors LiF:Ti, Mg (5 mm) - DTG-4 (Russia) are used. The bange holders DTU-01 from polystyrene (front thickness - 1 cm) have additional filter 1 mm of Al, that ensures measurements of equivalent doses from photon exposure in 10 mm deep of soft tissues. Until now, the calibration of TL-dosimeters has been carried out in free air using reference source 137Cs. The dose range of irradiation of control dosimeters were 0.1 - 100 mSv that enables to study the linearity of TL-detectors indications as well. Calibration coefficients are calculated for three parts of the dose range chosen in such a way that confidence interval for every part would be no more than 10 % for doses more than 0.5 mSv and increasing up to 40 % in the range 0.1-0.3 mSv. Minimal dose measured is about 0.05 mSv. The estimation of individual doses of occupational exposure obtained using calibration coefficients, we also account for daily coefficients of installation operation stability, sensibility coefficients for every group of TL-detectors, and indications of background dosimeters of every hospital during monitoring. In 1996-1998 the center participated in the program 'Intercomparison for Individual Monitoring of External Exposure from Photon Radiation' for Eastern European countries. Results of phase 'Type-Test' Intercomparison (irradiation on phantom in terms of Hp(10) in the photon energy range 33 keV-6.3 MeV under 3 angles) showed that dosimeters DTU-01 enable to determine dose Hp(10) with errors less than 10-15 % for photon radiation energy more than 100 keV, but for energy 33 to 83 keV the errors were increased up to 60-100 % (especially for angle 60o). From results of the phase 'Simulated Workplace Field' Intercomparison it was shown that under combined irradiation (AP plus rotational) of dosimeters DTU-01 on the phantom by irradiation with different photon energies the errors of measurements of Hp(10) were less than 25 %, only under irradiation by photons with energy 57 KeV was 34 %. At present, the main goal is implementation of operational quantity Hp(10) for photon irradiation energies less than 100 keV and harmonization of personal monitoring within EU and ICRU recommendation. In the frame of IAEA TC-project the x-ray Unit PANTAK and Unit OB-6 (137Cs), PMMA slab phantom (30x30x15 cm) were obtained. These units produced photon reference radiations in accordance to ISO standard 4037-1. The technique of calibration of personal dosimeters in operational dose units Hp(10) is testing and estimating. (author)