It can be concluded that the effective shielding area is an important parameter in thyroid gland dose reduction. Lead frontal collars with large effective shielding areas (>~300 cm but not necessarily very large) are appropriate for an optimized thyroid gland dose reduction particularly for the critical patients in dental panoramic imaging. Regardless of the shape and thickness, using the Pb-equivalent shields is not justifiable in dental panoramic imaging.
The type testing of measuring devices is one of the most important parts of a quality management system in a personal dosimetry services program. In this study, based upon the International Electrotechnical Commission (IEC) 62387 criteria, a reader-testing program was performed for a home-made personal thermoluminescent dosimetry (TLD) reader. The stability of the reader, the effects of light exposure, temperature and fluctuations of primary power supply on TLD read-outs as the main parameters were investigated in this program. Moreover, this study assesses some important criteria of dosimetry system including the non-linearity of response, reusability, after effect and overload that may include significant contribution in the performance of a reader. The results showed that the TLD reader met all requirements of the IEC for the reader tests by a large margin.
The Iran Nuclear Regulatory Authority has investigated overexposure cases in industrial radiography over a period of three years. Radiographers with thermoluminescence dosimeter (TLD) records of more than 4 mSv in any 2 month routine monitoring period were asked to fill in a questionnaire explaining their points of view of the reasons for such relatively high doses. The responses showed that more than 50% of the radiographers did not agree with their recorded TLD doses, although the majority of the alternative explanations were weak. The main causes of overexposures were found to be difficult working conditions and ignoring safety principles while accidents or device failures were a minor contribution. Also, there was poor correlation between the TLDs and direct reading dosimeters worn by the radiographers, indicating that personal monitoring instructions were not being implemented appropriately.
Irradiated dose to the organs at risk surrounding the paranasal sinuses was compared in cone beam computed tomography (CBCT) and multi spiral computed tomography with respect to the organs’ relative positions to the imaging field. A head and neck Alderson-Rando phantom equipped with thermoluminescence dosemeters pellets was irradiated according to three routine CBCT protocols and one protocol in multi spiral computed tomography. Dose value of organs outside the imaging field as well as those measured dose of organs inside the field were assessed. The highest measured doses were obtained from CT scan for most of the organs investigated in this study, whereas the lowest one was associated with the low-resolution mode of CBCT. Also, statistical analysis showed no significant differences between the dose values of out-of-field organs in all CBCT modes, whereas significant differences were observed between the radiation doses of CT and CBCT modes for all organs at risk inside and outside of the imaging field. CBCT is recommended on the basis of having a lower dose; however, the image qualities were the same in the two employed modalities, so the approach of lower dose can be made.
This report presents status of external and internal dose assessment of workers and introducing the structure of National Dose Registry System of Iran (NDRSI). As well as types of individual dosemeters in use, techniques for internal dose assessment are presented. Results obtained from the International Atomic Energy Agency intercomparison programme on measurement of personal dose equivalent H(p) (10) and consistency of the measured doses with the delivered doses are shown. Also, implementation of dosimetry standards, establishment of quality management system, authorisation and approval procedure of dosimetry service providers are discussed.
In this work several studies have been conducted on hard x-ray emissions of Damavand tokamak based on radiation dosimetry using the Thermoluminescence method. The goal was to understand interactions of runaway electrons with plasma particles, vessel wall, and plasma facing components. Total of 354 GR-200 (LiF:Mg,Cu,P) thermoluminescence dosimeter (TLD) crystals have been placed on 118 points--three TLDs per point--to map hard x-ray radiation doses on the exterior of the vacuum vessel. Results show two distinctive levels of x-ray radiations doses on the exterior of the vessel. The low-dose area on which measured dose is about 0.5 mSv/shot. In the low-dose area there is no particular component inside the vessel. On the contrary, on high-dose area of the vessel, x-ray radiations dose exceeds 30 mSv/shot. The high-dose area coincides with the position of limiters, magnetic probe ducts, and vacuum vessel intersections. Among the high-dose areas, the highest level of dose is measured in the position of the limiter, which could be due to its direct contact with the plasma column and with runaway electrons. Direct collisions of runaway electrons with the vessel wall and plasma facing components make a major contribution for production of hard x-ray photons in Damavand tokamak.
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