D Datta

Beta response of CaSO4:Dy based thermoluminescent dosimeter badge and its angular dependence studies for personnel monitoring applications

Studies on the response of 0.4 mm and 0.8 mm thick Teflon embedded CaSO4:Dy discs are carried out using different beta sources having energy ranging from 0.224 to 3.54 MeV. Angular dependence of the response for 0.4 and 0.8 mm thick thermoluminescence (TL) discs was also studied for 85 Kr and 90 Sr/90 Y beta sources. The ratio of the response of the open disc to that under 1.5 mm thick Perspex filter (DOpen/DPerspex) was estimated for 0.4 and 0.8 mm thick TL discs in the energy range from 0.689 to 3.54 MeV. The evaluation of (DOpen/DPerspex) ratio is necessary as same is used to estimate the energy of beta particles and to apply appropriate correction factor while evaluating beta dose. In addition to above, the optical density and transmission characteristics for 0.4 and 0.8 mm thick Teflon and CaSO4:Dy Teflon embedded discs are also reported.

BETA DOSE EVALUATION ALGORITHM FOR CaSO4:Dy BYSED TLD BADGE USED IN COUNTRYWIDE PERSONNEL MONITORING PROGRAMME IN INDIA

The prevalent algorithm for the estimation of beta dose, which in turn is used for estimation of skin dose for exposures involving beta radiations was observed to significantly overestimate the dose in individual monitoring based on CaSO4:Dy TLD badge in India. A new algorithm has been developed by estimating the correction factor from the response of dosemeter to different beta sources at various angles of incidence. The correction factor was observed to vary linearly with the ratio of the responses of dosemeter element without filter (D3) and dosemeter element under Perspex filter (D2). The correction factor determined using the ratio of D3 and D2 was applied to the response (D3) of dosemeter element without filter for estimation of beta dose. Protocol for identification of beta in the mixed gamma beta fields was defined such that it resulted in nearly same correction factor for given beta source in both gamma beta mixed fields and pure beta fields. The beta dose evaluation algorithm has provided the beta dose estimation within the required accuracy for >90% cases obtained from national quality assurance test data from different laboratories.

Study on the measurement of photo-neutron for15 MV photon beam from medical linear accelerator under different irradiation geometries using passive detectors

AIM OF STUDY The photo-neutron dose equivalents of 15 MV Elekta precise accelerators were measured for different depths in phantom, for various field sizes, at different distances from the isocenter in the patient plane and for various wedged fields. MATERIALS AND METHODS Fast and thermal neutrons are measured using passive detectors such as Columbia Resin-39 and pair of thermoluminescent dosimetry (TLD) 600 and TLD 700 detector from Elekta medical linear accelerator. RESULTS It is found that fast photo-neutron dose rate decreases as the depth increases, with a maximum of 0.57 ± 0.08 mSv/Gy photon dose at surface and minimum of 0.09 ± 0.02 mSv/Gy photon dose at 15 cm depth of water equivalent phantom with 10 cm backscatter. Photo neutrons decreases from 1.28 ± 0.03 mSv/Gy to 0.063 ± 0.032 when measured at isocenter and at 100 cm far from the field edge along the longitudinal direction in the patient plane. Fast and thermal neutron doses increases from 0.65 ± 0.05 mSv/Gy to 1.08 ± 0.07 mSv/Gy as the field size increases; from 5 cm × 5 cm to 30 cm × 30 cm for fast neutrons. With increase in wedge field angle from 0° to 60°, it is observed that the fast neutron dose increases from 0.42 ± 0.03 mSv/Gy to 0.95 ± 0.05 mSv/Gy.s CONCLUSIONS Measurements indicate the photo-neutrons at few field sizes are slightly higher than the International Electrotechnical Commission standard specifications. Photo-neutrons from Omni wedged fields are studied in details. These studies of the photo-neutron energy response will enlighten the neutron dose to radiation therapy patients and are expected to further improve radiation protection guidelines.

DEVELOPMENT OF AN ALGORITHM TO ESTIMATE EYE LENS DOSE IN TERMS OF OPERATIONAL QUANTITY Hp(3) USING HEAD TLD BADGE

In view of the recommendations of International Commission on Radiological Protection for reduction of the occupational annual dose limit for eye lens from 150 mSv to 20 mSv/y, questions have been raised on the adequacy of monitoring for the quantities Hp(10) and Hp(0.07). As an immediate requirement, in the present situation, where there is no exclusive eye lens dosemeter in India, the existing chest TLD badge was modified to be used as head badge (head dosemeter) by including a strap to enable wearing on the forehead. In order to estimate the eye lens dose in terms of the operational quantity Hp(3), the prevalent algorithm of chest badge was also modified. The modified algorithm was applied to estimate Hp(3) for dosemeters irradiated to various beta and photon radiations including mixtures. The Q values (estimated/delivered dose equivalent) were found to be within ±20% for most of the photon beams.

FLUKA-BASED MONTE CARLO INVESTIGATION OF MICRODOSIMETRIC DISTRIBUTIONS OF TELECOBALT BEAM

FLUKA-based Monte Carlo calculations of microdosimetric distributions in water phantom involving a walled spherical Tissue-Equivalent Proportional Counter filled with tissue-equivalent propane gas have been studied for an indigenously developed telecobalt machine. The simulated site size considered in the study was 2 μm. In the Monte Carlo calculations, field size was varied from 10 cm × 10 cm to 35 cm × 35 cm and the depth was varied as 5-20 cm. The study also includes calculation of microdosimetric distributions with a 30° wedge filter. The efficiency of the calculations was improved up to a factor of 26 by choosing appropriate cut off values for production and transport of electrons. The calculated microdosimetric distributions of telecobalt machine is distinctly different from that of a bare 60Co source which is attributed to the influence of scattered photons from the machine head and the water phantom.

DEEP, SHALLOW AND EYE LENS DOSES FROM 106Ru/106Rh—A COMPARSION

106Ru/106Rh is unique amongst other commonly used beta sources such as 147Pm, 85Kr, 204Tl, 32P, natU and 90Sr/90Y in the sense that it is capable of simultaneously delivering shallow/skin, eye lens and deep/whole body doses (WBDs) and they differ from each other substantially. In view of this, the investigation of various quantities defined for individual monitoring is possible and this makes 106Ru/106Rh beta source, a classical example in radiation protection and dosimetry. This led us to estimate skin, eye lens and WBDs for 106Ru/106Rh beta source. Optically stimulated luminescence based ultra-thin α-Al2O3:C disc dosimeters were used in the present study. Typical values (relative) of the eye lens and whole body/deep doses with respective to the skin dose (100%) were experimentally measured as ~66 ± 4.6% and 17 ± 3.9%, respectively. The study shows that 106Ru/106Rh beta source is capable of delivering even WBD which is not the case with other beta sources.