Compliance of Bhabhatron-II telecobalt unit with IEC standard - Radiation safety.

Bhabha Atomic Research Centre, Mumbai, India designed and developed a telecobalt unit, which was named as Bhabhatron-II. In this paper, the results pertaining to radiation safety of indigenously developed Bhabhatron-II telecobalt unit are reported. The various tests were carried out as per requirements of International Electrotechnical Commission standard and acceptance criteria developed nationally. Various devices such as CaSO4:Dy based thermoluminescent dosimeters, farmer type ionization chamber, water phantom and radiographic films were used. All the parameters pertaining to radiation leakage/transmission were within the tolerance limits as per IEC-60601-2-11 standard except the collimator transmission through X collimators (upper jaw), which marginally exceeds the tolerance limit.

Study on the response of indigenously developed CaSO4:Dy phosphor-based neutron dosemeter.

In the present paper, we report the indigenous development of a neutron-sensitive thermoluminescent dosemeter (TLD) based on CaSO4:Dy Teflon TL disc. The study includes indigenous development of a neutron dosemeter and its response in terms of operational quantity to different energies of neutrons under various irradiation conditions. It was found that the thermal neutron sensitivity of the CaSO4:Dy Teflon neutron disc is about one-third of TLD-600. However, the thermal neutron sensitivity with respect to CaSO4:Dy Teflon gamma disc is about 42 times for in-air irradiation and about 84 times for on-phantom irradiation condition. This newly developed neutron dosemeter can be used as a routine TLD with a slight change in the design of the TLD badge system in the mixed fields of gamma and neutrons of energy up to 500 keV for radiation workers engaged in nuclear fuel cycle operation and other applications.

Estimation of bremsstrahlung photon energy in the environment of high-energy electron accelerator using CaSO4:Dy based TL dosemeter.

High-energy bremsstrahlung X rays constitute the major radiation hazard to working personnel around the high-energy electron accelerators. Thermoluminescent (TL) dosemeter system based on CaSO4:Dy Teflon disc used in the routine individual monitoring was used to estimate the bremsstruhlung photon energy at different locations of the experimental hall of 450-MeV synchrotron accelerator. The response of TL discs under different filter regions of the dosemeter system undergo change with photon energy due to the lack of build up and interaction of photon in the metal filters. This change in the response of the discs used to estimate the energy of the bremsstrauhlung photon in conjunction with suitable calibration curve generated using known photon energy from medical linear accelerator. The photon energies estimated were in the range 1-4 MeV, depending on the locations.

Response of CaSO4:Dy based TL dosemeter system to high-energy photon beams: theoretical simulation.

Response of thermoluminescence (TL) discs under different filter regions of a CaSO(4):Dy based TL dosemeter system was simulated to high-energy photon beams in the energy range of 1.25 MeV to 24 MV ( approximately 9 MeV). This was done using FLUKA Monte Carlo code and also experimentally verified for some energy points. Response of disc D1 under metal filter combination was found to increase with photon energy, whereas that for the discs under polystyrene filter and open window regions of the dosemeter decreases continuously. The changes in the response of the discs under polystyrene filter and open window were attributed to the lack of build-up material. The increase in the response of disc D1 was due to the contribution from secondary electrons produced through Compton and pair production processes mainly arising out from the metal filter combination. The knowledge of the change in the response of individual discs and the ratio of discs' responses under different filter regions of the dosemeter system could be used for the measurement of energy of bremsstrahlung radiation that exists in and around high-energy electron accelerator and could be used for accurate evaluation of personal dose equivalent in high-energy photon field.

Studies on the response of the TLD badge for high-energy photons.

Absorbed tissue dose measurements are carried out for high-energy photon beams using CaSO4:Dy thermo-luminescence dosemeter (TLD) badge and the results are also verified using ionisation chamber used in radiation therapy. The photon beams generated using linear accelerator at 6 and 18 MV photon beam energies have been used and the absorbed doses are measured at the surface as well as at various depths. It has been found that the depth at which maximum dose is delivered increases with the increase in photon energy and the depth of maximum absorbed dose in tissue occurs beyond 10 mm. It has also been found that the evaluation of the absorbed dose (or Hp(10) as well) using thermoluminescence readout of disc D1 clearly shows that the current TLD badge provides a reasonable estimate of the effective dose for photon fields from 6 to 18 MV linacs for anterior-posterior incidence. The paper also provides information regarding the misinterpretation of radiation pattern in multi-element/filter TLD badge.

Development of an algorithm for TLD badge system for dosimetry in the field of X and gamma radiation in terms of Hp(10).

In view of the introduction of International Commission on Radiation Units and Measurements operational quantities Hp(10) and Hp(0.07), defined for individual monitoring, it became necessary to develop an algorithm that gives direct response of the dosemeter in terms of the operational quantities. Hence, for this purpose and also to improve the accuracy in dose estimation especially in the mixed fields of X ray and gamma, an algorithm was developed based on higher-order polynomial fit of the data points generated from the dose-response of discs under different filter regions of the present TL dosemeter system for known delivered doses. Study on the response of the BARC TL dosemeter system based on CaSO(4):Dy Teflon thermoluminescence dosemeter discs in the mixed fields of X and gamma radiation was carried out to ensure that the accuracies are within the prescribed limits recommended by the international organisations. The prevalent algorithm, based on the ratios of the disc response under various filters regions of the dosemeter to pure photons, was tested for different proportion of two radiations in case of mixed field dosimetry. It was found that the accuracy for few fields is beyond the acceptable limit in case of prevalent algorithm. The new proposed algorithm was also tested in mixed fields of photon fields and to pure photon fields of varied angles. It was found that the response of the dosemeter in mixed fields of photons and its angular response are satisfactory. The new algorithm can be used to record and report the personal dose in terms of Hp(10) as per the international recommendation for the present TL dosemeter.

Correlation of phase transition with the change in TL characteristics in CaSO4:Dy phosphor--effect of thermal treatment.

The study is aimed at optimising the glow curve structure of CaSO4:Dy phosphor for dosimetric purpose and also to understand the observed changes owing to thermal treatment in the thermoluminescence (TL) sensitivity and glow curve structure. The reversible changes in the intensities of the lower temperature satellite peak and the main dosimetric glow peak with temperature of thermal treatment in the temperature range 400-700 degrees C indicates the interconversion of defect complexes responsible for the glow peaks in CaSO4:Dy. Phosphor samples subjected to thermal treatment in temperature range beyond 800 degrees C, showed irreversible changes in the intensities of the lower temperature and dosimetric peaks. The changes in TL characteristics of the phosphor for thermal treatments >800 degrees C are attributed to the partial phase transition in the phosphor as confirmed by the XRD and TG/DTA analysis of the phosphor.

Studies on automatic hot gas reader used in the countrywide personnel monitoring programme.

In India, approximately 58,000 radiation workers are monitored using locally made CaSO4:Dy teflon embedded thermoluminescence dosemeter (TLD) badge system. The automatic hot gas readers developed locally are also used in TL measurements. The hot gas reader system has many advantages over the manual readers used previously and has completely replaced the manual reader system in all TLD personnel monitoring units in India. In the present study, the new reader system is studied and a theoretical attempt has been made to interpret the experimentally obtained results. The glow curves are generated theoretically and are also plotted experimentally. It has been found that the heat capacity of the heating gas, which is responsible for the transfer of heat, has a role in deciding the position of peak and is verified experimentally using different gas flow rates of nitrogen and argon as heating gases in the reader. The theoretical study may also be helpful in fitting the experimentally obtained glow curves and, therefore, the elimination of unwanted non-radiation-induced contributions, such as dark current, electronic spikes, light leakage and triboluminescence that generally distort the glow curve shape, can be achieved.