Response of TLD Badge to Mixed Fields of Photons of Energies above 6 MeV and Beta Radiation Encountered in Nuclear Power Plants

INTRODUCTION In India, official and centralized personnel monitoring service using film badge was started in 1952 by Bhabha Atomic Research Centre, Bombay (1). With the indigenous development of a TLD badge system (2) based on CaSO4:Dy Teflon disc dosimeter (3) exhibiting evident superiorities over film badge, replacement of film based individual dosimeter by TLD was initiated in 1975. At present about 30,000 radiation workers are monitored by using the TLD badge. Individual monitoring is the most stringent and crucial requirement for radiation workers in Nuclear Power Plants in view of the regulatory controls on one hand and the commercial aspects of power generation on the other hand. Dose budgeting of the plant is assessed by the results of TLD monitoring and therefore the quality of performance of TLD service is often reviewed for any dosimetric inaccuracies. In India, most of the power reactors (presently 10 out of 12 producing power) are pressurized heavy water reactors. In nuclear power plants, especially CANDU type pressurized heavy water reactor systems, gamma rays of energies higher than 6 MeV could significantly contribute to personal doses of radiation worker, especially to those carrying out maintenance operation inside containment close to coolant and moderator system. High energy gamma rays are produced from the disintegration of N (Nitrogen-16) which is produced by the interaction of neutrons (energies above 10 MeV) with O of coolant via O(n, p)N reaction. N has a half-life of 7.13 s and decays by beta emission followed by the emission of photons of energy 6.13 MeV (about 68%) and 7.12 MeV (about 5%). N transported from the reactor core with water or steam, results in external radiation dose due to high energy photons, mainly 6.13 MeV gamma rays. Recently, questions have been raised about the response of CaSO4:Dy Teflon TLD disc and the TLD badge to high energy gamma rays and also about its response in mixed fields of gamma rays and beta radiation (4). In general, the previous studies have shown considerable variability of responses of survey instruments and personnel dosimeters to gamma rays of energy above 6 MeV. Both under-response as well as over-response were reported. GM detectors were found to exhibit large over response (5, 6, 7) and in the case of photographic film badge (8, 9), an over response of more than 250% has been reported. An under-response of about 8% in the LiF TLD measurements as compared to the response of ionization chamber has also been reported (7, 10). Efforts have also been made to develop calibration factors for high-energy gamma rays (7,11,12). Recent study (13) concluded that the problem of calibration essentially lies in the availability of an appropriate build-up on the dosimeter in a badge for both photons and electrons. It was therefore decided to evaluate the response of the present TLD badge system to high energy gamma rays encountered in nuclear reactors and to understand the intricacies of personal dose evaluation in the mixed fields of high energy photons and beta radiation.