Response of TLD badge to mixed fields of photons of energies above 6 MeV and beta radiation encountered in nuclear power plants

Response of TLD badge in use for our countrywide personnel monitoring was evaluated for high-energy photon beams (10 MV, 15 MV and 18 MV) from medical Linear Accelerators with beam output measurements within an accuracy better than ±3%. The badge has three CaSO4:Dy teflon TLD discs (each of dia. 13.5 mm and thickness 0.8 mm) clipped on an aluminum card kept in a cassette having three regions (i) combined metal filter of 1 mm thick Cu and 1 mm thick Al, (ii) 1.6 mm thick plastic filter and (iii) an open window area on either side sandwiching the TLD card. Mainly discs under metal filter and open window are used to estimate the dose due to gamma rays and beta rays, respectively while ratios of responses of discs under plastic filter and open window/metal filter in an algorithm are used for arriving at the correction factors for evaluation of beta and gamma ray doses. In nuclear power plants, especially the CANDU pressurized heavy water reactor systems, gamma ray of energy higher than 6 MeV (produced from disintegration of 16N via the reaction 16O(n p)16N) do contribute significantly to personal dose equivalent Hp(10) in addition to situations where beta radiation contributes to personal dose equivalent Hp(0.07). The values of absorbed doses at 10 mm in phantom were obtained from the measured values of dose at Dmax traceable to national standard and irradiations of dosimeters were made on a plastic phantom of size 25x25x25 cm3. The bare dosimeter disc was found to exhibit no photon energy dependence for high energy photons of energy above 300 keV (up to 7 MeV). However, the response of the badge was found to increase by about 10% above 6 MeV as compared to that of Co-60 gamma rays due to the influence of metal filters used in the badge for compensation of photon energy (below 200keV) response. In the case of LiF TLD-100 ribbons sandwiched between 0.3 mm thick Cu filters, no over-response was observed. A significant complication in the evaluation of dose due to beta radiation was encountered in mixed fields of beta radiation and high energy photons of energy above 6 MeV, because the dose evaluation involves subtraction of TL signal due to high energy gamma rays (TLD under metal filter) from the response of the dosimeter responding to both gamma rays and beta radiation (TLD under open window). This was mainly because the TLDs under plastic filter and open window were found to under respond to the extent of 40 to 60% to high-energy gamma rays. This complication is not limited only to our badge but also applicable to the other personnel monitoring badges where the method of subtraction of response due to penetrating radiation from the combined response due to penetrating and non-penetrating radiation is used for arriving at Hp(0.07). This aspect has therefore been studied in detail for our system and the inaccuracy involved in the measurements is evaluated. (author)