THE RESPONSE OF LIF TLDS TO CS-137 AND CO-60 GAMMA-RAYS

The radiation absorption characteristics of LiF thermoluminescence dos emeters (TLDs) are such that the TLD response per unit air kerma is ex pected to be almost the same for Cs-137 and Co-60 gamma rays. However, various reports in the literature suggest that the response per unit air kerma is several per cent greater for Cs-137 than for Co-60. The r esponse of LiF per unit air kerma has been measured under well defined conditions using Cs-137 and Co-60 gamma rays. Measurements were carri ed out with LiF chips mounted in Lucite and aluminium holders as well as in commercial holders. For the Lucite and aluminium holders, the ef fect on the TLD response of photon attenuation and scattering was esti mated by varying the wall thickness of the holder. After correcting fo r the effect of the Lucite and aluminium holders, the response per uni t air kerma was 2.5 +/- 0.7% greater for Cs-137 than for Co-60. For TL Ds mounted in a commercial holder which uses aluminium for build-up, t he response to Cs-137 was 5.8 +/- 0.6% greater than to Co-60. For a co mmercial holder made largely of plastic, the corresponding result was 2.2 +/- 0.9%. Our results show that the holder can play an important r ole in enhancing the response per unit air kerma of LiF TLDs to Cs-137 relative to Co-60 gamma rays. However, parr of the enhancement may be due to a change in the TLD response with LET.