Response of LiF powder to /sup 125/I photons

/sup 125/I seeds are being used to treat a variety of diseases, including brain tumors. However, the uncertainty in the energy response of LiF for /sup 125/I photons relative to megavoltage radiation limits the accuracy of dosimetry with this material. An array of 12 40-mCi /sup 125/I seeds was used to irradiate LiF powder in throwaway capsules, and an ion chamber was used to measure the absorbed dose. A comparison of the response of these LiF dosimeters with the response of capsules given a similar dose of /sup 60/Co radiation has yielded values of 1.39 +- 0.03 (dose to water) and 1.32 +- 0.03 (dose to muscle)= for the LiF response for /sup 125/I relative to that for /sup 60/Co. Details of the procedure and sources of uncertainty are discussed.