MANY MATERIALS in which irradiation causes excitation of electrons into energy states, which are stable at room temperature, have been found useful for thermoluminescence dosimetry. On subsequent heating of such a material, the de-excitation of the electrons results in the emission of light, and the light intensity may be plotted as a function of time or temperature to give a glow curve. Dose previously absorbed by the material can then be related to the integrated light output. In recent years lithium fluoride has become the most popular thermoluminescent material and is now available commercially as a microcrystalline powder. Cameron et al. (1–4:) and Kenney et al. (6) found that these inert, insoluble, and nontoxic crystals of LiF are relatively free from mechanically induced luminescence, have an almost linear response to amounts of radiation from a few milliroentgens to several kiloroentgens, and, because of the low atomic number of both lithium and fluorine, possess an energy dependence small as compared to most other dosimeters. An annealing procedure has been developed (2), whereby the LiF powder may be reused many times. Karzmark et al. (5) confirmed most of this work and found no dose-rate dependence up to 2 × 108 rads/ sec. Marrone and Attix (7) found permanent damage in the γ-ray responsiveness of the LiF phosphor for exposures of the order of 104 R. The LiF and the associated read-out apparatus employed in the work reported herein were purchased from Controls for Radiation, Inc. (Con-Rad). The LiF powder was purchased as Con-Rad type N (natural abundances of Li isotopes) and Con-Rad type 7 (enriched in Li7). Type N was used in all work unless otherwise stated. With a volumetric aluminum dispenser 60 mg of LiF powder was weighed out into a polyethylene capsule. Five capsules were then placed in a phantom of unit density rubber, the phantom being so designed that the capsules of LiF could be replaced by ion chambers or a chemical dosimeter. After irradiation the LiF was tipped into five separate stainless-steel planchets, and each in turn was inserted into a Con-Rad #4100 read-out instrument. A current of about 150 amperes was passed through the planchet, heating it to about 300 °C in ten seconds. The light given off by the phosphor was detected by a photo-multiplier whose output was stored on a 10/μF polystyrene capacitor. The final voltage on the capacitor was read by an electrometer voltmeter and displayed on a digital voltmeter. The voltmeter reading was therefore proportional to the integrated light output. We have adopted Cameron's annealing procedure (3): one hour at 400 °C followed by twenty-four hours at 78 °C prior to re-use of the phosphor. Accuracy One hundred capsules of LiF were irradiated with Co60 gamma rays to about 100 rads.