Effects of atmosphere and grain size on thermoluminescence sensitivity of annealed LiF crystals

A study has been made of the thermoluminescence sensitivity of lithium fluoride crystals as a function of both grain size and the surrounding atmosphere when given prolonged annealing at high temperatures. Optical grade lithium fluoride and dosimetry grade lithium fluoride (Harshaw TLD-100) do not show the same response to high-temperature annealing. For a given technique of quenching from high temperature, the thermoluminescence sensitivity of optical grade lithium fluoride increases with temperature when annealed in the absence of air, whereas the thermoluminescence sensitivity of TLD-100 stays roughly constant. When annealed in air the sensitivity of TLD-100 decreases sharply with increasing temperature. Optical grade crystals when annealed in air show an increase in optical absorption in the visible spectrum, and infra-red absorption bands are found due to the presence of OH anions which are a known thermoluminescence poison. Optical grade crystals show a generally decreasing sensitivity with increasing grain size.