The possibility of using thermoluminescence for in vivo and in vitro direct dose measurements in tiny body cavities (for example, bone marrow) suggested investigation of the response to beta radiation of LiF and CaF2 powder and single crystals of different thicknesses (down to microns). Several beta-ray sources were made in the form of gelatin or polyester resin molds. The emitters consisted of ${}^{3}{\rm H},{}^{63}{\rm Ni},{}^{35}{\rm S},{}^{45}{\rm Ca},{}^{204}{\rm Tl}$ , and ${}^{90}{\rm Sr}({}^{90}{\rm Y})$ , and the absorbed dose rate was computed from the known concentration in each case. As expected, the thermoluminescent output per unit weight and per unit absorbed dose was essentially the same for ${}^{204}{\rm Tl}$ and ${}^{60}{\rm Sr}$ as for60 Co but dropped for the lower-energy beta-ray emitters. Among the factors considered which might account for the drop in light output were (1) the relation of the beta-ray ...