[en] Rare earth oxyhalides activated with terbium form thermoluminescent materials. Their preparation and use in thermoluminescent dosimetry, which uses the proportional relationship between the relative intensity of the emitted light and the dose of the ionising radiation, is described. The sensitivities of various thermoluminescent materials to ionising radiations and the stability of retention of the absorbed radiations are given. These show the rare earth oxyhalides to be suitable for long term thermoluminescent dosimetry applications such as in medical radiography and nuclear power plant operations. (U.K.)