This invention relates to a thermoluminescent phosphor for the measurement of low radiation doses, including calcium sulphate (CaSO.sub.4), Dysprosium (Dy) and manganese (Mn), wherein Dy and Mn are present as dopants. A process for the preparation of a thermoluminescent phosphor is also provided. The process includes the steps of: separately dissolving calcium sulphate (CaSO4), Dysprosium chloride (DyCh) and Manganese chloride (MnC) in hot concentrated sulphuric acid, to obtain sulphuric acid solutions of CaSO4, DyCb and MnCb; mixing the solutions; and followed by slow evaporation of the solvent to obtain a powder of microcrystalline phosphor of CaSO4:Dy, Mn.