Ionizing radiation, such as X-rays, alpha rays, beta rays, and gamma rays, are undetectable by the human senses and therefore require a measuring device to detect, measure and record the amount of radiation. Dosimeters may measure an individual's or an object's radiation dose and also cumulative dose which is accumulated over a period of time. A thermoluminescent dosimeter (TLD) is a type of radiation dosimeter which measures ionizing radiation dose by measuring the amount of visible light emitted from a crystal in the detector when the crystal is heated after its irradiation. The amount of light emitted depends upon the amount of radiation exposure received by it. TLD is extensively used for radiation monitoring of workers engaged in radiation area and environmental radiation measurement. Calcium Sulphate (CaSO4) is one of the most investigated materials which is used as TLD. It has been experimentally modified with several dopant to increase/modify its Thermoluminescent (TL) sensitivity, the Dysprosium (Dy) doping, in particularly has been the most successful in the series. CaSO4:Dy is always the choice when it comes to radiation exposures for personnel monitoring. It remains one of the most attractive TLD materials because of ease preparation in large quantity with essentially the same Thermoluminescent (TL) properties and its high sensitivity. The stability of its response even in adverse climatic conditions further adds to its demand in monitoring doses in the field. The need for further enhancing the Thermally Stimulated Luminescence (TSL) sensitivity of CaSO4:Dy is felt in order to cater to the dose range used in the area of radiological medical imaging, nuclear medicine, interventional fluoroscopy x-rays in medical use like urology, orthopaedics, cardiology, anaesthesiology, dental and medical radiation research use which are getting smaller and smaller with the advent of safe and smart technology. For example, radiation doses involved in medical imaging (like Radiography, Computed Tomography (CT), Fluoroscopy during angioplasty and angiography, Mammography etc.) has come down significantly as compared to the radiation doses involved in these medical imaging devices a decade ago.
Literature survey shows that the first synthetic CaSO4, activated with Manganese (Mn), showed high sensitivity, but its glow curve exhibited a single peak at low temperature (90° C.). Therefore, fading of radiation dose recorded of this material was very pronounced (40-85% in the first 3 days after exposures). The credit for the development of a highly sensitive CaSO4 TLD material goes to Yamashita et al. Later Nambi et al. have studied the TL properties of CaSO4 phosphors doped individually by different rare earth (RE) elements and concluded that Dysprosium (Dy) and Thulium (Tm) are the most efficient activators and that the optimum concentration of the dopant is 0.1%.
With advancement in technology, the amount of radiation dose in radiation use in medicine and medical imaging has become smaller. Therefore, ultrasensitive TL dosimeters and more sensitive phosphors compared to CaSO4:Dy for low doses of radiation, are the need of the hour.