DYSPROSIUM DOPED CaSO4 FOR HIGH SENSITIVITY X AND GAMMA-RAYS DETECTORS

The crystalline substances are the most used for Thermoluminescent Dosimetry (TLD) purposes. The crystalline structure allows for a controlled impurification with activators in order to improve their dosimetric parameters. The activators introduced into the crystalline network [1], enhance the number of electrons and hole traps inside the forbidden band. In the first step, immediately after the irradiation, the electrons and hole traps are populated, due to mobile charge carriers released by ionization; in the second step of heating the trap depopulation occurs and a light signal is emitted. CaSO4 becomes thermoluminescent if an activator, e.g. dysprosium, is introduced in the crystalline structure. The energy of the ionizing radiation, stored in the thermoluminophore after its irradiation can be maintained for a 3 months period, with a fading value of <5%. The main requirements imposed to a TL phosphor are the followings: (i) A high efficiency or sensitivity, defined as the ratio between ETL and mD, ηTL=ETL/mD, where: ETL is the energy emitted as luminescence, m is the phosphor mass and D is the absorbed dose value. (ii) Good dosimetric properties, such as linear response for a large interval of doses. (iii) Reproducibility and insensitivity to temperature, light, humidity etc. (iv) Easy to be handled Generally, one thermoluminophore, cannot accomplish all these requirements at the same time. This is the reason why the choice of the luminophore and activator and the construction of the dosimeters are very important parameters in designing a dosimetric system. The energy stored in the CaSO4 luminophore is released as a light signal after its heating at a 360 C temperature in a special oven, included in a TL -Reader; in our case a commercial TL Reader, VICTOREEN 2800 type was used; the light spectrum contains two peaks at 478 nm and 571 nm. The CaSO4:Dy dosimeters are used in a dosimetric system for the measurement of environmental absorbed dose in air produced by X and γ rays within the energy range 30 keV-3000 keV. They are exposed to the environment for a 30 days period. Due to their high sensitivity [1] CaSO4: Dy detectors are constitutive elements of the dosimetric systems used for the environmental survey of the Nuclear Power Plants, specially designed localities, or other Nuclear Objectives. The present paper presents a new technology for getting CaSO4: Dy type TL detectors, as well as the results obtained during their testing.