Some aspects of radiation-induced effects on thermoluminescence mechanisms of natural barite for dosimetric utilization

Abstract

During the past 50 years thermoluminescence (TL) phosphors of high effective atomic number have become a subject of interest in many fields of dose assessment and radiation dosimetry, such as in medical dosimetry, processing level dosimetry, radiation personnel dosimetry and in other fields of application such as agriculture, archaeology, radiation physics and solid-state physics [1-12]. However, many of the commercial TL phosphor types have been discontinued because of their relatively high cost and the damage induced in them by high-level radiation and heat-treatment. Therefore, great efforts and intensive studies have been directed towards the induced effects of y-irradiation and thermal treatment on the TL behaviour produced from natural materials [12, 13]. None of these studies has explained in detail the TL mechanisms and characteristic behaviour of barite as a high atomic number TL material. Since different natural barite types contain a wide variety of activator elements as dopants, it was necessary to carry out the present kinetic studies on TL mechanisms as a comparison between some naturally occurring barite types and the synthetic laboratory-prepared equivalent compositions. The aim of this work was to identify the dopant activator elements which show the main effective response of the colour centres, in order to develop an effective TL detector of high atomic number and low-cost, with wide utilization range in the different fields of radiochemistry, radiophysics, radiobiology and radiobiophysics. Measurement of the induced TL and the TL emission spectra by the effect of y-irradiation on both Egyptian naturally collected barite samples (denoted "black", "colourless" and "yellowish" barite) and laboratory-prepared synthetic samples of equivalent chemical compositions (Table I) was performed using a Harshaw TL measuring system and a TL emission optical spectroscopic monochromator system. Both barite species were prepared in the laboratory in disc form of about 5 mm diameter and 1.0 mm thickness and were of weight 10 rag. The synthetic barite was prepared by coprecipitation of the different dopant activators (as shown in Table II) with BaSO4 (which was precipitated by the addition of dilute sulphuric acid to barium chloride). The prepared disc samples were treated thermally before y-irradiation between room temperature and 700 °C for 1 h at 100 °C intervals. The annealed samples were irradiated to g-ray doses in the range 1-10 Gy from a 6°Co y-cell. In Fig. 1 the TL glow curves of natural barite samples thermally annealed at 500 °C before yirradiation to 1 Gy are shown. The figure shows a glow curve structure of a main dosimetry glow peak at temperature positions 185-200, 160 and 135-150 °C, as well as a second-order peak at 115, 235 and 255 °C, corresponding to "black", "colourless" and "yellowish" barite, respectively. The attributed changes in the glow curve structure and peak temperature positions are mainly ascribed to the variation of the doped activator types and concentrations within the main host-lattice components (Table I). The cause of the colour centre created in the different barite types can be predicted