A hole trapped centre responsible for high temperature thermoluminescence in sodium borate glass

Abstract

Thermoluminescence (TL) is now widely used for radiation dosimetry and B203 glass is no doubt one of the most advantageous materials for this purpose from the point of view that its effective atomic number is very close to that of human tissue. However, according to our previous investigation [1], pure B~O3 glass has fatal shortages: (a) The temperature of the main glow peak is too low to keep the radiation memory against preservation at room temperature (RT). (b) Pure B~O3 is very hygroscopic and cannot be used in wet air. These problems can be solved by the addition of alkali oxides into borate. According to Nasipuri et aL [2], sodium borate glasses containing less than about 33 mol% Na20 showed better characteristics for TL materials, though the effective atomic number slightly increased. Also though they mentioned that the TL output of the glasses is much weaker compared to commercial phosphors, and this shortage can be easily overcome by the addition of luminescent impurities, such as copper ions. Therefore, to explore the mechanisms of TL, or to find out the trapping centres responsible for-the TL in these base materials without impurities seems to be required. This is the reason why the following investigation on a borate glass containing sodium was undertaken. Samples were made from a mixture of reagent grade orthoboric acid (H3BO3) and the corresponding sodium carbonate calculated to form 80mo1% B203-20mol% NazO. They were melted at 1100 ° C for 1 bin a platinum crucible and cast into a brass mould and annealed at 250 ° C for 1 h to remove strain. Samples 7 mm x 7 mmx 2 mm were polished for TL measurement. Smaller samples of about 2 mmx 2 mm x 3 mm was used for ESR measurement. After the samples were irradiated by 7-rays up to 10 v R at RT, they were kept at RT for 60 days before the TL and ESR measurements. Fig. I shows the TL glow curves of a 7-irradiated Na~O (20 tool %)-B203 (80 mol %) glass measured with a heating rate of 20°Cmin -1. A real line indicates the whole glow curve which starts at about 350 K and finishes at about 515 K. The glow curve consists of at least three components. The dotted line shows the last component, which was obtained by the second heating after the sample was heated firstly up to 450 K followed by cooling to RT. The peak temperature, Tin, of this last component is 477.5 K, and the two half-maximum temperatures are T 1 = 452.0 K and T2 = 496.0 K, respectively. The activation energy for the last glow peak can be determined by using the Chen's formula [3]