Quartz as a heat-resistant dosimeter.

Radiation dose may be measured by means of the increase which it produces in the thermoluminescence sensitivity of quartz. The measurement is possible even if the quartz was heated after the dose was received (for example for several hours at temperatures up to 300°C). This makes the quartz suitable as a heat-resistant dosimeter. THERMOLUMINESCENCE (TL) phosphors and film badges are the principal existing dosimeters. However the records they store are partly erased by heating for more than a few hours a t temperatures above about 50°C [see, for example, Ref. 1 for the behaviour of LiF (TLD-loo)]. Observations upon the radiationsensitive T L peak in quartz a t about 100°C (for a heating rate of 20"C/sec) show that this material may be used as a heat-resistant dosimeter. It is possible to use such a low temperature T L peak by measuring the increase in T L sensitivity which results from thermal annealing of the exposure dose. @) The dosimetry does not, then, involve the measurement of the stored TL due to the exposure dose, but requires the measurement of the change in sensitivity produced by that dose. The quartz used is prepared by firing geological a-quartz a t 1000°C for 18 hr. To determine the T L sensitivity, So, of the material so produced (called hereafter the virgin material), a 1 rad test dose is given to a sample, and the resulting TL measured. Throughout this work, all doses are of beta radiation from 'OSr plaque sources. The T L is measured by heating the irradiated material at 20"C/sec, and the light detection system is an EM1 96359 photomultiplier plus Corning (7-59) and Chance Pilkington (HA3) filters (the sensitisation being greatest in the region of violet emission). At the end of the period of dosimetry, during which the quartz received the exposure dose, the quartz is thermally annealed by heating at PO"C/sec to 550°C. The final sensitivity, S, is obtained by giving this sample the same test dose as used on the virgin material, and measuring the resulting TL. The sensitivity ratio 8 567 S/So is a function of exposure dose, as shown in Fig. 1, curve a. The same growth of the sensitivity ratio is obtained if the 100°C traps filled by the exposure dose are still filled when the anneal is made, or have been emptied by drainage at room temperature before the anneal. Thus it appears that the peak retains a memory of its radiation history independent of the time elapsed between the exposure dose and the anneal. The temperature of 550°C is decided upon from evaluation of the sensitivity ratio as a function of maximum temperature to which the sample is annealed. The greatest ratio occurs a t 550°C. For example, the data for an exposure dose of 1 krad is shown in Fig. 2, curve a. The behaviour of S/S, shown in Figs. 1 and 2 is very sensitive to the firing temperature, and must be established for each batch of quartz unless the firing temperature is reproduced within about 5°C. For different quartzes fired under identical conditions, these sensitivity characteristics may differ considerably. As yet, no annealing procedure has been found which will restore the sensitivity of material which has received an exposure dose back to the sensitivity of the virgin material. The additional effects of long periods of heating on the sensitivity ratio were tested by irradiating samples of the virgin material with 1 krad, and then heating them at 300°C for up to 10 hr. They were then given the 550°C anneal, and their sensitivity measured. No destruction of the enhancement was observed. In fact, for the longer heatings an additional sensitivity was observed, reaching +30 % for 10 hr. Therefore, if much heating of the dosimeter occurs during use, the value deduced for the exposure dose will be an over-estimate. 568 QUARTZ AS A HEAT-RESISTANT DOSIMETER FIG. 1. Variation of sensitivity ratio with exposure dose for 100°C TL peak in geological a-quartz fired for 18 hr at (a) 1000eC and (b) 7OOOC. (The thermal anneals after application of exposure dose were made by heating at 20°C/sec to (a) 55OoC and (b) 400°C. ) FIG. 2. Variation of sensitivity ratio with temperature to which sample is annealed for 100°C TL peak in geological a-quartz, fired for 18 hr at (a) 1000°C and given 1 krad exposure dose and (b) 700°C and given 54 rad exposure dose. If there has been little heat treatment of the dosimeter in use (for example, heating to 200°C for an hr) the sensitivity increase gives exposure dose directly. The range of measurement is 5 to 600 rads, with an uncertainty of f5 rads or &5 %, whichever is larger. The minimum detectable dose is decreased to 0.3 rads if the geological quartz is fired at 700°C for 18 hr. This material is usable for exposure doses up to about 300 rads, where the uncertainty is f5 %. The sensitivity saturates thereafter, as shown in Fig. 1, curve 6 , for a 400°C anneal. This annealing temperature is used because it gives the maximum enhancement in sensitivity ratio as illustrated in Fig. 2, curve b. That curve shows the relative sensitivity to a 0.2 rad test dose for the 700°C fired material which has received a 54 rad exposure dose and has been annealed to different temperatures. Under conditions of excessive heating, this material is not as suitable as that fired at 1000°C , because its maximum sensitisation occurs at a lower annealing temperature (see Fig. 2). Investigations are now being made on quartz to determine the physical mechanism of the radiation sensitisation phenomenon. Then it is hoped to develop a suitable synthetic quartz which will be much more independent of heating received after the exposure dose and before the annealing, and which may be used over a greater dose range than the quartzes described above. REFERENCES 1. D. W. ZIMMERMAN, C. R. RHYMER and J. R. 2. S. J. FLEMING, Unpublished thesis, Part IV. CAMERON, HeaZth Phys. 12, 525 (1966). Oxford University (1969).