Fading of TLD-100 as a Function of Crystal Size, Energy, and Dose

Abstract

The fading of TLD-100 as a function of crystal size, radiation energy, and total dose is being studied. A light source (tritium-activated phosphor) is used as a standard in a commercial reader (Model S-2LC, Madison Research and Development Laboratories, Inc.). The response of TLD-100 is recorded and then normalized to the variations of the “standard.” The fading indicated by these normalized readings is thought to represent a truer picture of the actual fading than results obtained without normalizing to a standard. The TLD-100 was separated into three different crystal sizes, using the U. S. Standard Sieve Series—powder that remained on the #100 and on the #200, and that which passed through the #200 Tyler equivalent mesh. A 50-50 mixture of the #100 and #200 also was used. Exposures of approximately 100 R from C060 and 200 kVp x rays were used to study the effect of different energies and crystal sizes; exposures of 50 R and 500 R from C060 were employed to study the effect of different exposures. The powder dispenser as furnished by Madison Research and Development Laboratories, Inc., was utilized. The photomultiplier tube voltage was 1,777 volts for the 100 R exposures, and 1,400 volts for the 50 R and 500 R C060 exposures. The heating cycle was terminated at the end of twenty-two seconds; peak temperature of the heater pan was 345° C. Readings of the light source and for some of the powder at various times after exposure of the powder are shown in Figure 1, A. The variations in the readings of the light source can be attributed to such causes as voltage changes and fatigue and thermal effects. Normalizing the readings for the powder to the readings of the light source gives values which represent the fading of the powder (Fig. 1, B). These normalized curves have fewer fluctuations and compare favorably with the fading curves of others (1), including a return to approximately the initial value at seven weeks. It appears that fading is not a function of radiation energy, crystal size, or total dose for this batch of powder. We are currently examining several different batches of powder in the same manner.