Techniques for reducing systematic errors in TLD readers

A new TLD reactor has been developed which employs several techniques to minimise errors during the readout of dosimeters. The light measurement system utilises variable negative feedback both to set the sensitivity to a desired level and to automatically maintain that level against system drifts. A stepped drawer system has been designed which eliminates light leaks through the drawer, while enabling mechanical tolerances to be relaxed to provide a smooth drawer action. Experiments to determine the optimum preheat zone in the heating cycle which is employed to reduce fading effects in lithium fluoride and lithium borate dosimeters have shown that the preheat temperature is very critical. A heating cycle control system employing a unique thermocouple cold junction compensation circuit has therefore been developed which is capable of very reproducible readout cycle temperatures. With this new instrument the reproducibility of readings from individual LiF/PTFE disc dosimeters irradiated to 1 rad is typically 0.5 to 1% (one standard deviation) over 10 readout cycles. (author)