The Influence of LED and Fluorescent Light on Unirradiated TLD Cards

Thermoluminescence dosimeters (TLDs) are used as standard personal dosimeters at nuclear facilities and laboratories around the world. TLD cards are known to be affected by light that causes a spurious signal which may increase the measured dose. These spurious signals arise from the teflon sheets holding the TLD dosimeters [1]. Therefore, TLDs are sealed in badges that protect them from light while being used by employees. However, TLD cards may be exposed to fluorescent light in the laboratory, while being handled before and after readout, and may give rise to false radiation readout at the lab. In the current study we measured the influence of different light conditions on unirradiated TLD cards.

The tested TLD cards were 4 elements 7776 LiF:Mg,Ti Type manufactured by RadPro (Poland). The cards were exposed to different types of fluorescent and LED lamps with different wavelength spectrum (2700 K-6500 K), exposure times and intensities and then were read by a 6600 Harshaw^TM reader. The heating profile was a linear heating rate of 25^OC/s from 50^OC to 300^OC and then the cards were kept at 300^OC for a total time of 13.3 s, as recommended by the manufacturer.

The results indicate that exposure to 6500 K fluorescent light for 1 hour (white color) with intensity of 1000 lux, adds approximately 80µSv of spurious signal to unirradiated TLD cards. Using 2700 K LED or fluorescent light (yellow color) reduces the spurious dose by approximately a third.

In conclusion, exposure of TLD cards to normal LED or fluorescent light in the dosimetry laboratory while handling the cards for more than few minutes can adds a significant spurious signal. Reducing the effect can be achieved by using work practices including: minimizing the time the cards are exposed to light, reducing the light intensity in the dosimetry lab and using yellow light (2700 K).

[1] The effect of visible light on Harshaw model 8801 thermoluminescent dosimeters, E. Sonder et al ORNL, September 1990.