Answer to Question #1726 Submitted to "Ask the Experts"

Q

Can we use properly annealed TLD-100 (LiF) for measuring indoor gamma radiation? Is it enough to keep the TLD chips in sealed polythene sachets? Or, does it require some other container for shielding betas? How long should the TLDs be kept to collect the indoor radiation before being read? Should one have controls kept at the laboratory?

A

Much of the answer as to the utility of LiF for an indoor gamma radiation monitor depends on the dose rate you expect. However, the adequacy of any given TLD material (for example, LiF, CaF, CaSO₄, AlO, etc.) for a specific application depends on the total dose expected, minimum detection level, energy response, fade characteristics, physical form, etc. With LiF one has a fairly flat energy response and acceptable fading over a few months, but most vendors state that the minimum reportable dose is normally about 10 mrem. There is something to consider when calculating how long the TLD needs to be deployed. For example, if one has a background of 10 µrem/hr dose rate over three months (2,160 hours), you would have about 22 mrem over the ratio control that got zero dose. With LiF there may not be a great deal of signal to noise at typical backgrounds of 5 to 10 µrem/hr. There are many good textbooks and Health Physics Journal articles that discuss environmental TLD measurements. A literature review would be recommended and, in particular, look for the DOE intercomparison studies with environmental TLDs. I believe there is an ANSI Standard on testing environmental TLDs being developed too for a possible NVLAP accreditation of TLD suppliers. Also, talk to the Health Physics Society Affiliates that manufacture the material. You may want to look at other materials such as AlO for low-level gamma measurements. On the issue of shielding betas, any deployed TLD materials should be in adequately sealed containers to protect the chips or power from humidity, ultraviolet/sun light, etc. If you have a contaminated space you're monitoring, and the betas are not of a concern, then yes, be sure there is a thick low Z material over the TLD to prevent thermoluminescent signal accumulation. Shielded controls should be kept and used for "blank" subtraction at end period readout. Also, "travel" controls may be needed if you're moving the TLDs through areas that may cause a signal enroute before or after deployment. Lastly, you should look hard at proper gamma calibrations and individual corrections on the TLDs used and consider placing several TLDs together and averaging results to better control and understand variance in readout signal. David J. Allard, CHP