Answer to Question #10222 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Personnel Monitoring (PM)

The following question was answered by an expert in the appropriate field:

I am in charge of an investigation about an x-ray radiation overdose to a worker (young female). The dose informed by dosimetry laboratory was 88.27 mSv over a quarter year. She works in a medical center, radiology department and her duties are related to mammography and two x-ray rooms taking osteoarticular/pulmonary radiographs. The type of dosimetry used is a thermoluminescent dosimeter (TLD), and her historical doses usually remain far below "permitted" doses. In the notification the dosimetry labortory says that the badge was in bad condition—as if it was washed by some detergent (she denies any kind of wash, even accidentally). My question is: if we assume that the TLD was washed before reading, is it possible to have a good reading of it? What else can increase dose to such level (excluding bad intention)?
I notice that the reported dose of 88.27 mSv is for a quarter year. I interpret this to mean that the affected individual was badged on a quarterly basis. While such an interval may be justified on the basis of historical dose data for the affected individual, I would not feel comfortable badging an occupational worker whose daily responsibilties involve taking diagnostic x rays over intervals this long. I would opt for monthly badging, which is much more the norm, at least in the United States. While individuals in the position you describe do not generally receive significant doses, there are occasions when unexpected things happen and inadvertantly large doses accrue to the dosimeter and occasionally to the worker. The sooner after an event that one learns of an elevated dose report, the more likely it is that the cause will be discovered. I am aware of several cases in which personal dosimeters have been left in locations subject to radiation from x-ray machines. If a badge gets in the direct beam, badge dose can accrue rapidly. In general, the sooner after a reported exposure a follow-up investigation is conducted, the more likely it is that the circumstances surrounding the apparent exposure will be determined.

Regarding the impact on a TLD badge going through a wash process, presumably in a washing machine, the outcome is variable but usually not good. The effects depend, at least in part, on the specific badge design, especially the encapsulation of the active TL elements, the specific phosphor(s) in the active elements, and the temperatures achieved during the wash cycle. Most badges used for personal dosimetry are designed so as to offer a fair degree of protection against water from getting directly to the phosphor. They are not, however, designed to assure protection against water damage when put through a machine wash cycle or other vigorous wash process. While all of the most popular phosphors, including LiF, Li2B4O7, CaSO4, and Al2O3 (used mostly in OSL systems) are not extremely soluble in water, they do dissolve to varying extents and the presence of detergents may hasten deterioration of protective coverings around the active elements. Warm- or hot-water temperatures also increase the damaging effects. Even if the badge remains intact, elevated temperatures will enhance signal fading that may be significant, depending on the phosphor and the readout cycle normally employed. At any rate, the known effects of passing a TLD badge through a washing cycle are negative and would be expected to produce a reduced reading compared to what would have been the case had the wash not occurred.

We should note that there have been cases of elevated readings on some TLDs resulting from chemical contaminants in some situations. In particular, it has been shown that exposure to hydrogen sulfide will result in elevated readings on Li2B4O7 dosimeters (see this link to a U.S. NRC information notice). I would not expect to find H2S in the washing machine water unless possibly certain sulfur-containing additives were being used to prevent yellowing of clothes by iron in the water. To my knowledge, the effects of chemical contaminants on TLD response have not been studied in any systematic fashion, and I cannot comment as to whether any other additives that are used in or along with laundry detergents might have any effects that would yield increased readings.

Exposure to ultraviolet (UV) light is also a source of excessive readings in some TLDs, and as part of your investigation you should attempt to determine whether the badge might have been exposed to any significant UV sources.

Attempting to determine cause in cases of this sort can be quite frustrating for all parties involved. At times they can be resolved with perseverance and guided detective work and cooperation by the affected individual in attempting to reconstruct events that might have resulted in the elevated reading. I wish you well as you pursue the answers in this case.

George Chabot, PhD
Answer posted on 13 July 2012. The information posted on this web page is intended as general reference information only. Specific facts and circumstances may affect the applicability of concepts, materials, and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice. To the best of our knowledge, answers are correct at the time they are posted. Be advised that over time, requirements could change, new data could be made available, and Internet links could change, affecting the correctness of the answers. Answers are the professional opinions of the expert responding to each question; they do not necessarily represent the position of the Health Physics Society.