Hello Yvan Dutil ,

By the term 'passive dosimeters', I assume you mean dosimeters where the sensor is separate from the readout device, such as TLDs, PICs, etc. Most passive dosimeters come with warnings such as don't expose TLDs to UV light, only read PICs using the charger/reader to prevent false readings due to geotropism, etc.

But one warning that is not heeded or sometimes not even mentioned is to be careful where the passive dosimeter is stored. Let me give you an example. I once worked in a high-bay facility that housed, within it, a concrete and steel enclosure that contained a high energy flash x-ray machine. The TLD reader sat on a metal desk outside of and well away from the enclosure. The unexposed TLDs were contained in a plastic bag inside the metal desk. Each day, before the TLDs were used to measure the dose in front of the flash x-ray machine, several of the unexposed TLDs were read to serve as blanks/controls. After several weeks, I happended to notice that the blank/control readings were monotonically increasing when plotted versus time. When I stored some of the same TLDs in my metal desk several miles away in my office, their readings versus time were basically flat except for a smaller amount of random noise. It turned out that the concrete and steel enclosure, housing the flash x-ray machine, did not have a roof, and that the radiation inside the enclosure was being scattered upwards and then downwards by the tens of feet of air and metal roof above the enclosure, i.e., skyshine.

In your situation, the question I would ask is where are the personal, passive dosimeters being stored when they are not being worn? They might be stored somewhere with a significant but unrecognized radiation background. I would be very surprised if the readings you are getting are true false positives.

Regards,

Tom Cuff

Dear Yvan Dutil ,

I must agree with Thomas Cuff ,

even in the case of a non-lead roof (or ceiling) of the x-ray cabinet, x-ray backscatter via such a roof is measurable in the vicinity of the x-ray cabinet even when several meters of distance are involved. Thus special care has to be taken to omit*) such unwanted radiation. Storage of the dosimeters in such areas has a) to be omitted or b) they should be stored in a special shielded box there.

So please check the radiation exposure at the place of dosimeter storage.

*) e.g. via sufficient height of the (lead) shielding of the walls of the cabinet.

best regards

G.M.

And what is about a 'bad joke' ?

A worker may intentionally put a dosimeter inside the x-ray cabinet in order to 'see' whether the 'dosimeter surveillance' is really 'working' ....

Such cases are often reported in x-ray safety courses/tutorials.

What about flight travels with such dosimeters. X-ray baggage inspection machines contribute to the dose readings; but only very a few of these events do not lead to an over-exposure....

Thomas Cuff Thank you. UV exposure makes sense. But the casing should should stop it. However, there is a possibility that IR laser can pass trough it and affect the reading.

When cases end up in our office, this is because they have pass the yearly limit (>20 mSv). Therefore, they are pretty large discrepancies.

Gerhard Martens Yes, this is certainly a possibility. Apparently, this happens time to time. Airport X-ray machine add dose, but this is a a very small dose per exposure.

In one case, we have checked the possibility that the dosimeters were exposed to thorium dust from TIG welding.

Hello Yvan Dutil ,

Thank you for your explanation. I should have explained that a UV false positive is in the case of a bare TLD. That was why the bare TLDs were stored in a metal desk so that they would not be exposed to UV from direct sunlight or UV leakage from overhead fluorescent lighting. It is hard for me to believe that the >20 mSv you are seeing is a false positive. I know of no other explanation for such high readings other than irradiation. I know it sounds crazy, but I worked on projects where people were inadvertently exposed to an x-ray source due to a failure in the interlock system. Most such x-ray systems have a SCRAM button, but this presupposes that the system operator has such presence of mind as to recognize that the system is behaving incorrectly and act on this in a split second.

The most famous case of interlock failure, I remember, was the Therac-25 linear accelerator (two modes: x-rays and electrons) made by AECL (Atomic Energy of Canada, Limited) used to treat tumors, which killed a patient and incapacitated others. Here the operator only noticed something was wrong when the patient started screaming in agony.

[1] Jonathan Jacky; Programmed for disaster; The Sciences; September/October 1989; pp. 22-27.

[2] Ivars Peterson; Fatal Defect; Times Books/Random House; 1995; pp. 27-33, 36-44, 46-48, 207.

As Gerhard Martens mentioned, you should map the x-ray radiation field around the suspected x-ray systems, and take into account where the passive dosimeters are stored. For example, a person might only work some fraction of 40 hours/week near an x-ray system, but they might store their dosimeter somewhere near the x-ray machine that is used in 2 or 3 shifts of 8 hours and weekends. For example, the passivie dosimeter might be clipped to their lab coat, which they leave in the lab at the end of their shift.

I think you need to know more about the circumstances surrounding each of these cases: 1) location of the dosimeters when not being worn, 2) radiation field around the x-ray system, 3) interlock system and any problems with it, ...

Regards,

Tom Cuff

Some partial answer I have already found. Chemical vapour exposure might create a false reading.

https://www.nrc.gov/reading-rm/doc-collections/gen-comm/info-notices/1985/in85081.html

Hello Yvan Dutil ,

Congratulations, NRC 85-81 is a very interesting find, but is it relevant to your situation? I mean, are you using: Panasonic 800 series badges with Li2B4O7 TLD material in one or more of its elements and with the Li2B4O7 material in the same state of aggregation in the presence of high levels of ambient H2S gas consistent with the worker's environment, etc.? It is still not clear to me how, if the Li2B4O7 TLD materials were sealed in the badge, the H2S gas penetrated through the sealant. Since the worker in NRC 85-81 was also exposed to high humidity, due to the nature of her or his job, and high humidity can also result in an artifact, how can we be sure that humidity did not also play a role? After all, NRC 85-81 also states that "Several agents (such as chemical contaminants, ultraviolet light or extreme humidity) are known to cause abnormal TLD responses." I know the report says the licensee conducted extensive tests with different levels of H2S, but where was this documented? There is also the question of why H2S and/or humidity did not cause a high artifact with the CaSO4 TLD material?

Regards,

Tom Cuff

Gerhard Martens One of my two cases as been solved. The worker was using is travel bag as a diffusion source when testing the X-ray security machine for leaks! Of course, his dosimeter was in the bag, not on his chest ;)

I will increase the probability of the branch "volontary exposure" in the root-cause analysis tree.

Thomas Cuff Gerhard Martens

Follow-up on the story, exhausting all mishaps possibility with the dosimeter, we asked for a biodosimetry test on the «most exposed» worker. The result came negative. So this a case on unexplained dosimeter exposure.

I am writing a short article about this cas for the CRPA-ACRP BULLETIN. If you do not mind, I would name you in the article as contributor to the discussion.

Ok Yvan Dutil ,

thanks

and best regards

G.M.

Hello Yvan Dutil ,

Thanks for the interesting question, and for your kind offer to mention my name in your article. I would be very pleased to have my name mentioned.

Regards,

Tom Cuff