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How to use TLD badges to measure radiation levels.

Now if we assume that the issue of how strong a radiation field has to be to be a menace to society can be solved then we have some more problems with the general public making and publishing measurements.

  1. If the general public is making measurements, then what methods are they using? Also how trustworthy are their instruments. Does Joe Public’s meter have a spurious response which either exaggerates or under reports something? How do we know if Joe Public’s machine is in working order? How do we know if the right tool for the job has been selected?
  2. How truthful is the analyst, one problem is if the analyst favours one answer then the analyst may reject “wrong answers”. If I am weighing myself and I want to show that I am under 80 kilos then if I weigh myself and get the answer 81 kilos, then if I repeat the measurement until random errors give me 79.9 kilos and record this (and this alone). Then I would be failing to tell the “truth, the whole truth and nothing but the truth”. It is OK for me to repeat a measurement which I do not like or trust, but I need to record all the readings. Then I need to give the average and the standard deviation.
  3. When the data is poorly documented (poor chain of evidence from the original measurement to what arrives in the newspaper) how do we know that the analyst is not faking data to suit a political end? While I have no evidence that in Japan such black lies have been told, I am aware of some cases where people have fabricated evidence to back up their pet theory (or the theory which keeps them on the gravy train).

These three issues combined with the technical difficulty of using some tools such as the ordinary Geiger counter make me nervous about data produced by a poorly defined group of citizens. I think that there is a way out of the problem. Get a bag of nails, a cheap hammer (from any hardware shop) and a stack of TLD badges which are normally used to check for radiation exposure in the workplace.

Then put the badges in a metal box and go around fixing them with nails to phone poles, fence posts and other wooden objects. Record the date each badge was put somewhere, and the date it was collected again.

The badges need then to be sent away to the health physics lab where the badge reader is, without any markings which give a clue to where a badge had been. The idea is that the person reading the badges should not know where they were exposed; all he/she should know is that the badge was used to check the radiation level in a place rather than on a person. The idea of doing this blind measurement is that it makes it impossible for the person with the badge reader to distort the results.

If possible include some duplicate badges to get an idea of the random errors, if it is possible include a few badges which have been exposed in locations where the utility company or the state have made measurements. This will allow the two different measuring methods to be compared with each other. This will deal with the possibility of a systematic error in the badge measurements.

The badge method solves problem two as the humble looking TLD badge is well understood and tends to be a very truthful recorder of gamma radiation levels.

Problem three should be largely solved as the person doing the measurements will have a limited and defined number of samples which can be taken per TLD badge. The one I wear at work has three capsules of solid which have a thicker window to measure the radiation which could get to my innards and one very thin windowed capsule for low energy gamma photons which mainly damage the top layers of the body.

It should not be possible for the analyst to keep on making measurements until the right answer is obtained, also it is hoped that a professional analyst would record all the data obtained from the badge. Also from what I know I think that the people who read the badges are trustworthy. I know of one case where a radiochemical worker’s film badge fell off on Friday and landed on top of a safe being used to store all the gamma emitting stock solutions, the worker on Monday could not find their film badge. The worker then finds it in the lab sticks it back on their lab coat and continues work (Without thinking of what has happened). At the end of the month the dosimeter service thought that the worker had a larger than normal radiation dose (in the 20 mSv range). In a second case I know of a film badge which was left in near a radioactive materials store in a totalitarian state, when the badge was examined the health protection people wrote an angry / shocked letter to the boss of the lab asking “what on earth is going on !? How has someone managed to get a 24 mSv dose”? Please do not bother asking me what country or lab the latter tale comes from, I will refuse to answer this question.

But the take home message is that if you can trust the film badge service in some totalitarian state then in a typical democracy the TLD badge service should be trustworthy. A good place to find a TLD service would be a large hospital, either they will do it themselves or they will know who to contact. Another good place to find out where the TLD badge service is the local dentist and any veterinary surgery which uses X-ray equipment. Industrial radiographers and another place would be any nuclear site

If the people fixing the badges to objects are truthful and take effort to document properly what they do then together with the original records from dosimeter reading service then many of the issues about the chain of evidence will go away. What is needed is some means of making sure that it can be proven how long the TLD badges stayed outside. I would advise any group using the TLD badge method to put the badges out as soon as possible after getting them and to hand them in for examination as soon as they collect them. This will reduce the possibility that someone will claim that the badges were left out for too long to increase the dose

Also document the placing of the badges using a photograph which provides some form of time/date stamping. If possible use a tamper evident fixing to make sure that the person who removes a badge is the first person to handle the badge after it is put out. This will prevent people moving badges into higher or lower dose rate areas to get the answers which they like to get. One method would be to put the badge in a clear plastic tube which has a tamper evident seal on both ends, then the tube could be fixed to the fence post with a wire which has a seal like the one used on an electricity meter. The idea being that each person placing badges will have a different (and hard to obtain) tool which puts a distinct mark on a lead seal. The tubes would then be taken (unopened) back to a central facility where they would be opened.

None of these methods will protect against a person who stacks bricks around the badge to get a lower dose, so my advice is to put the badge above the ground at torso height in a place where it would be difficult to put shielding around it. For example do not put the badge on a wall where someone could put a brick on top of it. Part way up a fence post is better.

Lastly you should get permission from whoever is in charge of a place before you start to leave TLD badges anywhere, also I would be inclined to write on the outside of the outer container what it is and who to contact about it.

I will say that gamma rays and high energy beta are easy to measure, but alpha contamination is much harder to detect. But the good news is that the majority of the radioactivity released by the Fukushima accident is beta/gamma emitters and not alpha emitters.

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One Response

  1. wow, awesome blog post. Really Cool.

    radiation monitoring

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