I have become aware that a group of citizens (Safecast) in Japan have started to make their own radiation measurements using “off the shelf Geiger counters”, from one of the pictures that one of the members of this organisation took it appears that they are using inspector alert GM tube based units. This is a device which can give some misleading measurements.
I have nothing against International Medcom, it is a general problem with GM tubes. It is possible to make dose rate measurements with a GM tube if only a single radioisotope is to be measured. The problem is that for different isotopes that a GM tube has a different efficiency. The great problem with a power reactor accident more than one radioisotope is released. This could lead to some “interesting” results, the isotope signature of the accident fallout will change with time. By now the iodines will have decayed away, but I imagine in the early days that the iodines will have been very important.
International Medcom’s own data indicates that for C-14 that the efficiency is only about 5 % while for P-32 it is about 30 %, the machine has a GM tube with a mica window. The great problem with the GM tube is that no matter what the energy of the incoming particle the electrical output of the tube will be the same. All a GM tube can do is count events.
Other tools (more expensive) such as the Babyline 81 use an ionization chamber and are known to be able to be linear over a wide range of photon energies. The great problem I see with the inspector alert unit is that I suspect that it is not energy compensated, to make a GM tube more usable over a wide range of gamma photon energy some shielding will be needed around the tube. I suspect that if a GM tube device has any sensitivity to C-14 beta then it will not have any metal shielding around the tube. If you want to read more then see this document. Also the fact that a device will respond to beta emitters may also cause it to give some misleading results under some conditions.
For example the different between the indoor and outdoor dose rate could be wrong. If you were to measure close to a Cs-137 contaminated lawn in the garden then the device would respond to both gamma and beta. If you then step into the house then assuming that nobody has walked radioactivity into the house then the detector will only be able to detect gamma. I am assuming that the beta is absorbed by the walls of the house.
My advice to anyone who wants to make their own measurements is to note down the count rate, if possible get two frying pans and a cheap wooden table. Put the detector on a wooden table to record the number of events per minute. I have to confess I did not dream up the use of the frying pan, I read this when I was a teenager in a book (with a yellow cover) which explained how to do radioactivity experiments at home. (Before you ask, I never tried out any of these experiments)
Then put a frying pan on the table and put the unit inside it, then cover it with an up turned frying pan. This will shield out almost all beta particles. Then get a new measurement under these conditions.
Also try on a regular basis check your detector against one which is more trustworthy, and or check your meter against a calibration source. If you have access to an old watch / clock with radium paint (not modern tritium paint) then you could use this as a mixed gamma / beta test source. As the half life of radium-226 is long then this check source will last a very long time.
Other check sources would include a bottle of thorium nitrate or if you know an industrial radiographer who uses Co-60 then depending on the activity of their source and the thickness of the shielding on the radiographer’s ‘camera’ you might be able to detect some gamma photons which leak through the walls of the camera. Do not try to get your local dentist to use the X-ray machine to check your detector, dental X-ray machines make photons with much lower energy than Cs-137. The main long lived gamma emitting isotope of interest in a nuclear accident is Cs-137.