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Uranium glass again and how to make a radiometric measurement I

Dear Reader,

I have checked most of the uranium glass with a geiger counter, the geiger counter uses a tube which is known as a Geiger-Muller tube which is a gas filled high voltage discharge tube which uses an avalanche effect to increase the number of free electrons and ions formed in the tube after the absorption of radiation by the gas inside the tube. It is important to understand that GM tubes are not all born equal, it is possible through careful design to optimise a tube for an application. I borrowed a GM tube based device to check my uranium glass, this first device was a bit of a disappointment.

It has a tube with a thin mica end window and it has some beta sensitivity, but it is not very sensitive. It was intended as a gamma / beta detector which has a full scale reading of 1000 rem per hour. I think that such a device is the tool of choice when dealing with a high dose rate event. It would be very suitable for nuclear warfare use assuming that the fragile GM tube survives the bomb detonation, it could be very useful when dealing with a industrial radiography accident such as a lost source or a radiotherapy accident which involves a lost source. This detector has a rate meter for GM tube events which goes as low as 1 count per second.

With this high dose rate meter I could not get any reading from my uranium glasses, except for a very dark green one which gave 2 counts per second, the background in my house was 1 count per second. It is hard to work out if 2 cps is different to 1 cps as radiation from radioactive decay and cosmic rays is occurring randomly.

So I then tried a different GM tube based device, I choose one which is optimised for looking for low to moderate levels of beta emitting radioactive contamination. Note while from personal experience I know it works for carbon-14 it will never work for tritium. The device is a “Radiation Alert Inspector” made by S.E. International INC (Summertown Tennessee). This device can give the count rate or it can be set to record the number of counts over a given time.

As I was dealing with very weak sources I choose a counting time of five minutes, I measured the background in my house three times. The results for the background were 248, 224 and 263 counts. The total number of counts in these three determinations of the background count rate were 735 counts, which makes the count rate in my house to be 0.82 counts per second, the ESD on this count rate is 0.03 counts per second which is about 3.7 %. This ESD is based on the number of events observed. So based on this count number we should expect 245 events (give or take 9 events) in five minutes.

I measured a glass object which does not fluoresce when exposed to UV light, I got a total of 224 counts in five minutes. The difference between the count number for this object and the background is 21 counts, the sum of the ESDs is 24, so this difference is unlikely to be significant in a statistical sense.

I then went and measured a green glass milk jug which fluoresces nicely with UV light, this gave a count of 580 events. The difference between this count number and the background count number is a staggering 335, while the sum of the ESDs is 33, as the difference is ten times the sum of the ESDs it is very real which suggests that the milk jug does contain something which is radioactive.

The lowest count number I got in five minutes on uranium glass was 441 on a pale green thin blown glass vase, this value still suggests that it is radioactive. The difference between the background and the sample is 196 and the sum of the ESDs is 30. This is still very convincing.

Now while I have done quite a trivial experiment I would like to ask other people who are considering doing independent radiation measurements to up their game a bit. I sometimes see data shown on the internet where the people making the measurements do not explain their experimental method fully or state the number of counts which they use to estimate the dose rate or radioactivity level. For example Greenpeace have been using NaI spectrometers in Japan, they used a thing called a “Georadis RT-30” which is a nice bit of kit. The only problem is that they did not give full details of how they obtained dose rates with these machines.

While I know that a NaI spectrometer will never give as good energy resolution as a high purity germanium detector this type of NaI detector can distinguish between different radionuclides (based on the gamma photon energy), what I would like Greenpeace to report are the gamma spectra and all the details such as the counting time, details of the dead time correction. This would allow the contribution of Cs-137 to the dose rate to be separated from the gamma rays from the uranium decay chains.

I would also like spectra obtained at different distances from a known cesium-137 source at different distances. This could be used to calibrate the spectrometers. I would also like to see the spectra obtained using natural uranium, natural thorium and uranium ore samples. This would allow me to see how well the machine is able to separate the signals from the different gamma photons.

When I write gamma spectrum I always mean a table or chart of counts per channel against channel number. The Greenpeace NaI spectrometer has 1024 channels so with some luck it should be able to separate the photopeak for Cs-137 (662 keV) from all the other gamma photons or at least allow a partial cleanup of the data.

Now I will not pretend that Greenpeace are neutral regarding the question of “should the world have nuclear power ?”. I know that they are opposed to nuclear power, the fact that they are opposed to it does not either disqualify them from commenting on nuclear issues or make them more trustworthy. I hold the view that if Greenpeace put in extra effort into their radiometric measurements then in the long run it will be good for them and the rest of society.

Firstly it would make their results more trustworthy, people would be more willing to accept their results as true.

Secondly it would avoid problems such as “I will not trust it until I have checked to see if another explanation exists for their observation”. For example if an antinuclear activist claimed that hot spot exists in Aberdeen (Scotland) as a result of a discharge from a Scottish nuclear reactor, then I would want to know that they had not been fooled by the high gamma background due to the rocks in Aberdeen (Granite). One way of proving to me that a high gamma level was not due to the granite is to show me the gamma spectrum.

In recent times the higher background radiation levels on some beaches in the western part of the USA have provoked great excitement. The radiation has been blamed by some on the Fukushima accident, however a close examination of the site indicates that the radiation is coming from daughters of uranium / radium rather than cesium 134 or cesium 137. As the Fukushima event released mainly cesium and iodine this radionuclide signature is not reasonable for the beach.

A person or group which has a track record of making hasty statements will carry much less weight than a group which takes its time and makes sure that its statements are correct.

The problem with making a statement which is quickly shown to be false is that the person or group which made the statement will lose credibility, so by taking additional care to improve the quality of the work which is behind a statement then in the long run you will be more persuasive. I will get onto another point about radiometric measurements soon.

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