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Radiation free food… more likely common sense free food !

Dear Reader,

It has come to my attention that Greenpeace have told us that a major Japanese food seller is moving towards food which is free of radioactive contamination. While at first this might seem like a sensible and noble goal, when you look at it in more detail it may well be the start of a very silly thing.

Now before I get started I want to go on the record and say that I do not want to consume food / drink with high levels of radioactivity and I also do not want my fellowman to eat food with high levels of radioactivity, but the great question is how do we set the limit for what is “too high ?”. The great problem is that if we set the limit too low then we will end up throwing out plenty of good food and then we all starve to death !

I hold the view that we must not try to get a zero limit for radioactivity in food, it is impossible to reach this limit. Even before 1900 food contained radioactivity as a result of the fact that it contains potassium-40. A diet which is free of potassium-40 will be fatal ! This is because if you get no potassium in your diet you will die ! All potassium contains some K-40 so as a result it is impossible to have non radioactive food.

Also due to the fact that scientific equipment has advanced over the years it is possible to measure many “dangerous” things at concentrations far below the level at which they can cause harm. A classic example which I like is uranium in glass, now imagine that I am sitting back drinking a cool beer poured out of a glass bottle into a glass with my feet up. Am I at risk of uranium poisoning ?

It so happens that glass contains uranium, and the uranium will slowly appear in a sample of super pure nitric acid which is left in a normal glass bottle. This observation of uranium leaching out of the glass bottle does not mean that the uranium will reach the level at which it causes a health problem.

What we need are sensible laws dictated by sound science rather than the views of extremists from either end of the political spectrum. What we need to do regarding radioactivity in food is to make a judgement based on the risk per unit of activity and the benefits of eating the food. One method is to choose a dose which is thought to be an acceptable risk to the general public, from this dose and some assumptions about the amount of food eaten by a person in a year and the dose per Bq it is then possible to calculate a limit for radioactivity in food.

I would rather stay out of the debate as to what is a reasonable dose per year to the public for the purposes of radioactivity in your diet. But I would like to point out a few things, the photo which Greenpeace publish showing someone waving a gamma detector over a fish is a very misleading picture. It might look good for the general public but if you know something about radiometric work you will understand why it is not good.

The tool which greenpeace are using is a tool which is good for hunting for small and very intense sources such as dropped radiographic or cancer treatment sources, if I was required to locate a lost gamma source then while I could do it with a simple non-energy dispersive counter such as cheap NaI counter I know that I could be able to do the job under some conditions faster if I had the gamma spectrometer which greenpeace’s person was holding. A better use of the tool would be to use it to survey the most contaminated land to get a gamma spectrum from the released radioactivity, a good gamma spectrum shows an important part of the isotope signature. From the isotope signature it can be possible to work out what has happened, I find it interesting that despite me asking Greenpeace twice they have not been willing to share with me a gamma spectrum in the form of a table of counts vs energy. This data could have allowed me at an early date to make an independent analysis of some of the important isotopes which were emitted during the accident.

I hold the view that if the following group of three spectra were to be released in the form of tables of counts vs channel number it would allow independent scientists and some of the more technically minded members of the general public to check what the isotope signature of the accident is if soil is contaminated by radioactivity released by an accident.

1. A spectrum taken with the detector well shielded, I would suggest during an accident that an aluminium sheet, covered in turn by a copper sheet which in turn is covered with a large volume of a dense (and non radioactive solid) should be used to get a background for the detector. One suggestion I would have is to surround the detector with jerry cans of water if lead bricks can not be obtained. Another suggestion would be sacks of potting compost (bagged before the accident).

2. With the shielding in place a small radioactive source which has many gamma lines should be placed close to the detector. I would suggest an old radium containing watch would do nicely. If you use such a watch then it would be a good idea to record the geometry used and the thickness of the watch glass. This is used to allow the relative sensitivity of the detector to different energies of gamma rays to be tested, also it allows the energy calibration of the detector to be checked.

3. The repeat the measurement with no shielding around the detector. This will give a spectrum of the soil contamination.

4. If the levels of low energy gamma radiation are very high, then repeat stage three with the copper and aluminium shields in place, this will give a more simple spectrum.

It is important to publish the counting time, the dead time and all the other parameters/settings used. I hope soon to tell you about some of the pitfalls which can exist.

The measurement of low levels of radioactivity in food is not a trivial matter, I hold the view that the use of only gamma detector is a bad way as it will miss Sr-89, Sr-90 and moderate levels of plutonium. What is needed is for a large amount of food to be be dried out and burnt to ash, the ash then needs to be gamma counted with a well defined geometry. This would allow things like Cs-137, Cs-134 and Zr-95 to be measured.

This is because the distance from the object to the detector and a series of other things can alter the results. I would also like to point out burning kilos of fish to make a little ash allows the detection limit to be made lower.

I would also suggest that the ash should be leached with acid to extract the non gamma emitting isotopes, to allow these to be chemically separated to allow better counting. For example to count low levels of plutonium with an alpha spectrometer it needs to be deposited onto a metal disk in a super thin layer, while to measure low levels of plutonium-239 by ICPMS often you will want to separate all the plutonium from the uranium which is in the sample. This is becuase the UH peak at 239 will mask the Pu peak at 239.


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