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Ghost peaks

Dear Reader,

I hold the view that for many things that “the best disinfectant is sunlight”, rather than leaving matters of public concern shrouded in secrecy we should where possible allow people to see the evidence and discuss the thing which troubles the public in a calm manner.

There are some things which in the public interest should never be made public, for example the exact location of some nuclear materials, which room in Buckingham Palace the Queen sleeps in, the combination on the safe containing controlled drugs in a hospital pharmacy and personal details of the guards in a prison should all stay confidential.

These facts have clear misuse potential, a second type of troublesome fact is the type of fact which great misunderstanding potential. For example if I was to tell you how much 40K radioactivity is in your body the number of 0s might make some people panic.

I can also tell you that I am going to encourage my dog to throw himself off a 1000,000,000 Å ledge onto the floor, before you start to worry about my dog (and call the RSPCA) I need to explain to you that 1000,000,000 Å is equal to 10 cm which is the height of a typical step. My dog tends to jump down my front door steps one at a time when it is time for his walk.

I could also give some of you a shock by telling you that I have added some 2-((2,3-dihydroxy-1,4-bis(hydroxymethyl)cyclopentyl)oxy)-6-(hydroxymethyl)tetrahydro-2H-pyran-3,4,5-triol to the mixture when I was cooking that cake that you have just eaten.

If you are worried that you are getting too much 2-((2,3-dihydroxy-1,4-bis(hydroxymethyl)cyclopentyl)oxy)-6-(hydroxymethyl)tetrahydro-2H-pyran-3,4,5-triol in your food, I suggest you might want to talk to a doctor or some other health care professional about lowering the amount of table sugar in your diet. The long sciency sounding name may have made some of you worry about what 2-((2,3-dihydroxy-1,4-bis(hydroxymethyl)cyclopentyl)oxy)-6-(hydroxymethyl)tetrahydro-2H-pyran-3,4,5-triol is. I have to confess that I do have 2-((2,3-dihydroxy-1,4-bis(hydroxymethyl)cyclopentyl)oxy)-6-(hydroxymethyl)tetrahydro-2H-pyran-3,4,5-triol in my diet.

The best way to deal with the facts which have misunderstanding potential is education, by understanding what the facts mean a person is able to distinguish between what is good for you (healthy), what is neither good for you or harmful and what is harmful to your wellbeing.

Now I am sure that you are aware that recently a dire event accident occurred at Fukushima in Japan. What happened was that as a result of a station black out a series of three nuclear reactors overheated and released radioactivity into the environment. Now I would like to remind you that the detonation of atom bombs is banned by international law.

Part of the way that this international law is enforced is the series of test stations which are designed to detect the radioactive signature of an atomic bomb. The basic idea is that if it is made very easy for the international community to detect an atomic bomb test then it will make it much harder for a rogue country to test an atomic bomb without being spotted.

The many test stations for radioactivity in the air are running non-stop watching the content of the air which we all breathe. Now it should not shock you too much to hear that this anti-bomb test network did detect the radioactivity released by the Japanese accident. If you want to read about it then I suggest you go here.

As well as applauding the French for releasing the data, I would like to point out a problem. It is possible even for “clever people” to make some horrible errors when looking at Gamma spectra.

One of the lines which was seen was at 1001 keV, now this line happens to relate to uranium-238. It is the case that every gamma emitting radioisotope have a series of gamma lines which are associated with the radionuclide. The key thing to do if you spot a line which relates to a radioisotope is to keep your wits about you.

Now I hope that you will never have to try to understand a gamma spectrum during a crisis but here are a few hints. After you have matched the line to a radioisotope then you need to look to see what other lines should be associated with the same radioisotope.

For example cobalt-60 has two lines associated with it. What happens is that cobalt-60 undergoes a beta decay to form an excited state of nickel-60 which decays by the emission of two gamma photons. These photons are at slightly different energies and it is believed that the two photons are emitted in random directions. So as the two are close in energy the detector has an equal chance of detecting. As a result for cobalt-60 you should see two lines in a gamma spectrum. If you only see one line then something odd is going on.

Now you have got past this test of seeing if the other peaks which should be present are present we will get onto the subject of ghost peaks.

When a gamma ray hits a detector it delivers its energy into the solid or liquid. Either light is made or charge carriers in a semiconductor are made. What happens next is that the electronics associated with the detector measures the amount of energy which was delivered to the detector in the event before recovering and getting ready to measure the next event.

Just after the detector has sensed the event it normally has a short time where it is unable to detect anything, this is known as the dead time. The dead time can make the radiation level appear to be lower than it truly is but we will deal with dead time another time (WARNING: dire pun !). But if two events occur very close to each other then the detector can be fooled into thinking that it was a single event with a higher energy than either of the two real events.

On slide nine of the slides in the link I gave you an example of this type of problem is shown, the spectrum gave one of the peaks for uranium-238 but in fact it was due to iodine-131.What happened was that two photons from different iodine-131 decays (to be exact the excited state of xenon-131 formed by the beta decay of the iodine-131) arrived together at the detector on a regular basis. This then caused the “pile up” which made the higher energy ghost peaks appear.

My own view is that three solutions exist to this problem.

  1. For liquid samples repeat the measurement with a less radioactive sample, what you could do is to dilute the original solution x      10 more and repeat the measurement. This reduction in the radioactivity will reduce the chance that a second photon will arrive at the detector      during the critical time where the second photon could be confused as being part of a bigger event.
  2. For a solid sample change the geometry of the counting, the way I would want to do this is to move the sample further away from the      detector. This reduction in the number of photons arriving at the detector each second would reduce the effect of the “pile up” effect more than it would reduce the real effect of the radiation on the detector.
  3. Lower the efficiency of the detector for lower energy photons by placing a sheet of aluminium between the sample and the detector. This would lower the efficiency of the detector for low energy events more than it would for high energy events. As a result you could get a new spectrum where you would hope to see that the “high energy” peak has now vanished or shrunk while the genuine high energy peaks would still remain strong.

A second problem is that some isotopes emit several photons in each decay, it is possible for this to create some other false peaks. This is an interesting problem which I was told about recently by a coworker.

If you were to place a radioactive source containing cobalt-60 inside a gamma detector which was a perfect uniform sphere which surrounds the source and has a 100 % detection efficiency. Then instead of seeing peaks at 1173 and 1332 keV we would see a single peak at 2505 keV. This is because at the moment that the 1173 keV photon is emitted by the excited state of the nickel-60 the other photon is also emitted. This is the “true coincidence summing effect”. The way to deal with this effect is to change the geometry of the experiment. If the distance between the detector and the sample is increased then the detector is less likely to be struck by both photons from the decaying nuclide.

Now another day I will comment on low energy ghost peaks, but I would say that if you are troubled deeply by ghost peaks then sometimes the best thing to do is to use a chemical separation to make the radioactive mixture less complex.


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