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Gamma spectroscopy under different conditions

Dear Reader,

While working on the project for SSM I had to make some measurements by means of gamma spectroscopy using a high purity germanium detector, note that it is not a high purity geranium detector. Being a main group chemist I do occasionally allow myself a joke. One such joke among main group chemists regards the fact that germanium sounds a little like a flowering plant which is popular in the UK.

What happens in a high purity germanium detector is that gamma photons enter a crystal of germanium which has a high voltage (2 to 3 kV) applied to it. When the photon deposits energy in the crystal this produces charge carriers (free electrons and holes). As a result of the event charge flows through the crystal, the electronics measures the amount of charge, and from that it is possible to work out the amount of energy which was deposited in the crystal during the event.

This sounds all very simple and easy but in real life it is not easy, for example some energy can enter the crystal in the form of the photon and some of this energy can then escape from the crystal as a result of Compton scattering or pair production.

Also the photons from the radioactive decay event must be able to enter the detector, also they have a finite probability of being able to interact with the crystal. The issue of penetration and overpenetration add a new layer of complexity, it causes the sensitivity of the detector to change greatly as a function of the energy of the photons.

Also the radioactivity can generate secondary radiations, for example beta particles which strike high Z (atomic number) materials can generate characteristic X-rays and braking radiation (Bremsstrahlung). During my efforts to make some measurements I created what I think is a good set of spectra which show off some of these effects.

My first problem was that I wanted to verify that I had iodine-131, I choose to put some methyl iodide which contained a lot of radioactivity into the detector. I knew that putting the vial into the detector would overload the detector, so I used a trick to cut down the gamma flux. I put the sample inside a lead pot. Or lead pig, this greatly reduces the photon flux near the vial. I knew that the activity calibration already made for that detector with a different geometry would not work, but all I wanted to do was to verify the identity of the radionuclide.

In some ways I got a better answer than the one which I would have had if I had used an unsheilded vial with less activity in it, I was more able to observe the higher energy gamma photons from the radionuclide. This was because the lower energy photons from the radioactivity were strongly attenuated by the lead pot.Here are the gamma spectra for iodine-131 recorded under different conditions.

Firstly here is the gamma spectrum for the radioactive iodine in a LSC vial.

i131lsclinear

i131lsclog

I have shown both a view of the graph with a log and a linear scale on the y axis. Now here are the graphs for the radioactive iodine inside a lead pot.

i131piglinear

i131piglog

What you should be able to see is that the high energy end of the spectrum is now more important, the moderate energy gamma line for I-131 at about 300 keV is much weaker, as a result the higher energy gamma lines are now more important in the spectrum.

It is also interesting that when we look at the low energy end of the spectrum that the lead pot changes things greatly.

i131lowenergy

What we have here are some x-ray peaks and a low energy gamma line. It is important to understand that the beta decay of the iodine forms a xenon atom in an excited state. The capture of an electron by the xenon atom can result in the generation of photons. In this case it is k line photons. The radioactive iodine also emits gamma rays with about 80 keV energy. Please keep in mind that the energy calibration of the spectrometer is a bit off, it tends to over report the energy of the events.

The lead pot was responsible for the generation of some lead K x-rays. What I suspect happened was that the gamma rays from the radioactive iodine with 300 or more keV interacted with the pot and generated plenty of fast moving electrons. This would have been by both the photoelectron and the Compton effects. Those electrons which were generated close to the surface of the lead pot were able to excite lead atoms, these lead atoms then emitted x-rays, here you can see the K alpha and K beta lead x-ray photons. The lead pot was able to screen out the 80 keV photons from the radioactive iodine so the X-ray generation must have occurred on or near the outer surface of the lead pot. One interesting experiment would be to put a pure high energy beta emitter such as strontium-90 inside a range of lead pots and then count these with the HPGe gamma spectrometer.

If I was to have covered the outside of the lead pot with a layer of copper then these lead x-rays would have been attenuated. One option for getting the best shielding out of a lead (or DU) object is to cover the object with copper, then aluminium and then finally plastic. The idea is that the secondary radiations from the lead (fast electrons and X-rays) will be captured in the copper. The copper being a lighter element is less able to generate x-rays from the electrons and it will also offer some attenuation of the lead x-rays. The lower energy x-rays generated in the copper will then be attenuated by the aluminium. Finally the very low energy x-rays generated in the aluminium will be captured in the plastic.

It is interesting that the lead shielding for the gamma spectrometers are lined with copper sheet to try to reduce the formation of secondary radiations by the action of cosmic rays (and other background radiation) and the radiation from the sample in the spectrometer on the lead shielding. It is also interesting to note that photographic film is more sensitive to high energy gamma rays if it is placed in contact with a thin lead sheet.

During the Fukushima event some people claimed that workers at the site were issued with lead sheets to make their dosimeters less sensitive, under some conditions by wrapping a dosimeter in lead sheet it would be possible to make it over report a dose of gamma rays. A lot will depend on the energy of the gamma rays, while a thin lead sheet will make a dosimeter under-report things like x-rays from a dental radiography set (70 kV tube) it will make the dosimeter over report the dose from a high energy gamma emitter such as cobalt-60. The old fashioned film badges which we used to use back in 2000 for gamma dose measurement of people had a series of metal filters designed to allow the badge to make better measurements of gamma photons with different energies.

Note that before you put anything into a gamma spectrometer it is important to wrap it in a plastic bag. If you have ever been a responsible dog owner you will know how to pick up dog poo (or other objects) using a plastic bag in such a way that you never touch the dirty side of the plastic bag. Put your hand into the bag, pick up the object and then turn the bag inside out and tie it shut. I did this with the lead pot containing the radioactive iodine and then just to be sure I bagged it a second time. I reason that no harm can come from using a second bag, and just in case something went wrong during the first bagging the second bag added while the singly bagged object was on a clean bench would reduce the potential for the transfer of radioactive contamination yet further.

My second problem was that I wanted to get a quench curve for liquid scintillation counting with large chemical amounts of methyl iodide present, any of my readers who are organic chemists will know how methyl iodide is a toxic and very volatile liquid which is difficult to pipette out. While my one of favorite pipettes (200 microlitre) is unable to measure out 20 or 50 microlitre volumes of methyl iodide a 10 microlitre pipette can dispense 5 microlitres it as the diameter of the tip is much smaller. But for 10, 20 and 50 microlitres I used a gas tight syringe designed for gas chromatography work.

Rather than trusting my pipetteing with this volatile liquid, I choose to use the gamma spectrometer to check my volumes. By making a measurement of the number of counts per second I was able to make an independent measurement of the radioactivity in the LSC vials. I will save the results of this for another day.

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