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Police act in cobalt-60 robbery case in Mexico

Dear Reader,

It appears that the police in Mexico have identified six people who they believe are connected with the van robbery which caused the large cobalt-60 source to go missing. One of these people has been reported to have shown some signs of radiation exposure.

Without being privy to all the evidence it is impossible for me to make an estimate of the person’s radiation dose and thus workout what is likely to happen to the person. But assuming that they survive then I am sure that authorities will be strongly considering some criminal charges against them.

While I hold the view that the unauthorised possession of a large radioactive source should be treated with great gravity, I asked my legal advisor if the hijackers should be charged with a violation of the nuclear / radioactivity laws of Mexico or not. My advisor told me that unless the state can show that the criminals knew that they were stealing a dangerous radioactive object then they should not be charged with anything other than normal robbery. My advisor commented that to commit the radioactivity crime that they needed to be aware of what they were doing was something different to a normal robbery.

Well while we are on the subject of cobalt-60 I would like to point out something to you, now firstly it is not a pure gamma emitter (gamma emission is always linked to beta or alpha decay or some other process which forms a nucleus in an excited state). The cobalt-60 undergoes a beta decay to form an excited state of nickel-60, this excited state then normally emits 2 gamma photons to drop down into the stable ground state of the nickel-60.

These two photons are emitted at the same time. In a typical gamma semiconductor spectrometer many of the photons from the sample are lost, they fly away because they miss the detector. The spectrometer can be made more sensitive by making the distance between the detector and the sample smaller, or by making the geometry closer to a perfect sphere which surrounds the sample. In the ideal world you might think that you could have a 4π geometry where no matter which way a photon fly from the sample it will be detected.

But there is a problem, if the two photons are captured by the detector then the spectrometer will register an event with the total energy of both photons, this will create a false peak with a higher energy than either of the two real photons. What happens inside the detector crystal is that the energy of the photon is used to form free electrons and holes in the block of semiconductor. The block of semiconductor is often a giant diode which is reverse biased with about 3000 volts. When the event occurs the charge carriers (holes and electrons) allow some charge to pass through the crystal. The spectrometer measures the amount of charge which flows during each event. The more energy deposited in the crystal by an event the greater the number of charge carriers produced by the event.

A second problem is that if the gamma photon undergoes a Compton scattering event where it delivers some of its energy to an electron then it can change direction and then leave the crystal. In this way the photon can deliver only a fraction of its energy to the crystal. This effect can result in a broad peak in the lower energy part of a spectrum when a high photon energy gamma emitter is present in the sample. Thus it can be very hard to measure a low energy gamma emitter like americium-241 when a high energy gamma emitter such as cobalt-60 is present. One solution is to use anticoincidence counting. I will write about this soon.


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