Ionising Radiation
Quantum and Nuclear

Photons from the nucleus and elsewhere

Physics Narrative for 14-16 Supporting Physics Teaching

Sources of high energy photons – not only nuclei

Photons shift energy chunk by chunk on emission. Whatever the emitter, the photons deplete a store little by little, with the precise quantity depending on the frequency of the photon. Their emission is the exact inverse of their absorption as they are both energy dependent.

The absorption depends on the match in energy between the process in the absorber and the energy provided by the photon. Emission is the same accounting process, but run backwards. Here the energy of the emitted photon, and so its frequency, depends on the energy made available by some rearrangement – the precise process depends on the details of the mechanism. But, of course, the particular value of an approach based on energy is that you need not think about the mechanisms.

But if you want to explore mechanisms, there are a couple of possibilities.

Rearrangements of the nucleus shuffle the components of the nucleus around. These components are called nucleons, and a simple model of the nucleus suggests that there are two kinds: protons (positively charged) and neutrons (not charged). There are very good (quantum mechanical) reasons not to expect to find any electrons in the nucleus.

As these photons do not carry away any charge, it is only a rearrangement rather than a more significant change. The forces between these nucleons are very large and therefore any small average change in the distance between the nucleons will result in large changes in energy. This is the origin of 'gamma' radiation: the high energy photons emitted from the nucleus. We write average – just to be cautious – as it turns out that the nucleus is not best thought of as a collection of static protons and neutrons glued together, but rather a place where there is continuous activity.

However, the nuclei do pretty much what they please – you cannot easily engineer any circumstances that will initiate the re-arrangement, and so produce photons on demand. Varying the temperature or pressure affects the atoms, and alters the rates of chemical reactions, as these depend on the electrons. The nuclei are both sheltered by these atomic electrons and bound by much larger internal forces, so they are not susceptible to having changes wrought by inter-atomic collisions. And that's all that altering the pressure and temperature have to offer.

If you do want an on-demand source of high-energy photons, you'll need to look outside the nucleus, and to other mechanisms. Remember that to emit a high-energy photon you need to provide a highly localised, carefully tuned change in an energy store over a very short timescale. One simple way is to smash very fast electrons into a metal target: this is how an X-ray machine works. The kinetic store of energy of tiny things is emptied very quickly, as the electrons are brought to a sudden halt. You can select the average energy of the emitted photons by fixing the energy in the kinetic store of each electron. Do this by accelerating the electrons with a known electric force for a known distance.

Ionising Radiation
is used in analyses relating to Radioactive dating
can be analysed using the quantity Half-Life Decay Constant Activity
features in Medical Physics
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