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Sources of ionising radiation - Physics narrative
Physics Narrative for 14-16
A Physics Narrative presents a storyline, showing a coherent path through a topic. The storyline developed here provides a series of coherent and rigorous explanations, while also providing insights into the teaching and learning challenges. It is aimed at teachers but at a level that could be used with students.
It is constructed from various kinds of nuggets: an introduction to the topic; sequenced expositions (comprehensive descriptions and explanations of an idea within this topic); and, sometimes optional extensions (those providing more information, and those taking you more deeply into the subject).
The ideas outlined within this subtopic include:
- Energetic radiations: gamma, beta, alpha
- Beams and power
- Models of atom and nucleus
- Energetic rearrangement
- Atomic electrons insufficient energy
- Transformations and nuclear changes
- Charged particle emitted
- No change in charges
- Fission and fusion
- Constituents and their arrangements in atoms: proton, neutron, electron, alpha
- Evidence for arrangements from scattering
Nuclear sources are required: electromagnetic stores outside the nucleus are insufficient
Both photons and particles can shift enough energy to atoms or molecules to ionise these targets. Although the mechanisms are different, the end result is the same and a significant quantity of energy must be shifted to a new store as the ionising radiation interacts with the atom in the absorber. To shift that much energy, the ionising radiation must come from a very energetic source.
That much is not new – it's a review of the radiations and how the irradiated absorbers are damaged, if enough energy is supplied. In this episode we'll be focusing on the sources of these ionising radiations. These sources turn out to be mostly nuclear, because of the quantities of energy that must be made available on emission.
Such sources, being nuclei, are embedded in atoms. The ionising radiation travels from the source atom and irradiates a different material. That process is the focus of this topic.
There is a completely different process where some of the atoms containing the unstable nuclei are themselves moved so that they are attached to a material. This is contamination, not irradiation.
The focus here will be on the nuclear origins of terrestrial ionising radiations.
Non-nuclear sources of ionising radiation
The concern here is with naturally occurring energetic photons and particles, originating in the nucleus. Understanding the man-made processes that generate the high-energy ionising radiations artificially requires more involved physics: there are also high-energy ionising radiations originating in the cosmos (cosmic rays), but the origins of many of these are speculated about, rather than known about.
Why are the sources of ionising radiations mostly nuclear?
High-energy photons can be produced in a number of other ways – such as medical X-rays. So far as the absorber is concerned, there may be no difference between inbound photons, depending on their source (these are characterised only by their frequency – photons carry no history about their origins with them).
Similarly there are now particle accelerators that can increase the energy in the kinetic store of charged particles to the same level as that from naturally occurring particulate ionising radiations. Again the artificial and the natural produce indistinguishable effects on absorbers and therefore detectors cannot be built that distinguish between them.
Photons - remote workers
A recap on photons: radiating shifts energy from the source to the absorber
Photons have a frequency – they are a radiation, one way of describing the essential process of
do like me, later that connects a source with a detector. This frequency is a vibration in an electric field and a linked vibration in a magnetic field, as photons are an electromagnetic radiation.
Photons also shift a quanta of energy on emission and on absorption: exactly how much depends on the frequency.
Photons can be detected by destroying them; but not in any other way. So any knowledge of what they get up to between emission and absorption is, in principle, limited. So we suggest extreme caution in visualising the photon's journey from the source to the detector.
Another process involving electric fields that needs great care is thinking about the action of an electric circuit. Just like photons, circuit loops seem to rely on the mediating action of fields. Energy is shifted by the remote action of the electric field acting on the charged particles only when they are in the filament. This is perhaps even more obvious in the case of alternating currents, as introduced in the SPT: Electricity and energy topic. Changes in one place (the battery) in the circuit affect what happens in another (the filament). There is even a slight delay (remember the message from the big circuit in the SPT: Electric circuits topic?). So there are very significant similarities between the action of a simple alternating current electric circuit and the action of an electromagnetic vibration as both shift energy from store to store.
do like me, but later action links what happens in the battery with what happens in the filament.
do like me, but later action links what happens in the source with what happens in the detector.
Perhaps you are even thinking that a very high-frequency rope loop could be used to model electromagnetic radiation by greatly reducing the number of linked electric and magnetic loops between source and detector. This is a thought worth exploring (look back at SPT: Electricity and energy topic, episode 03 to get started).
So a photon is a discrete remote worker, shifting energy from one store to another to another, quantum by quantum. Just as with electric circuits, the fruitful approach is to concentrate on what happens at the source (battery) and the detector (filament).
Photons from the nucleus and elsewhere
Physics Narrative for 14-16
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.
High-speed particles from the nucleus and elsewhere
Physics Narrative for 14-16
Sources of high energy particles – not only the nucleus
Particles that ionise must also shift significant quantities of energy, and it is this that (again) suggests that they are nuclear in origin. Shuffling electrons around in atoms or molecules simply does not produce enough changes in the electromagnetic store (only attojoules (about 2 × 10-18 joule) for completely moving an electron from the atom).
A typical alpha particle has an energy of about a picojoule (about 1 × 10-12 joule). A typical beta particle has an energy of about a tenth of a picojoule (about 1 × 10-13 joule).
The energy available from the electromagnetic store of the atomic electrons is about a million times too small. As with ionising photons, the rate of emission cannot be altered by changing the environmental factors. This is another clue that the origins are nuclear and not atomic, for the same reasons: that temperature and pressure affect only the atomic electrons and not the nuclei.
There are ways of producing particles that are identical to those emitted from nuclei. Here, identical means having an effect on absorbers that is indistinguishable from the effect produced by ionising particles of nuclear origin. That these effects are identical makes it impossible to build detectors that do distinguish between the particles on the basis of their sources. These artificially accelerated particles (surprisingly) come from particle accelerators. A simple particle accelerator uses an electric force to accelerate the charged particle: place a concentration of charge of opposite sign to the particle and there is an attractive force. To produce a larger acceleration, produce a larger electric force, by setting up a higher concentration of charge. There are subtleties beyond the simple physics, and the engineering of particle accelerators is extremely complex. Physics 16–19 refines these simple physics ideas, calculating the forces on the charged particles and so their ejected energy.
Activity changes over time
Exponential changes – fixed chance leads to reducing activity
There is a very precise and important result that follows from the inability to alter the chance that an ionising radiation is emitted from a nucleus. Exponential decay over time is a consequence of the constant fractional decay of the nuclei. This is often modelled by throwing multi-sided dice. The lower the chance of decay in any one period, the greater the number of sides the dice should have. This change in chance for a decay, and the number of nuclei you start with, both rescale the graph but do not alter its shape. The shape of the curve is controlled by very exact mathematics – the rate of change at any time (the activity of the source) depends on the number present at that time. The change in the number of nuclei depends only on the number of nuclei present and on the kind of nuclei. Each kind of nucleus has a fixed and immutable chance of decaying in each interval – there is absolutely nothing we can do to change it. It was this that led Einstein to the famous statement:
God does not play dice. Here it appears that he was wrong.
There are significant consequences. If we make something radioactive then there is no way to reduce the activity. If there is a nuclear accident, the only clean-up remedy is to seal off the unstable nuclei and wait.
Emission can change elements
One of the most fundamental principles in the universe is the conservation of charge. You may have already met it, albeit implicitly, in the SPT: Electric circuits topic. The charge does not get lost as the charged particles drift round the loop. Look at any section of loop over time and the number of charged particles entering will be the same as that leaving: you cannot manufacture charge. At the nuclear level the same rule applies. So, when a nucleus emits a charged particle, we ought to expect some corresponding change in the nucleus.
If a positively charged particle is emitted from the nucleus, then the charge on the nucleus is reduced: if a negatively charged particle is emitted, then the charge on the nucleus is augmented. The charge (of the nucleus and emitted ionising particle) before the emission is the same as the charge (of the nucleus and emitted ionising particle) after the emission. As the remaining atom will, after a short interval, be neutral again, you might also expect the number of atomic electrons to change as a result of the nuclear change. So the chemistry will change, as this depends, in turn, on the number of atomic electrons. The atom is now a new element, with new behaviours and a new structure.
Emission of gamma radiation does not result in transmutation, as there is no change in the charge of the nucleus, so there is no need for a gaining or shedding of one or more atomic electrons to achieve a neutral atom.
Stability depends on the nuclear store, which depends on the arrangement of neutrons and protons
The number of protons in the nucleus sets the element: the number of neutrons affects the stability. This is not too surprising: if you tried to cram lots of positively charged particles together, without any nuclear glue, you'd expect the resulting electrical forces to make the nucleus rather unstable.
Moves towards a more stable, lower energy system result in a number of different emissions, depending on the nucleus. There are huge numbers of possible nuclei, constructed from selected numbers of neutrons and protons, and only a few are truly stable. Some are very unstable, perhaps even decaying so rapidly that you'd have a real difficulty in convincing yourself that you'd actually managed to make anything. The more unstable, the greater the fraction that decays in each interval of time (picoseconds to teraseconds), and so the shorter the half-life. A number of the more common emissions are given here, but you can see from the graphic that there are many, many more. In each case there is movement towards a more stable, lower-energy state, where the nucleons are more tightly glued together afterwards.
How well glued together the nucleons are can be measured by the energy that you'd need to provide to release each nucleon: this is the binding energy per nucleon. Energy is shifted from the nuclear store as more stable nuclei are formed: there is less energy in the nuclear store after the change and this energy turns up elsewhere.
Fission and fusion
Physics Narrative for 14-16
Two possibilities for nuclei rearrangements
There are two cataclysmic possibilities for rearrangements of nuclei that result in a movement towards a lower-energy, more stable state: a large nucleus splits, or two small nuclei combine. As there is movement towards a lower-energy state, the binding energy per nucleon increases: the resultant nuclei that are the outputs are further down the energy hill than the nuclei you started with.
The most stable nucleus of the lot is iron: so nuclei with fewer protons than iron can fuse, or join, and end up in a more stable state. Nuclei with more protons than iron can fission, or split, to attain a more stable state.
In each of these cases there is a significant drop in the binding energy. Energy from the nuclear store is shifted to the kinetic stores of the outputs from the fission or fusion. The energy in the nuclear store is now lower, that's why we suggest a drop. As the binding energy is now lower, so the nucleus is more tightly bound. You'd have to provide more energy to pull the nucleus apart than before the energy was shifted to the kinetic stores.
There is a large quantity of energy shifted from the nuclear store per fission or fusion, compared with much smaller changes in the chemical store per reaction. So these nuclear changes enable a small quantity of matter to be a significant energy resource: for good or ill; for power stations or bombs.
Ionising radiations, unstable nuclei and rates of decay
Ionising radiations are emitted by unstable nuclei called
radioactive nuclei (for historical reasons).
Contamination (atoms containing unstable nuclei moving to a material) and irradiation (ionising radiations travel from the unstable nuclei to a material) are fundamentally different. Contamination may lead to irradiation as the unstable nuclei can still emit ionising radiations in their new locations: irradiation will not lead to contamination.
Photons and charged particles can both be emitted by unstable nuclei, which depends on the nucleus. The energy of these emissions also depends on the emitting nucleus.
Ionising particles deplete their kinetic store of energy as they ionise. Photons may ionise as they are absorbed.
As the number of unstable nuclei decrease by becoming stable, so the activity of that source will decrease.