Arriving photons change the absorber

Arriving photons fill a store, drip by drip

When photons are absorbed there is always some change in the absorber. Absorption is the end for the photon: it is destroyed and the energy it was shifting is made available to the absorber. As a result of this new supply of energy, new processes are made possible in the absorber. Exactly what these are depends on the absorber. In episode 02 you saw how the interactions of photons and matter depended on both the energy (and so the frequency) of the incident photon, and on the material of the absorber. Two kinds of interactions were introduced: threshold and lock-and-key. Here we'll concentrate on developing the model of the atom a little, so that the you can develop an energy-based account of what happens during absorption. We'll start with the lock-and-key account.

It's a fact that only certain frequencies are absorbed by particular materials. So only certain quantities of energy are absorbed: larger or smaller amounts, corresponding to higher or lower-frequency photons, are rejected. So whatever model we build of the atom, as the constituent of the absorbing material, will be quantum in nature, as the photon is quantum in nature. The mechanics of atoms, developed in the early 20th century, was quantum mechanics. This is rather complicated and involves some behaviour that is very different from the world at the scale of humans (a few metres and a few seconds). This should not be so surprising as the scale of atoms is a thousand million times smaller (a few nanometres). Here we'll aim for a suggestive, rather than a comprehensive, model but with no deliberate dead-ends embedded in the model.

You should also be pretty confident with the idea, from earlier work on ionisation, that the atoms contain both positive and negative charged particles. That might give the beginnings of a model with which to think about the energy: if these charged particles have their separation altered then the energy in the electromagnetic store is changed. This can work both ways: if the separation between the oppositely charged particles is reduced by a discrete amount then the energy in the electromagnetic store is depleted by a corresponding discrete amount, and this could correspond neatly with the discrete energy shifted by an emitted photon. Conversely, the separation between the oppositely charged particles could be increased by a discrete amount then the energy in the electromagnetic store is augmented by a corresponding discrete amount, and this could correspond neatly with the discrete energy shifted by an absorbed photon.

This is neat and leaves only the small challenge of a plausible mechanism to explain why only certain separations of the charged particles seem to be permitted. This certainly is a topic for further study, involving wave mechanics, so much of the work in this topic will be useful to make a start in that area, but the mathematics is quite hard, so we don't make any progress along that route here. It took very smart people some time to develop a mechanism, and even then, almost a century on, there is still debate about how we should interpret that mechanism, as it has such apparently counter-intuitive implications.

A simple model describes each element as an energy map: a one-dimensional chart of the energy that can be stored (the energy levels). Each kind of atom is distinct (it's a separate element), and so has its own distinctive map. Indeed, these maps are like fingerprints, serving to identify the atoms. The identification takes place remotely: simply look at the discrete frequencies (and so colours) that are emitted or absorbed by the atom. This quiver of photons will uniquely identify the atom, wherever it is in the universe.

A photon absorbed: energy stored in the atom.

A photon emitted: energy released from the store in the atom.

Up and down the ladder of energy

The components of an atom are not static, and what has been learnt about atoms over the last 100 years suggests that simple static pictures are simply misleading.

So picturing the negatively charged particles in atoms, electrons, as small hard spheres with a minus in the middle somewhere is not helpful. Adding an orbital motion about the positively charged particles concentrated in the nucleus does not make the picture any more helpful.

The energy ladder picture is both economical and precise. Adding the idea that this energy is stored by separating the positively and negatively charged particles, so the electrons move further away from the nucleus, is generally true and adds an ability to visualise a physical arrangement that might underpin the energy description, but at the risk of being correct in only some cases. Quantum mechanics, the most tested theory of the atom that we have, treats the electron as a delocalised entity, so any simple picturing will not do. So we suggest that you avoid diagrams that seek to be a more literal picture of the situation. It turns out that the resources at our disposal for making images of the world garnered from illustrating and otherwise representing scenes at scales of millimetres to kilometres just do not work at the scale of nanometres: the world down there is very different.

But arriving photons do change the absorber, firstly by altering the energy stored in the atom by rearranging the outlying constituents of the atom – the electrons. This depends on the lock-and-key mechanism by which photons shift energy to or from the atom's store. The second way in which they can change the absorber is through the threshold effect, where the energy provided is enough to lift the electron right off the top of the energy ladder and so ionise the atom.

To lift the electron right out of the atoms requires more energy: the photons must be more energetic to ionise the absorber. The only difference between ionising and non-ionising photons is frequency. Higher-frequency photons shift more energy on interaction, and so are able to strip electrons from atoms, leaving ions behind.

Activity and absorption: fixed chance of absorption for each thickness

Photons passing through a thickness of absorber have a fixed chance of being absorbed. This reduces the activity of the beam, but also brings changes in the absorber. These changes can be exploited to detect the photons in the beam: you just tune the material or structure of the absorber to get a particular change triggered by the absorption, then look for that change.

However, as the pattern of absorption shown here makes plain, the number of changes that you get depends on the initial activity of the beam; the material of the absorber; and the thicknesses of absorber already traversed.

The pattern of absorption is also important when considering safety, as the activity of the beam is only reduced by a constant fraction as each additional slice of absorber is added. This fraction can be altered by increasing the thickness of each slice or altering the material from which the slices are cut.

Ionising: any way of increasing the energy in the store will do

It's not only photons that can ionise. What's needed to produce an ionisation is some way of augmenting the energy store of an atom so that the electron can be stripped from that atom, climbing off the top of the energy ladder. Fast-moving, small, charged massive particles are another effective way of supplying the required energy.

The fast moving and massive facets ensure that the particles have energy in their kinetic store to shift to the stores of the atoms. Why small? Well, the delivery needs to be to individual atoms, so the collision needs to be with a single atom and not with a whole surface composed of many atoms. And charged – that's again not so hard: the passing, fast-moving, particle must exert a force on the charged parts of the atom in order to rearrange them, and electric forces are large enough to do this in the short time available. Some of the energy in the kinetic store of the particle is shifted as a result of this interaction: the particle is slowed down. This is very different from the photon, where the photon is destroyed as it ionises an atom.

As the ways in which atoms are ionised are very different for particles and photons, so the distribution of energy through the material is very different. This has consequences for safety and for therapeutic applications of ionising radiation.