Particles that ionise
Physics Narrative for 14-16
Fast-moving charged particles ionise effectively
Fast-moving charged particles can rather easily shift energy to atoms and so strip electrons – that is, ionise the atoms. There is a force between the charged particle and the charged particles in the atoms that is large enough to shift lots of energy to the store associated with the atom. The larger this force, the more energy that can be shifted during the short time for which the fast-moving particle is close to the atom. The size of the force depends both on the charged particles (on the passing particle and the charged particles in the atoms) and on the distance between them (see the SPT: Forces topic). Since atoms have their positive charge concentrated in the centre shielded by the surrounding electrons, and it's the electrons that are to be stripped from the atom, so the charge at one end of the interaction are always the same: the charge on an electron. At the other end the charge will depend on the ionising particle.
Remembering that the larger the charge, the larger the force, you can see that particles with larger charges are likely to be more ionising. If they also have a larger mass, then they will be deflected less by the force of the interaction, and so remain closer to the atom for longer. So it is that high mass, highly charged particles do a lot of damage. But there is an upside: each joule of damage done is a joule shifted from the kinetic store of the particle, so slowing it down. Such particles therefore tend to be short range, as the energy in their kinetic store is dissipated rather rapidly.
Lower-mass, less highly charged particles may do less damage per millimetre of absorber traversed, but they do travel further, and so may be more dangerous for biological tissue, as they do manage to travel from the source through the air, and get as far as the tissue, rather than only ionising the air and puttering to a halt before they reach it.
So what are the more common particles we actually find?
History plays a part here, as the first two letters of the greek alphabet were used to name the first two ionising charged particles that were detected: alpha and beta. Later this pair were shown to be nuclear in origin (more on this in episode 05), and so described by reference to what could be found in nuclei. It turns out that the alpha particle has the same charge and mass as a helium nucleus, and the beta particle the same charge and mass as an electron. Putting this together with the discussion about the factors that affect ionising, you'd expect the alpha particle to be the short-range, highly ionising radiation, and the beta particle to be the longer-range radiation, and, as a consequence, ionising rather less for each millimetre traversed. The energy in the kinetic store as the radiation is emitted sets the range: the discussion so far alters the rate at which that store is depleted. Here again there is a difference between the two kinds of radiation: beta particles are emitted with a range of energies from a single source, typically varying from some maximum down to zero. Across different sources the maximum energy varies from 3 attojoule–3 picojoule (300 aJ–3 picojoule). Alpha particles, by contrast, are all of the same energy from a given source (
mono-energetic), and this energy varies from 0.4–1.0 picojoule. Alpha particles are much more damaging than beta because:
- They shift more energy to the target per millimetre (more charge and more mass).
- There is more energy in their kinetic store to shift.
The third kind of ionising radiation to be discovered was named gamma, after the third letter of the greek alphabet, and it turned out to have no mass and no charge. In fact, it's the photon, but this time a very-high-frequency example, and again emitted from the nucleus (again, more on this in episode 05).
Detecting alpha, beta and gamma radiations
These are the three
traditional types of radiation, and it is easily possible to distinguish between them because they have very different charges and masses, as well as differing in their typical ionisation per millimetre of absorber, and so in their range.
The varying charge results in differing forces when they are placed in an electric field. These forces produce varying accelerations, depending on the mass of the particle. The accumulated velocities from these accelerations lead to varying paths, which can be detected.
The movement of the positive or negative charges results in a current – a flow of charges. These currents are anti-parallel for the alpha and beta particles and so result in forces in opposite directions when the particles pass through a magnetic field. Again, the differing masses affect the resulting accelerations, and the curvature of the tracks enables the experimenters to distinguish between the particles.
The same techniques are used in distinguishing between the many other ionising particles of nuclear origins that have been detected and identified in the century following the first discovery of these particle-like ionising radiations: protons, neutrons, positrons, and many others. The detectors built into the LHC use these same techniques to work out what is produced in the very energetic collisions between the two beams of particles.