Nature of ionising radiations
Teaching Guidance for 14-16
Students' models of each of the radiations will develop through this topic. They will start with an idea of a generalized invisible radiation. As they see more evidence for the nature of the radiations, their model will become more sophisticated. This will be reflected in the developing language that you use to describe the radiations:
- the radiations come from radioactive materials and cause ionisation: they are ionising radiations.
- natural radioactive materials produce three types of ionising radiation: alpha radiation, beta radiation and gamma radiation.
- alpha radiation and beta radiation are made up of streams of charged particles, alpha particles and beta particles; gamma radiation is an electromagnetic wave.
- an alpha particle is a helium ion (an atom that has lost two electrons), He2+; a beta particle is a fast moving electron, e-.
- an alpha particle is a helium nucleus (because it only has two electrons per atom); all three radiations originate in the nuclei of atoms.
Eventually, the properties and nature of alpha, beta and gamma radiations can be summarized as follows.
|property||highly ionising||fairly ionising||weakly ionising (depends on intensity)|
|short range in air (3 to 5 cm)||medium range in air (~15 cm)||long range (inverse square law)|
|stopped by paper||stopped by lead or thick aluminium||attenuated by thick lead|
|deflected slightly in magnetic field||deflected in magnetic field||Undeflected in electric and magnetic fields|
|deflected in electric field||deflected in electric field|
|nature||positive charge||negative charge||no charge|
|large mass (same as helium nucleus)||small mass|
|identity||helium nucleus||fast moving electron||high frequency electromagnetic wave|
At each stage in this developing picture, you can link the properties of the type of radiation with its nature. Alpha radiation is highly ionising because of the large momentum, though relatively modest speed (~10 7m/s) of the alpha particles and their double positive charge. But, given its propensity to interact with atoms (in the air and solids), it has a shorter range and lower penetrating power than the other two types of radiation.
Beta radiation is made up of a stream of beta particles moving extremely fast (about 98% the speed of light). They have less momentum than alpha particles and are less ionising, tending to pass through the air and matter more easily than alpha particles.
Beta particles are noticeably deflected in a magnetic field, much more so than alpha particles, whose deflection cannot easily be measured in a school laboratory. This is because the beta particles have a smaller momentum and experience a bigger force because they are moving faster (although they also have a smaller charge, their speed is more than twice as much as that of an alpha particle).
The deflection of alpha particles can be more noticeable in an electric field. Here the force depends on the charge but not on the speed.
Gamma radiation is an electromagnetic wave. This means it has no charge and is not deflected by magnetic or electric fields. It is weakly ionising and its effects on matter depend among other factors on the intensity of the radiation.