What is ionising radiation?

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What is ionising radiation?

Ionising radiation comes in three varieties:

α (alpha) particles
β (beta) particles
γ (gamma) rays.

All of these forms of radiation are energetic enough to pull electrons away from atoms. The atoms that have had electrons removed in this way are now charged particles, or ions, and hence the name ionising radiation.

The fact that these radiations are ionising allows them to be detected and discriminated from other forms of radiation (such as infra-red or radiowaves). Detectors such as ionisation chambers, Geiger-Muller tubes and cloud chambers all rely on the ionising properties of these radiations to produce measurable effects.

One alpha particle can ionise 10,000 atoms. However, because it puts all its energy into ionising others, it very quickly runs out of energy itself. Hence alpha particles can't penetrate through much.

The fact that they are strongly ionising makes them very dangerous to life, however. Think about it! Your body is a finely tuned machine, designed to carry out complex chemical reactions between neutral atoms. You start turning those neutral atoms into charged ions and suddenly the reactions don't work. That's how radiation disrupts the function of living things.

In order to quantify the effects of ionising radiation on tissue we define a quantity called the absorbed dose. The absorbed dose is the energy absorbed per kilogram of tissue. It is measured in units called grays (1 gray = 1 J/Kg).

Alpha Particles.

Alpha particles are strongly ionising but can be stopped by paper or skin. They have a strong positive charge (+2) and a mass of 4 (i.e. 4 times the mass of a proton)

An alpha particle is in fact the same as a helium nucleus - 2 protons and 2 neutrons.

Beta particles.

Beta particles are electrons - but they are called beta particles to identify that they came from the nucleus of the atom.

How do you get an electron from the nucleus? A neutron splits up and becomes a proton and an electron. The proton remains behind in the nucleus, the electron is emitted.

Beta particles are also strongly ionising (perhaps 1 beta particle will cause 100 ionisations).

Because it is less charged it doesn't ionise as well, but it therefore doesn't run out of energy so quickly → more penetration.

However, it is less damaging to us for some reason. (It's not as ionising!)

Gamma Rays.

Gamma rays are very poor at ionising (about 1 to 1) but they are very difficult to stop (they are very penetrating). As they are not good ionisers, they are less dangerous to life.

They are in fact pure energy (at the shortest wavelength end of the E-M spectrum) and gamma emission accompanies most emissions of beta or alpha particles.

Type of radiation: Symbol: Formula: Penetrating Power: Mass: Charge: Speed:
Alpha particle: α Copyright S-cool Stopped by paper or skin 4 +2 Slow
Beta particle β Copyright S-cool Stopped by thin metal Negligible -1 Fast
Gamma rays γ
0 δ
reduced by many cms of lead or a few metres of concrete No mass No charge Speed of light

In radioactive decay, both alpha and gamma radiation are emitted from a given nuclide with a definite energy characteristic of the nuclide. This is not a property of beta particles. Measurements show a range of energies for the beta particles emitted from a given radioactive substance (see below)

What is ionising radiation?

This spread (or spectrum) of energies seemed to contradict the conservation of energy. Why did some particles emerge with less energy than others? To avoid this problem it was suggested that a new particle, the neutrino, was responsible for carrying off the missing energy. We now know of the existence of several types of neutrino each uncharged and with a very small mass - possibly zero.