Nuclear radiation

Types of radioactive decay

An unstable nucleus can decay by emitting an alpha particle, a ß- (beta minus) particle, a ß+ (positron), a gamma ray or in some cases a single neutron.

Alpha particle

If the nucleus is unstably large, it will emit a 'package' of two protons and two neutrons called an alpha particle.

Helium ion, two neutrons, two protons and no electrons.

An alpha particle is also a helium-4 nucleus, so it is written as 42He. It is also sometimes written as 42α.

Alpha decay causes the mass number of the nucleus to decrease by four and the atomic number of the nucleus to decrease by two.

Beta minus decay

If the nucleus has too many neutrons, a neutron will turn into a proton and emit a fast-moving electron. This electron is called a beta minus (β-) particle - this process is known as beta radiation.

A beta particle has a relative mass of zero, so its mass number is zero. As the beta particle is an electron, it can be written as 0-1e. However, sometimes it is also written as 0-1β.

The beta particle is an electron but it has come from the nucleus, not the outside of the atom.

Electrons are not normally expected to be found in the nucleus but neutrons can split into a positive proton (same mass but positive charge) and an electron (which has a negative charge to balance the positive charge) which is then ejected at high speed and carries away a lot of energy.

Beta decay causes the atomic number of the nucleus to increase by one and the mass number remains the same.

Positron (ß+) emission

If the nucleus has too few neutrons, a proton will turn into a neutron and emit a fast-moving positron. This positron can be called a beta plus (β+) particle - this process is known as positron emission.

A positron is the antimatter version of an electron. It has the same relative mass of zero, so its mass number is zero, but a +1 relative charge. It can be written as 0+1e, however sometimes it is also written as 0+1β.

Beta plus decay - positron emission - causes the atomic number of the nucleus to decrease by one and the mass number remains the same.

A re-arrangement of the particles in a nucleus can move the nucleus to a lower energy state. The difference in energy is emitted as a very high frequency electromagnetic wave called a gamma ray.

After emitting an alpha or beta particle, the nucleus will often still have excess energy and will again lose energy. A nuclear re-arrangement will emit the excess energy as a gamma ray.

Gamma ray emission causes no change in the number of particles in the nucleus meaning both the atomic number and mass number remain the same.

Neutron emission

Occasionally it is possible for a neutron to be emitted by radioactive decay. This can occur naturally, ie absorption of cosmic rays high up in the atmosphere can result in neutron emission, although this is rare at the Earth's surface. Or it can occur artificially, eg the work done by James Chadwick firing alpha particles at beryllium resulted in neutrons being emitted from that.

A further example of neutron emission is in nuclear fission reactions, where neutrons are released from the parent nucleus as it splits.

Neutron emission causes the mass number of the nucleus to decrease by one and the atomic number remains the same.

Properties of nuclear radiations

The different types of radiation are often compared in terms of their penetrating power, their ionising power and how far they can travel in the air.

SymbolPenetrating powerIonising powerRange in air
AlphaSkin/paperHigh < 5 centimetre
Beta3 mm aluminium foilLow≈ 1 metre (m)
GammaLead/concreteVery low> 1 kilometre (km)
Alpha, beta and gamma rays passing through a hand, beta and gamma rays passing through aluminium and gamma rays stopping at lead.

All types of radioactive decay can be detected by photographic film, or a Geiger-Muller tube (G-M tube). The photographic film is chemically changed by the radiations so it can be developed to see if there has been exposure. In a G-M tube, the radiations ionise the gas inside and the resulting charged particles move across the chamber and get counted as charges rather like an ammeter.