Nuclear radiation

Types of radioactive decay

An unstable nucleus can decay by emitting an alpha particle, a beta particle, a gamma ray or, in some cases, a single neutron.

Alpha particle

If the nucleus has too few neutrons, 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 _{2}^{4}{He} or sometimes _{2}^{4}{\alpha}.

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 particle

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 (β) particle. This process is known as beta radiation.

A beta particle has a relative mass of zero, so its mass number is zero; and as the beta particle is an electron, it can be written as _{-1}^{~0}{e}. Sometimes it is also written as _{-1}^{~0}{\beta}.

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). The electron is then ejected at high speed and carries away a lot of energy.

_{0}^{1}\textrm{n} \rightarrow _{1}^{1}\textrm{p}~+ _{-1}^{~0}\textrm{e}

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

Gamma ray

After emitting an alpha or beta particle, the nucleus will often still be too ‘hot’ and will lose energy in a similar way to a hot gas cooling down. A hot gas cools by emitting infrared radiation, which is an electromagnetic wave.

High energy particles will emit energy as they drop to lower energy levels. Since energy levels in the nucleus are much higher than those in the gas, the nucleus will cool down by emitting a more energetic electromagnetic wave called a gamma ray.

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

Neutron emission

Occasionally it is possible for a neutron (sometimes written as _{0}^{1}\textrm{n}) to be emitted by radioactive decay. This can occur naturally, eg 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 unstable parent nucleus as it splits.

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

Symbol Penetrating powerIonising powerRange in air
Alpha α Skin/paper High< 5 centimetre (cm)
Betaβ3 mm aluminium foil Low≈ 1 metre (m)
GammaγLead/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 a Geiger-Muller tube (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.

The G-M tube is usually connected to a counter that registers the current from each ionisation and also releases an audible click. This set up is known as a Geiger counter.

Activity

The activity of a source is defined as the rate at which a source of unstable nuclei decays and is measured in decays per second.

The unit for activity is the Becquerel (Bq) where 1 Bq = 1 decay per second.