Radioactive Decay and Nuclear Radiation

Unstable atoms can spontaneously undergo radioactive decay. During this process, the unstable nucleus of the atom emits (gives out) nuclear radiation to become more stable. So, a substance is considered radioactive when it emits nuclear radiation.

The main types of nuclear radiation emitted by unstable atoms are:

  • Alpha (α)
  • Beta (β)
  • Gamma (γ)
  • In some cases, a neutron (n) can be emitted

All types of nuclear radiation emitted by unstable nuclei are ionising. This means they can knock electrons out of the orbits of other atoms. When that atom loses electrons, it becomes a positively charged ion.

Types of Nuclear Radiation

Alpha particles

Alpha particles are made up of two neutrons and two protons.

An alpha particle is the same as a helium-4 nucleus. So, alpha particles are often written as \mathrm{}_{2}^{4}He. However, alpha particles are different from helium atoms because they do not have electrons. This means that they have a +2 overall charge, from their two positively charged protons.

  • The symbol for alpha is α, so alpha particles can also be written as \mathrm{}_{2}^{4}\alpha

Penetration of alpha particles

As alpha particles are quite large, they do not penetrate far into materials. A single sheet of paper is capable of stopping them.

Alpha particles only travel a few centimetres in the air, as they are easily stopped upon colliding with air molecules. However, as alpha particles are large and have a strong positive charge, they are strongly ionising.

Beta particles

A beta particle is a high-energy, high-speed electron that is released by an unstable nucleus. However, these electrons are not from energy levels around the nucleus. They are formed when a neutron, in the nucleus, splits to form a proton and an electron. The proton stays in the nucleus and the electron is emitted at a very high speed.

As a beta particle is an electron, it can be represented as \mathrm{}_{-1}^{\:0}e. However, it can also be written as \mathrm{}_{-1}^{\:0}\beta.

Penetration of beta particles

Beta particles are negatively charged, with a charge of -1. As they have a very small mass, they are moderately ionising. They can be stopped by a thin sheet of aluminium.

Gamma rays

Gamma rays are waves of electromagnetic radiation with a short wavelength and a high frequency. They are often emitted alongside alpha and beta particles, as a way of the nucleus getting rid of extra energy, to become more stable.

Gamma rays have no mass, which means they typically don’t collide with other atoms. They usually pass straight through materials, so they have very low ionising power.

  • Out of all the electromagnetic waves, gamma rays have the highest energy

Penetration of gamma rays

Because they typically don’t collide with atoms, gamma rays can penetrate deep into materials and travel long distances through the air. It often requires thick sheets of lead or several meters of concrete to absorb gamma rays.

Neutron emission

If a nucleus contains too many neutrons, it can become unstable. So, the unstable nucleus releases a neutron to become more stable.

As neutrons have a relative mass of 1, neutron emission decreases the mass number by 1 and the atomic number remains unchanged.

Properties of Nuclear Radiations

The table below summarises the properties of alpha, beta, gamma and neutron radiation.

NatureChargeRange in airPenetrationIonisation
Alpha (α)Two protons and two neutrons (helium nucleus)+2A few centimetresStopped by skin or paperHigh
Beta (β)A high-speed electron-1A few metresStopped by a thin layer of aluminiumLow
Gamma (γ)Electromagnetic radiation0Hundreds of metresStopped by several layers of lead or concreteVery low
Neutron (n)A subatomic particle in the nucleus of an atom0Hundreds or thousands of metresStopped by a hydrogen-rich material, such as water or concreteNeutrons can only ionise matter indirectly

Different nuclear radiations penetrate materials in different ways, so they are stopped in different ways.

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