Nuclear decay is a spontaneous random process by which an unstable atomic nucleus loses its energy by emission of particles or electromagnetic radiation.

An atom that has an unstable nuclei is said to be radioactive.

The radiation emitted by a radioactive nucleus is spontaneous and random in direction.

Thus, for any given radioactive atom:

• • we cannot predict when it will decay,
  • we cannot predict in which direction the radiation will be emitted,
  • its decay will not be altered by heating, cooling, undergoing chemical reactions, etc.

Three Types of Nuclear Emission

There are three types of radiation, namely α, β and γ radiation, all of which have different properties which are summarised below:

1. Alpha Decay

Alpha decay involves the emission of an alpha particle (α-particle) from the nucleus.
An alpha particle contains 2 protons and 2 neutrons. It can therefore be thought of as a very fast moving helium (He) nucleus.
When a nucleus undergoes alpha decay, its nucleon number decreases by 4 and its proton number decreases by 2.

e.g. Radium (Ra) decays via alpha decay to form radon (Rn):

MikeRun, CC BY-SA 4.0
via Wikimedia Commons

The nuclear equation for this decay would be:

In general we get:

2. Beta Decay

Beta decay involves the emission of a beta particle (β-particle) from the nucleus.
Beta particles are electrons. They are not orbital electrons in the atom, they are emitted from the nucleus itself.
When a nucleus undergoes beta decay, its nucleon number remains the same and its proton number increase by 1.

e.g. Caesium (Cs) decays via beta decay to form Barium (Ba):

MikeRun, CC BY-SA 4.0
via Wikimedia Commons

The nuclear equation for this decay would be:

In general we get:

This can be thought of as the decay of a single neutron within the nucleus.

 

Notice in these above equations that:

• mass is conserved – the number of nucleons before & after the decay remain the same
• charge is conserved – the proton number before & after the decay remain the same

3. Gamma Radiation

After many decays the nucleus is left in an excited state (indicated by a *). It drops to a lower state (loses energy) by emitting a high energy electromagnetic wave – a gamma ray (γ-ray). This does not affect the composition of the nucleus.

In general we get:

 

 

Behaviour of the Different Types of Radiation

Ionising Strength

Ionisation is the ability to eject electrons from atoms to form ions. All three types of radiation have the ability to ionise atoms around them. This leads to them being referred to as ionising nuclear radiation.

Alpha particles, being the most massive of the types of radiation, are able to cause the most ionisation. Being large they have the most kinetic energy and so will come into contact with a large number of surrounding particles in a short distance before they lose most of their energy.

Beta particles are the next most ionising particles. Finally, gamma rays (being a type of electromagnetic waves and not a particle) are the least ionising.

Penetrating power

Due to the different nature of the three types of radiation they can each penetrate materials to different depths.

Alpha particles, being the largest of the three types of radiation, are stopped very easily as they will collide with many atoms around them.
Alpha particles will be stopped by a piece of paper or the outer layer of your skin. In air they can only travel a few centimetres.

Beta particles, being much smaller than the alpha, can penetrate much further into materials than alpha particles.
Beta particles will take several millimetres of aluminium to stop them.

Gamma rays are a type of electromagnetic wave that are able to pass through most materials with ease.
It will take several centimetres of lead or several metres of concrete to stop gamma rays.

 

 

Summary of The Three Nuclear Emissions:

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Links
YouTube Video: Alpha, Beta & Gamma Radiation
YouTube Video: Nuclear Decay Equations