In 1986, a nuclear power plant accident in Chornobyl, Ukraine unleashed a huge amount of radioactive material into the surrounding environment. Many people living in the area were affected by the high intensity of ionising radiation released by the exposed radioactive material. For example, the number of cases of certain diseases, such as thyroid cancer, increased dramatically. Ionising radiation can cause many other harmful effects on the human body, even in lower doses. This is why it is very important that we control substances that emit ionising radiation carefully, such as in nuclear power plants. Read on to find about about the various properties and hazards of ionising radiation and why they can be so dangerous.

Ionising radiation definition

Ionising radiation is a form of radiation that has a high enough energy to ionise atoms, which it does by removing an electron from an atom.

In the case of ionising radiation, an electron is knocked off from the atom, which causes it to become a positive ion, otherwise known as a cation. When any form of ionising radiation impacts an atom, it can transfer some of its energy to it. If this quantity of energy is great enough, then ionisation will occur.

Types of ionising radiation

Ionising radiation can either come in the form of particles or electromagnetic waves. There are several types of ionising radiation, which can either come in the form of particles or electromagnetic waves:

• Alpha particles
• Beta particles
• X-rays
• Gamma rays
• Neutrons

The different types of ionising radiation vary in their physical properties, ionising powers (their probability of ionising an atom), and penetrating powers (the distance they can travel through a material before being absorbed).

The image above shows both the ionising and penetrating powers of different types of radiation. Greater ionising power means a higher ability to damage atoms and therefore molecules. It requires energy to ionise an atom which then causes the radiation particles to lose energy and slow down. This process continues to take place until the radiation loses all of its energy and is eventually absorbed. The greater the ionising power of a type of radiation, the lower its penetrating power.

Alpha particles

An alpha particle is made up of two protons and two neutrons, which is the same as a helium nucleus. Protons are positively charged and neutrons have no charge so an alpha particle has an overall positive charge. An alpha particle is the least penetrating type of radiation and will be absorbed by a few sheets of paper. Alpha particles have the highest ionising power out of all of the types of radiation due to their comparatively large mass.

Beta particles

A beta particle is a fast-moving electron. It is negatively charged and has a much smaller mass than an alpha particle, being about \( 8000 \) times lighter. Beta particles are created when a neutron transforms into a proton and an electron. This electron is then emitted from the nucleus. Beta particles are less ionising than alpha particles due to their smaller mass and hence are more penetrating, they can be stopped by a thin sheet of aluminium.

X-rays and gamma rays

X-rays and gamma rays are forms of electromagnetic radiation. They have no mass so are less ionising and more penetrating than both alpha and beta particles. In fact, they do not directly ionise atoms (due to their lack of charge) but can cause atoms to emit other particles (alpha and beta particles) which then cause ionisation. X-rays and gamma rays need a dense material such as lead with high thickness to be fully absorbed. It's interesting to note that gamma rays will actually travel further than X-rays before full absorption as they have higher energy.

Neutrons

Neutron radiation is made up of fast-travelling free neutrons. As they are neutral particles, they do not interact electrically with electrons, meaning that they are weakly ionising and have great penetrating power. They will be absorbed by a very thick material containing hydrogen atoms, such as water or concrete.

Ionising radiation sources

All types of ionising radiation are released by unstable atoms. Atoms are unstable when they have excess energy or excess mass. These unstable atoms are radioactive.

Radioactive decay

Radioactive decay is the process by which an unstable nucleus emits radiation to attain a more stable state. Alpha, beta, gamma, and X-ray radiation are released by atoms undergoing radioactive decay. After decay, the unstable nucleus will fundamentally change depending on the number and type of particles emitted. For instance, Uranium-238 can undergo alpha decay to become Thorium-234.

In general, very heavy nuclei such as uranium and plutonium will undergo alpha decay to become more stable, releasing an alpha particle in the process. Beta decay, which involves the release of a beta particle, occurs in nuclei that have too many neutrons relative to protons. Gamma rays and X-rays are also produced in both alpha and beta decay, they carry away some of the excess energy from the nucleus.

Nuclear reactions

Nuclear Reactions are processes in which one or more atomic nuclei are fundamentally changed either from a collision between two nuclei or between one atomic nucleus and a subatomic particle.

Nuclear fission

The first example of this is nuclear fission, where a parent nucleus is split into two daughter nuclei and other fission products such as neutrons and electrons. The mass of the daughter nuclei and the emitted neutrons is always less than the parent nucleus, this missing mass is released as energy, usually in the form of gamma radiation.

Nuclear fusion

The second possible method of heating is through nuclear fusion, where two light atomic nuclei are forced together and merged into a single nucleus. Similarly to fission, the resultant nucleus from a fusion reaction has a lower mass than the two original nuclei. The leftover mass is released as energy.

Ionising radiation effects

Reducing the risk of harm from ionising radiation

The risk of harmful effects caused by ionising radiation increases the longer that someone is exposed to it. While using radioactive sources, many precautions can be taken to reduce these risks:

• Wear face masks to prevent inhalation of radioactive materials.
• Handle radioactive material with tongs to keep a safe distance from them.
• Wear protective clothing such as hazmat suits or lead aprons.
• When the radioactive materials are not in use, keep them in a lead-lined box.
• Limit the amount of time that you are exposed to ionising radiation.

Difference between ionising and non-ionising radiation

Radiation that does not cause the ionisation of atoms is called non-ionising radiation. This type of radiation can still cause atoms to move and vibrate, but it does not have enough energy to ionise them. To understand why different types of radiation are non-ionising, it helps to consider the electromagnetic spectrum, which is shown below. The electromagnetic spectrum splits up the different frequencies (and wavelengths) into different categories of electromagnetic radiation. As mentioned earlier in the article, the energy of radiation is directly proportional to its frequency.

Gamma radiation has the highest frequency of the entire electromagnetic spectrum (except cosmic rays which we do not typically experience on Earth), and hence the highest energy, whereas radio waves have the lowest frequency and therefore the lowest energy. It should be reassuring to you that we have already discussed the ionising properties of gamma radiation and x-ray radiation. The other types of radiation are lower in energy and they do not have enough energy to cause ionisation.