A level revision

Ionising radiation

Ionising radiation comes in 3 forms: alpha (), beta () and gamma (). Ionising radiation does exactly what it says on tin: it ionises the atoms of the material they are passing through. To be able to ionise atoms the radiation needs to be energetic which is why radio waves and visible light are not ionising - they do not have enough energy. X-rays, UV light and beams of protons or neutrons do have enough energy.

Alpha radiation

Alpha radiation is a helium nucleus, with two protons and two neutrons. In equations it is written as

Alpha radiation is the most ionising form of radiation because of this they have a definite range in air as they quickly give their energy up to other particles.

Outside the body alpha radiation does not pose much risk as they will rarely make it to the body and even then will be stopped by the skin. Inside the body however and alpha radiation poses significant risk. It will ionise the particles of nearby cells causing them damage.

Beta radiation

Beta radiation is a high speed electron which is emitted from a nucleus when a neutron turns into a proton. When a proton turns into a neutron a high speed positron is emitted. Beta radiation is written as for an electron and for a positron.

Beta radiation is less ionising than alpha radiation and so has a longer variable range in air, its range however is still limited.

Like with alpha radiation beta radiation only causes serious risk and damage inside the body as it ionises and damages cells.

Gamma radiation

Gamma radiation is the result of a nucleus losing energy and moving to a lower energy level. When this occurs the energy is emitted in the form of gamma radiation as a photon. Gamma radiation is represented using the Greek letter

Gamma radiation is less ionising than both beta and alpha radiation, it travels much further and so requires more shielding to protect people as it can cause harm, even outside the body.

Half thickness

As gamma radiation is absorbed from outside the body protection is needed. Generally the intensity of radiation decreases exponentially with the thickness of a material. The half thickness for a material is therefore the thickness of material needed to reduce the intensity of radiation by half. For example, if 1cm of lead reduces the intensity of x-rays (a form of gamma radiation) by a half by how many times does 10cm of shielding reduce the intensity by? The answer is 1000 times, as 10cm is 10 half thicknesses the reduction is given by 2¹⁰

Intensity and the thickness of material are related by the equation:

Where I is the intensity of the radiation, I₀ is the original intensity (usually 100%, i.e. the intensity without any shielding), is the absorption coefficient and x is the thickness of the material.