Alpha, beta and gamma radiation

Alpha, beta and gamma radiation differ in what they consist of, their electric charge and how easily they pass through materials. They can all ionise atoms: they transfer energy that can remove electrons from atoms, leaving charged particles called ions.

Nature and properties of the radiation

Alpha radiation consists of alpha particles. Each particle is a helium nucleus containing two protons and two neutrons, with charge $$+2e$$. Alpha radiation is strongly ionising but has a short range in air. Paper or the outer layer of skin stops it.

Beta-minus radiation consists of fast-moving electrons. Each beta-minus particle has charge $$-e$$. Beta radiation is less strongly ionising than alpha and is more penetrating. A few millimetres of aluminium can stop many beta particles.

Gamma radiation consists of high-energy electromagnetic photons. Gamma photons have no electric charge. Gamma radiation is generally weakly ionising but highly penetrating. Thick lead or concrete reduces its intensity; it does not necessarily stop all gamma radiation.

Identifying radiation using absorbers

An absorption experiment compares the count rate detected from a source with different materials placed between the source and detector. Keep the source and detector in the same positions so that changes in count rate can be compared fairly.

First measure the background count rate without the source. Subtract it from the readings taken with the source to estimate the count rate due to the source alone. Then compare the corrected readings:

• A large reduction when paper is added suggests alpha radiation.
• If paper has little effect but aluminium causes a reduction, this suggests beta radiation.
• Radiation that remains detectable through aluminium but is reduced by thick lead or concrete suggests gamma radiation.

These observations can indicate more than one type of radiation if the source emits a mixture.

Radiation exposure hazards and safe handling

Ionising radiation can remove electrons from atoms in living cells. This can damage cells and their DNA, leading to tissue damage or increasing the risk of cancer. The hazard depends on the type of radiation, the amount of exposure and whether the radioactive source is outside or inside the body.

Radiation hazards inside and outside the body

Alpha radiation is strongly ionising: it deposits its energy over a very short distance. Alpha radiation from a source outside the body is largely stopped by the skin. However, if an alpha-emitting substance is inhaled or swallowed, it can deposit energy directly in living tissue, making it especially hazardous.

Beta radiation can penetrate the skin and damage tissue. Gamma radiation is more penetrating and can irradiate tissues from a source outside the body. Both beta- and gamma-emitting substances can also expose tissues if they enter the body.

Reducing exposure when handling sources

Exposure can be reduced by spending less time near a source, keeping more distance from it and using suitable shielding. Tongs increase the distance between the source and the person handling it. Shielding must suit the radiation: for example, plastic can shield beta radiation, while thick lead or concrete can reduce gamma radiation.

Containment helps prevent radioactive material from being inhaled or swallowed. This is particularly important for alpha emitters because they are much more hazardous inside the body than outside it.

Gamma intensity and distance

For a gamma source that approximates a point source, the radiation spreads over a larger area as distance increases. Its intensity follows the inverse-square law:

Here, $$I$$ is the gamma radiation intensity and $$r$$ is the distance from the source. For example, doubling the distance reduces the intensity to one quarter of its original value.

Radioactive thickness measurement

The thickness of a moving sheet can be monitored by placing a radioactive source on one side and a radiation detector on the other. As the sheet passes between them, it absorbs some of the radiation. The detector measures the count rate: the number of radiation counts it records per unit time.

The count rate depends on how much material the radiation passes through. A thicker sheet generally absorbs more radiation, so less reaches the detector and the count rate falls. A thinner sheet generally absorbs less radiation, so more reaches the detector and the count rate rises.

Using the count rate to control thickness

The desired sheet thickness corresponds to a target count rate. If the measured count rate falls below the target, the sheet may be too thick; if it rises above the target, the sheet may be too thin. The manufacturing process can then be adjusted to bring the sheet back towards the required thickness.

Choosing radiation for thickness measurement

The radiation must be absorbed enough for a change in thickness to produce a measurable change in count rate. If the sheet stops the radiation completely, the detector cannot distinguish the thicknesses from the transmitted radiation. If almost all the radiation passes through, a change in thickness has too little effect on the count rate to measure reliably.

This method can be used for sheets such as paper, aluminium foil and steel. The source is selected to suit the material and its thickness, so that some radiation reaches the detector while some is absorbed by the sheet.

Inverse-square law for gamma radiation

Gamma radiation from a source spreads out as it travels. If the source is approximately a point and emits uniformly in all directions, the radiation passes through the surface of an expanding sphere. As the distance from the source increases, the same radiation is spread over a larger area, so the intensity decreases.

The area of the sphere increases with the square of its radius. The intensity therefore follows the inverse-square law:

Here, $$I$$ is the gamma radiation intensity, $$x$$ is the distance from the source, and $$k$$ is a constant for the source and measurement conditions.

For example, doubling the distance increases the area over which the radiation is spread by a factor of four. The intensity is therefore one quarter of its original value, provided other relevant conditions remain unchanged.

Testing the inverse-square law

Measure the detector count rate at several distances from the source, keeping the source and detector arrangement consistent. Measure the background count rate without the source, then subtract it from each reading to obtain the corrected count rate.

If the corrected count rate is proportional to intensity, plot it against $$1/x^2$$. Agreement with the inverse-square law gives an approximately straight line through the origin: when $$1/x^2$$ doubles, the corrected count rate should also double.

The measurements may not agree exactly with the law. The source and detector have finite sizes, so their separation is less well represented by a single distance when they are close together. Absorption can reduce the radiation reaching the detector, and random counting fluctuations can cause readings to vary.

Background radiation and count-rate correction

Background radiation is ionising radiation present in the environment independently of the source being investigated. It comes from natural sources, including cosmic radiation from space, radioactive substances in rocks and soil, and radon gas. Artificial sources, such as medical uses of radioactive materials, can also contribute.

A detector records events caused by background radiation as well as events caused by the source. A measurement taken with the source present therefore includes both contributions.

Correcting a count rate for background radiation

First, measure the background count with the source absent. Then measure the count with the source present. For each measurement, calculate the count rate by dividing the count by the counting time:

Subtract the background count rate from the count rate measured with the source present:

The corrected count rate is an estimate of the source’s contribution. Use the same time unit for both rates. If the counting times differ, convert the counts to rates before subtracting them.

For example, suppose the background count is $$120$$ in $$60\ \text{s}$$, giving $$2$$ counts per second. With the source present, the detector records $$500$$ counts in $$100\ \text{s}$$, giving $$5$$ counts per second. The corrected count rate is $$5 - 2 = 3$$ counts per second.

Radioactive counts fluctuate, so repeated measurements do not usually give exactly the same rate. Longer counting periods generally give a more reliable estimate of the mean count rate.

Risks and benefits of medical radiation

Ionising radiation can give doctors information needed to diagnose a condition or be used to treat disease. However, it can also damage healthy cells and increase the risk of cancer developing later. A decision to use it therefore balances the expected medical benefit against the risk of harm to the patient.

Justifying medical radiation

A medical use of radiation is justified when its expected benefit for the patient outweighs its associated risk. For example, the benefit of finding a serious condition may outweigh the risk from the exposure needed to obtain a useful diagnostic image. In treatment, radiation may damage diseased cells, but nearby healthy cells can also be affected.

The balance depends on the patient's clinical need and how much benefit the procedure is likely to provide. It also depends on the amount of radiation and where the body is exposed, as well as whether an alternative could achieve the medical purpose with less risk.

Keeping exposure as low as reasonably practicable

Once a use is justified, exposure is kept as low as reasonably practicable while still achieving its medical purpose. Reducing exposure must not make a necessary diagnostic image too poor to interpret or a treatment ineffective. The aim is therefore not simply the lowest possible exposure, but an exposure low enough to limit unnecessary risk and sufficient to achieve the intended benefit.