Radiation properties, background and detection

An alpha particle is a helium nucleus, not a complete helium atom: it has no surrounding electrons. A beta-minus electron comes from the nucleus, rather than from the electron shells. A positron has the same mass as an electron but a positive charge. Gamma radiation is a wave, not a particle made of protons and neutrons.

Ionising power and penetrating power

Alpha, beta-minus, beta-plus and gamma radiation are all ionising. They can transfer enough energy to remove electrons from atoms. An atom that loses an electron is left as a positively charged ion.

Ionising power describes how readily radiation produces ions as it passes through matter. Penetrating power describes how well it passes through a material. These are different properties: radiation that produces many ions along a short path transfers its energy quickly and is therefore stopped relatively easily.

Alpha radiation is the most strongly ionising of these radiations. Alpha particles are relatively massive and have a charge of +2. They lose energy rapidly through interactions with matter, so they travel only a few centimetres in air and are stopped by paper.

Beta radiation is less ionising than alpha but more penetrating. Both beta-minus and beta-plus radiation pass through paper, but a few millimetres of aluminium can stop them.

Gamma radiation is the least ionising but the most penetrating. Its lack of charge does not prevent it from transferring energy and causing ionisation. Gamma rays pass through paper and thin aluminium. Lead or thick concrete reduces their intensity; shielding does not simply stop every gamma ray at one fixed thickness.

The key comparison is therefore: from alpha → beta → gamma, penetrating power increases while ionising power decreases.

Background radiation is always present

A radiation detector can register radiation even when no radioactive source has been placed beside it. This is because background radiation is present around us in the environment. It has both natural and human-made origins.

Natural sources on Earth include radioactive substances in rocks and soil. Uranium decay produces radon gas, which can escape from the ground into the air. Radioactive substances also occur naturally in food, drink and living organisms: examples include potassium-40 in foods and carbon-14 in biological material. Small amounts of naturally occurring radioactive material in food are not, by themselves, a reason to avoid it.

Radiation also arrives from space as cosmic rays. These originate from the Sun and other high-energy events, such as supernovae. Incoming particles can interact with Earth's atmosphere and produce further radiation.

Human activities add to radiation exposure. Examples include medical X-rays, CT scans and radioactive tracers, as well as radioactive material released by nuclear weapons testing or nuclear accidents. Background radiation is therefore not solely a result of human activity: natural sources have always existed.

Photographic film records exposure over time

Photographic film becomes darker when it absorbs radiation and is developed. Greater exposure produces greater darkening. This allows film to record radiation exposure accumulated over a period of time, rather than giving an immediate reading.

People who work with radiation, such as radiographers, can wear a film badge. The film is protected from visible light by a light-proof covering. Different regions of the film sit behind different absorbing materials, such as paper, aluminium and lead. Comparing the darkening in these regions helps assess the amount and type of radiation received.

The pattern can be understood using penetrating power. Alpha is unlikely to reach the film because the paper covering stops it. Beta can reach less shielded regions but is absorbed by the aluminium. Gamma can affect regions behind these materials, although lead reduces the amount reaching the film. The badge is checked regularly to monitor the wearer's exposure.

A Geiger–Müller tube detects radiation immediately

A Geiger–Müller tube is connected to a counter. When the tube detects radiation, it produces an electrical pulse. The counter records the pulses and may make a click for each detection or display a count rate. More frequent clicks mean a higher count rate.

Count rate is the number of detections per unit time. It tells us how much radiation the detector is registering, not the total number of emissions from the source: some radiation never reaches the tube. Moving the detector farther from a source generally lowers its count rate, so distance matters when comparing readings.

To measure radiation from a particular source, first take a background reading with that source absent. Then measure with the source present under the same conditions. Subtract the background count rate:

$$\text{corrected count rate} = \text{measured count rate} - \text{background count rate}$$

Both rates must use the same units. The corrected rate estimates the detections due to the source rather than radiation already present in the environment. Unlike a film badge, the Geiger–Müller counter provides an immediate indication of radiation being detected.

• Unstable nuclei emit α, β⁻, β⁺, γ or neutrons.
• Decay is random: the exact decay time of an individual nucleus cannot be predicted.
• α: helium nucleus, two protons and two neutrons, charge +2.
• β⁻: electron emitted from the nucleus, charge −1.
• β⁺: positron, charge +1.
• γ: electromagnetic radiation, no charge.
• Neutron: no charge.

Penetration versus ionisation

Alpha, both beta types and gamma are ionising: they can remove electrons from atoms.

Background origins

• Earth: radioactive rocks and soil, radon gas, food and living organisms.
• Space: cosmic rays from the Sun and other cosmic sources.
• Human-made: medical radiation, nuclear weapons fallout and nuclear accidents.

Detection

• Photographic film: darkens after exposure and development; film badges record accumulated exposure. Different filters help distinguish radiation types.
• Geiger–Müller tube: produces electrical pulses recorded as clicks or a count rate; gives an immediate reading.

$$\text{corrected count rate} = \text{measured count rate} - \text{background count rate}$$