Describe methods for measuring and detecting radioactivity limited to…

The idea

Two methods are used to detect and measure radioactivity. Photographic film darkens when ionising radiation reaches it, so a film badge worn by a worker shows how much radiation that person has received; the darker the film, the greater the dose. A Geiger–Müller (GM) tube detects radiation when ionising particles enter the tube and produce a pulse of current, which is counted and displayed as a reading or as clicks. The GM tube gives a count rate, often in counts per minute, so it measures activity rather than dose. A good answer explains what each device does and how its output is interpreted, and keeps to these two methods only.

What earns the marks

• Explains that photographic film darkens when exposed to ionising radiation, so the degree of darkening indicates the amount of radiation received.
• Describes a film badge worn by a person to monitor the dose of radiation that person receives.
• Explains that a Geiger–Müller tube produces a pulse of current when ionising radiation enters it.
• States that a Geiger–Müller tube gives a count rate, such as counts per minute, which measures the activity detected.

Common mistakes and how to put them right

• Claims a Geiger–Müller tube measures the absorbed dose directly.

  Put it right: Link dose monitoring to the film badge instead.

• Says photographic film glows or changes colour.

  Put it right: Describe it darkening as radiation exposes it.

• Introduces other detectors such as cloud chambers as if required.

  Put it right: Keep to photographic film and the Geiger–Müller tube.

Exam tips

• Link each device to what it actually measures: film badge for dose, GM tube for count rate.
• Use the term 'count rate' and give a unit such as counts per minute when describing the GM tube.
• Explain the mechanism briefly, for example film darkening or a pulse of current, to gain credit for 'describe'.