Practical skills: (l) use of ionising radiation, including detectors.

The idea

This practical skill involves handling ionising radiation sources safely and using detectors to measure radiation. You should know that alpha, beta and gamma radiation have different penetrating and ionising powers, and that detectors such as the Geiger-Müller tube, cloud chamber or spark counter are used to detect them. A typical investigation measures the count rate from a source at various distances or through different absorbers to determine the type of radiation or the half-life of a source. You must apply the inverse square law for gamma radiation intensity and correct for background radiation by measuring count rate without the source and subtracting it. Safety procedures include minimising exposure time, maximising distance and using shielding.

What earns the marks

• Identifies appropriate detectors for ionising radiation, such as a Geiger-Müller tube for count-rate measurement or a cloud chamber for visualising tracks.
• Describes how to correct for background radiation by measuring the count rate without the source and subtracting it from readings taken with the source.
• Explains how absorption experiments distinguish alpha, beta and gamma radiation by their differing penetrating powers through paper, aluminium and lead.
• Applies the inverse square law to gamma radiation count rates at different distances from a point source.
• States safety precautions: minimising exposure time, maximising distance from the source, and using appropriate shielding.

Common mistakes and how to put them right

• Forgetting to subtract background count rate.

  Put it right: Measure background count rate separately and subtract it from all readings taken with the source present.

• Confusing count rate with activity.

  Put it right: Count rate is the number of detected counts per second, while activity is the number of nuclear decays per second; the detector may not detect every decay.

• Assuming alpha radiation requires thick lead shielding.

  Put it right: Alpha particles are stopped by a few centimetres of air or a sheet of paper, so lead shielding is unnecessary for alpha.

Exam tips

• Always mention background radiation correction when describing count-rate experiments, as it is a standard assessed point.
• State the safety precautions explicitly: time, distance and shielding.
• When comparing penetrating powers, give specific absorber materials and approximate thicknesses to show precise knowledge.