Cambridge Notes, Past Papers, Revision Questions

IGCSE Physics 0625 – Detection of Radioactivity

5.2.1 Detection of Radioactivity

Learning Objective

Know that ionising nuclear radiation can be measured using a detector connected to a counter.

Why Detect Radioactivity?

Ionising radiation (α, β, γ) can be hazardous, but it also provides valuable information in scientific, medical and industrial applications. Measuring the intensity of radiation allows us to:

Assess safety levels and apply appropriate shielding.

Determine the activity of a radioactive source.

Monitor environmental contamination.

Quantify decay processes in experiments.

Basic Principle of Detection

All detectors work on the principle that ionising radiation deposits energy in a material, producing ion pairs or excited states that can be counted. The detector is linked to an electronic counter which records each event as a “count”.

Common Types of Radiation Detectors

Detector	Radiation Detected	How It Works	Typical Use
Ionisation Chamber	α, β, γ (all)	Gas-filled chamber; radiation ionises the gas, producing a continuous current proportional to radiation intensity.	Measuring high dose rates, laboratory calibrations.
Geiger‑Müller (GM) Tube	β, γ (α only if thin window)	Gas-filled tube at high voltage; a single ionisation event triggers an avalanche, giving a large, identical pulse for each particle.	General‑purpose counting, survey meters, classroom demonstrations.
Scintillation Detector	α, β, γ (depends on scintillator)	Radiation excites a scintillating material; emitted light is converted to an electrical pulse by a photomultiplier.	Medical imaging, low‑level environmental monitoring.
Semiconductor Detector (e.g., Si, Ge)	α, β, γ (high resolution)	Radiation creates electron‑hole pairs in a semiconductor crystal; the charge is collected as a pulse.	Spectroscopy, precise energy measurements.

Connecting a Detector to a Counter

The basic circuit is:

Detector produces a small electrical signal when radiation is absorbed.

Signal is amplified (often within the detector assembly).

Amplified pulse is sent to a counter (electronic or digital).

The counter increments its display for each pulse, giving a count rate.

The count rate \$R\$ is related to the activity \$A\$ of the source by the detection efficiency \$\varepsilon\$:

\$R = \varepsilon \, A\$

where \$0 \le \varepsilon \le 1\$ depends on detector type, geometry and radiation energy.

Units of Radioactivity

Counts per minute (cpm) – direct reading from a counter.

Becquerel (Bq) – one decay per second; \$1\ \text{Bq}=60\ \text{cpm}\$ (approximately, ignoring efficiency).

Safety Considerations

Always use the lowest activity source needed for the experiment.

Maintain a safe distance; intensity follows the inverse‑square law \$I \propto \frac{1}{r^{2}}\$.

Use appropriate shielding: paper for α, thin aluminium for β, lead for γ.

Never point a source at people or the detector’s sensitive window.

Sample Question

A sealed source emits \$2.0\times10^{5}\$ decays per second. A GM tube placed 10 cm away records 1500 cpm. Calculate the detection efficiency of the GM tube for this source.

Solution:

Convert activity to counts per minute: \$A = 2.0\times10^{5}\ \text{s}^{-1}\times60\ \text{s/min}=1.2\times10^{7}\ \text{cpm}\$.

Use \$R = \varepsilon A \;\Rightarrow\; \varepsilon = \frac{R}{A}= \frac{1500}{1.2\times10^{7}} \approx 1.25\times10^{-4}\$ (or 0.0125 %).

Key Points to Remember

All detectors convert ionising events into electrical pulses.

The counter tallies pulses, giving a count rate that reflects radiation intensity.

Detection efficiency links the measured count rate to the true activity of the source.

Choose the detector type that best matches the radiation type and required sensitivity.

Suggested diagram: Schematic of a GM tube connected to a counter, showing the high‑voltage supply, pulse amplifier and digital display.