Cambridge Syllabus Notes

5.2.1 Detection of Radioactivity

Learning Objective (Core)

Know that ionising nuclear radiation can be measured using a detector‑counter system (a detector connected to a counter).

Why Detect Radioactivity? (Core)

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 a Detector‑Counter System (Core)

All detectors convert the energy deposited by ionising radiation into an electrical pulse. The counter tallies these pulses, giving a count rate that is directly proportional to the intensity of the radiation incident on the detector.

Typical circuit (core steps)

Radiation interacts with the detector → a small electrical pulse is produced.
The pulse is amplified (often inside the detector housing).
The amplified pulse is sent to a counter (electronic or digital).
The counter increments its display for each pulse, providing a count rate R (counts s⁻¹ or cpm).

Extended content: The relationship between the measured count rate R and the true activity A of the source can be written as

R = ε A

where ε (0 ≤ ε ≤ 1) is the detection efficiency. Efficiency depends on detector type, geometry, and radiation energy. This equation is part of the *supplementary* material and may be omitted for a core‑only lesson.

Block diagram: Detector → Amplifier → Counter — Block diagram of a detector‑counter system.

Units & Terminology (Core)

Unit	Definition	Core / Extended
Counts per minute (cpm)	Number of pulses recorded by the counter in one minute.	Core
Becquerel (Bq)	One nuclear decay per second (1 Bq = 1 s⁻¹).	Extended (used when converting activity to count rate)
Inverse‑square law	Radiation intensity varies with distance as I ∝ 1/r², where r is the distance from a point source.	Core

Common Types of Radiation Detectors (Extended)

Detector	Radiation Detected	How It Works	Typical Use
Ionisation Chamber	α, β, γ (all)	Gas‑filled chamber; ionisation produces a continuous current proportional to intensity.	High dose‑rate measurements, laboratory calibrations.
Geiger‑Müller (GM) Tube	β, γ (α only with thin mica window)	Gas‑filled tube at high voltage; a single ionisation event triggers an avalanche, giving a large, identical pulse.	General‑purpose counting, survey meters, classroom demos.
Scintillation Detector	α, β, γ (depends on scintillator)	Radiation excites a crystal or liquid; emitted light is converted to an electrical pulse by a photomultiplier.	Medical imaging, low‑level environmental monitoring.
Semiconductor Detector (Si, Ge)	α, β, γ (high resolution)	Radiation creates electron‑hole pairs; charge is collected as a pulse.	Spectroscopy, precise energy measurements.
Proportional Counter	β, γ (weak α)	Gas‑filled tube at lower voltage than a GM tube; pulse height ∝ deposited energy.	Laboratory work where limited energy discrimination is useful.
Cloud Chamber (Wilson Chamber)	α (visible tracks), also β and γ indirectly	Supersaturated vapour condenses along ionisation trails, making particle paths visible.	Demonstrations of particle tracks, qualitative studies.

Safety Considerations (Core)

Use the lowest activity source needed for the experiment.
Maintain a safe distance; intensity follows the inverse‑square law.
Shield appropriately: paper for α, thin aluminium for β, lead for γ.
Never point a source at people or at the detector’s sensitive window.
Wear suitable personal protective equipment (gloves, lab coat, lead apron where required).
Display the trefoil radiation warning symbol on all containers and workstations.
Store sources in lead containers when not in use and keep them in a locked cabinet.

Practical Experiment – Detecting Radioactivity (Core)

This short activity demonstrates that a detector‑counter system can register the presence of a radioactive source.

Measure background. Place the detector away from any source, record counts for a fixed time (e.g., 5 min), and calculate the background count rate R_bg (cpm).
Introduce the source. Position a known source at a fixed distance (e.g., 10 cm) from the detector’s window.
Record counts. Count for the same time interval as the background measurement and obtain the total count rate R_total (cpm).
Subtract background. Net count rate R = R_total – R_bg (cpm).
Interpretation. The increase in count rate shows that the detector is responding to the radiation from the source.

Extension – Determining Detection Efficiency (Extended)

The same set‑up can be used to calculate the detector’s efficiency ε if the source activity A (in Bq) is known, using the equation R = ε A. This activity‑based calculation is optional and intended for higher‑ability students.

Sample Question – Efficiency Calculation (Extended)

Problem: A sealed source emits 2.0 × 10⁵ decays s⁻¹. 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 × 10⁵ s⁻¹ × 60 s min⁻¹ = 1.2 × 10⁷ cpm
Use R = ε A → ε = R / A = 1500 / 1.2 × 10⁷ ≈ 1.25 × 10⁻⁴ (0.0125 %).

This demonstrates how the extended efficiency concept links the measured count rate to the true activity of a source.

Key Points to Remember (Core)

All detectors convert ionising events into electrical pulses; the counter tallies these pulses.
For a given detector, the recorded count rate is directly proportional to the radiation intensity incident on it.
Choose the detector that matches the radiation type, required sensitivity, and experimental aim.
Apply the safety rules – PPE, shielding, distance, and proper storage.

Schematic of a GM tube connected to a counter with high‑voltage supply, pulse amplifier and digital display — Schematic of a GM tube connected to a counter (detector‑counter system).

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