Know that ionising nuclear radiation can be measured using a detector connected to a counter
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.
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 Rbg (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 Rtotal (cpm).
- Subtract background. Net count rate R = Rtotal – Rbg (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.
