You are viewing a free preview of this lesson.
Subscribe to unlock all 10 lessons in this course and every other course on LearningBro.
Nuclear radiation — alpha (α), beta (β), and gamma (γ) — is invisible, odourless, and cannot be felt. Yet it interacts with matter in predictable ways, and these interactions form the basis of all detection methods. This lesson covers the key detectors you need to know for Edexcel A-Level Physics, along with the important concepts of background radiation, count rate, and the inverse square law for gamma radiation.
| Property | Alpha (α) | Beta (β⁻) | Gamma (γ) |
|---|---|---|---|
| Nature | ⁴He nucleus (2p + 2n) | High-speed electron | Electromagnetic wave |
| Charge | +2e | −e | 0 |
| Mass | ~4 u (~6.64 × 10⁻²⁷ kg) | ~1/1836 u (~9.11 × 10⁻³¹ kg) | 0 |
| Speed | ~5% of c (~1.5 × 10⁷ m s⁻¹) | Up to ~99% of c | c (3 × 10⁸ m s⁻¹) |
| Range in air | ~3–8 cm | ~15 cm to 1 m | Inverse square law — no definite range |
| Stopped by | Paper / skin / few cm air | ~3 mm aluminium | Several cm of lead / thick concrete |
| Ionising ability | Very high (~10⁴ ion pairs per cm) | Moderate (~10² ion pairs per cm) | Low (~1 ion pair per cm) |
| Deflection in E/B fields | Deflected (low charge-to-mass ratio) | Deflected strongly (high q/m) | Not deflected |
| Energy spectrum | Discrete (specific energies) | Continuous (with neutrino) | Discrete |
The Geiger-Müller (GM) tube is the most commonly used radiation detector at A-Level. It consists of:
flowchart TD
A["What do you need\nto detect?"] --> B["Just count\nradiation events?"]
A --> C["Identify radiation\ntype?"]
A --> D["Measure radiation\nenergy?"]
A --> E["See individual\nparticle tracks?"]
B --> B1["Use GM tube\n+ counter"]
C --> C1["Use GM tube with\nabsorber method:\n1. No absorber → total count\n2. Paper → removes α\n3. Al sheet → removes β\n4. Remaining = γ"]
D --> D1["Use scintillation\ndetector with\nphotomultiplier"]
E --> E1["Use cloud chamber\nor bubble chamber"]
A cloud chamber contains supersaturated vapour (usually alcohol). When an ionising particle passes through, it ionises vapour molecules along its path. These ions act as condensation nuclei, and tiny droplets form along the track, making the particle's path visible.
Film darkens when exposed to ionising radiation, just as it darkens with visible light. This was how Becquerel first discovered radioactivity in 1896. Today, film badges are worn by workers who may be exposed to radiation:
| Filter | What passes through | What it tells you |
|---|---|---|
| Open window | α, β, γ | Total radiation exposure |
| Thin plastic | β, γ (blocks α) | Exposure excluding alpha |
| Aluminium (3 mm) | γ only (blocks α and β) | Gamma exposure only |
| Lead (thick) | Very high-energy γ / neutrons | High-energy radiation |
| Cadmium | Absorbs thermal neutrons | Neutron exposure |
A scintillation detector uses a material (often sodium iodide, NaI, or a plastic scintillator) that emits a tiny flash of visible light when struck by ionising radiation. A photomultiplier tube detects and amplifies this flash into a measurable electrical pulse.
Advantages over GM tubes:
Background radiation is ionising radiation that is always present in the environment. Sources include:
When measuring the activity of a radioactive source, you must subtract the background count rate to find the count rate due to the source alone:
Corrected count rate = Measured count rate − Background count rate
The background count rate is measured by running the GM tube without the source present, typically for several minutes, then dividing the total count by the time to get a reliable average.
Common exam mistake: Students apply the inverse square law to the raw (uncorrected) count rate. You MUST subtract the background first, then apply the inverse square law, then add the background back if you need the actual detector reading.
The count rate is always less than the activity because:
Gamma radiation spreads out uniformly from a point source in all directions (isotropically). As it travels further from the source, the same radiation is spread over a larger area, so the intensity decreases.
At distance r from a point source, the radiation passes through the surface of a sphere of area 4πr². Therefore:
I = P/(4πr²)
where I is the intensity (W m⁻²) and P is the power of the source.
This gives the inverse square law:
I ∝ 1/r²
Or equivalently, for two distances r₁ and r₂:
I₁/I₂ = r₂²/r₁²
Since count rate is proportional to intensity (for a given detector):
C₁/C₂ = r₂²/r₁²
A GM tube records 540 counts in 3 minutes with a gamma source at 0.20 m. The background is 30 counts per minute. Find the corrected count rate at 0.50 m.
Step 1: Measured count rate = 540/3 = 180 cpm Step 2: Corrected count rate at 0.20 m = 180 − 30 = 150 cpm Step 3: At 0.50 m: C₂ = C₁ × (r₁/r₂)² = 150 × (0.20/0.50)² = 150 × 0.16 = 24 cpm Step 4: Actual GM reading at 0.50 m = 24 + 30 = 54 cpm
A gamma source gives a corrected count rate of 1600 counts per minute at 10 cm. At what distance will the corrected count rate fall to 100 counts per minute?
C₁/C₂ = r₂²/r₁²
1600/100 = r₂²/0.10²
16 = r₂²/0.01
r₂² = 0.16
r₂ = 0.40 m = 40 cm
| Source | How to minimise |
|---|---|
| Random nature of decay | Take measurements over longer times; repeat and average |
| Distance measurement (where is the source inside its housing?) | Introduce a correction factor x₀; plot C vs 1/(r + x₀)² |
| Background fluctuations | Measure background for several minutes |
| GM tube dead time | Keep count rates below ~5000 cpm |
| Absorption by air | Negligible for gamma at lab distances |
A GM tube is placed near a radioactive source. The following readings are taken:
| Condition | Count rate (cpm) |
|---|---|
| Background only (no source) | 28 |
| Source present, no absorber | 1250 |
| Source + paper between | 450 |
| Source + 3 mm aluminium | 55 |
| Source + thick lead | 30 |
Analysis:
Actually, let us be more careful: after lead, count rate = 30 ≈ background (28). So essentially all radiation is accounted for.
The source emits all three types: primarily alpha (dominant), significant beta, and a small amount of gamma.
Edexcel 9PH0 specification, Topic 11 — Nuclear and Particle Physics covers the detection of ionising radiation, including the use of Geiger–Müller (GM) tubes to measure count rate, the identification of alpha, beta and gamma radiation by their distinct absorption characteristics, the requirement to subtract background radiation from raw counts, and the inverse-square law for the intensity of a gamma source with distance (refer to the official specification document for exact wording). Topic 11 sits alongside Topic 12 (space) in Paper 2, but detection content is genuinely synoptic: it is examined wherever practical interpretation of count-rate data appears, and it underpins the dose calculations that link to medical-physics options. The Edexcel formula booklet provides I=x2k in the proportional form for inverse-square problems, but the half-life decay law N=N0e−λt and the relationship A=λN must be applied confidently when interpreting detector data.
Question (8 marks): A small unsealed source is being investigated with a GM tube and ratemeter. The background count rate (no source present) is measured over five minutes and recorded as 144 counts. With the source placed 5.0 cm from the GM tube window, the count rate is 928 counts per minute. A sheet of paper is placed between the source and the tube; the count rate falls to 312 counts per minute. A 3 mm aluminium sheet is then substituted for the paper; the count rate falls further to 64 counts per minute. A 5 mm lead sheet is then substituted; the count rate is 32 counts per minute.
(a) Calculate the background count rate, in counts per minute, and use it to find the corrected count rate for each absorber. (3)
(b) Identify the type(s) of radiation emitted by the source, justifying each conclusion from the data. (5)
Solution with mark scheme:
(a) Step 1 — convert background to counts per minute. 144/5=28.8 cpm ≈29 cpm.
M1 — dividing total counts by total time. Common error: forgetting to convert and subtracting 144 directly from each per-minute reading.
Step 2 — corrected count rates (raw − background):
M1 — subtracting background from each reading.
A1 — all four corrected values correct (one slip, lose A1 only).
(b) Step 3 — interpret the paper test. The corrected count rate falls from 899 cpm to 283 cpm when paper is inserted. Paper stops alpha radiation but is essentially transparent to beta and gamma. The drop of 899−283=616 cpm represents the alpha component.
M1 — recognising paper absorbs alpha only.
B1 — alpha radiation present, justified by paper test.
Step 4 — interpret the aluminium test. Going from paper (283 cpm, no alpha) to 3 mm aluminium (35 cpm). A few millimetres of aluminium absorbs beta but transmits gamma. The drop of 283−35=248 cpm represents the beta component.
B1 — beta radiation present, justified by aluminium test.
Step 5 — interpret the lead test. Going from aluminium (35 cpm) to 5 mm lead (3 cpm). The remaining 35 cpm above background must be gamma; lead attenuates but does not entirely block gamma. A residual 3 cpm remains, consistent with weak gamma penetration.
B1 — gamma radiation present, justified by lead test (must explicitly note that lead attenuates rather than blocks gamma).
Total: 8 marks (M3, B3, A1, B1).
Question (6 marks): A gamma source of constant activity is placed at varying distances x from a GM tube. After background correction, the count rate C at four distances is recorded:
| x / cm | 10 | 20 | 30 | 40 |
|---|---|---|---|---|
| C / cpm | 480 | 120 | 53 | 30 |
(a) State the inverse-square law for gamma intensity, defining all symbols. (2)
(b) By plotting an appropriate linear graph, demonstrate that the data are consistent with an inverse-square law and determine the constant k such that C=k/x2. (4)
Mark scheme decomposition by AO:
(a)
Subscribe to continue reading
Get full access to this lesson and all 10 lessons in this course.