IGCSE CP12 – Radiation Penetrating Power | Edexcel International GCSE Physics

Theory — Ionising Radiation

Alpha, beta and gamma are three types of ionising radiation emitted by unstable nuclei. They differ in nature, charge, penetrating power and ionising ability.

Properties of the Three Radiations

Property	α Alpha	β Beta	γ Gamma
Nature	Helium nucleus (²₄He)	Fast electron	EM wave (photon)
Charge	+2	−1	0
Stopped by	Paper / skin	Few mm aluminium	Several cm lead
Ionising power	Strongest	Medium	Weakest
Range in air	~5 cm	~1 m	Effectively infinite

Why the Difference in Penetration?

Alpha is a large, doubly-charged particle. It interacts strongly with matter — it ionises atoms rapidly, losing energy quickly. It is stopped by just a sheet of paper or a few centimetres of air.

Beta is a fast, light electron. It ionises less strongly and so penetrates further — stopped by a few millimetres of aluminium or a metre of air.

Gamma is electromagnetic radiation (a high-energy photon) with no mass and no charge. It interacts only rarely with matter and can penetrate many centimetres of lead. Even thick lead only reduces, not eliminates, gamma.

Background Radiation

Background radiation is always present — from cosmic rays, rocks (especially granite containing uranium), building materials, food, and medical procedures. It must be measured before the experiment and subtracted from all readings. Typical background: 15–25 counts per minute.

Procedure

⚠ Safety Precautions

Never handle sources directly — use tongs or forceps at all times
Keep sources in lead-lined containers when not in use
Point sources away from the body and minimise exposure time
Maintain at least 30 cm distance between source and body
Wash hands thoroughly after the experiment

Equipment

Radioactive source (unknown) in holder · Geiger-Müller tube + counter · Absorbers: paper, aluminium sheet (1mm, 2mm, 5mm), lead sheet (1mm, 5mm, 10mm) · Ruler · Clamp stand · Lead-lined container

1

Measure background radiation

With no source present, run the GM counter for 1 minute. Repeat 3 times and find the average. This is your background count rate (counts per minute, cpm). Subtract this from all subsequent readings.

2

Position the source

Place the source exactly 3 cm from the GM tube using the ruler. Record the count rate with no absorber — this is the baseline reading.

💡 Conduct all readings from the same location to avoid variation in background radiation.

3

Test each absorber in order

Place absorbers one at a time between the source and the tube. Record the corrected count rate (subtract background) for each absorber. Start with paper, then increase aluminium thickness, then try lead.

4

Identify the radiation type

If radiation is stopped by paper → alpha. If it passes paper but is stopped by a few mm of aluminium → beta. If it passes thick aluminium and is only reduced (not stopped) by lead → gamma.

💡 Some sources emit more than one type — if count rate drops significantly with paper but is not zero, the source may emit both alpha and beta/gamma.

☢️ Step 1 — Press ▶ Measure Background first. Then select a radiation source and test each absorber.

Step 1 — Background

Background—

StatusNot measured

Step 2 — Source

Step 3 — Absorber

GM Counter

000

counts per minute (cpm)

Radiation Identified

—

Data Table

Results of penetration experiment.

Absorber	Thickness	Raw count / cpm	Background / cpm	Corrected count / cpm	Result
No readings yet — run the simulation.

Questions

Question 1

A student measures a background count rate of 18 cpm. With a radioactive source 3 cm away, the count rate is 340 cpm. When paper is placed between source and tube, the count rate drops to 22 cpm. (a) What is the corrected count rate without the paper? (b) What type of radiation is the source most likely emitting? Explain your reasoning.

(a) Corrected count without paper = 340 − 18 = 322 cpm. (b) The source is most likely emitting alpha radiation. The corrected count rate without the paper is 322 cpm, but when paper is inserted the count rate drops to 22 − 18 = 4 cpm corrected — essentially zero (just background). This shows that paper is sufficient to stop virtually all the radiation. Only alpha particles are stopped by paper; beta particles require several millimetres of aluminium, and gamma rays can penetrate many centimetres of lead. The near-complete absorption by paper is characteristic of alpha radiation.

Question 2

Explain why background radiation must be measured and subtracted from all readings in this experiment. Why should background be measured three times and averaged?

Background radiation is always present — it comes from cosmic rays, naturally occurring radioactive isotopes in rocks and building materials, and other environmental sources. If background is not subtracted, the count rate recorded will be higher than it actually is for the source alone. This could lead to false conclusions — for example, concluding that an absorber has not stopped all radiation when actually only background is being detected. Measuring background three times and averaging gives a more reliable estimate. Radioactive decay (and background radiation) is a random process — count rates fluctuate statistically. A single measurement could be unusually high or low by chance. Three measurements and an average reduces the effect of random variation, giving a more accurate baseline to subtract.

Question 3

A radioactive source gives a corrected count rate of 850 cpm through no absorber, 830 cpm through paper, and 180 cpm through 5 mm of aluminium. It still gives a count rate of 120 cpm through 10 mm of lead. What type(s) of radiation is the source emitting? Explain each piece of evidence.

The source emits beta and gamma radiation. Evidence: (1) The count rate barely changes with paper (850 → 830 cpm) — this rules out alpha, which would be stopped completely by paper, and confirms no significant alpha component. (2) The count rate drops significantly with 5 mm aluminium (830 → 180 cpm) — this 78% reduction shows that beta particles are present and being absorbed by the aluminium. (3) Even through 10 mm of lead, a significant count rate remains (120 cpm) — gamma rays can penetrate lead and are only reduced, not stopped. The remaining count after thick lead is characteristic of gamma radiation. In summary: no significant change with paper → no alpha; large drop with aluminium → beta present; significant count through thick lead → gamma present.