Radiation Absorption Virtual Lab: Alpha, Beta and Gamma

Updated 2026-10-07

This radiation absorption virtual lab matches the GCSE work on the penetration of alpha, beta and gamma radiation, and the AQA A-level Physics required practical on the inverse-square law for gamma rays. Students measure the background count with a Geiger–Müller tube, then count a beta source through paper and aluminum of increasing thickness, compare alpha and gamma, and correct every count rate for background. Counts are random, as with real sources, so students also meet the √N uncertainty. No school needs a sealed source or a radiation risk assessment to run it.

Geiger counter lab – absorption of α, β and γ radiation and the inverse-square law

Curriculum links

AQA GCSE Physics 4.4.2.1 (properties of alpha, beta and gamma radiation): penetration through materials, range in air and ionizing power.
AQA A-level Physics 3.8.1.2, required practical 12: the inverse-square law for gamma radiation, with a correction for background and for the unknown source position.
IB Physics E.3 (radioactive decay) and Cambridge IGCSE Physics 5.2 (detection and properties of radiation).

Simulic is not affiliated with or endorsed by AQA, the IB, Cambridge or any exam board.

Before the lab (5 min)

Ask students to commit to a prediction, on paper or as question 1 of the class link:

"Which is the thinnest absorber that stops almost all the beta radiation from the source?"

Students often pick lead "because radiation needs lead", or paper "because it's like alpha". Ask for a reason with each answer.

Method in the simulation

Stay on the Absorption tab. Set Speed to Instant and Count time to 100 s so every count is quick and precise.

Press Count background. Record the background rate B.
Choose Source β (Sr-90 type), Absorber None and Distance d 3 cm. Press Start count. The sim shows N, N/t and the corrected rate C = N/t − B.
Choose Paper (0.1 mm), then Aluminium at 0.5, 1, 2, 3 and 4 mm. Count each one and record C.
Switch to γ (Co-60 type): count with no absorber, then with Lead 10 mm and 20 mm.
Switch to α (Am-241 type) with no absorber at d = 2, 3 and 4 cm, then with paper at 3 cm.
Plot C against aluminum thickness for beta.

Source	Absorber	Thickness (mm)	d (cm)	C = N/t − B (s⁻¹)
Background	—	—	—	—
β	none / paper / aluminum	0 / 0.1 / 0.5 / 1 / 2 / 3 / 4	3
γ	none / lead	0 / 10 / 20	3
α	none / paper	0 / 0.1	2 / 3 / 4

the absorption bench with the beta source, 1 mm of aluminum and the results table after a 100 s count

Expected results

Typical 100 s counts from the simulation (each repeat differs a little):

Background: about 0.40 counts/s, 24 per minute. A single 100 s count gives 0.31 to 0.51.
Beta at 3 cm: no absorber 159 s⁻¹; paper 151; aluminum 0.5 mm 65, 1 mm 26.5, 2 mm 4.1, 3 mm 0.4, 4 mm 0. Through 1 mm, about 16.6% gets through (15.9 to 17.3% in 30 trials).
Gamma at 3 cm: no absorber 164 s⁻¹; 8 mm of aluminum still lets 145 through; 10 mm of lead about 84 (51%), 20 mm about 43.
Alpha: 48 s⁻¹ at 2 cm, 22 at 3 cm and almost none at 4 cm; a sheet of paper stops it completely.

With 4 mm or more of aluminum, the raw beta reading equals the background, so C is zero. Without the background correction, students would wrongly conclude that some beta gets through.

Inverse square (A-level). On the Inverse square tab, count the gamma source at 5, 10, 20 and 30 cm. C falls from about 72 to 21, 5.8 and 2.6 s⁻¹. Doubling d from 10 to 20 cm divides C by about 3.7, not 4, because the source sits behind the ruler's zero. The graph of 1/√C against d is a straight line; the sim's best fit gives e ≈ 0.9 to 1.2 cm (true value 0.9 cm).

Questions for students

(Prediction, asked again after the lab) Which thinnest absorber stops almost all the beta radiation?
When you compare absorbers for the beta source, what must stay the same?
Beta source, d = 3 cm, no absorber, 100 s: what is the corrected count rate?
Add 1 mm of aluminum. What percentage of the beta count rate gets through?
With 5 mm of aluminum the counter still clicks. Explain why, and why background is subtracted.

Answers for teachers: (1) A few millimeters of aluminum. (2) The source and its distance from the tube. (3) About 159 s⁻¹ (accept 155 to 164). (4) About 16.6% (accept 15 to 18.5%). (5) The beta particles are all stopped; the clicks are background radiation (about 0.4 per second, from rocks, buildings and cosmic rays). It is present in every reading, so it must be measured and subtracted.

Common misconceptions

"Gamma rays are stopped by lead." Lead only reduces them: 10 mm halves the rate, 20 mm leaves a quarter.
"A zero reading means zero counts." The tube always counts background, so the raw reading never drops below about 0.4 s⁻¹.
"A repeat count should give the same number." Decay is random. A longer count gives a larger N and a smaller percentage uncertainty, 1/√N.

Extension

Fields tab: compare how alpha and beta bend in magnetic and electric fields, and why gamma goes straight.
Half-value thickness: count gamma through 0 to 30 mm of lead and find the thickness that halves C (about 10 mm).

FAQ

Is it safe to replace the real practical?

The simulation is safe, but it does not replace hands-on work where your exam board requires it. Use it to plan the method, practice background correction and analyze data. See virtual labs vs physical labs.

Why do my students get different numbers?

Each count is random (Poisson), like a real source. That is a feature: compare results across the class and discuss uncertainty.

Can I pin the settings for my class?

Yes. On your class link, set the starting values for the source, absorber, thickness, distance and count time. The note under the simulation lists the model values, such as the background rate.