Geiger counter lab – absorption of α, β and γ radiation and the inverse-square law
PhysicsQuantum & Nuclear PhysicsAges 17–18
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Sign in to playA virtual practical with a Geiger–Müller tube: measure and subtract the background count, then count α, β and γ sources through paper, aluminium and lead of chosen thickness and over different distances in air. Counts are random (Poisson) with a √N uncertainty and a results table. The Inverse square screen plots 1/√C against d for a γ source to find the hidden source offset e; the Fields screen shows α, β and γ passing through electric and magnetic fields.
Lesson: Ionising radiation: penetration of α, β and γ, the Geiger counter, background count and the inverse-square law for γ radiation
What it shows
Alpha, beta and gamma radiation differ in how far they penetrate. Alpha particles are stopped by a sheet of paper or a few centimetres of air, beta particles by a few millimetres of aluminium, while gamma rays are only reduced by lead, falling exponentially with its thickness. A Geiger–Müller tube also detects background radiation, which must be measured and subtracted. Radioactive decay is random, so every count varies; counting for longer makes N larger and the percentage uncertainty 1/√N smaller. For gamma rays the corrected count rate falls with the inverse square of distance, so 1/√C against d is a straight line.
How to use
Press Count background, then choose a Source, an Absorber and its Thickness, set Distance d and Count time, and press Start count; each result goes into the table. The Instant speed collects data quickly. On the Inverse square tab, count the γ source at several distances and read e from the 1/√C against d graph. On the Fields tab, choose Magnetic or Electric and change the field strength.
Parameters you can change
- Screen Absorption, Inverse square (γ), Deflection in fields
- Radioactive source α (Am-241 type), β (Sr-90 type), γ (Co-60 type)
- Absorber None, Paper, Aluminium, Lead
- Absorber thickness 0.1–50 mm
- Source-to-tube distance d 1–60 cm
- Count time 10 s, 30 s, 60 s, 100 s, 300 s
- Field type (Fields screen) Magnetic field, Electric field
- Magnetic flux density B 0–200 mT
- Voltage V across the plates 0–50 kV
Questions to explore
- Why must the background count rate be measured and subtracted before comparing absorbers?
- About how many millimetres of aluminium stop the beta particles completely, and how does lead reduce gamma radiation?
- Why is the graph of 1/√C against d a straight line that does not pass through the origin?
