PHY095 · Quantum & Nuclear Physics · Class 12 + College
Geiger counter counting
Objective: To understand how a Geiger–Müller counter measures activity, using a digital simulation only.
🧰 Apparatus
- (Digital only) simulated GM tube and counter
- Stopwatch
- Results table
📐 Formulas and SI units
A = λN (Bq = counts/s for a perfect counter)
corrected rate = measured − background
💡 Theory
A GM tube contains low-pressure gas. Each ionising particle that enters starts an avalanche of ions, giving one pulse (a click) on the counter. The count rate is proportional to the activity A = λN of the source. Counts are random, so we count for a fixed time and subtract the background count.
🪜 Procedure
- Note the background count rate with no source (simulated).
- Set λ and N for the simulated source.
- Count for 10 s and calculate the count rate.
- Repeat three times and average.
- Compare with A = λN.
📋 Observations Simulated values
✅ Result and conclusion
10 000 nuclei with λ = 0.01 /s give 100 counts per second (1000 in 10 s) in the simulation.
🔬 In a real lab
Real counters catch only some of the radiation (geometry and efficiency) and miss counts at high rates (dead time).
⚠️ Safety
This experiment is digital only. No radioactive sources are used; the source in the scene is simulated.
❓ Quick check
1. Each click of a GM counter means…
2. Background radiation should be…
3. Doubling N makes the count rate…
🏋️ Practice
Easyλ = 0.002 /s, N = 50 000. A?
Answer: 100 Bq
MediumMeasured 130 /s, background 0.5 /s. Corrected?
Answer: 129.5 /s
ChallengeWhy count for a long time?
Answer: Random counts average out