The Geiger-Muller (GM) Tube

⚛️ The Geiger-Muller (GM) Tube

Figure 2: Inside the GM tube — radiation ionises argon gas, freed electrons accelerate to the central anode, producing a pulse of current the counter registers.

The Geiger-Muller (GM) tube is the most important radiation detector in GCSE physics. It measures the count rate — the number of radioactive decay events detected per unit time.

How It Works — Step by Step

1. Radiation enters the tube through a thin mica window (alpha particles need a thin window as they cannot penetrate thick materials)
2. Inside the tube is a low-pressure inert gas (typically argon)
3. When radiation passes through, it ionises the gas atoms — knocking electrons off them to create positive ions and free electrons
4. A high voltage (about 400 V) is applied between the central wire (anode, positive) and the outer cylinder (cathode, negative)
5. The free electrons are accelerated towards the anode; positive ions move towards the cathode
6. This movement of charge creates a brief pulse of electric current
7. Each pulse is detected and counted by the electronic counter, producing a "click" or incrementing the count display

What GM Tubes Measure

A GM tube measures count rate in counts per minute (cpm) or counts per second (cps). This is proportional to the activity (number of decays per second) of the source, though the tube does not detect every single decay — its efficiency varies with radiation type and energy.

GM tubes detect all three types of radiation — alpha, beta, and gamma — but with different efficiencies. They are most efficient for beta particles and less efficient for alpha (which may be absorbed before reaching the gas) and gamma (which may pass straight through without ionising).