Background Radiation

Geiger Muller tube

Definition

A Geiger-Muller (GM) tube is a device used to detect and measure ionizing radiation, such as alpha particles, beta particles, and gamma rays. It is the key component of a Geiger counter.

Diagram

How the GM Tube works

Construction:

The GM tube is a sealed metal or glass tube filled with a low-pressure inert gas (e.g., argon or neon) mixed with a small amount of a halogen gas.

Inside the tube is a central wire electrode surrounded by the gas, with the tube wall often acting as the other electrode.

Principle:

When ionising radiation enters the tube, it ionises the gas inside.

The ions and electrons produced cause a brief electrical discharge or pulse as they move between the electrodes under a high-voltage electric field.

Detection:

Each pulse corresponds to a single ionizing event, which is counted by the connected electronics.

How to measure background radiation

its random nature means that you should:

  • measure over long periods of time. e.g 1000s

  • Repeat and calculate the mean value

Small variations in count rate are expected due to radioactive being a random process

Key Notes

  • No anomalies as it is a random process

  • Greater time reduces effect of random nature of decay

  • Background count must always be measure at least 3 times

  • Background count must always be measured over 60 or 100 seconds

  • Then the mean value should be calculated

  • The background account must be subtracted from overall readings

Typical background count is 0.5 s-1

Results Interpretation

  • If the count rate drops significantly with paper: The source emits alpha radiation.

  • If the count rate drops with aluminium but not paper: The source emits beta radiation.

  • If the count rate is only reduced by lead: The source emits gamma radiation