Geiger–Müller Counter

Introduction

A Geiger–Müller counter is an instrument used to detect ionising radiation, most commonly beta and gamma radiation. Special tubes with thin windows can also detect alpha particles, which are helium nuclei.

The detector operates through ionisation of a low-pressure gas inside the Geiger–Müller tube.

Operating Principle

The ionisation chamber contains a gas at a pressure below atmospheric pressure and two electrodes. A high voltage, often several hundred volts and historically described in the original article as approximately 1 kV, is applied between them.

When ionising radiation enters the tube, it ionises the gas and triggers an avalanche discharge. The resulting electrical pulse is detected and amplified by the electronic circuit. The instrument can display an instantaneous radiation value, historically often in mR/h or R/h, or simply count pulses.

Many instruments also include an audible click for every detected event.

Dead Time

A limitation of Geiger–Müller counters is that they cannot register every event in a very intense radiation field. After each discharge, the detector requires a short recovery period during which another particle cannot be counted. This interval is called the detector's dead time.

Practical Instrument

The original article recommended that anyone working with radioactive materials or high-voltage vacuum devices should have a suitable radiation detector, because some vacuum devices can generate X-rays under high voltage.

New radiation meters may cost several thousand Czech crowns, but older instruments can sometimes be obtained inexpensively. The author used an IT-65 radiometer purchased for approximately CZK 200 and reported being satisfied with it.

The specified measuring range was approximately 0.05 mR/h to 500 R/h. The normal background measured at home was below about 0.1 mR/h, while an improvised X-ray experiment produced approximately 10 mR/h in the original measurement.

Radioactive Household Items

Older luminous watch hands, clock dials and signs may contain radium-based luminous paint. These items can remain radioactive even after they no longer glow. Although the amount of radium may be only micrograms, damaged or flaking paint can represent a serious contamination hazard.

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