INTRODUCTION:
It was invented by the German physicist Hans Geiger (co-discoverer of the atom nucleus) and later improved by his student Walther Muller, therefore the name Geiger-Muller counter.

GEIGER MULLER COUNTER:
A Geiger counter is an avalanche detector and is used to measure and detect forms of ionizing radiation (which includes alpha α particles, beta β particles and gamma γ rays)

PRINCIPLE:
It relys on runaway multiplication of electrons.

CONSTRUCTION:
Geiger-Muller tube, is basically a chamber filled with low pressure gas(inert gas or a mix of organic vapor and halogens.)The tube contains two electrodes, the anode and the cathode, which are usually coated with graphite. The anode is represented by a wire in the center of the cylindrical chamber while the cathode forms the lateral area. One end of the cylinder, through which the radiation enters the chamber, is sealed by a mica window.
A potential difference of +1,000 volts relative to the tube is maintained between the electrodes, therefore creating a strong electric field near the wire.

WORKING:

• Ionization in the gas is caused by the entry of photon or a particular radiation coming through the mica window. 
• The positively charged ions are attracted to their appropriate electrode (i.e. cation to cathode, anion to anode) and they gain sufficient energy to eject electrons from the gas atoms as they pass through the gas. This causes more atoms to ionize. 
• Therefore electrons are produced continuously by this process and rapid gas multiplication takes place.
• The effect of "gas multiplication" is that more than one million electrons are collected by the central electrode for every ion produced in the primary absorption process.When this happens an electrical current is established between the two electrodes.

This current can then be easily collected, amplified and measured or counted and played in the form of an acoustic signal made out of clicks each of which should correspond to the detection of a single ion.

Therefore a Geiger counter is able to detect low-energy radiation because even one ionized particle produces a full pulse on the central wire.