The electron avalanche in the detector

A charged particle passing through a gas ionises the atoms of the gas. The atoms split into a negatively charged electron and a positively charged ion. In an electrical field the electrons will move towards the anode and the ions towards the cathode. At the anode an avalanche of electrons is produced indicating the passage of the original particle.


The particle ionises the gas In Charpak's invention the anode consists of a large number of parallel wires, normally a hundredth of a millimetre in diameter and one or a few millimetres apart. The cathode consists of an electrically conductive plane on each side of the densely packed anode wires.	The charges move In the electrical field the liberated electrons rapidly move towards the anode wire and the ions move towards the cathode planes. The electrons are accelerated in the strong field near the anode wire.	The electron avalanche More electrons are liberated which in their turn ionise the gas – an avalanche of charges is produced, giving rise to an electric pulse on the anode wire. The proportional chamber is so called because the pulse is proportional to the original amount of ions.
Each anode wire can handle several hundred thousand signals per second. This is of great importance when rare particle collisions are studied. Sometimes only one particle collision in a million is particularly interesting.