the GM counter

The tube consists of a metal outer tube connected to the negative (it’s now called a cathode) and the metal pin in the middle is connected to the positive (called the anode). Between the two is gas under a low pressure. The voltage is about 900V but since the gas doesn’t conduct, nothing happens. Something does start to happen when a particle enters through the window on the right. In the next few picture, we’ll just look at the top half.

Situation 1. Nothing happens yet. The atoms are neutral so they’re not attracted to either the positive of negative side.

Situation 2. An atom has been hit by a particle and is not ionised. An electron is forced off (green) and the atom is now a positive ion (red). The electron is attracted to the anode (- and +) and the ion to the cathode (+ and -)

Situation 3. This is what makes this so beautiful, the positive and negative particles ionise all the other particles that are in their way, which in turn ionise even more particles that start to move etc. This chain reaction is called a “Townsend discharge” or better, Townsend Avalanche.

Situation 4. Everything is quiet again. All the ionised particles have reach the point they were attracted to. At the positive anode, electrons are caries away and at the negative cathode, the ion get their electrons back. For a moment, a current has flown between the anode and cathode and this is what the yellow box measure. All the ions are atoms again and the tube is ready to detect the next particle.