4.3.1 Gas-filled detectors

Gas-filled detectors consist of a chamber filled with a gas (often noble gases) and two voltage plates known as electrodes. The positive electrode is called the anode and is often in the center of the chamber and electrically insulated from the outer casing. The outer casing of the chamber is often the negative electrode or cathode.

Incoming radiation interacts with the walls of the chamber or the gas particles and produces ion pairs. When a voltage is applied between the electrodes, positive ions are attracted towards the negatively charged cathode, and the electrons are attracted towards the positively charged anode. A charge builds up on the anode, causing a voltage change in the circuit. This change in voltage is referred to as a pulse, and the presence of this pulse causes a current to flow in the external circuit. By detecting either this pulse or current, we can detect the presence of ionizing radiation.

Depending on the voltage applied Gas filled detectors have a different response (see Figure below).