Chapter 10 from BASICS OF NUCLEAR PHYSICS AND OF RADIATION DETECTION AND MEASUREMENT - An open-access textbook for nuclear and radiochemistry students by Jukka Lehto

Photons and particles emitted in radioactive decay ionize gas molecules which phenomenon is utilized in detection and measurement of radiation. In detectors based on the gas ionization, the ionizable gas is inside a metal chamber, which has typically a cylinder shape and is called tube. A voltage is applied to the tube so that the metal wall acts as cathode and a metal wire in the middle of the tube as anode (Figure X.1).

Figure X.1. Gas ionization detector (http://www.equipcoservices.com/support/tutorials/introduction-to-radiation-monitors/).

Gamma radiation penetrates the tube wall and ionizes the filling gas whereas beta and alpha radiations are not able to penetrate the wall. For the detection of alpha and beta active sources they either need to be placed inside the tube or the tube needs to have a penetrable window made of glass, mica or plastic. For the detection of external alpha radiation the window thickness should be very small. The filling gas is typically noble gas, such as argon, that the radiation ionizes to Ar⁺ ions. Due to electric field applied between the electrodes these argon cations transfer towards the cathode, the tube wall, while the electrons transfer towards the anode, the metal wire in the middle of the tube. From the anode wire the electrons are transported through an external circuit to the tube wall where they neutralize Ar⁺ ions back to Ar atoms. The electrons going through the external circuit are registered as an electric pulse representing an individual radiation absorption event. Thus the number of electric pulses corresponds to the number of radiation absorptions in the tube which in turn corresponds to the number particles or photons hitting the tube, i.e. the number of pulses corresponds to the activity of the source detected. As will be explained below the height of a pulse corresponds to the energy of a particle or a photon being absorbed in the tube in the case of two modes of gas ionization detectors (ionization chamber and proportional counter) but not in the third mode (Geiger-Műller counter). Depending on the voltage applied across the tube there are three types of gas ionization detectors (Figure X.2).

• Ionization chamber
• Proportional counter
• Geiger-Műller counter

Figure X.2. Operation ranges of three gas ionization detectors as a function of high voltage applied across the tube (http://www.canberra.com/literature/fundamental-principles/).