What Is a Proportional Counter?
A proportional counter is a type of radiation detector. It detects radiation by measuring the number of electrons (electric current) produced by the ionization of gas molecules when energetic particles pass through a gas.
Ionization chambers and GM (Geiger-Müller) counters are radiation detectors based on similar principles, but they differ in the magnitude of the voltage applied to measure the current. Ionization chambers operate at lower voltages than proportional counters, while GM counters operate at higher voltages than proportional counters.
Applications of Proportional Counters
Proportional counters detect radiation such as X-rays, α-rays, β-rays, γ-rays, and neutrons and measure radiation counts and energy. Because of their simple structure and relatively easy manufacturing, proportional counters are often used as detectors in academic research and other fields.
Generally, they are used to measure radiation dose in the air, especially for neutron detection and β-ray contamination detection. Proportional counters for X-rays are also sometimes incorporated as detectors in X-ray fluorescence film thickness meters, which are used for non-contact measurement of plating thickness, etc., because of their larger incident window and higher efficiency compared to semiconductor detectors.
Principle of Proportional Counters
The proportional counter is usually a cylindrical aluminum vessel filled with a mixture of inert gases such as argon, helium, or neon, and methane or butane and operated by applying high voltage to wires stretched inside the tube. The cylindrical vessel has an incident window through which radiation enters the tube.
When the incident radiation passes through the gas, the gas along its path is ionized to produce electrons and cations. The electrons produced are called primary electrons. The primary electrons are accelerated by the electric field in the tube and ionize the gas anew, and these ionized electrons cause further ionization, resulting in a phenomenon called gas amplification in which the number of electrons increases explosively. The amplified electrons flow into the wire inside the tube and are detected as an electrical pulse.
Since the number of amplified electrons is proportional to the energy of the incident radiation, this detector is called a proportional counter. The amplification rate of electrons depends on the type of gas and the applied voltage, but if the gas and voltage are known, the energy of the incident radiation can be measured.
An ionization chamber operates on a similar principle to the proportional counter but operates at a lower voltage and therefore does not amplify the gas. Therefore, proportional counters have the advantage of higher sensitivity than ionization chambers. On the other hand, proportional counters have the disadvantage that they cannot measure the number of primary electrons as accurately as an ionization chamber, and their energy resolution is inferior.