What Is a Proportional Counter?
A proportional counter tube is a type of radiation detector. It detects radiation by measuring the number of electrons (electric current), utilizing the fact that when energetic particles pass through a gas, the gas molecules become ionized, producing electrons.
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 a lower voltage range than proportional counters, while GM counters operate at a higher voltage range than proportional counters.
Applications of Proportional Counters
Proportional counters detect radiation types such as X-rays, α-rays, β-rays, γ-rays, and neutrons, and measure radiation counts and energy. Due to their simple structure and relatively straightforward manufacturing process, they are often employed as detectors in academic research and other fields.
Typically, they are used to measure radiation dose in the air, particularly for neutron detection and β-ray contamination detection. Proportional counter tubes for X-rays are also occasionally integrated as detectors in X-ray fluorescence film thickness meters. These meters are used for non-contact measurements of plating thickness, among other applications, owing to their large incident window and higher efficiency compared to semiconductor detectors.
Principles of Proportional Counters
Proportional counter tubes are typically cylindrical aluminum vessels filled with a mixture of inert gases such as argon, helium, neon, methane, and butane. They operate by applying a high voltage to wires stretched inside the tube. The cylindrical vessel features an incident window through which radiation enters the tube.
When incident radiation passes through the gas, the gas along its path becomes ionized, producing electrons and cations. The electrons produced are referred to as primary electrons. These primary electrons get accelerated by the electric field within the tube and further ionize the gas, leading to a phenomenon known as “gas amplification,” in which the number of electrons increases significantly. 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 tube. The amplification rate of electrons depends on the type of gas and the applied voltage. However, 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. Consequently, proportional counters have the advantage of higher sensitivity compared to ionization chambers. However, proportional counters may not measure the number of primary electrons as accurately as an ionization chamber, and their energy resolution may be inferior.