3.2.4.2 Proportional counters

Proportional counters are built with a thin positively charged anode wire in the center of a cylindrical chamber filled with gas. Most of the gas volume will act as an ionization chamber, but in the region closest to the wire the electric field is high enough to make the electrons ionize gas molecules. This will create an avalanche effect greatly increasing the output signal. Since every electron cause an avalanche of approximately the same size the collected charge is proportional to the number of ion pairs created by the absorbed x-ray. This makes it possible to measure the energy of each incoming photon, although with poor resolution.

Gas flow proportional detectors are normally used to detect longer wavelength (low energies). They are typically filled with a gas mixture of 90% argon and 10% methane (called as "P 10"). The argon is ionized by incoming X-ray photons, and the electric field multiplies this charge into a measurable pulse. The methane is to suppress the formation of fluorescent photons caused by recombination of the argon ions with stray electrons. When very long wavelengths (over 5 nm) are to be detected, the argon may be replaced with neon or helium.