1. Proportional Counters

In a proportional counter a potential difference of typically 2 keV is maintained across a gas-filled chamber. When an atom of the gas absorbs an X-ray a photoelectron is ejected along with Auger electrons. These electrons ionize more atoms resulting in an electron cloud whose size is proportional to the X-ray energy. The electron cloud drifts towards the anode. When it reaches the high-field region near the anode the electrons gain enough kinetic energy to collisionally excite an avalanche of electrons. This results in an amplification of the signal by a factor known as the gas gain G which is typically 10³-10⁵. In order to maximize the photoelectric cross-section for X-rays a high-Z gas like Ar or Xe is used. The energy resolution of a proportional counter is limited by $\Delta$E² = FN where F $\sim$ 0.1 is the Fano factor and N is the number of electron-ion pairs produced. It also depends on G. N depends on the mean energy required to produce an electron which is 26.2 eV for Ar. The Fano factor takes into account the fact that, for a given X-ray photon, the creation of each ion pair is not statistically independent.

There are special problems associated with operating proportional counters in space. The window must have have a low X-ray absorption cross-section and must also be able to withstand the pressure difference between the enclosed gas and a vacuum. The window materials that are usually used are beryllium or aluminized mylar. The particle background rate is proportional to the volume. For background rejection a guard detector, usually another gas cell surrounded by a shield opaque to X-rays, is used to identify cosmic ray events. Another way to identify charged-particle events is by rise-time discrimination. The absorption of an X-ray photon produces a localized electron cloud and thus a pulse with a fast rise-time. A charged particle will leave an ionization trail in the detector which results in a broader pulse with a slower rise-time. Events with slow risetimes can be tagged by the detector electronics for rejection. CH₄ or another hydrocarbon is added to the gas mixture to quench the discharge quickly. However for high counting rates the detector will have a significant dead time. The detector's response to monochromatic X-rays will have an escape peak at a lower energy due to the finite probability that the absorbing atom will emit a K$\alpha$ X-ray photon that escapes the gas cell instead of an Auger electron. Position sensitivity may be obtained by using multiple anodes and measuring electron drift times.