Proportional counters are photon counting devices, meaning that the detection of each photon (an event) results in a discrete signal in the associated electronics. A typical counter, for example as shown, consists of a gas-filled chamber fitted with one or more x-ray transparent windows.
A basic proportional counter tube. Devices of this type are available commercially.
Photons penetrate the window and pass into the gas inside where interactions with the gas atoms result in the creation of a number of ion pairs (electrons and partially ionised gas atoms). Anodes in the detector volume are held at a positive potential with respect to the rest of the detector. The anodes are usually thin metal wires, and their electric field causes the electrons to drift towards the anodes where the field strength is highest. The energy of the electrons increases, and collisions with other gas atoms cause further ionisation producing more electrons. These secondary electrons themselves drift and acquire enough energy to cause further ionisation (and electrons), and so a large cloud of electrons arrives at the anode in a process known as an avalanche. The quantity of charge produced in the avalanche is great enough to be detectable in an amplifier connected to the anode.
The avalanche is localised – it covers a small portion of the length of the anode – and it is therefore possible to devise instruments which determine the position of the incident photon.
Charge division on the anode is one way of implementing a position sensitive proportional counter. This typically uses a resistive anode and determines the position of the avalanche by sensing either the amplitudes or the rise times of the signals received at either end.
Cross-section of a position-sensitive proportional counter with multiple anodes.
Another method of position sensing uses charge division, but in a different way. In a planar proportional counter, such as a multi-wire proportional counter, a cathode surface facing the anodes will receive an induced charge signal during the avalanche. By incorporating a progressive geometry readout on the surface, such as the wedge and strip pattern or wedge and wedge pattern, the position of the avalanche can be determined by comparison of the induced signal amplitude. This method has been used to achieve both one and two-dimensional imaging in proportional counters within space instrumentation.
Sketch of the counter developed for use on the Yohkoh BCS instrument.
Although proportional counters have been used as X-ray detectors for many years they continue to be refined and improved. A pair of one dimensional counters was used as part of a Bragg Crystal Spectrometer experiment on board the Japanese satellite Yohkoh (launched 1990). Similar detectors were employed in the ReSIK experiment on the Russian KORONAS mission (launched August 2001).