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Proportional Counters

All X-ray proportional counters consist of a windowed gas cell, subdivided into a number of low- and high-electric field regions by some arrangement of electrodes. The signals induced on these electrodes by the motions of electrons and ions in the counting gas mixture contain information on the energies, arrival times, and interaction positions of the photons transmitted by the window. At energies less than 50 keV, X-rays interact with gas molecules via the photoelectric effect, with the immediate release of a primary photo-electron, followed by a cascade of Auger electrons and/or fluorescent photons. To date, gas proportional counters have been the 'workhorses' of X-ray astronomy.

In a proportional counter, X-rays are detected by photoionization of the counter gas. The absorption cross section of the gas determines the energy sensitivity of the detector, with the greatest sensitivity lying at energies just above the absorption edges. Photons deposit all of their energy within a short distance within the detector, so that only one cell is activated. A charged particle ionizes the gas through collisions, hence leaving a trail of ionized particles through more than one cell. (In addition to rejecting charged particles by the number of cells they activate, such signals can also be rejected because the shape of their analog pulse differs from those of X-rays). Once a gas atom is ionized by photons or charged particles, the free electron is attracted to the positive anode wire at the center of the cell. This electron then ionizes more atoms through collisions. Subsequent electrons may recombine with ionized atoms, emitting a photon, which photo-ionizes more atoms. Hence there is a cascade of electrons to the anode, which results in an electrical impulse at the anode wire.

Background Rejection

Background rejection is critical in non-imaging X-ray proportional counters. In all gas counters, there are 3 distinct categories of background rejection techniques: energy selection (rejecting all events which deposit energies outside the X-ray bandpass), rise-time discrimination, and anti-coincidence within a sub-divided gas cell (a technique which completely replaced the early technique of surrounding the active gas cell with a shield of plastic scintillator). Energy selection and the anti-coincidence method can, in practice, both reduce the raw background rates by a factor of 100. A factor of 100 is what is usually required. The rise-time discrimination method becomes less effective as the X-ray energy increases. For 6 keV photons, background reduction factor of 30 has been achieved with this method.

Whether the satellite is in a high-Earth orbit (thus the environment is influenced by solar cycle and the isotropic interplanetary cosmic ray flux) or in low-Earth orbit (where the influences are primarily from cosmic rays, albedo electrons, Compton scattered photons, trapped electrons, and induced radioactivity -- leading to a background heavily dependent on satellite latitude), how to suppress the background differs. In low-Earth orbit, the raw background count rates are an order of magnitude less than in interplanetary space. However, they are still of the same order of magnitude as the brightest cosmic sources.

Timing Resolution

The intrinsic timing resolution of a wire chamber is usually limited by the anode-cathode spacing and the positive ion mobility. These physical factors may limit the resolution to the microsecond level. In the past, the temporal resolution was actually limited by the accuracy of the spacecraft clock and telemetry rate rather than the physics of the detector. However, this has changed and microsecond data are now possible.

Current Trends in Proportional Counter Development

In recent years, there have been 3 main foci of proportional counter development: large area, low background collimated detectors; imaging detectors; and enhanced energy resolution detectors.

Large Area, Low Background, Collimated Detectors

The point source detection sensitivity for non-imaging detectors is proportional to the square-root of the the counter area. It is also limited by source confusion in its field-of-view. Sensitivity to time variations, however, scales linearly with counter area. Thus, non-imaging large area proportional counters continue to be developed for high resolution timing studies of bright celestial sources (the Proportional Counter Array on the Rossi X-ray Timing Explorer, for example).

Imaging Proportional Counters

Electronic position encoding can be done either digitally or in an analog fashion. A digital encoder associates a preamplifier and counting circuit with each pixel, or resolution element, of the field. An analog system estimates event coordinates from the properties of voltage waveforms at different output electrodes. Digital schemes are capable of handling higher count rates, but are more complex systems. In X-ray astronomy, where count rates are typically low and system simplicity is a good thing, the analog readout, in which it is the centroid of the induced charge distribution which is encoded, is more common.

Proportional Counters with Enhanced Energy Resolution

The first gas scintillation proportional counter had an energy resolution of Delta E/E = 0.20/sqrt(E) which surpassed the old avalanche detectors by a factor of two. In less than 10 years, GSPC spectrometers were major players on X-ray missions.

Techniques have developed which improve the resolution, such as electron counting, multistep avalanches, and imaging GSPC which are coupled to a microchannel plate.

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