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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