SXRP consists of two separate polarization analyzers: a thin mosaic graphite crystal that makes use of the polarization dependence of Bragg reflection, and a metallic lithium target that exploits the polarization dependence of Thomson scattering. The graphite crystal and lithium target are surrounded by four imaging proportional counters that detect the Bragg reflected and the Thomson scattered x-rays. The entire polarimeter assembly rotates about the optical axis of the telescope.
The Bragg polarization analyzer consists of a thin graphite crystal mounted above the lithium scattering target. A graphite crystal oriented at 45 degrees with respect to an incoming x-ray beam will reflect only those x-rays with energies satisfying the Bragg condition and with electric vectors lying in the plane of the crystal. If the incident beam is polarized, the intensity of the reflected beam will be modulated at twice the rotation frequency of the polarimeter. The Bragg polarimeter is sensitive in two narrow energy bands, the first order and second order Bragg peaks at 2.6 and 5.2 keV.
The angular distribution of x-rays scattered from the lithium target depends on the polarization of the incident x-ray according to the Thomson scattering cross-section. Maximum scattering occurs when the photon is scattered through an azimuthal angle perpendicular to the photon electric vector. Therefore, the intensity of scattered radiation for a polarized beam is, again, modulated at twice the rotation frequency. The energy band pass for the lithium polarimeter extends from 5 keV, limited by photoelectric absorption, up to 15 keV, limited by the reflectivity of the SODART telescope.
The IPCs are part of the scattering polarimeter subassembly of the SXRP and are used to detect x-rays which have passed through a polarization analyzer, either reflected from a graphite crystal or scattered in a metallic lithium target. There are four IPCs, forming a box which surrounds the graphite crystal and lithium scattering target. X-rays reflected from the graphite crystal fall on a small part of one of the IPCs. There is a small thin window which is devoted to imaging x-rays reflected from the graphite crystal. The remainder of the window area on each counter is used to detect x-rays scattered from the lithium.
To match the energy band pass of the graphite and lithium polarization analyzer, the IPCs must be efficient in an energy band extending from 2 to 15 keV. The x-ray energy is not significantly changed by Thomson scattering or Bragg reflection. Therefore, the accuracy with which we can measure the energy of incoming x-rays is determined by the energy resolution of the IPCs.
Since photons are scattered from the lithium over all angles, the detectors must intercept a large fraction of the solid angle; also, the distance from the target to the detector window must be at least 90 mm in order to reduce false polarization signatures due to spacecraft pointing errors. These two conditions lead to an active area requirement of 100 cm2 for each counter, leading to a total active area of 400 cm2.
A multiwire proportional counter with a single amplification stage, a wedge and strip cathode for position sensing, and a rear anticoincidence region are used. Each counter has an active area of 10 cm by 11 cm sealed with a beryllium window to eliminate the need for a gas system. The drift region is 3 cm. To have good efficiency up to 15 keV, the gas mixture contains a large fraction of xenon - 50% Xenon - 40% Argon - 10% Methane.
Position sensing of the X-rays scattered by the lithium increases the effective modulation factor because it leads to more accurate determination of the scattering angle. Non-imaging detectors used in the same geometry would decrease the polarization sensitivity by a factor of 3. In addition, imaging allows continuous measurement of the background and its possible polarization signature.
A crucial parameter of the IPCs is their background rejection efficiency. The use of a polarization analyzer, Bragg crystal or Thomson scattering target, greatly reduces the number of detected X-rays. Low count rates imply a corresponding increase in sensitivity to background. For the lithium scattering polarimeter, the large active area required further increases the deleterious effects of the background. A low background rate is crucial for the operation of a sensitive polarimeter. The detectors employ five sided anticoincidence (rear and four sides) and anode pulse shape discrimination to reduce the background count rate.
The entire SXRP is contained in a single package with an envelope of 470 cm by 604 cm by 400 cm (not including the aperture cover) and an estimated total mass of 60 kg. The polarization analyzers and the four detectors are mounted on a rotating platform within the SXRP instrument. During normal observations, this platform under goes continuous rotation at a typical rate of 0.5 rpm. In the case of failure of a drive motor, a parafin actuator can be used to change the rotation platform drive to a redundant motor.
All of the preamps, signal processing and digitization electronics, and high voltage power supplies are mounted on the rotating platform. Only low voltage power and digital signal are passed, via a slip ring, between the rotating and non-rotating sections of the SXRP.
The non-rotating section of the SXRP contains a 96 MB mass memory, BIUS interface and control microprocessors, the rotation drive motors, and the low voltage power supplies. The SXRP electronics is divided into two separate, but non redundant parts. Each half of the electronics processes the data from two of the four IPCs. The rotation drive can be operated from either half of the SXRP. Both halves of the SXRP must function to provide optimal performance. However, if either half fails the SXRP will still function but with reduced performance. This design greatly reduces the potential for single point failures of the entire instrument.