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Jet-X at MSSL |
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SXRP |
SXRP (Stellat X-ray polarimeter) 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 (see ). 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 45x 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 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 IPCsmust 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.
We have chosen to use a multiwire proportional counter with a single amplification stage, a wedge and strip cathode for position sensing, and a rear anticoincidence region. 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 us to continuously measure 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.
Taken from the MOR v1.9 (for complete document go here)
Polarisation sensitivities for SXRP (taken from the SXRP Homepage)
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