RHESSI as a Hard X-Ray Polarimeter |
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| Authors: | McConnell Mark L Ryan James M Smith David M Lin Robert P Emslie A Gordon |
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| Institution: | (1) Space Science Center, University of New Hampshire, Durham, NH, 03824, U.S.A;(2) Space Sciences Laboratory, University of California, Berkeley, CA, 94720, U.S.A;(3) Physics Department and Space Sciences Laboratory, University of California, Berkeley, CA, 94720, U.S.A;(4) Physics Department, University of Alabama, Huntsvile, AL, 35899, U.S.A |
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| Abstract: | Although designed primarily as a hard X-ray imager and spectrometer, the Ramaty High-Energy Solar Spectroscopic Imager (RHESSI)
is also capable of measuring the polarization of hard X-rays (20–100 keV) from solar flares. This capability arises from the
inclusion of a small unobstructed Be scattering element that is strategically located within the cryostat that houses the
array of nine germanium detectors. The Ge detectors are segmented, with both a front and rear active volume. Low-energy photons
(below about 100 keV) can reach a rear segment of a Ge detector only indirectly, by scattering. Low-energy photons from the
Sun have a direct path to the Be and have a high probability of Compton scattering into a rear segment of a Ge detector. The
azimuthal distribution of these scattered photons carries with it a signature of the linear polarization of the incident flux.
Sensitivity estimates, based on Monte Carlo simulations and in-flight background measurements, indicate that a 20–100 keV
polarization sensitivity of less than a few percent can be achieved for X-class flares. |
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