Gamma-Ray Imager Polarimeter for Solar Flares Project

<p> We propose here to develop the <em>Gamma-Ray Imager/Polarimeter for Solar flares </em>(<em>GRIPS</em>), the next-generation instrument for high-energy solar observations. <em>GRIPS </em>will provide a nearoptimal combination of high-resolution imaging, spectroscopy, and polarimetry of solar-flare gamma-ray/hard X-ray emissions from ~20 keV to &gt;~10 MeV (see Table 1). The key new technology (already developed for non-solar astrophysics by collaborator Steven Boggs) is that of a 3D position-sensitive germanium detector (3D-GeD) that allows the position and energy deposition of every photon interaction to be recorded individually, even within the same detector.

With the 3D-GeDs, <em>GRIPS </em>can use a new single-grid imaging system, called the Multi- Pitch Rotating Modulator (MPRM), that provides a near-ideal point response function with twice the throughput per cm2 of detector area compared to the rotating modulation collimator (RMC) imaging of <em>RHESSI</em>. In addition, since Compton scattering (the dominant interaction at &gt;~150 keV) of the incident photons can be tracked by the 3D-GeDs, much of the background can be rejected and the polarization of photons can be determined.</p> <p> The proposed balloon-borne <em>GRIPS </em>consists of a Spectrometer/Polarimeter with sixteen 3DGeDs in four stacked 2_2 planes, together with a single rotating MPRM grid placed 8 meters away, to provide a full-Sun field-of-view and angular resolution (12.5-arcsec FWHM at gamma ray energies) sufficient to separate the footpoints of 2.2 MeV sources for almost all flares.</p> <p> The spacecraft-borne version of <em>GRIPS </em>is envisioned to have sixty-four 3D-GeDs in four stacked 4_4 planes, together with a single grid ~20 meters away, in a spinning spacecraft.