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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 >~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 >~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>
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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. Such a space <em>GRIPS </em>would provide a tremendous leap forward for high-energy solar physics, with order of magnitude increases in effective area for gamma-ray line (and hard X-ray) imaging, and much better angular resolution and image quality (see Table 1). A space <em>GRIPS </em>will allow the 0.511 MeV positron-annihilation line and the weaker prompt nuclear de-excitation lines to be imaged for the first time. Together with line shifts and shapes from the simultaneous high-resolution spectroscopy, such an instrument will provide powerful new diagnostics of the flare energy release and particle acceleration process. Furthermore, precise polarization measurements would be obtained for the first time to constrain models and determine beaming of energetic electrons.</p>