The Gamma-Ray Imager/Polarimeter for Solar flares (GRIPS) instrument is a balloon-borne telescope designed to study solar- are particle acceleration and transport. We describe GRIPS's first Antarctic long-duration flight in January 2016 and report preliminary calibration and science results. Electron and ion dynamics, particle abundances and the ambient plasma conditions in solar flares can be understood by examining hard X-ray (HXR) and gamma-ray emission (20 keV to 10 MeV). Enhanced imaging, spectroscopy and polarimetry of are emissions in this energy range are needed to study particle acceleration and transport questions. The GRIPS instrument is specifically designed to answer questions including: What causes the spatial separation between energetic electrons producing hard X-rays and energetic ions producing gamma-ray lines? How anisotropic are the relativistic electrons, and why can they dominate in the corona? How do the compositions of accelerated and ambient material vary with space and time, and why? GRIPS's key technological improvements over the current solar state of the art at HXR/gamma-ray energies, the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), include 3D position-sensitive germanium detectors (3D-GeDs) and a single-grid modulation collimator, the multi-pitch rotating modulator (MPRM). The 3D-GeDs have spectral FWHM resolution of a few hundred keV and spatial resolution <1 mm3. For photons that Compton scatter, usually ⪆150 keV, the energy deposition sites can be tracked, providing polarization measurements as well as enhanced background reduction through Compton imaging. Each of GRIPS's detectors has 298 electrode strips read out with ASIC/FPGA electronics. In GRIPS's energy range, indirect imaging methods provide higher resolution than focusing optics or Compton imaging techniques. The MPRM gridimaging system has a single-grid design which provides twice the throughput of a bi-grid imaging system like RHESSI. The grid is composed of 2.5 cm deep tungsten-copper slats, and quasi-continuous FWHM angular coverage from 12.5-162 arcsecs are achieved by varying the slit pitch between 1-13 mm. This angular resolution is capable of imaging the separate magnetic loop footpoint emissions in a variety of are sizes. In comparison, RHESSI's 35-arcsec resolution at similar energies makes the footpoints resolvable in only the largest ares.
Hard X-ray and gamma-ray emission during solar flares encode information about electron/ion dynamics and provide a proxy to deduce solar atmospheric parameters. Enhanced imaging, spectroscopy and polarimetry of HXR/gamma-ray are emissions over ~20 keV to greater than or approx. equal to 10MeV is needed to study particle transport; the Gamma-Ray Imager/Polarimeter for Solar Flares (GRIPS) instrument is designed to meet these goals. GRIPS' key technological improvements over the current solar state of the art in HXR/gamma-ray energies (RHESSI) include 3D position-sensitive germanium detectors (3D-GeDs) and a single-grid modulation collimator, the Multi-Pitch Rotating Modulator (MPRM). The 3D-GeDs allow GRIPS to reconstruct Compton-scatter tracks of energy deposition, providing enhanced background reduction and polarization measurements. Each of GRIPS' sixteen detectors has 298 electrode strips, each of which has dedicated ASIC/FPGA electronics. In GRIPS' energy range, indirect Fourier imaging provides higher resolution than focusing optics or Compton imaging techniques. The MPRM grid-imaging system has a single-grid design which provides 2x the throughput of a bigrid imaging system like RHESSI. Quasi-continuous resolution from 12.5 - 162 arcsecs is achieved by varying the grid pitch between 1 - 13mm. This spatial resolution will be capable of imaging the separate footpoints in a variety of flare sizes. In comparison, RHESSI's minimum 35 arcsec resolution at the same energy makes footpoints resolvable
in only the largest flares. We discuss GRIPS' science goals, the instrument overall, and recent developments in GRIPS' detector and imaging systems. GRIPS is scheduled for an engineering flight from Fort Sumner in September 2014, followed by two long-duration balloon flights from Antarctica in 2015/16.
The balloon-borne Gamma-Ray Imager/Polarimeter for Solar flares (GRIPS) instrument will provide a near-optimal
combination of high-resolution imaging, spectroscopy, and polarimetry of solar-flare gamma-ray/hard X-ray emissions
from ~20 keV to >~10 MeV. GRIPS will address questions raised by recent solar flare observations regarding particle
acceleration and energy release, such as: What causes the spatial separation between energetic electrons producing hard
X-rays and energetic ions producing gamma-ray lines? How anisotropic are the relativistic electrons, and why can they
dominate in the corona? How do the compositions of accelerated and ambient material vary with space and time, and
why? The spectrometer/polarimeter consists of sixteen 3D position-sensitive germanium detectors (3D-GeDs), where
each energy deposition is individually recorded with an energy resolution of a few keV FWHM and a spatial resolution
of <0.1 mm3. Imaging is accomplished by a single multi-pitch rotating modulator (MPRM), a 2.5-cm thick tungstenalloy
slit/slat grid with pitches that range quasi-continuously from 1 to 13 mm. The MPRM is situated 8 meters from the
spectrometer to provide excellent image quality and unparalleled angular resolution at gamma-ray energies (12.5 arcsec
FWHM), sufficient to separate 2.2 MeV footpoint sources for almost all flares. Polarimetry is accomplished by
analyzing the anisotropy of reconstructed Compton scattering in the 3D-GeDs (i.e., as an active scatterer), with an
estimated minimum detectable polarization of a few percent at 150–650 keV in an X-class flare. GRIPS is scheduled for
a continental-US engineering test flight in fall 2013, followed by long or ultra-long duration balloon flights in
Antarctica.
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