Monogrid Imaging with Pixelized Detectors: The GRIPS Imaging Concept

Abstract We describe the imaging concept used by the Gamma-Ray Imager/Polarimeter for Solar flares (GRIPS) project. In its high-altitude balloon payload form, GRIPS is equipped with a rotating grid (Multi-Pitch Rotating Modulator, MPRM) with 13 pitches ranging from $\approx12.5$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mo>≈</mml:mo> <mml:mn>12.5</mml:mn> </mml:math> ” to $\approx162.5$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mo>≈</mml:mo> <mml:mn>162.5</mml:mn> </mml:math> ” FWHM angular resolutions. An array of spatially-differentiated (pixelized) germanium strip detectors (GeD) allows the localization of incoming photons from $\sim20$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mo>∼</mml:mo> <mml:mn>20</mml:mn> </mml:math> keV to $&gt;10$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mo>&gt;</mml:mo> <mml:mn>10</mml:mn> </mml:math> MeV, to within the strip pitch of $\sim0.5$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mo>∼</mml:mo> <mml:mn>0.5</mml:mn> </mml:math> mm or better. At $\gtrsim100$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mo>≳</mml:mo> <mml:mn>100</mml:mn> </mml:math> keV, Compton reconstruction can also be used to reject non-solar photons and determine polarization. In this introductory paper, we concentrate on the optical response (point-spread-function) of the imaging grid from low to high energies. We also introduce several detector effects, which become progressively more important at higher energies.