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In recent years, a number of purpose-built scintillator-based polarimetershave studied bright astronomical sources for the first time in the hard X-rayband (tens to hundreds of keV). The addition of polarimetry can help datainterpretation by resolving model-dependent degeneracies. The typicalinstrument approach is that incident X-rays scatter off a plastic scintillatorinto an adjacent scintillator cell. In all missions to date, the scintillatorsare read out using traditional vacuum tube photo-multipliers (PMTs). The adventof solid-state PMTs ("silicon PM" or "MPPC") is attractive for space-basedinstruments since the devices are compact, robust and require a low biasvoltage. We have characterised the plastic scintillator, EJ-248M, opticallycoupled to a multi-pixel photon counter (MPPC) and read out with the CitirocASIC. A light-yield of 1.6 photoelectrons/keV has been obtained, with a lowenergy detection threshold of $\lesssim$5 keV at room temperature. We have alsoconstructed an MPPC-based polarimeter-demonstrator in order to investigate thefeasibility of such an approach for future instruments. Incident X-rays scatterfrom a plastic-scintillator bar to surrounding cerium-doped GAGG (GadoliniumAluminium Gallium Garnet) scintillators yielding time-coincident signals in thescintillators. We have determined the polarimetric response of this set-upusing both unpolarised and polarised $\sim$50 keV X-rays. We observe a clearasymmetry in the GAGG counting rates for the polarised beam. The low-energydetection threshold in the plastic scintillator can be further reduced using acoincidence technique. The demonstrated polarimeter design shows promise as aspace-based Compton polarimeter and we discuss ways in which our polarimetercan be adapted for such a mission.

A Compton polarimeter using scintillators read out with MPPCs through Citiroc ASIC