The development of gamma-ray burst polarimeter for a small satellite

We are developing a gamma-ray burst polarimeter for a small satellite. It is Compton-scattering-type polarimeter which consists of segmented plastic scintillator and segmented GAGG scintillator. The scattering position in the plastic scintillator and the absorption position in the GAGG scintillator for incident hard X rays can be read out by multi-anode photomultipliers and avalanche photodiodes, respectively. The detection efficiency and the modulation factor amount to about 21% and 39% at 60 keV, respectively. The geometrical area for one module is about 300 cm 2 . If two modules will be installed, the polarimeters will measure the polarization for about thirty-five GRBs in two years. Through the observation of the polarization, the radiation mechanism of gamma-ray bursts will be clarified.