Evaluation of a bread board model gamma-ray burst polarimeter toward installation on the international space station

To elucidate the radiation mechanism of gamma ray bursts, we are developing the polarimeter which consists of segmented plastic scintillators and GAGG(Ce) scintillators attached with multi-anode photomultipliers and avalanche photodiodes, respectively. Constructing the bread board model with 36 pieces of plastic scintillator and 24 pieces of GAGG(Ce) scintillator, we have carried out the beam experiments at the KEK Photon Factory in Japan. At the same time, we carried out also computer simulation and we compared the experimental data with results of simulations. As the results, it was recognized that the modulation factor by the experiments is consistent with that by the simulations. On the other hand, though the detection efficiency obtained by the experiments is in rough agreement with that by the simulation, the value of the detection efficiency can not be well explained reasonably. This discrepancy is likely due to the incomplete energy calibration of the plastic scintillators.