Basic performance of unitized Compton scattering type polarimeter

We have been developing a Compton scattering type polarimeter which consists of nine unit counters to detect the polarization of hard X-rays from stellar objects such as the Crab Nebula. We constructed one unit counter which consists of one plastic scintillator (7×7×5 cm³) and four phoswich scintillators (6.7×1.1×32.2 cm³) made of NaI(Tl)+CsI(Na). The characteristics of the unit counter were measured in detail to determine the detection efficiency (20.3%) and the modulation factor (27.5%) at 53.4 keV. The background spectrum of the unit counter in the laboratory is also measured. Based on the results, the background from the outside of the active collimator is reduced by 50.5% when used with the PSD. On the basis of these results, the performance of the planned polarimeter was estimated by computer simulations. The results showed that the polarimeter can detect the polarization of hard X-rays from the Crab Nebula at a MDP (minimum detectable polarization) of 14.0% with a 3-hour balloon observation