Basic Performance of PHENEX: A Polarimeter for High ENErgy X rays

We have been developing an instrument named "PHENEX (Polarimetry for High ENErgy X rays)" to measure polarization in the hard X-ray region. The PHENEX polarimeter consists of detector modules called "unit counters". We have investigated the performance of the unit counter, using a highly polarized beam in KEK-PF BL14A. From these measurements, we obtained a detection efficiency of 20% and a modulation factor (analyzing power for polarization) of 53% for 80 keV X rays. Assembling four unit counters, a balloon-flight version was constructed and a preliminary observation of the Crab Nebula was carried out on Jun. 13th, 2006. During this flight, the PHENEX polarimeter observed the Crab for about one hour and detected its hard X rays with a significance of 9sigma. From this result, we expect that a PHENEX polarimeter with nine "unit counters" would achieve a 17% minimum detectable polarization of the Crab Nebula with 3sigma significance in a 3-hour observation.