POLAR – space-borne Gamma Ray Burst polarimeter

A compact detector dedicated to the measurement of GRB gamma-ray photon polarization is presented. The gamma-ray space detector consists of an array of light, plastic scintillator bars. Contrary to classical spectrometers, a light scintillator is chosen as a detection medium to guarantee the prevalence of Compton scattering in the gamma-ray interactions. Using the Compton effect is a standard method of determining the polarization of incoming gamma-rays in the 10-1000 keV energy range. A segmented detector connected to segmented light sensors and dedicated electronics capable of recognizing time coincidences, gives a unique chance of measuring the polarization of gamma-rays coming from strong, transient sources like GRBs and the strongest persistent sources. POLAR is a Compton polarimeter sensitive to gamma-rays in the energy range 50-500 keV. It consists of an array of scintillator bars optically coupled to Multi-Anode Photo Multipliers. In POLAR photons undergo Compton scattering in a target made out of 1600 scintillator bars. The azimuthal distribution of the scattered photons inside the target provides information on the photon polarization. The design, results of tests with a demonstration model and the current status of the project are presented.