POLAR: Design of a novel X-ray polarimeter based on plastic scintillators and multi-anode photomultipliers

POLAR [1] is a space-borne hard X-ray polarimeter whose design has been optimized to measure the level of linear polarization of gamma-ray bursts (GRB) in the energy range 50-500 keV. In POLAR, the GRB photons undergo Compton scattering in a target made out of 1600 plastic scintillator bars (6x6x200 mm3 each). The azimuthal distribution of the scattered photons inside the target provides the information on the GRB polarization. The target is divided into 5x5 units, each one consisting of 8x8 scintillator bars optically coupled with a 64 channel multi-anode photomultiplier (MAPM, Hamamatsu H8500). POLAR, thanks to its large modulation factor (μ100 40%), its large effective area (Aeff = 400 cm2), and its large field of view (around 1/3 of the sky) will be able to determine the degree and angle of polarization of a strong GRB of energy 10-5 erg cm-2 s1 with a minimum detectable polarization of less than 10% (3σ). To be able to measure polarization of photons with energy as low as 50 keV, an energy threshold for each single channel of maximum 5 keV is required. This introduces strong constraints in the photon collection efficiency. To improve it, detailed studies of the scintillator bar surfaces and the available wrapping materials have been performed using both Monte Carlo simulations (GEANT4) and laboratory measurements. At present, a POLAR demonstration model (2 of the 25 units of the final design) is being tested in the laboratory. The engineering-qualification model will be ready in 2010. Investigations conducted by the POLAR instrument will provide very valuable information on how the X and γ-rays are emitted in GRBs. This way, an important contribution to the explanation of the particle acceleration will be achieved. We will present the design and status of POLAR with the newest results acquired from Monte Carlo simulations and from the demonstration model laboratory measurements. (authors)