Polarimetry of cosmic gamma-ray sources above e+e− pair creation threshold

We then study the dilution of the asymmetry that parametrize the degradation of the precision due to experimental effects such as multiple scattering. In a detector made with a succession of converter slabs and tracker foils, the dependence of the dilution is found to be different from that predicted assuming a given (the most probable) value of the pair opening angle. The limitations of a slab detector are avoided by the use of an active target, in which conversion and tracking are performed by the same device, in which case the dilution of the measurement of P is found to be manageable. Based on a realistic sizing of the detector, and for an effective exposure of 1 year, we estimate the precision for a Crab-like source on the full energy range to be approximately 1.4%.