WPOL is a wide field camera which aims to monitor the X-ray/gamma-ray sources and measures their polarimetric properties. This camera will be used in space to alert a main instrument in case of transient events (gamma-ray bursts, black hole binaries state transition, supernovae, …), and to map the X-ray/gamma-ray polarized sources in our Galaxy, which has never been done up to now. It will be proposed, as an accompanying instrument of the AstroMeV mission, in the context of the next medium mission ESA call (M4). This concept is based upon coded mask imaging, with a detector unit composed of two planes of Silicon double sided stripped detectors (DSSD), a passive collimator and a tungsten mask. Mapping is done on the first plane through mask imaging, and polarization is measured by studying Compton scattering events between the two planes, and inside the mask pattern corresponding to the observed source. The source direction being known through the mask pattern, and the scattered photon direction being measured between the two planes, only the determination of the first energy deposit is needed to compute the whole Compton scattering kinetics and in particular, determine the source photon energy.

View of one WPOL detection plane with 4 DSSD
Schematic view of one DSSD plus its front-end electronics
In the concept presented above, the detector unit is composed of two planes of 4 x 4 DSSD modules, each having a sensitive area of 92x92 mm2. The first plane has a thickness of 0.5 mm and an interstrip pitch of 0.180 mm. The second plane has a thickness of 1.5 mm and an interstrip pitch of 0.360 mm. The mask is composed of a tungsten sheet of 540 x 540 x 0.150 mm placed 400 mm above the detector unit, which define a total field of view of 2 steradian (zero sensitivity limit). With a mask element size of 0.54 mm, and mask – detector distance of 400 mm, the camera will have a angular resolution of 4 arcminutes, which lead to a localization precision of tenths of arcseconds for bright sources. This camera will have an overall weight of 14 kg, and power consumption around 40 W. This is one possibility of this camera concept and the parameters have to be optimized according to the mission profile and allocations. In particular, a third calorimeter layer, possibly in LaBr3 in a similar way as in the PACT proposal, may be added to enlarge the energy range. It will have the drawback to ask for more power, for ~ 4 kg extra mass for the detector unit and possibly a new mask design.

This detector plane concept is based upon well-known technologies (DSSD, IdefX ASIC, ...), and is studied in a R&D program lead by APC, AIM, IRAP, IPNO and CSNSM. Prototypes of these detector planes should be tested in the lab in 2014 – 2015 and potentially during a balloon flight. The coded mask will be implemented in the same way as the SVOM/ÉCLAIR ones.