Compton telescope with a coded aperture mask:: Imaging with the INTEGRAL/IBIS compton mode

Compton telescopes provide good sensitivity over a wide field of view in the difficult energy range from a few hundred keV to several MeV. Their angular resolution is, however, poor and strongly energy dependent. We present a novel experimental design associating a coded mask and a Compton detection unit to overcome these pitfalls. It maintains the Compton performance while improving the angular resolution by at least an order of magnitude in the field of view subtended by the mask. This improvement is obtained at the expense only of efficiency, which is reduced by a factor of 2. In addition, the background correction benefits from the coded- mask technique, that is, simultaneous measurement of the source and background. This design is implemented and tested using the IBIS telescope on board the INTEGRAL satellite to construct images with 12' resolution over a 29 degrees x 29 degrees field of view in the energy range from 200 keV to a few MeV. The details of the analysis method and the resulting telescope performance, particularly in terms of sensitivity, are presented.