The Integral payload

Introduction Gamma-ray astronomy is more complicated than other branches of astronomy because at high energies matter is transparent, and therefore source detection and imaging cannot be accomplished with standard optical-like technologies. In addition, the weakness of the source fluxes calls for instruments with large detector areas, which tends to make them large and heavy. The first problem to be solved is how to make an image of a source when the incoming light cannot be focussed, i.e. it passes through most materials without being deviated. The solution adopted for Integral is to use a method known as the ‘coded-mask technique’ (Fig. 1). The concept is simple, even if the associated mathematics is far from trivial. A coded mask can be seen as a chessboard where the black squares are made of very thick and heavy material with high atomic number, and the white squares are either empty or made of very light and thin material. The incoming gammaray radiation is stopped by the black elements, but is unaffected by the white elements. Any source will cast a shadow on the detector, placed a few metres below the mask. From the kind of shadow produced and knowing the geometric characteristics of the mask (the socalled ‘code’), the position and shape of the source in the sky can be reconstructed on the ground. Of course, the presence of several sources in the sky, the effect of the external and internal backgrounds, and the fact that the geometry of the mask is much more complex than a simple chessboard, make the image reconstruction a complex mathematical exercise.