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The wide field X/gamma-ray cameras.

Our design is derived from the WATCH instruments that were flown on the GRANAT and EURECA satellites [74,75]. The instruments make use of the Rotation Collimator Modulation Technique (RMC). The results of these instruments have been discussed elsewhere [75-80].

The RMC principle is based on the modulation of the intensity of a source in the instrument FOV, due to the presence of a rotating shadow mask placed in front of the scintillator.

The innovation in the design of the six wide field x/gamma-ray cameras (WIFICAM) for GOYA is the use of independent read-out of the scintillator bars in the detection plane. This permits the use of a nonumiform mask pattern, thereby allowing a significant suppression of the sidelobes typical of previous RMC instruments. The shadow masks are made of tungsten (2 mm thick), absorbing 90% of the photons with energies of 200 keV.

The instrument employs CdZnTe crystals, so photons down to 5 keV can be detected. The coaxial CZT crystals can be manufactured 30 cm long and 1 cm in diameter. 12 strips will be use for each camera, providing a 360 square cm collecting area.

The modulation of the X-ray signal is achieved by the rotation (at 1 Hz) of a single platform carrying all six WIFICAM cameras, rather than by rotation of the individual shadow grids (as was done in the WATCH instruments). This solution has been chosen to reduce the vibration noise associated with the moving parts, possibly interfering with the precise optical and X-ray observations with the pointed telescopes.

The design will permit to calculate on board in a few seconds a precise GRB position.



Alberto J. Castro-Tirado
Wed Mar 5 18:34:03 MET 1997