Advantages of hybrid positron sources with granular converters

The interest in crystal effects and more precisely for channeling radiation in axially oriented crystals is underlined. An association of a photon radiator using a crystal followed by an amorphous converter may have advantages concerning heating considerations. Such a set of targets, known as a hybrid source has been already investigated and experimental results gathered at KEK are presented. As the heating load with the accompanying thermal stresses have to be studied, variants for the amorphous converter have been considered and also realized: these variants are using small spheres of high Z material, as tungsten: they are providing a better heat dissipation and lower thermo-mechanical effects, leading to better lifespan. These converters, granular, are described and compared to the bulk converters generally used. Their characteristics concerning the yield and the thermal load are analyzed. Experiments carried out at KEK with such converters provided results which are also related. Such granular targets can also be used in conventional sources.