EXPERIMENTAL INVESTIGATION OF PHOTON MULTIPLICITY AND RADIATION COOLING FOR 150 GEV ELECTRONS POSITRONS TRAVERSING DIAMOND AND SI CRYSTALS/

Detailed experimental investigations of photon multiplicities for 150 GeV electrons/positrons traversing thin diamond and Si crystals have b een performed, Along axial directions up to 10 photons are emitted in 1.5 mm diamond for a radiative energy loss larger than 4 GeV, This cor responds to a mean free path for photon emission of about two orders o f magnitude shorter than in an amorphous target. This is in agreement with an enhanced radiative energy loss of similar to 30 times that in amorphous targets, The strongly enhanced photon emission leads to radi ation cooling which can result in particles exiting the crystal with a reduced angle to the axis. For incidences along planar directions the average multiplicity is still above one, even for the thinnest crysta ls used in the present experiment, so a single-photon spectrum can onl y be obtained for thicknesses less than or equal to 50 mu m, which, on the other hand, is comparable to the coherence lengths for GeV photon s, leading to destruction of the coherent effects.