MEASUREMENT OF PAIR-PRODUCTION BY HIGH-ENERGY PHOTONS IN AN ALIGNED TUNGSTEN CRYSTAL

A new measurement has been made of the rate of pair-production in a 3. 2 mm thick tungsten crystal, exposed to photons with energies in the r ange 10 to 150 GeV, for angles of incidence up to 10 mrad from the cry stal axis. A strong enhancement of the pair-production rate is observe d when the beam is aligned along the [100] crystal axis, as compared t o a random orientation. This effect can be exploited in the NA48 CP-vi olation experiment by using a thin crystal rather than an amorphous ma terial to convert photons, thus minimising the scattering of kaons in the converter.