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 range 10 to 150 GeV, for angles of incidence up to 10 mrad from the crystal axis. A strong enhancement of the pair-production rate is observed when the beam is aligned along the 〈100〉 crystal axis, as compared to a random orientation. This effect can be exploited in the NA48 CP-violation experiment by using a thin crystal rather than an amorphous material to convert photons, thus minimising the scattering of kaons in the converter.