PAIR PRODUCTION BY 5-150 GEV PHOTONS IN THE STRONG CRYSTALLINE FIELDSOF GERMANIUM, TUNGSTEN AND IRIDIUM

Experimental results for pair production by 20-150 GeV photons in a Ge crystal cooled to 100 K are presented. The results include total and differential enhancements for incidence of the photons along the (1 1 O) plane with angles 0-4.5 mrad to the (1 10) axis. For the first time , a comparison can be made with theoretical results which for incidenc e along the plane predict pronounced structural changes of a resonance type in the pair production yield as a function of angle to the axis. These changes originate from the coherent interaction with the strong field of the atomic strings in the crystal. Furthermore, results for pair production by 5-55 GeV photons in W and Ir crystals at temperatur es between 100 and 300 K are presented. These results are for total en hancements with photons incident along the strongest axes in these mat erials, i,e, the (111) axis in W and the (110) axis in Ir. A clear dis agreement with theoretical curves calls for improved calculations. (C) 1998 Elsevier Science B.V.