Pair production by 5-150 GeV photons in the strong crystalline fields of 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 0) plane with angles 0-4.5 mrad to the 〈110〉 axis. For the first time, a comparison can be made with theoretical results which for incidence 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 temperatures between 100 and 300 K are presented. These results are for total enhancements with photons incident along the strongest axes in these materials, i.e. the 〈111〉 axis in W and the 〈110〉 axis in Ir. A clear disagreement with theoretical curves calls for improved calculations.