On the theory of electromagnetic cascades in thin targets and photon multiplicity measurements

The partial probabilities of different scenarios of electromagnetic shower processes are calculated in analytical form. It has been shown that these results are important for measurements of the photon multiplicities which occur during above 100 GeV electron radiation in oriented crystals or in the case of interaction of relativistic electrons with powerful Petawatt laser beams. Quantitative analysis of optimal experimental conditions has been performed. The results are applicable for above few GeV electrons and gamma quanta.