A study of high-energy gamma rays at the top of the atmosphere

Abstract

Electromagnetic cascades with energy 0.7·1011≤E0≤1.2·1012eV have been investigated in several stacks of nuclear emulsion exposed at an altitude above 100 000 feet for between 7.5 and 15 h. The energy of 96 events has been measured by using the shower theory of Kamata and Nishimura. Extended evaluations from the theory were made for cascades initiated by an electron pair and by a single γ-ray. The scanning losses were measured and the detector biases calculated in order to determine the absolute intensity of individual γ-rays entering the stack at this altitude. The integral energy spectrum of individual γ-rays was found to obey a power law with an exponent of 1.9−0.2+0.3 in the region 0.7·1011≤E0≤1012 eV. The observed angular distribution of individual γ-rays at a depth into the atmosphere of ∼ 6.5 g/cm2 could be interpreted by a simple calculation and was found to have no significant energy dependence and to show no indication of a sizable contribution from primary γ-rays. The longitudinal development and lateral distribution of these cascades were compared with the average behavior predicted by the shower theory, and the agreement was very satisfactory.