Observation of the suppression effect on bremsstrahlung

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Abstract
The distance y between the primary pair of a photon-initiated cascade and its first daughter pair has been measured on 58 electromagnetic showers of energy below 1000 Bev, and on 47 above 1000 Bev. An average value of y of 10 mm was found for the low energy group, in agreement with the predictions of the Bethe—Heitler and Migdal theories. For the high energy cascades the average value for y was 13.5 mm, in agreement with the one predicted by Migdal's calculations, and in disagreement with the value predicted by the Bethe-Heitler theory.