Bothe, Walther Max Planck Institute for Medical Research, Max Planck Society;
https://journals.aps.org/pr/pdf/10.1103/PhysRev.94.1402 (Any fulltext)
https://doi.org/10.1103/PhysRev.94.1402 (Any fulltext)
Bothe, W., & Krämer, H. (1954). Shower structure in the higher shower maxima. Physical Review, 94(5), 1402. doi:10.1103/PhysRev.94.1402.