661 tures, such as occurs in stellar atmospheres and in thermonuc1ear processes, will not be considered 1. Because photoelectric absorption predominates completely at low photon energies, and penetration theory is elementary under these conditions, attention is directed in this artic1e to photon energies above ""20 kev. On the high energy side, this artic1e does not cover the cascade shower processes which are dealt 2 with in cosmic ray studies • In this connection it is recalled that the cascade shower process, which involves electrons and positrons besides X rays, becomes predominant above 10 Mev in heavy elements, and above 100 Mev in light ones. Theories developed for the study of cascade showers in cosmic rays rely on assumptions about the prob ability of interactions with matter which are adequate only at energies of the order of 1 Gev or more. Below this energy there is a gap in which penetration phenomena are qualitatively known and understood but have not yet been calculated in detail. A few detailed experimental studies which have been made at energies up to 300 Mev will be reviewed in this article.

Autorentext
Prof. Edoardo Amaldi (5 September 1908 5 December 1989) was an Italian physicist. He graduated under the supervision of Enrico Fermi. After the second world war, Amaldi held the chair of 'General Physics' at the Sapienza University of Rome. He was the co-founder of the Italian National Institute for Nuclear Physics, and of CERN and ESRO. He pioneered the European search for gravitational waves.

Inhalt
The Production and Slowing Down of Neutrons.- A. Introduction.- B. Reactions with emission of neutrons and neutron sources.- C. The slowing down of neutrons of energy larger than a few ev.- D. Slowing down and diffusion of neutrons in the chemical region, i.e. E less than or of the order of 1 ev.- E. The diffusion of thermal neutrons.- Penetration and Diffusion of X Rays.- A. Elementary processes.- B. General theory.- C. Calculation of flux distributions in infinite homogeneous media.- D. Effects of boundaries and inhomogeneities.- E. Experimental aspects.- Appendix. Notation, units and normalization.- Sachverzeichnis (Deutsch/Englisch).- Subject Index (English/German).