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.
The Production and Slowing Down of Neutrons.- A. Introduction.- I. The discovery of the neutron.- II. The properties of the neutron as a fundamental particle and its role in nuclear structure.- III. General remarks on the interaction of neutrons with matter. The first neutron reactions.- IV. The artificial radioactivity. Various reactions produced by neutrons and their general description.- V. The effect of hydrogeneous substances. The mechanism of the slowing down of neutrons.- VI. A few important properties of slow neutrons and the quantum mechanical representation of processes produced by neutrons.- VII. The velocities reached by slow neutrons.- VIII. Selective capture of slow neutrons.- IX. The main processes involved in the slowing down of neutrons.- B. Reactions with emission of neutrons and neutron sources.- I. The application of the conservation laws to nuclear reactions.- II. The excitation curves of nuclear reactions.- III. The (?, n) reactions.- IV. The (?, n) reactions.- V. The (p, n) reactions.- VI. The (d, n) reactions.- VII. Neutrons in nature.- C. The slowing down of neutrons of energy larger than a few ev.- I. General principles and elementary approach to the problem of slowing down of neutrons.- II. The integral spectrum of the neutrons during the slowing down in an infinite medium.- III. The spatial distribution of neutrons in a medium as a function of the energy.- IV. Experimental data on the space and time distributions of the flux of neutrons of energy ? 1 ev in an infinite medium.- V. Measurements of integral effects produced by epi-cadmium neutrons.- D. Slowing down and diffusion of neutrons in the chemical region, i.e. E less than or of the order of 1 ev.- I. Survey of the main features of the problem.- II. The scattering of slow neutrons by molecules.- III. The scattering of slow neutrons by polycrystalline materials.- IV. The slowing down and diffusion of neutrons.- E. The diffusion of thermal neutrons.- I. General features and mathematical aspects of the diffusion of thermal neutrons.- II. Measurements of the diffusion of thermal neutrons.- III. Detection and absorption 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).
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