LIV. Nuclear reactions produced by slow negative π-mesons

Abstract

Reactions produced by the nuclear capture of slow negative π-mesons have been studied by the photographic method. The disintegrations of the light and heavy elements of the emulsion have been distinguished by methods which are described in detail. Because of the large number of "stars" observed-more than 2500-it has been possible to make a statistical approach to the study of the disintegrations. The general features of the "stars" produced by the disintegration of heavy nuclei are in satisfactory agreement with the predictions of the evaporation theory. In addition, the emission of fast, singly-charged particles has been observed, most of which must be attributed to an alternative process to that of evaporation. Preliminary results have been obtained for the frequency of emission and the energy "spectrum" of these particles. The conclusion is reached that in the initial process of capture-the so-called "primary" process-the energy corresponding to the rest-mass of the φ^- meson is shared among a larger number of nucleons than has commonly been assumed hitherto. Observations on "stars" produced by the disintegration of light nuclei indicate that there is a wide variety in the characteristics of the disintegrations which appear to be strongly dependent on the manner in which the primary distribution of energy takes place. A detailed analysis has been made of some of the disintegrations due to ₁₂⁶C and ₁₄⁷N nuclei, by applying the conservation laws of mass energy and momentum. In the majority of cases, the primary interaction results in the production of one or two fast neutrons and an excited residual nucleus which then disintegrates. In a few eases, however, the primary interaction involves the nucleus as a whole, resulting in its total disruption and the ejection of fast α-partieles.