Energy Spectrum ofπMesons Produced by Interaction of Cosmic Radiation with Emulsion Nuclei

Abstract

An analysis of 958 $\ensuremath{\pi}$ mesons ejected from stars which terminate their range in $G5$ emulsion exposed in the stratosphere shows a predominance of negative mesons. The average ratio from all emulsion interactions over a pion kinetic energy ⋜ 45 Mev is $\frac{{\ensuremath{\pi}}^{\ensuremath{-}}}{{\ensuremath{\pi}}^{+}}=5.17\ifmmode\pm\else\textpm\fi{}0.41$. Further study shows that this ratio varies with the charge of the target nuclei and is strongly dependent on the kinetic energy of the emitted mesons. By classifying the stars on the basis of the sum of the charges on the evaporation tracks a large fraction of the stars can be attributed to the disintegration of heavy (Ag---Br---I) or light (C---N---O) target nuclei. On this basis 537 pions originate from heavy nuclei and 297 from light nuclei. The residual group of 124 mesons arise from peripheral collisions with heavy or light target nuclei, interactions with hydrogen, photomeson production, and fundamental particle decay processes. The variation of the charge ratio with pion kinetic energy is evaluated for both the light and heavy emulsion nuclei and compared with the yield from targets of similar charge employed in cyclotron studies. The energy spectrum of the mesons produced by cosmic ray interactions on emulsion nuclei from 1.5 to 33 Mev shows good agreement with early Bristol measurements of shower particles originating from stars and with more recent studies of ejected mesons by the Russian group. The form of the differential energy spectrum appears to be largely independent of geomagnetic latitude and altitude of the exposure when the meson yield is expressed in units of particles per shower star per Mev.