The chemical composition of the primary cosmic radiation above the earth’s atmosphere

Abstract

The chemical composition of nuclei heavier than beryllium in the primary cosmic radiation has been studied as a function of atmospheric depth between 8 and 40 g/cm2 by making observations on their tracks recorded in a vertical and a horizontal stack of nuclear emulsions flown from Texas, U.S.A., at an atmospheric depth of 6.6 g/cm2. It is shown that it is useful and advantageous to divide theH-group of nuclei (which consists of all elements with charge Z ≥ 10) into three subgroups: theH 1 H2 and H3-nuclei comprising charge values 20/28, 16/19 and 10/15 respectively. This division is necessary because (a) of the rarity of the H2-nuclei in the primary radiation; (b) and also in that it helps to keep the mean mass number of all the charge groups (H 1H2,H 3, M, L almost constant with atmospheric depth. Very strong evidence is given to show that the ratioH/M of the number ofH-nuclei to theM-nuclei, (C, N, O, F), slowly increases with atmospheric depth between 8 and 40 g/cm2; the value at the top of the atmosphere is 0.30±0.02. The proportion of boron nuclei compared to theS-nuclei (Z≥6) outside the earth’s atmosphere has also been obtained; the value is (14±5)%. From this the ratio L(0)/S(0) is estimated to be 0.24±0.09. The absorption mean free paths Λi′ of the H1 H3,H andM-nuclei in air have been found to be 16.3±4.7, 54.9±16.2, 54.3±20.6 and 29.7±2.9 g/cm2 respectively. While the absorption mean free path obtained for theM-nuclei agrees well with the values obtained by other workers, the values for the heavy groups are found to be much larger than those of other workers. It is shown that it is extremely likely that ΛII > ΛM′ for atmospheric depths less than 40 g/cm2. The flux of the H1 H3,H, M and boron nuclei at the top of the atmosphere is found to be 0.71±0.23, 1.42±0.14, 2.07±0.28, 6.69±0.39 and 1.66±0.58 particles/m2 s sr respectively; the flux of the H2-nuclei is estimated to be ≤ 0.11 particles/m2 s sr.