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On the Presence of Lithium, Beryllium, and Boron Nuclei in Primary Cosmic Radiation
The first ascents of sensitive photographic plates to altitudes of 25–30 km, carried out in 1947–1948[^1], showed that, in addition to protons, which constitute the principal share of primary cosmic radiation (p. c. r.), it contains nuclei of heavier elements, beginning with helium and extending to elements with \(Z = 27\text{–}28\) (nickel, iron). The authors of the first studies asserted that the relative abundance of Li, Be, and B nuclei in p. c. r. is very small. Thus, for example, according to data[^2], at geomagnetic latitude \(\lambda = 30^\circ N\) and at an atmospheric depth corresponding to a pressure of \(20 \text{ g cm}^{-2}\), the flux of Be, Li, and B nuclei amounts to only about 20% of the flux of C, N, O nuclei. This fact was associated by the authors with the good agreement between the astrophysical data on the low abundance of Be, Li, and B in stellar matter. Obviously, this question is of great importance for the problem of the origin of p. c. r. and of the transformations undergone by it before it enters the Earth’s atmosphere. Suppose, for example, that, owing to the low abundance of Be, Li, and B nuclei, they do not take part in the creation of p. c. r. occurring somewhere in the universe in accelerator processes unknown to us. In that case these nuclei may nevertheless appear in the composition of the p. c. r. observed in experiments at the boundary of the Earth’s atmosphere.
Indeed, primary cosmic radiation interacts with matter in interstellar space, and the collisions of heavy nuclei with protons of the rarefied interstellar gas must give rise to fragments, including $\alpha$-particles and nuclei of Be, Li, and B. The effective cross sections for the formation of such fragments are more or less well known from observations of collisions of primary protons with heavy nuclei in photographic emulsions. Therefore, by observing the flux of Be, Li, and B nuclei at the boundary of the Earth’s atmosphere, one can estimate the amount of matter traversed by the primary cosmic radiation from the place of its origin to the moment it enters the Earth’s atmosphere, and, consequently, estimate the path traversed.
If, in the composition of the primary cosmic radiation observed at the boundary of the atmosphere, Be, Li, and B nuclei are practically absent, this will indicate that equilibrium between the heavy nuclei of the primary cosmic radiation and their fragments has not had time to become established, and that, consequently, the composition of the observed primary cosmic radiation is close to the relative abundance of the various nuclei that undergo acceleration. Conversely, the presence of a noticeable flux of Be, Li, and B nuclei will indicate that the path traversed by the primary cosmic radiation is comparable with the mean free path for the formation of these nuclei as fragments.
In the paper under review it is shown that in the primary cosmic radiation at latitude $55^\circ N$ Be, Li, and B nuclei are present in the same, if not in larger, quantity as carbon, nitrogen, and oxygen nuclei. This extremely important result was obtained by a method somewhat different from the method of charge determination used in paper $^1$. Recall that in paper $^1$, to determine the charge spectrum of primary particles, their range and the density of grains along the track were measured. In the paper under review, to determine the charge of a particle, the number of $\delta$-electrons $N_\delta$ per unit length of the particle track was measured, along with the mean scattering angle $\bar{\alpha}$ experienced by the particle in the photoemulsion. From the Bethe–Bloch formula for ionization energy losses it follows that
\[ N_\delta = Z^2 \frac{K}{\beta^2}, \tag{1} \]
where $Z$ is the charge of the nucleus, $\beta = \dfrac{v}{c}$ is the particle velocity, and $K$ is a constant.
For the plates used, the constant $K$ was determined by counting the number of $\delta$-particles in the tracks of singly charged particles—protons and $\pi$-mesons. From Williams’ formula for multiple scattering it follows that
\[ \bar{\alpha} = Z \frac{S}{A p \beta}, \tag{2} \]
where $S$ is a constant, and $p$ is the particle momentum $P$ divided by the number of nucleons in the nucleus $\left(\dfrac{P}{A}\right)$. Equations (1) and (2) relate two variables, $Z$ and $\beta$. Measurements of $N_\delta$ and $\bar{\alpha}$ thus make it possible to determine the charge $Z$ of the particle and its velocity, and, consequently, its energy.
The results of measuring the charge spectrum, carried out near $\lambda = 55^\circ N$ at an altitude corresponding to the residual amount of matter above the plates (air pressure plus packaging), $t \simeq 50 \ \mathrm{g\,cm}^{-2}$, are shown in Fig. B. A distinctive feature of the obtained charge spectrum (about 900 particles were included in it) is its high degree of resolution: the spectrum clearly shows maxima corresponding to Li $(Z = 3)$, Be, B, C, N, O, etc. This resolution, obtained without any additional corrections, proves the correctness of the method used. From the presented spectrum it follows that the intensity of the flux of Be, Li, B nuclei
... in cosmic rays, and of the same order as the intensity of the nuclei C, N, O. In order to determine to what extent the result obtained is distorted by the presence of Li, Be, and B nuclei arising as fragments in the residual layer of the atmosphere above the plates, it would have been desirable to make measurements at as high altitudes as possible. The results of these measurements are given in Figs. C and D. They were made at a residual pressure equal, respectively, to 30 and \(20\ \mathrm{g\,cm^{-2}}\). We see that a considerable increase in the altitude of ascent did not have any noticeable effect on the relative abundance of Be, Li, and B nuclei in cosmic rays.
The authors measured the charge spectrum of cosmic rays also by another method, determining \(\alpha\) and the grain density \(d\) in the tracks of multiply charged particles. These measurements were made under conditions in which the total amount of matter above the plates (air, packaging) was about \(t \simeq 70\ \mathrm{g\,cm^{-2}}\). The charge spectrum obtained in these measurements is shown in Fig. A. It follows from it that the spectrum obtained by measuring \(\alpha\) and \(d\) practically does not differ from the charge spectra obtained by measuring \(\alpha\) and \(N_\delta\). Thus, from the work considered it follows that at latitude \(55^\circ N\) the flux of Li, Be, and B has no lower intensity than the flux of C, N, O. Measurements of charge spectra were continued by the authors up to \(Z = 28\). From the spectra obtained it follows that the number of nuclei with a given value of \(Z\) is proportional to the number of stable isotopes of the given nucleus. Thus, for example, in the interval \(8 < Z < 14\) the number of nuclei with an even value of \(Z\) is approximately three times greater than the number of nuclei with an odd value of \(Z\). The authors regard the value of the result obtained for the question of the origin of cosmic rays as unclear. In particular, it is of great interest to determine what amount of interstellar matter cosmic rays traverse before entering the Earth’s atmosphere. From the spectra obtained it follows that the ratio of the number of nuclei with charge \(20 < Z < 30\) to all nuclei with \(Z > 1\) is equal to 150. Knowing this ratio and the order of magnitude of the effective cross section for fragment formation, and assuming that all nuclei with \(Z = 3, 4, 5\) are fragments, the authors obtain that the amount of interstellar matter traversed by cosmic rays does not exceed \(10\ \mathrm{g\,cm^{-2}}\).
Charge spectrum: A) \(t = 70\ \mathrm{g/cm^2}\); B) \(t = 50\ \mathrm{g/cm^2}\); C) \(t = 30\ \mathrm{g/cm^2}\); D) \(t = 20\ \mathrm{g/cm^2}\).
A. V.
Cited Literature
- Bradt and Peters, Phys. Rev., 74, 1828 (1948).
- Peters, Progress in Cosmic Ray Physics, 1952, p. 191.
- Dainton, Fowler, Kent, Phil. Mag., 43, No. 342, 129 (1952).