MESONS OF COSMIC RADIATION NEAR SEA LEVEL

Until now there have not been sufficiently reliable and accurate measurements of the intensity of the electronic component, especially at low altitudes, which is of considerable interest from the standpoint of the existence and origin of the nonequilibrium part. For this purpose Kraushaar¹ carried out measurements of the absolute intensity, energy spectrum, and angular distribution of slow mesons at altitudes of 260 m (New York) and 3240 m (Echo Lake). The indicated data are quite sufficient for solving the problem posed, since there exist, on the one hand, reliable data on the intensity of the total ionizing radiation and separately of its hard component,² and, on the other hand, sufficiently accurate methods for calculating the intensity of the equilibrium soft component (taking into account decay electrons and δ-electrons from mesons). Carrying out quantitative investigations by two different methods (the delayed-coincidence method and the “telescope” method with an additional anticoincidence group of counters below) in different experimental configurations, the author obtains very useful information on slow mesons both from the standpoint of the research technique and from the standpoint of the final results.

We shall give here the following most essential results:

1) The number of stopped mesons, recalculated (by extrapolating the altitude variation) to sea level, under a filter of thickness 105 g/cm² of air equivalent*) is \(8.0 \cdot 10^{-6}\ \mathrm{g}^{-1}\ \mathrm{sec}^{-1}\), including, for the vertical flux, \(5.5 \cdot 10^{-6}\ \mathrm{sterad}^{-1}\ \mathrm{g}^{-1}\ \mathrm{sec}^{-1}\).

2) The angular distribution of slow mesons (with ranges from 15 to 90 g/cm²) at altitudes of 260 and 3240 m is well described by the expressions:

\[ i_1(\theta) = i_1(0)\cos^{3.3}\theta \qquad (H = 260\ \text{m}), \]

\[ i_2(\theta) = i_2(0)\cos^{3.1}\theta \qquad (H = 3240\ \text{m}). \]

At an altitude of 3240 m there was observed (in contrast to the altitude of 260 m) an excess of the result obtained by the “telescope” method over the corresponding result in the delayed-coincidence method, which indicates the presence of some fraction of slow protons. The number of these protons is estimated at 20% of the corresponding number of stopped mesons, or about \(4 \cdot 10^{-6}\ \mathrm{sterad}^{-1}\ \mathrm{g}^{-1}\ \mathrm{sec}^{-1}\).

*) As the corresponding experiments of the author showed, the number of stopped mesons at a given altitude depends only very weakly on the thickness of the filter above the apparatus, up to thicknesses of 300 g/cm².

Returning to the problem formulated at the beginning, we must, following the author, subtract from the total intensity of the soft component at sea level the data for the number of slow mesons (integrated over the corresponding interval of the spectrum).

If the resulting difference is compared with the nonequilibrium soft component, calculated on the assumption that mesons transfer \(1/3\) of their energy to decay electrons\(^3\), it turns out that about 25% of the soft component must be attributed to some additional process of generation. Referring to the angular and altitude dependence also considered by the author for the entire soft component, the author concludes that this process is nuclear interactions of high-energy nucleons. Thus the concept recently put forward by Soviet authors\(^4\), that the entire nonequilibrium soft component in the atmosphere is produced in electron-nuclear showers, is once again confirmed.

V. A.

CITED LITERATURE

1. W. L. Kraushaar, Phys. Rev. 76, 1045—1058 (1949).
2. K. Greisen, Phys. Rev. 63, 326 (1943).
3. G. B. Zhdanov and A. A. Khaidarov, DAN 65, 287 (1949).
4. N. G. Birger, V. I. Veksler, N. A. Dobrotin et al., ZhETF 19, 826 (1949).