From Current Literature

Properties of Nuclear-Active Particles in Atmospheric Showers of Cosmic Rays

In 1948, Soviet researchers discovered¹ the presence in extensive atmospheric showers of particles possessing strong nuclear interaction (the so-called nuclear-active particles). It was indicated that these particles are very numerous and are concentrated in considerable numbers near the axes of showers. Nuclear-active particles constituted from one quarter to one half of the number of all penetrating particles and about 0.5% of the number of all shower particles.

Subsequently, the appearance of nuclear-active particles in showers found its explanation as a result of the discovery² of the nuclear-cascade process, which plays an essential role in the formation and development of atmospheric showers.³

The works reviewed below⁴˒⁵ were devoted to the investigation of the nature and spatial distribution of nuclear-active particles in showers.

In work⁴, carried out at an altitude of 3260 m above sea level, the detector of nuclear-active particles was a group of boron proportional counters embedded in a block of paraffin. This block was placed at various distances from another installation—the so-called “shower-axis detector.” In previous works by the authors it had been shown that, in the interaction of nuclear-active particles with lead placed inside such a block, electron-nuclear showers are generated, containing a large number (about 60) of neutrons with energies of several MeV. These neutrons are slowed down in the paraffin and are registered by the boron counters (for details see, for example, the review previously published in our journal⁶).

The “shower-axis detector” consisted of a system of Geiger counters connected in coincidence and placed in the cavity of a lead block with an upper shielding thickness of 9 cm. Such a system is selective for various shower particles and predominantly selects the central regions of showers, which contain electrons of high energy and sufficiently dense fluxes of nuclear-active particles.⁷˒⁵ Above the paraffin block were placed three groups of Geiger counters of different area, making it possible to estimate the “shower density” in the air at the point where the nuclear-active particles fell.

According to the authors’ estimate, at small distances (~10 m) between the two detectors the nuclear-active particles amounted to about 1% of the number of all ionizing particles in the showers, and their fraction decreases only very slightly when the detectors are separated by large distances (up to 96 m). Thus, the spatial distribution of nuclear-active particles in extensive atmospheric showers apparently differs little from the distribution of electrons.

FROM CURRENT LITERATURE

The authors of the second paper\(^5\), relating to an altitude of 4260 m, arrive at an analogous conclusion.

In this work the detector of nuclear-active particles was a lead block with several groups of fast Geiger counters placed inside it.

Showers containing at least two penetrating particles were recorded.

In most cases the inclusion of the counters in a hodoscopic system made it possible to distinguish coincidences caused by electron-nuclear showers from lead from cases in which two penetrating particles from the air fell on the apparatus. In addition, it became possible to estimate the ratio between the number of charged and neutral particles generating electron-nuclear showers.

At distances of 7 and 26 m from this detector other hodoscopic counters were placed, making it possible to estimate the shower density at the indicated distances from the point of incidence of the nuclear-active particle.

Analysis of the hodoscopic photographs showed that in most cases the shower axes passed close to the detector of nuclear-active particles (at distances of \(\sim 5\)–10 m from it); however, the latter appeared often and at a considerable distance from the shower core. This corresponds to a fairly broad spatial distribution of nuclear-active particles, differing little from the distribution of electrons. Approximately it may be assumed that, with distance from the shower axis, the flux density of nuclear-active particles decreases as \(r^{-1}\). The following regularity is also observed: as the density of the penetrating shower in the detector increases, the density of the air accompaniment also increases. This means that nuclear-active particles of higher energy are located in regions close to the shower axes, or else form part of more powerful showers. The authors estimate in several ways the ratio between penetrating nuclear-active (\(A\)) and nuclear-passive (\(\Pi\)) particles. A more accurate method consisted in estimating the cases of appearance in the lead block of secondary particles upon registration by the detector of a penetrating particle accompanied by a discharge in at least two remote hodoscopic counters. After corrections for \(\delta\)-electrons and random coincidences had been introduced, the ratio

\[ \frac{A}{\Pi} = 0.6 \]

was obtained, in agreement with a rougher estimate made by another method and with the data of the Soviet authors cited above.

The electron-nuclear showers recorded by the apparatus were caused mainly by charged particles. The number of ionizing nuclear-active particles was \(1.5 \pm 0.3\) times greater than the number of neutral particles. This ratio was obtained with allowance for the correction introduced for the possible upward emission of ionizing shower particles formed in lead by neutral particles. In conclusion the authors present their considerations on the possibility of explaining the obtained law of the spatial distribution of nuclear-active particles in showers on the basis of a nuclear-cascade process leading to the knocking out from the shower core of active particles of relatively low energy. In this case the nuclear-active particles observed at a distance of \(\sim 10\) m from the shower axis will be collected from a height of only a few hundred meters above the apparatus.

L. Eidus

CITED LITERATURE

1. G. T. Zatsepin and L. I. Sarycheva, DAN 69, 635 (1949); S. A. Azimov, N. Bifger, and A. Gorbunov, DAN 65, 625 (1949); G. Ya. Artyukhov, G. T. Zatsepin, L. I. Sarycheva, L. Kh. Eidus, DAN 69, 153 (1949).

1. N. G. Birger, V. I. Veksler, N. A. Dobrotin, G. T. Zatsepin, L. V. Kurnosova, A. L. Lyubimov, I. L. Rozental, L. Kh. Eidus, ZhETF 19, 826 (1949).
2. G. T. Zatsepin, DAN 67, 993 (1949).
3. G. Cocconi, V. Cocconi-Tongiorgi, Phys. Rev. 79, 730 (1950).
4. K. Greisen, W. Walker, S. R. Walker, Phys. Rev. 80, 535 (1950).
5. UFN 41, 211 (1950).
6. G. Cocconi, V. Tongiorgi and K. Greisen, Phys. Rev. 76, 1020 (1949).