FROM CURRENT LITERATURE

NEUTRONS IN AUGER SHOWERS

Auger showed that if two Geiger–Müller counters are arranged in a horizontal plane, then the number of coincidences recorded by such a system decreases slowly as the distance between the counters is increased, and even at distances on the order of tens of meters exceeds by many times the background of random coincidences. This phenomenon is explained by the presence in the air of showers of genetically related charged particles that pierce the entire atmosphere and cover enormous areas, in some cases reaching several tens of thousands of square meters (Auger showers). The study of Auger showers, carried out during the past ten years both in our country and abroad, has shown that, apparently, this relatively rare phenomenon in cosmic rays consists in the cascade multiplication in the atmosphere of electrons of very high energy arriving from outer space. The energy of these primary electrons, for large showers containing many particles and covering large areas, reaches \(10^{16}\)—\(10^{17}\) eV. It is, however, not excluded that mesons or fast protons also occur in Auger showers among the penetrating particles.^1 Although the number of these particles is small—no more than 2–3% of the total number of particles in the shower—a detailed study of the particles entering into the composition of Auger showers is of great interest, for the following reason.

Fig. 1.

Auger showers consist mainly of electrons and \(\gamma\)-quanta, the number of which increases as the shower develops as a result of radiation processes and pair production, which are purely electromagnetic in character. Meanwhile, the protons and mesons—particles that are essentially nuclear—and the presence of such particles in Auger showers possibly indicates that they may be produced as a result of the interaction of the \(\gamma\)-quanta and electrons of Auger showers with nuclei. In this connection, two papers by Tongiorgi (Vanna Tongiorgi), which appeared in April and July 1948,^2 are of interest; they report the presence of neutrons in Auger showers. The scheme of the author’s apparatus is shown in Fig. 1. Three groups of counters \(a\), \(b\), \(c\), each with an area of \(2000\ \mathrm{cm}^2\), connected for coincidences, were placed at the vertices of an equilateral triangle whose side was \(4\ \mathrm{m}\). This group of counters recorded Auger showers. Along one of the heights of the triangle, at a distance of \(120\ \mathrm{cm}\) from one another, two identical blocks \(N_1\) and \(N_2\) were placed for recording slow neutrons. The blocks consisted of four neutron counters filled with \(\mathrm{BF}_3\) and embedded in paraffin, with dimensions \(45 \times 45 \times 50\ \mathrm{cm}^3\). The efficiency of the neutron counters was 30%. The registration of Auger-shower coincidences

with neutrons \((abcN_1\) and \(abcN_2)\) is associated with a substantial difficulty: the counters \(N_1\) and \(N_2\) can be excited not only by neutrons but also by electrons of Auger showers. With sufficient particle density in an Auger shower, such a pulse may turn out to be of the same order as the pulse from a particle arising when a neutron is absorbed in a counter. To get around this difficulty, the author made use of the fact that the diffusion time of neutrons in a paraffin block is of the order of 200 μsec. Therefore, if one records not simultaneous coincidences \(abcN_1\) or \(abcN_2\), but coincidences \(N_1\) or \(N_2\) with the coincidences \(abc\) shifted by several microseconds, then in this way one can completely get rid of false coincidences and record only coincidences of Auger showers with neutrons. The result obtained by the author is that in Auger showers there is one neutron per 30–40 electrons. In order to clarify the question of the origin of the neutrons associated with Auger showers, various absorbers were placed above one of the blocks with the counters \(N_1\). The arrangements of these experiments and the corresponding numbers of coincidences are shown in Fig. 2. We see that when an additional layer of paraffin 25 cm thick, absorbing neutrons with energies up to 10 MeV (experiment III), is placed above the paraffin block with the counters \(N_1\), the number of coincidences decreases by

\[ \frac{0.12}{0.018} \simeq 7 \]

times. This indicates that the majority of neutrons recorded by the apparatus (Fig. 1) are produced in the air and have energies below 10 MeV. If a layer of lead \(\Sigma\), 4 cm thick, is placed above block \(N_1\) (experiment II), the number of coincidences \(abcN_1\) increases by a factor of 2.3. It follows from this that particles of Auger showers generate neutrons in lead. Experiment IV shows that these neutron-generating particles are only slightly absorbed in 5 cm of paraffin. We turn to experiment V. The lead plate \(S\), placed above a 25-centimeter layer of paraffin, absorbs almost the entire soft component and lets the hard component pass. The intensity in experiment V differs hardly at all from the intensity in experiments IV and II. This points to two possibilities:

1) Neutrons are produced in the lead \(\Sigma\). In this case their energy must be large, since they must pass through 40 cm of paraffin in order to enter the counters \(N_1\).

2) Neutrons are produced by the hard component, and their energy is, for the most part, less than 10 MeV. The choice between these two hypotheses is made by experiment VI. If the lead \(\Sigma\) is removed, the number of coincidences falls by comparison with V by 8 times. This indicates that the hard part of Auger showers generates neutrons in lead whose energy is less than 10 MeV.

A. V.

Fig. 2.

CITED LITERATURE

1. G. Cocconi, A. Loverdo and V. Tongiorgi, Phys. Rev. 70, 846, 852 (1946).
2. V. Tongiorgi, Phys. Rev. 73, 923 (1948); V. Tongiorgi and Cocconi, Phys. Rev. 74, 226 (1948).