FROM CURRENT LITERATURE

NEUTRONS IN COSMIC RAYS

About a decade and a half ago it was discovered that, in the composition of cosmic radiation, along with charged particles, there are also neutrons. However, only in the last two or three years has more detailed information been obtained on the nature of the processes of neutron generation in cosmic radiation and on the properties of the particles that generate neutrons of various energies.

The most vivid proof of the connection between the neutrons present in the atmosphere and cosmic radiation is provided by the sharp increase in the number of recorded neutrons when the altitude of the observation site above sea level is changed. At the same time, the study of the altitude dependence makes it possible to establish the character of the processes leading to the generation of neutrons of a given energy.

The change in the number of thermal neutrons \((E_n < 0.4\ \text{eV})\) in the atmosphere from sea level to an altitude with a residual pressure of \(55\ \text{g}/\text{cm}^2\) was investigated in the work of Yuan¹. The registration of thermal neutrons was carried out, as usual, by means of boron counters*). Two such counters were raised by means of balloon sondes into the upper layers of the atmosphere. One of the counters was shielded from the influence of thermal neutrons by a cadmium screen. With the aid of radio equipment, signals recorded on the ground were transmitted from the balloon sondes. The difference in the readings of the two counters gave the number of thermal neutrons at the corresponding altitude. As is seen from Fig. 1, the number of thermal neutrons grows with altitude approximately exponentially, with an exponent \(\mu\) corresponding to

\[ \frac{1}{\mu}=150\ \text{g}/\text{cm}^2, \]

and reaches a maximum at an altitude of \(122\ \text{g}/\text{cm}^2\) residual pressure (about \(15\ \text{km}\) above sea level).

The presence of a maximum is in itself quite natural, in view of the fact that neutrons, because of their instability, cannot be primary particles of cosmic radiation and, consequently, their intensity from the boundary of the atmosphere can only increase.

The same author showed² that at an altitude of \(8\ \text{km}\) above sea level the number of thermal neutrons depends sharply on the geographic latitude of the observation site, increasing almost threefold when moving—

* In all the experiments described below, boron counters of increased efficiency were used, achieved by enriching \(\mathrm{BF}_3\) with the boron isotope \(\mathrm{B}^{10}\).

displacements from 20 to 55° N latitude. A similar latitudinal effect is characteristic of processes connected with the main mass of primary cosmic-ray particles deflected by the Earth’s magnetic field. The stronger deflection of particles incident at the equator, as compared with higher latitudes, leads to a relatively smaller primary cosmic-ray intensity in the equatorial region. The presence of a latitudinal effect for the intensity of thermal neutrons confirms that they are a secondary product of the interaction of cosmic-ray particles with the substance of the atmosphere.

Fig. 1. Dependence of the number of thermal neutrons in the atmosphere on the altitude of the observation site.

Along with the investigation of thermal neutrons, a considerable number of experiments were devoted to the detection in cosmic radiation of neutrons with higher energies and to the study of the mechanism of their generation (in the subsequent exposition, when speaking of neutrons, we shall mean neutrons with energies of the order of several MeV or tens of MeV. Cases of thermal, as well as fast, neutrons \((>40\text{–}50\ \text{MeV})\) will be specified separately).

In particular, in the work of Cocconi, Tongiorgi, and Greisen³, carried out at an altitude of 3260 m above sea level, the generation of neutrons was detected in local showers arising in the thickness of a lead block placed above a neutron detector. As is known, under thick layers of lead one observes, in addition to radiation and δ-showers caused by the soft component, also other showers, the intensity of which increases sharply with altitude above sea level⁴. These so-called electron-nuclear showers were extensively studied by Soviet physicists⁵. A distinctive feature of electron-nuclear showers produced in lead and other materials by particles,

possessing strong nuclear interaction, is the simultaneous appearance of both soft (electrons, photons) and penetrating particles.

Studies of electron-nuclear showers carried out with Wilson's chamber\(^6\) showed that they also contain a large number of particles causing nuclear disintegrations. In all cases when the track of a particle that had caused a disintegration could be visible in the chamber, it could be asserted that the disintegration was caused by a non-ionizing particle.

In the work under consideration by Cocconi, Tongiorgi, and Greisen, showers generated under a great thickness of lead by penetrating particles were recorded by means of Geiger counters. The neutron detector consisted of two groups of boron counters immersed in a paraffin block, which served to slow the neutrons falling on it to thermal velocities. Coincidences of pulses from the neutron counters \((N)\) with pulses from three groups of Geiger counters \((S)\), which recorded showers, were registered. Three other groups of Geiger counters \((A)\), placed around the apparatus at distances of several meters from it and connected in anticoincidence with the counters \(S\), served to exclude extensive atmospheric showers.

To implement coincidences of discharges in the neutron counters with pulses from coincidences \((S-A)\), the latter were delayed for durations of \(150\ \mu\mathrm{sec}\), during which the neutrons were slowed in the paraffin and could be registered by the counters.

In this way an intense generation of neutrons was detected both in lead and in other materials (Fe and C). In a known number of cases, when showers were formed, both groups of neutron counters were triggered. This made it possible for the authors to estimate the average number of neutrons per shower. It turned out that a large number of neutrons are born simultaneously, and this number depends on the material of the block. Thus, the average number of neutrons born in lead is close to 30, while in iron and graphite it is respectively equal to 18 and 10. Lying at the basis of this calculation, the assumption of isotropy in the distribution of neutrons upon the occurrence of a local shower found its confirmation in another work by the same authors\(^7\). The neutron detectors in this work were placed under the lead block and above it (Fig. 2). It turned out that the effect does not depend on the position of the detector.

Isotropy in the distribution of the emitted particles is characteristic of processes of “evaporation” of excited nuclei (nuclear disintegrations of the type of ordinary “stars”). The energy with which we are dealing in these processes is usually relatively small. The generation of “electron-nuclear” showers with the formation of new penetrating particles proceeds in processes of nuclear interaction with much greater energy losses by the generating particles. This circumstance prompted the authors of the work\(^3\) to assume the existence of two processes of different energy connected with the formation of electron-nuclear showers: the birth of new particles in the collision of the generating particles with the nuclei of matter and the “evaporation” of nuclei excited in the process. The large number of simultaneously emitted neutrons led the authors to the conclusion that not only the nucleus on which the birth of penetrating particles occurred is excited, but also the nuclei surrounding it. However, further experiments cast doubt on this assumption. Indeed, upon excitation of other nuclei the number of emitted neutrons should depend substantially on the amount of material surrounding the center of shower generation, in other words, on the thickness

lead layer $\Sigma$ (Fig. 2). However, when the thickness of the layer $\Sigma$ was decreased from 8 cm to 0.6 cm, the number of neutrons decreased only by a factor of two, remaining appreciable in magnitude ($\sim 10 \pm 2$).

Thus, if neutrons are emitted by several nuclei, then in any case there exists an intermediate agent causing the excitation of these nuclei, with a very short range.

The formation of neutrons occurs, as was indicated above, not only in heavy substances, but also in substances with a small atomic number (graphite), and, consequently, also in the atmosphere. Indeed,

Fig. 2. Arrangement for studying the angular distribution of neutrons arising in a lead block. 1, 2, 3, 4—groups of counters selecting showers caused by one charged particle (operation of only one counter from group 1) and containing at least three penetrating particles (coincidence 1—2—3). $H_1$ and $H_2$—two groups of neutron counters.

confirmation was found$^{8}$ of the presence in air of neutrons with energies of 2–15 MeV.

However, the formation of an electron-nuclear shower near a neutron detector is a rather rare phenomenon. Much more frequent is another nuclear process, distinguished by a smaller release of energy, namely the process of “evaporation” of nuclei excited in a collision, not entailing the birth of new particles.

As shown by Tongiorgi$^{8}$, the principal contribution to the number of neutrons registered by the detector is made by the process of star formation. The methods and apparatus used in this work do not differ in the main from those given below in the description of another work by the same author (see Fig. 3). Careful analysis of the results of measurements made at various heights above sea level led the author to the conclusion that the component generating these nuclear disintegrations consists mainly of fast neutrons. In favor of this assertion there speaks, first of all, the sharp increase of the observed effect with height above sea level (corresponding to a mean free path of $132 \pm 15\ \mathrm{g/cm^2}$), which excludes the possibility of explaining the generation of neutrons by $\mu$-mesons. The weak absorption of the gene-

of the recording component in lead also excludes photons and electrons from the number of possible generating agents.

An estimate of the flux of this component makes it possible to choose among the remaining constituent parts of cosmic radiation (neutrons, protons, and other nuclear-active particles). According to an approximate estimate, the flux of the generating component at an altitude of 3200 m amounts to about 20% relative to the entire ionizing component, and at an altitude of 4300 m above sea level—about 40%.

The installation used was sensitive to neutrons of a limited energy interval. Therefore these figures characterized only the lower limit of the number of generating particles. Since the fraction of nuclear-active charged particles (i.e., protons, as well as mesons strongly interacting with nuclei) in the total flux of ionizing particles at these altitudes is small, the conclusion that neutrons are the main part of the generating component is convincing and is in agreement with the results of other investigators (see the review by V. L. Ginzburg^9, pp. 215, 221).

Above the neutron detector, layers of lead and other substances were placed in turn. It turned out that in different materials the generation of neutrons occurs with a cross section depending on the atomic number of the absorber substance, and this dependence is well expressed by the law \(A^{2/3}\). In this case, neutrons are usually born not singly but in groups. On average, in Tongiorgi’s lead apparatus, 8 neutrons appeared simultaneously. This number decreased for lighter elements, reaching \(\sim 1.6\) for graphite. Thus, the smaller magnitude of the energy characteristic of the process of nuclear “evaporation,” as compared with the energy of electron-nuclear showers, is manifested also in the smaller number of emitted neutrons.

Tongiorgi’s data are well confirmed by the results of another work^10.

The registration of neutrons in this work was carried out with the aid of an ionization chamber filled with \(\mathrm{BF}_3\) and surrounded by a layer of paraffin. The intensity of neutron generation in various substances was compared. An estimate was also made of the average number of neutrons born simultaneously, under the assumption that their source is nuclear disintegrations, and that the generation of the latter occurs proportionally to the geometrical cross section of the nuclei of the given substance. The ratio obtained,

\[ \frac{\nu_{\mathrm{Pb}}}{\nu_{\mathrm{C}}}=6.4 \]

(\(\nu\) is the average number of neutrons appearing simultaneously as a result of nuclear disintegration), is in agreement with Tongiorgi’s result.

From observations of nuclear disintegrations in photographic plates one can determine the absolute number of “stars” appearing in the emulsion of the plate per unit time.

Knowing the dependence of the number of generated neutrons on the substance, the authors were able to explain the whole observed effect, considering these nuclear disintegrations to be the main source of the neutrons registered by them. For this it proved sufficient to assume that in each of the “stars” there are born on average from 2.6 to 4.6 neutrons. If one recalls that the average number of strongly ionizing particles falling to one “star” in the emulsion is \(9 \sim 3\)—4, then the number of neutrons obtained is reasonable. In turn, the identical order of magnitude of the numbers of neutrons and ionizing particles falling to one nuclear disintegration serves as a justification for the assumption made that the main mass of neutrons is born in nuclear disintegrations. As in Tongiorgi’s work, the authors observed a sharp increase in the number of counts ...

detector with altitude (the absorption coefficient $\mu$ in the atmosphere corresponded to $\dfrac{1}{\mu}\sim 130—160\ \mathrm{g/cm^2}$.)

The authors investigated, in greater detail than in work$^{8}$, the altitude dependence of neutrons generated in absorbers with different atomic numbers. It turned out that a smaller atomic number corresponds to a sharper increase with altitude (thus for paraffin $\dfrac{1}{\mu}\sim 130—140\ \mathrm{g/cm^2}$, and for lead—about $160\ \mathrm{g/cm^2}$). The authors do not explain the observed difference in absorption. It is possible that it is explained by the more effective capture, by heavy lead nuclei, of negative $\mu$-mesons, whose fraction in the penetrating component increases as one approaches sea level. As is known, negative $\mu$-mesons that stop in matter have time to decay only in the case of a small atomic number of this substance. The larger the charge of the daughter nucleus, the greater the role played by the parallel process of capture of these mesons. Nuclei excited in this way expend their excitation energy by emitting one or several particles, among which there may also be neutrons. At present there is no complete information on the decay products of nuclei that have captured negative $\mu$-mesons. It is known that in captures occurring in a photographic emulsion, and also in Wilson chambers, slow protons characteristic of ordinary “stars” are not observed. If neutrons are emitted in such a process, then as sea level is approached this source of neutrons may become substantial in comparison with the processes caused by nucleons and other nuclear-active particles, the number of which falling into the absorber is larger in the high layers of the atmosphere.

Indeed, an analysis carried out by Sard, Conforto, and others$^{11,12}$ of neutrons associated with penetrating particles that stopped in a layer of lead agrees with the assumption that these neutrons are produced when negative $\mu$-mesons are captured by the nuclei of the absorber (the absolute number of the latter is known from other experiments). For this it is sufficient to assume that, upon capture of each negative $\mu$-meson, on the average 2–3 neutrons are generated, which agrees with the data of the works already discussed by us above. Thus these experiments, in turn, shed light on the nature of the particles emitted upon capture of negative $\mu$-mesons.

Thus, several sources of neutrons of moderate energy are found. Apparently, the main mass of them is generated by nuclear-active particles in “stars” (small energy release), and also in the process with a much larger energy release—during the formation of electron-nuclear showers. It is possible that the process of capture by nuclei of matter of negative $\mu$-mesons also contributes to the total number of neutrons, especially at low altitudes.

In recent years, a number of Soviet and foreign physicists have discovered the presence of a large number of penetrating nuclear-active particles also in the composition of broad atmospheric showers of cosmic rays$^{13}$. In connection with these particles it was natural to expect the appearance of neutrons also in broad showers.

The pages of our journal have already described$^{14}$ the first experiments, carried out by Tonkiordzhi, which led to the discovery at sea level of neutrons genetically associated with broad atmospheric showers. In these experiments, coincidences were recorded between pulses from neutron counters and pulses from three Geiger counters located near the neutron detector and registering the passage of a broad shower (for a brief description of the radio-engineering method of coincidence registration, see $^{14}$).

FROM CURRENT LITERATURE

At present, detailed results have been published of further investigations of the processes of neutron formation in extensive showers. These investigations were carried out at various altitudes up to 4300 m above sea level.^15 The neutron detector (Fig. 3, position I), analogous to that used in the work,^8 was successively surrounded by various absorbers (positions II–VI). In position I, mainly neutrons arriving from the air were recorded. (Careful analysis showed^16 that the generation of neutrons in the paraffin of the detector itself could be neglected.) The use of different groups of Geiger counters, differing in area, made it possible

Fig. 3.

to estimate the fraction of neutrons in showers of different densities. It turned out that, to a first approximation, the density of neutrons is proportional to the total density of charged particles and amounts to approximately 1–2% of it. Adding to I an additional layer of paraffin sharply reduced the counting rate, thereby confirming the quite small energy of the arriving neutrons (on average about 3 MeV). Comparison of the magnitude of the effect in I and III, and also in the II and IV positions, indicated active generation of neutrons in the lead layer Σ.

By replacing the lead in layer Σ with iron and graphite, the authors confirmed the generation of neutrons in these substances as well.

The density of the flux of generated particles increased with increasing density of the recorded extensive showers. At the same time, the generating component proved to be highly penetrating. The addition of 7.5 cm of lead (layer Σ′) had almost no effect on the detector counting rate (compare IV and VI). Thus the main fraction of the particles of extensive showers—electrons and photons—was excluded from the number of possible agents generating neutrons. The possibility of explaining the birth of the main part of the neutrons by the action of photons is also contradicted by the low probability of a process of the type \((\gamma, n)\) for any substantially large value of \(\gamma\), the number of neutrons produced. Meanwhile, as the authors showed, in extensive showers the generating component produces in lead, simultaneously, on average about 60 neutrons. This number was obtained by dividing the neutron counters into two groups (Fig. 4) and estimating the fraction of cases in which both groups were triggered simultaneously. (We note that the average number of neutrons generated in paraffin was close to 1.) The large number of neutrons born in lead could be explained by the simultaneous incidence on the apparatus of several generat—

Fig. 4.

ing particles. However, an analysis of the ratio of the areas of the Geiger counters and the paraffin block of the detector itself makes it possible to conclude that all neutrons are generated by one particle. In this case the inference suggests itself of a greater average energy of nuclear-active particles associated with extensive showers in comparison with particles not accompanied by a sufficiently dense flux of ionizing particles. (Compare \(v=60\) with \(v=8\) for neutrons not accompanied by extensive showers.) This conclusion is in agreement with a number of works by Soviet authors devoted to the investigation of the properties of nuclear-active particles \(^{17}\).

An estimate was also made of the flux of the generating component. The difference in the effect of III and IV, caused by the generation of neutrons in layer \(\Sigma\), could be attributed both to the action of nuclear-active particles and to the action of \(\mu\)-mesons. It turned out that in the latter case one would have to require that the number of \(\mu\)-mesons in the shower amount to about 23% of the number of electrons, which is in sharp contradiction with the experimental data. Such a large percentage is connected with the known fact of the weak absorption of \(\mu\)-mesons in matter. Thus the Tongiorgi data excluded \(\mu\)-mesons from the number of possible generating particles. At the same time, attributing the production of neutrons to the action of nuclear-active particles, one can explain the observed effect. In this case the fraction of such particles in the shower could not exceed 2–3% of the number of electrons. It is known from various experiments that charged penetrating particles in extensive showers amount to about 2% of the number of electrons. The majority of them are nuclear-active \(^{13}\). The generation of the remaining part of the neutrons could be assigned to fast neutrons, whose presence in extensive showers was detected, in particular, in other works of Cocconi’s group. Thus, Brown and McKay \(^{18}\) observed the generation, in lead plates placed inside a Wilson chamber, of electron-nuclear showers caused by neutral particles. The chamber was controlled by trunks of extensive showers passing at a distance of several meters from it. The fairly large statistics of the observed cases allowed the authors to estimate the fraction of fast neutrons in extensive showers as 40% of the number of all penetrating particles.

Thus, the experimental data agree, as to order of magnitude, with the assumption that neutrons in extensive showers are generated by nuclear-active particles, in particular by fast neutrons.

The presence of a considerable number of neutrons in extensive atmospheric showers, their generation by nuclear-active particles of high energy, and the ability of fast neutral particles to form electron-nuclear showers containing, in turn, neutral and strongly ionizing particles—all these experimental facts find their explanation within the framework of the hypothesis of the nuclear-cascade process \(^{19}\), developed in recent years by Soviet physicists \(^{5}\).

In their turn, these facts constitute additional confirmation of the objective existence of the nuclear-cascade process and of its role in the various phenomena observed in cosmic rays.

L. E.

CITED LITERATURE

1. Luke C. L. Yuan, Phys. Rev. 74, 504 (1948).
2. Luke C. L. Yuan, Phys. Rev. 76, 1267 (1949).
3. G. Cocconi, V. Cocconi-Tongiorgi a. Greisen, Phys. Rev. 74, 1867 (1948).
4. G. Zhdanov and A. Lyubimov, DAN USSR 55, 119 (1947).
5. N. Birger, V. Veksler, N. Dobrotin, G. Zatsepin,

L. Kurnosova, A. Lyubimov, I. Rozental, L. Eidus, JETP 19, 826 (1949).

1. N. Birger and L. Eidus, Dokl. Akad. Nauk SSSR 65, 819 (1949).
2. G. Cocconi, V. Cocconi-Tongiorgi, Phys. Rev. 76, 318 (1949).
3. V. Cocconi-Tongiorgi, Phys. Rev. 76, 517 (1949).
4. V. L. Ginzburg, collection Meson, Gostekhizdat, 1947.
5. C. G. Montgomery and Tobey, Phys. Rev. 76, 1478 (1949).
6. R. D. Sard, W. B. Ittner, A. M. Conforto, M. Crouch, Phys. Rev. 74, 97 (1948).
7. R. D. Sard, A. M. Conforto, M. Crouch, Phys. Rev. 76, 1134 (1949).
8. G. T. Zatsepin and L. I. Sarycheva, Dokl. Akad. Nauk SSSR 69, 635 (1949).
9. Uspekhi Fizicheskikh Nauk 37, 114 (1949).
10. V. Cocconi-Tongiorgi, Phys. Rev. 75, 1532 (1949).
11. J. S. Levinger, Phys. Rev. 75, 1540 (1949).
12. L. N. Korolev, A. L. Lyubimov, A. T. Nevraev, Dokl. Akad. Nauk SSSR 68, 273 (1949).
13. Brown and McKay, Phys. Rev. 76, 1034 (1949).
14. G. T. Zatsepin, Dokl. Akad. Nauk SSSR 67, 993 (1949).