THE NATURE OF COSMIC RADIATION *)

D. V. Skobeltsyn

In the last 2–3 years, two groups of young physicists, under the direction of Prof. S. N. Vernov and Doctor of Physical and Mathematical Sciences N. A. Dobrotin, have carried out a very extensive program of work on the study of cosmic radiation, encompassing a broad range of questions that have been developed by various methods and in different laboratories.

These numerous works (numbering in the dozens), to the results of which the present review communication is devoted, were conducted chiefly at the P. N. Lebedev Physical Institute of the Academy of Sciences of the USSR, in some—and quite substantial—part jointly with Moscow University and with the participation of the Academy of Sciences of the Uzbek SSR.

In addition to the leaders I have named, I should also like to mention Corresponding Member of the Academy of Sciences of the USSR V. I. Veksler, who made a very substantial contribution to the development of the high-altitude work of the group of the Physical Institute at its initial stage.

I believe that this collective work may serve as an indicative example of those exceptional opportunities which we, thanks to the leadership of the Party and the Government and to the broad support invariably given to us, are now able to realize.

Cosmic radiation, consisting of particles that have accumulated somewhere in the depths of world space an enormous energy, extraordinary by our terrestrial standards, has for more than 40 years attracted the attention of numerous investigators, who are concentrating ever more intense efforts on this advanced front of nuclear physics.

However, despite outstanding successes and numerous discoveries that have enriched our knowledge with new facts and new concepts of exceptional importance, until very recently we did not have any sufficiently clear picture or

*) Report at the Annual Meeting of the Academy of Sciences of the USSR on February 2, 1950.

D. V. SKOBELTSYN

even the basic scheme of the phenomena of cosmic radiation. Moreover, even the question of the very nature of primary cosmic radiation remained, until recently, a matter of debate.

It is not difficult to indicate the reason for this state of affairs. The phenomena caused by cosmic radiation in the atmosphere are extremely complex. As it penetrates into the depths of the atmosphere, cosmic radiation undergoes many successive transformations. In the lower layers of the atmosphere the observer is dealing, for the most part, no longer with the particles of the primary radiation, but with particles generated by them, and moreover with particles that have originated from the primary particles not in the first, but in some quite remote generation.

Until recently, primary attention was directed to the study of cosmic radiation in the lower layers of the atmosphere, while the stratosphere remained little accessible for experimentation. It is therefore not surprising that attempts to construct a general conception of the phenomena of cosmic radiation did not lead to satisfactory results.

For such constructions there was lacking a firm foundation, since the primary link in the chain of transformations undergone by cosmic radiation in the atmosphere remained undisclosed.

At the present time, as a result of the work to be discussed below, we already have a quite reliable basis for establishing the basic scheme of these transformations. As a result of this work, data have been obtained that make it possible finally to resolve the question of the nature of primary radiation, and, what is especially important, we can now establish in general outline the picture of those phenomena that are caused in the stratosphere by the primary cosmic agent, which provides us with reliable positions for further revealing the mechanism of the processes of transformation of cosmic radiation in the atmosphere in all their complexity.

In order to make clear the circle of ideas underlying the indicated works, I shall have to dwell briefly on those hypotheses that proved untenable and that have now been consigned to the archives, but that for quite a long time enjoyed general recognition.

One such hypothesis, which had seemed very soundly grounded, was the hypothesis of the electronic nature of primary cosmic radiation.

The assumption that the primary particles of cosmic radiation are electrons arose from the need to explain how the high energy of the particles of cosmic radiation can be reconciled with their relatively great absorption in the atmosphere.

The point is that, after it became possible to measure the energy of primary cosmic particles (and the subsequent exposition may perhaps give some idea of how this was done—

it at once became clear that the process of absorption of these particles in the atmosphere cannot be reduced to simple braking associated with the fact that charged particles, moving in a gas or in some other medium, ionize it, tearing electrons away from those atoms with which they collide on their path and expending a certain amount of energy on this. At not very high energy the braking of a charged particle is entirely determined by the expenditure of its energy on ionization. Such “ionization” braking can be calculated exactly.

Thus, if one takes into account the energy of the primary particles and calculates what fraction of it these particles would lose in passing through the Earth’s atmosphere if in doing so they were subjected only to ionization braking, it turns out that most of all particles could freely penetrate through the entire thickness of the atmosphere, whereas in reality only a small fraction of all the particles present in the stratosphere reaches the Earth’s surface.

Another mechanism of absorption is known, however, which leads to a much more rapid dissipation of energy. This mechanism, operating only at very high particle energies, is characteristic of such “light” particles as electrons. Owing to the ability of a fast-moving electron to radiate its energy when colliding with the atoms of the retarding medium, the “radiation braking” associated with this ability will, at high velocities, predominate over ionization braking.

The intervention of new mechanisms in the phenomenon of braking at high particle energy, however, is not limited to this. If only a single radiation mechanism of absorption were at work, then a fast electron moving in a medium would very soon become “overgrown” with a swarm of accompanying particles, identical in their nature with atoms of light, namely, a swarm of high-energy photons, and its energy would very quickly be transformed into the energy of electromagnetic radiation. But high-energy photons also turn out to be strongly absorbed. In being absorbed, they give rise to “pairs” of particles consisting of one negative and one positive electron. The successive repetition of the processes of the generation of photons by electrons, and conversely—of electrons by photons accompanying the motion of the primary electron—leads, as a result of the repetition of many cascades (and of such “cascade multiplication”), to powerful showers of particles if the primary energy is very large, or else to “showers” consisting of several or several tens of particles at lower energy.

The formation of showers is, in general, characteristic of cosmic radiation. As we shall see, besides the electromagnetic mechanism of shower formation just described, which may be called

classical; in cosmic radiation we also encounter other showers of a different nature and different origin.

On the basis of cascade theory, it seemed possible to give a complete explanation of the course of the absorption curve of the predominant—soft—component in the atmosphere, if one assumes that the primary cosmic radiation is a flux of high-energy electrons. Such an electron, entering the stratosphere from outer space, must produce an intense cascade process.

By virtue of cascade multiplication, instead of the primary high-energy electron there appear many other electrons of lower energies generated by it. Therefore, if a flux of such electrons were entering the Earth’s atmosphere from outer space, then the number of particles and the ionization caused by them would at first increase as they advanced into the depths of the atmosphere, which would create the characteristic so-called “transition” maximum. Further, however, the energy of the shower that has formed rapidly degrades and is expended on ionization, now produced not by one but by many particles constituting this shower.

The energy of the primary rays is known. Using the shower, or cascade, theory, it was possible to calculate the absorption curve just described and to ascertain that there is quite good quantitative agreement between the course of the curve calculated in this way and the absorption curve obtained from observational data, which shows how the energy of cosmic radiation is expended and how the flux of particles—and consequently also the intensity of this radiation—weakens as it penetrates into the atmosphere.

The curve that gives the course of absorption of cosmic rays below sea level has an entirely different form. It has been established that here we are dealing not with electrons, but with mesons—particles first discovered in cosmic radiation, whose mass has a value intermediate between the mass of the electron and the mass of the proton. Such particles—mesons—do not undergo cascade multiplication, and the absorption curve of the corresponding—penetrating or hard—component is very flat.

After all these preliminary remarks, one can outline in general terms the state of the question of the nature of cosmic radiation at the stage that preceded the works which are the subject of my report.

For quite a long time the following scheme of the chain of transformations undergone by cosmic radiation in the atmosphere, which I mentioned at the beginning, enjoyed general recognition: primary electron → cascade electrons and photons → mesons. The theory, based, to be sure, on more or less arbitrary hypotheses, made it possible to foresee the formation of mesons through the absorption of photons.

About 10 years ago, however, doubts arose as to the correctness of this scheme, and it was opposed by the assumption that the flux of primary particles consists of positively charged elementary nuclear particles—protons. These doubts and this hypothesis were in part based on direct experimental data. Thus M. Schein in America carried out a series of observations in the stratosphere and found that his results contradicted the assumption of the electronic nature of the primary radiation and the scheme indicated above. He proposed the following scheme: primary protons → mesons → electrons.

Schein’s experiments were not beyond reproach, and his results seemed unconvincing to many. Moreover, the scheme he proposed led to difficulties, of which we shall mention the two most important.

Experiments are possible that allow one to determine directly the sign of the charge of the particles of the primary component, as well as their energy. The point is that primary cosmic particles, if they are charged, on approaching the Earth encounter on their path a definite obstacle in the form of its magnetic field. The surface of the Earth at any point can be reached only by those particles which have a certain definite, very high energy. If this blocking action of the magnetic field is likened to a certain barrier (we are speaking of an energy barrier), then it turns out that the height of this barrier for points on the Earth’s surface at different latitudes is not the same. Investigation of the intensity of cosmic radiation at different latitudes thus makes it possible, on the basis of the theory of this so-called “geomagnetic” effect, to obtain an estimate of the energy of the primary particles and, moreover, to establish the distribution of these particles by energy.

As for the sign of the charge of the particles, its determination proves possible because at equatorial latitudes the directions from the west and from the east are not equally accessible to the majority of particles of the primary cosmic radiation. If the particles of cosmic radiation are positively charged, then near the equator they will penetrate into the stratosphere predominantly from the west, and conversely—from the east if their charge is negative.

In 1939 the well-known American specialist Johnson carried out difficult observations in the stratosphere in search of the described effect of the so-called “azimuthal asymmetry,” and did not discover it (in any case, according to Johnson’s data, this effect did not exceed approximately one tenth of the value that could have been expected).

Johnson’s results, which remained unrefuted and, so to speak, unique for 10 years, were in direct contradiction with Schein’s hypothesis concerning the proton nature of the primary radiation.

Another difficulty encountered by Schein’s hypothesis and the scheme indicated lay in the fact that no mechanism was known which would make it possible to explain the formation of the electrons observed in large numbers already in the very upper layers of the atmosphere. It remained completely unclear how this so-called soft component, predominating in the upper layers of the atmosphere and consisting of electrons and their accompanying photons, arises. Schein himself assumed that such a mechanism might be the decay of mesons, one of whose products turns out to be electrons. However, a more fundamental study of the question showed with complete clarity the untenability of such an explanation of the generation of the soft component, because of the relative slowness of the decay process of fast-moving mesons.

In order to clarify all these questions, two or three years ago Prof. S. N. Vernov began a large-scale study of cosmic rays in the stratosphere, conceived on a broad scale and carried out on a wide front.

The results of these successfully conducted investigations decisively settle the question of the nature of the primary radiation.

The stratospheric investigations in question are based on the use of a method of observing cosmic rays by transmitting signals by radio, a method now brought to a high degree of perfection and virtuoso technique.

S. N. Vernov, who began his investigations almost 20 years ago, has priority in the invention of this method of observing cosmic rays, which subsequently, especially in recent times, has come to be widely used in America as well.

I shall begin with the simplest experiment, whose result by itself already quite unambiguously rules out the hypothesis of the electron nature of the primary radiation. It is based on observations of the effect caused by the cascade multiplication of electrons.

What is involved is cascade multiplication that will take place when high-energy electrons pass through layers of lead. The task was to subject an instrument capable of detecting such an effect to the action of primary particles; for this it was necessary to carry out this, as well as other experiments, at very high altitudes.

In application to the problem posed, the requirement had to be met that high-energy electrons, if they come from outer space, should not have time to undergo cascade multiplication in the layer of the stratosphere located above the level to which the given instrument could be raised. This condition was fulfilled, since the observations were carried out at an altitude of 25–27 km above sea level.

To such altitudes the instruments are raised by radiosonde balloons. Particles of cosmic radiation were recorded by a miniature

with a Geiger–Müller particle counter, in which the entry of each particle causes a rapidly extinguished electric discharge. The ionization pulse caused by such a discharge is transmitted by a radio transmitter as the corresponding radio signal to the Earth, where it is registered by recording on a moving photographic film by a receiving device, which makes it possible to count the particles that have passed in a certain time through the counter located in the stratosphere.

In this experiment the counter, by means of a special mechanism actuated automatically and controlled by this very counter, was moved and alternately introduced inside a hollow lead sphere and then, after a certain number of pulses had been counted, was moved back out of this sphere, subsequently counting for some time the particles not filtered through lead, and so on.

Electrons passing through lead undergo cascade multiplication, producing that transition maximum in the absorption curve of which I have already had occasion to speak. If the radiation incident on the apparatus contains electrons, then the number of counts per unit time in the counter located inside the lead sphere will be several times greater than that observed in the counter when it is outside this sphere. Using the multiplication effect in lead, it proved possible to develop a method of analyzing the electron component, making it possible, from the magnitude of the effect, to judge the energy of the particles composing it. I am touching only in passing on an extensive series of works that were carried out using not only this method but also two other methods of solving the same problem. As a result of all these investigations it was established that in the stratosphere there are electrons in large numbers. Their energy, however, proved to be approximately 10 times less than the minimum energy that primary cosmic electrons would have to possess in order, overcoming the blocking action of the Earth’s magnetic field, to reach the surface of the Earth.

The observed electrons could not have come from cosmic space. They therefore originated already in the Earth’s own atmosphere; the primary electrons, however, were not detected. The results of the experiments described, however, in no way contradict the assumption that the primary component is made up of high-energy protons.

The hypothesis of protons as the particles constituting the primary cosmic radiation nevertheless still encountered a contradiction with Johnson’s experiments, which I mentioned. In order to clarify the question and to obtain new data on particles of higher energies than those with which we have on average to deal in our latitudes (even if only in the stratosphere), there was organi-

organized. The instruments on the balloon-sondes were launched from the deck of a Soviet vessel in the ocean near the equator.

In the present case, the discussion concerns far more complex experiments, which were carried out by the group of S. N. Vernov (N. L. Grigorov, S. P. Sokolov, and others), under the organizational direction also of N. A. Dobrotin.

In this case the particles of cosmic radiation were recorded (also, of course, with transmission of signals by radio) by the method of coincidences in particle counters forming a system conventionally called a telescope, since it records radiation coming from a definite direction.

Fig. 1 schematically shows a series of cylindrical counters forming this telescope, whose axis was set at an angle of \(60^\circ\) to the direction toward the zenith. This angle remained constant, while the azimuth was changed by \(180^\circ\) at definite time intervals. The telescope axis was alternately directed either to the west or to the east. These shifts were made automatically and rapidly, while the intervals of time during which the instrument was in each of the two orientations specified for it were regulated, as in other cases, by the counting device itself. The shifts from one position to the other were carried out after a definite number of counts obtained in the given specified position had accumulated.

Fig. 1.

As for setting the axis of the telescope, which always, independently of the rotation of the instrument associated with its flight, had to be directed either east or west, this was accomplished (and rigidly maintained as constant) by a system of photocells controlled by a sunbeam, which served as the reference for the entire system.

In Fig. 1 the results of the observations are shown. The curves show the intensity of cosmic radiation as a function of altitude, respectively for the western and eastern azimuths. The intensity of the radiation coming from the west proved to be greater than that observed in the eastern azimuth, and exceeded the latter by 60% of the mean value, in agreement with what is given by the calculation for radiation consisting of positive particles. It was thus established that the particles of primary cosmic radiation are positively charged. Johnson’s work, which had been discussed so much, turned out to be incorrect.

Thus, as a result of the work described above, which should of course be regarded as a major event in our science, the question of the nature of primary radiation has finally and with the utmost reliability been resolved: high-energy protons must be recognized as its carrier. There is no room left for other assumptions, unless one admits hypotheses about some new particles not observed under terrestrial conditions, for which there are no grounds. True, the thesis concerning the nature of the primary component should be formulated in a more general form, namely as the assertion that the carriers of the energy of primary radiation, originating in the depths of cosmic space, are positively charged nuclei of atoms, with an overwhelming predominance of the simplest nuclei—the nuclei of the most widespread atoms—hydrogen, i.e. protons.

As is known, at present, by the method of photographic plates (in the photosensitive layer of which traces of nuclear particles can be detected), at great altitudes in the stratosphere traces have already been observed many times of more complex multiply charged nuclear particles as well. Thus, for example, in Fig. 2 there is reproduced a microphotograph of the trace of a particle—possibly the nucleus of an oxygen atom—obtained as a result of observations carried out in the stratosphere with plates studied by G. M. Belovitsky and others at the Physical Institute of the Academy of Sciences of the USSR, in the laboratory of Corresponding Member of the Academy of Sciences of the USSR I. M. Frank.

But if the question of what particles, born in world space, and with what energy reach the boundary of the atmosphere may be considered solved, then the question arises with still greater sharpness as to the nature of those processes which are caused by these particles already in the upper layers of the stratosphere and which account for their very intense absorption.

As a result of the entire body of work I have described, which made it possible to carry out, so to speak, a concentric advance along many directions on this front of science, we now have an answer to the question just posed.

Let us leave the stratosphere for a time and descend to lower altitudes, in order to become acquainted with the results of broad investigations undertaken under expeditionary conditions in the mountains.

Unfortunately, the limited size of my report does not allow me to devote to these works—no less remarkable in scale and experimental skill—the place they deserve by their scope and significance. I must state in general that my exposition cannot be exhaustive. Moreover, I must again depart from the main line of the exposition for some, albeit brief, explanatory remarks.

I have already mentioned cascade electron showers and the cascade theory of these showers. To understand what follows it is necessary to bear in mind the following.

In such cascade showers we are dealing with processes caused by the collision of fast electrons, and also of photons, with the nuclei of the atoms of the medium in which they move. Here, however, the processes in question take place outside the atomic nuclei, at some distance from them, and are caused by forces of electromagnetic character, whose nature is well known to us.

The study of these classical cascade phenomena is of exceptional interest. But since the atomic nuclei in these phenomena play only a passive role, their study does not provide any new paths toward an approach to the problem of the atomic nucleus and, in particular, toward resolving the question of the nature of nuclear forces.

Particles of high energies, however, as calculation shows, repeatedly come into close contact with the atomic nuclei themselves, penetrating them. Such particles may serve, as it were, as probes for investigating intranuclear interactions and the phenomena caused by them. Fast electrons, judging from the entire body of available data, turn out to be nuclear-passive, i.e., particles that do not interact with nuclear matter. For this reason they are also unsuitable for use as such probes.

It may, however, be assumed a priori that the particles which constitute nuclear matter itself, i.e., protons and neutrons—often denoted by the single name “nucleons”—such particles of high energies may prove to be effective projectiles, the bombardment of atomic nuclei by which may be accompanied by peculiar processes depending on the specific nature of the powerful nuclear forces not yet studied by us.

During the mountain expedition of 1945–1946, in the course of the work of V. I. Veksler, G. B. Zhdanov, A. L. Lyubimov, and others in

Fig. 2.

Fig. 3.

Fig. 4.

Fig. 5.

Fig. 6.

Fig. 7.

as a result of extremely simple but successfully designed experiments, a very important discovery was made.

These observations showed that at great altitudes, along with the ordinary cascade showers, in the phenomena of cosmic radiation a very substantial role is also played by peculiar showers of an entirely different kind, which we shall for the time being call “special” showers.

The distinctive feature of these “special” showers consists in the fact that they include particles capable of penetrating through great thicknesses of lead. Layers of lead several centimeters thick prove to be completely transparent for such particles. Moreover, filtration through relatively great thicknesses of lead of the radiation that generates these showers does not substantially affect the intensity of their generation. With ordinary cascade showers just the opposite is observed: shower formation ceases—is suppressed—if the radiation is filtered through 10–20 cm of lead, while the particles themselves that make up the cascade showers are practically completely absorbed by layers of lead several centimeters thick.

As a result of persistent and many-sided study of this new phenomenon, carried out under the general direction of N. A. Dobrotin by the collective efforts of a large group of workers, who created new, very powerful and complex instruments for the investigation of these phenomena, it has now been established that “special” showers arise as the result of a nuclear process of an “explosive” character. Such a nuclear explosion is accompanied not only by the disintegration of the nucleus struck by the “cosmic projectile” that has collided with it (and by the complete breakup of the latter into its constituent parts), but also by the birth of many new particles as a result of such a catastrophic process.

From the totality of the data of which I shall speak further, it follows with a high degree of probability that processes of this kind and such “special” showers arise as the result of the action on atomic nuclei either of the primary particles of cosmic radiation themselves directly, or of other nuclear particles, produced by cosmic radiation already in the Earth’s atmosphere (in particular, neutrons), which have, however, acquired a sufficiently high energy, of the order of the energy of the primary particles.

I shall now describe, though only very briefly, the technique that made it possible to carry out a thorough and many-sided study of all these phenomena.

In describing the experiments of S. N. Vernov in the stratosphere, I mentioned corpuscular telescopes, consisting of two or three Geiger–Müller counters and making it possible to register cosmic particles passing, during a certain time, in some more or less precisely specified direction. The appearance of a single—

temporal coincident pulses in such two or three counters, which constitute a telescope, is a signal announcing the passage of a particle in a given direction. For experiments on the study of “special” showers, much more complex systems were created, composed of dozens of counters—the so-called hodoscopes. Signals arising from the appearance of coincident pulses in a certain, comparatively simple, suitably chosen system of several counters serve in this case to “control” the hodoscope. If this signal, caused by some shower, is received, then the apparatus makes it possible to detect what occurred (simultaneously with the phenomenon that produced the given signal) in each of the counters comprising the hodoscope separately; i.e., the apparatus makes it possible to learn—or rather directly to see—in which of the counters a discharge took place as a result of the passage through it of one, or perhaps several, particles. This can be seen directly with the eye because a small neon lamp is connected with each counter and flashes in the event that, in the given counter, a discharge has occurred caused by a particle entering it.

A system of lamps fixed on a separate panel, so arranged that their positions reproduce with some similarity the positions of the counters, is photographed every time a signal is received from the controlling system.

Figs. 3 and 4 reproduce photographs of panels with flashing neon lamps. In the first of these figures one can see the passage of a single particle, which left a trace in the form of a chain of flashing lamps arranged along a straight line; in the second figure the result of the passage of a shower of many particles is recorded.

Besides the hodoscopic method, another method was also employed, still more effective in the sense of making possible direct observation of the entire picture of the phenomenon: the method of the so-called controlled Wilson chamber.

In this case the pulse from the controlling system of counters actuates the Wilson chamber. Inside the chamber, in the path of the particles of the shower under investigation, alternating plates of lead and graphite are placed. The tracks of the shower particles that have passed through the chamber (which is filled with droplets of mist) can be directly photographed. Figs. 5 and 6 show pictures of showers obtained in this way in the work of N. G. Birger, S. A. Azimov, and others. All these showers, originating in blocks of lead surrounding the chamber and consisting in some cases of many particles that fill the entire volume of the chamber with a dense stream, are “special” showers. The point is that the apparatus is surrounded on all sides by massive shielding of lead, which completely suppresses showers of electrons arriving from outside and, on the other hand,

rons, showers photographed in these pictures, for the most part themselves consist of penetrating particles capable of passing freely, and without causing cascade multiplication, through considerable thicknesses of many centimeters of lead, as is shown especially clearly by Fig. 5.

I could, on the basis of these few photographs, point out a whole series of interesting features, already sufficient to substantiate certain conclusions about the composition and nature of the special showers, which, however, were made on the basis of far more extensive material obtained by applying various methods and by a comprehensive study of the phenomenon as a result of numerous complicated experiments.

I shall confine myself to presenting only the most important of these conclusions. Particles detected in the composition of “special” showers may be divided into three groups. The first group consists of strongly ionizing and comparatively slow particles—products of the disintegration of the atomic nucleus in which the “special” shower arose. These particles are nucleons which have received comparatively small energy and are nothing other than the constituent parts of the atomic nucleus, scattered in various directions, after it has broken up as a result of the explosive nuclear process that has occurred within it. Here we are dealing rather with a by-product of the explosive shower, and the picture observed in this part is probably identical with that which has already been observed many times before in the photosensitive layer of special photographic plates. Fig. 7 shows an example of the pattern of such a star-like splitting, produced in a subsequently developed photographic plate, which is taken from the collection of the Physical Institute of the Academy of Sciences of the USSR. It may be said that here there is a purely “nuclear” shower in the sense that it is composed of particles of nuclear matter—individual nucleons, and in some cases perhaps also alpha particles.

The greater part of the energy of the “special” shower is concentrated, however, in two other components of it, which are of greatest interest. These components—the second and the third—are, on the one hand, electrons of sufficiently high energies, and, on the other, penetrating particles.

The unexpected discovery of electrons in the composition of “special” showers is of exceptional fundamental importance. The presence of this electronic component indicates that here we are dealing with some essentially new mechanism, one which until now had not at all been envisaged by theory. Taking into account this circumstance—the dual nature of “special” showers—we shall henceforth call them electron-nuclear showers.

Of no less substantial interest is another result of the careful study of electron-nuclear showers. It has been established that in the penetrating component of these showers there are

“nuclear-active particles,” i.e., such particles that themselves can produce new electron-nuclear showers. The existence of these particles in the composition of showers leads to the possibility, in the region of very high energies, of a new cascade process, by its nature entirely different from the one of which I have already had occasion to speak repeatedly—namely, the nuclear-cascade process, which reduces to the fact that a particle arising as the result of a nuclear explosion creates a new explosion, accompanied by an electron-nuclear shower, and so on.

The question of the nature of the particles composing electron-nuclear showers will be the subject of further investigations. As regards nuclear-active particles, the natural supposition is that they may be the same protons and neutrons, if as a result of the explosive process they have acquired a very large energy. Along with nucleons, “heavy” mesons of high energies may also possess the property of inducing nuclear processes, i.e., nuclear activity.

It may also be regarded as established that, in the penetrating component of electron-nuclear showers, ordinary mesons, “inactive” in the nuclear sense, are also present in considerable numbers. At present there is already direct evidence for the existence of these latter particles in the composition of “special” showers.

The circumstance that electrons of sufficiently high energies are generated in explosive showers is of primary importance for the explanation of the whole complex of phenomena of cosmic radiation. I have already mentioned that the assumption of the proton nature of the primary radiation seemed little acceptable in view of the fact that it was difficult to understand in what way primary protons could create those high-energy electrons which are observed in large numbers in the uppermost layers of the atmosphere. As we now know, this difficulty arose because the mechanism of formation of the electronic component which has now been discovered in electron-nuclear showers was not known and, naturally, was not taken into account. The discoveries I have noted make it possible to put forward the thesis that the main electronic, as well as mesonic, components observed in the atmosphere originate in the stratosphere, as part of electron-nuclear showers. We shall return to the development of this thesis.

For the present, I would like to dwell on one consequence of the proposition just stated, which leads to the possibility of observing electron-nuclear showers of exceptionally high energies also under such conditions when we are dealing with phenomena of an entirely different, far larger scale. I refer to those giant showers which were discovered about 15 years ago by the French physicist Auger. These showers, consisting of hundreds of thousands or millions or even tens of millions of particles, develop in the

ever thicker layers of the atmosphere. For studying them, the confines of a laboratory room are already inadequate, and observations have to be transferred to the field.

Until recently it was assumed that showers of this kind, covering areas on the order of, in some cases, a square kilometer, and perhaps even larger, arise as the result of the penetration into the atmosphere of cosmic electrons of exceptionally high energies and are then produced by the path of classical cascade multiplication.

In view of the discoveries of which I have spoken, however, it should be assumed that in reality, in atmospheric showers, we are dealing with grandiose mixed electron-nuclear showers, initiated in the stratosphere by primary protons of very high energy. From this point of view, the study of atmospheric showers acquires a new and topical interest. The presence of the penetrating meson component of these showers, established by long-standing observations (and thoroughly studied in an extensive series of works under mountain conditions), becomes immediately understandable. The works I have just mentioned, which yielded very valuable new results, were carried out under the direct guidance of our young but already highly competent specialist—G. T. Zatsepin, who also proposed the new conception of the nuclear-cascade process underlying this phenomenon. I am unable to devote any sufficiently large place to these works in my report.

Up to now I have not given any estimates of the order of magnitude of the energies characteristic of cosmic-ray phenomena. A measure of this quantity may be taken to be the voltage that would be required in order, by means of the forces of an electric field, to accelerate an electron to a speed corresponding to a given energy. If one uses, as is customary, such a measure, then the average energy of the particles of primary radiation turns out to be approximately equal to 10 billion electron-volts.

As for the total energy released in large atmospheric showers, it is a million or tens of millions of times greater than this average energy of cosmic radiation and is expressed in electron-volts by a number equal to \(10\) to the sixteenth or even to the eighteenth power.

For reasons of the length of my report, I am compelled henceforth to resort to an even more summary style of exposition.

If the general conception of cosmic-ray phenomena outlined as a result of what I have set forth is correct, and if, in particular, it is true that protons possessing energy on the order of the energies of the primary radiation can very effectively produce electron-nuclear showers, then we must expect that in the upper layers

of the stratosphere, the formation of mixed—electron-nuclear—showers can be observed incomparably more often than at mountain altitudes.

This prediction was fully confirmed. A collaborator of S. N. Vernov’s group, A. N. Charakhchyan, created remarkable apparatus that made it possible to carry out hodoscopic observations in the stratosphere with the transmission of signals by radio. His hodoscopic installations are not as bulky and complex as those used in the mountains, but their principle of operation is the same.

As we have seen, hodoscopic photographs make it possible to observe directly what takes place in the various counters of a hodoscopic system. In the present case this also proves possible, and the observer on the Earth “sees” the picture caused by a phenomenon that has occurred in the stratosphere. Thus something like television is achieved. Such a comparison with television may be all the more justified in that certain elements of television technology are present here. On a moving photographic film, a fluorescent screen is photographed practically continuously; it is essentially identical with the one on which the image is obtained in television sets. The signal is recorded by a controlled spot from a narrow beam of cathode rays, which traverses the width of the film in the horizontal direction in \(1/25\) sec., then returns very rapidly to its initial position and immediately repeats its run again. Thus, here on the screen a definite time sweep is carried out. During this same interval—the time of the cathode-ray spot’s run across the width of the film—in the installation located in the stratosphere, a rapidly rotating motor switches the transmitter of the sending apparatus successively from one counter of the hodoscopic system to another. Thus during this time all the counters of the hodoscope are viewed in succession. Each counter corresponds to a definite place in the sweep obtained on the screen. If the given counter did not operate when a shower passed through a certain control system, a vertical projection appears on the screen at the definite place corresponding to that counter. Counters that did operate are marked by the fact that such a projection is absent. If, however, the control system did not operate, then a straight line without any projections is traced, and barographic signals are recorded. Each time a shower selected by the control system has passed through the hodoscope, this is detected by the disappearance of a certain number of projections, and the entire picture of the shower is thus revealed.

The apparatus described made it possible to observe electron-nuclear showers caused by primary particles of cosmic radiation in the stratosphere. This phenomenon could later be used as a kind of indicator of the presence, at a given level,

in the stratosphere of primary particles. From the number of special showers observed per unit time, it proved possible to judge the magnitude of the flux of primary particles proportional to this number. Thus, S. N. Vernov and K. I. Dobrotin-Alekseeva succeeded in tracing how this flux weakens as it penetrates into the depths of the atmosphere. From observations at different altitudes, by a method whose description I am compelled to omit so as not to increase the length of my report, they obtained a fundamental result. For the first time, the absorption curve

[Figure: vertical axis \(N\); horizontal axis “Height above sea level.”]

Fig. 8.

of primary cosmic radiation in the atmosphere was obtained, and it was shown that this radiation is very rapidly absorbed in the atmosphere. The cause of such rapid absorption is the interaction of primary cosmic radiation with nuclear matter and catastrophic processes of an explosive character, connected with the transformation, in a single elementary act, of colossal portions of energy, which give rise to electron-nuclear showers. Fig. 8 shows this absorption curve. It is unlike the curve giving the course of the total intensity of cosmic radiation in the atmosphere, of which I spoke at the beginning of my report.

This difference is highly characteristic. The curve mentioned earlier relates to a complex set of radiations produced by the primary agent. Here, however, the absorption curve is precisely that of this primary agent—the primary cosmic radiation.

In the scheme of the phenomena of cosmic radiation which I presented at the beginning, it is now necessary, taking all these

facts, make a new rearrangement, after which the scheme looks as follows:

\[ \text{Primary protons} \to \left\{ \begin{array}{l} \text{electrons}\\ \text{mesons}\\ \text{nuclear-active particles} \end{array} \right\} \to \left\{ \begin{array}{l} \text{cascade}\\ \text{processes} \end{array} \right. \]

This rearrangement means a substantial change in our conceptions.

What is essential is that, as has now been definitively established, the primary source of the phenomena caused by primary cosmic radiation and developing in the atmosphere turns out to be a nuclear process of an explosive character, which we are now able to observe and study. Essentially new, too, is the already indisputably established fact that the result of such a nuclear explosive process is the formation of intense streams of high-energy electrons. Relying on this fact, we may now regard the basic scheme of the phenomena of cosmic radiation as finally deciphered. At the same time, this fact raises the question of some new mechanism of energy conversion, one not yet interpreted by theory.

One of the possible mechanisms that theory is able to propose deserves special attention. We have in mind the assumption of the existence of new particles, not previously observed—extremely unstable neutral mesons—which decay with the formation of photons. Recently this hypothesis has been receiving ever greater confirmation. From the observations that I have set forth it follows that, if in explosive showers, together with charged mesons, such neutral mesons are also formed (from which, subsequently, in the second generation, the electrons observed as part of mixed showers arise), then the lifetime of these latter cannot be more than a negligible fraction of a second, expressed by a fraction whose numerator is unity and whose denominator is equal to \(10\) to the tenth power.

In all probability, however, the lifetime of such particles, if they exist, is still considerably smaller.

From the experimental material obtained, even at this stage of the study of the question, one can already draw a whole series of highly interesting conclusions concerning the conditions under which collisions of the primary particles of cosmic radiation with atomic nuclei are accompanied by the explosive processes described. From the magnitude of the measured absorption coefficient of the primary radiation in air it follows, for example, that at sufficiently high particle energy practically every collision with the nuclei of air atoms is accompanied by such an explosive process. Moreover, there are grounds for asserting that the nuclear interactions of the colliding particles are so intense that the explosion occurs already

in the surface layer of the nucleons of the target nucleus into which the cosmic particle that has collided with it penetrates.

It should not, of course, be thought that we shall be able, either now or in the near future, to establish any sufficiently distinct and detailed picture of what is taking place, or to obtain any exact data on the nature of the phenomenon and on the laws governing it. Even with a more detailed and profound exposition I could not, in this respect, report much more than what I have already said. At the present stage this does not appear possible.

It would be quite erroneous, however, to conclude from this that the exceptionally strenuous labor and the great resources expended in carrying out the investigations I have described have gone unrewarded. On the contrary, the results obtained have surpassed all our expectations, since the discoveries made are opening new paths into a still so little-studied domain of phenomena and toward such new summits of knowledge, mastery of which ultimately leads to a reconstruction of our basic conceptions of the physical picture of the world.

I consider it possible to say that, as a result of the work described, Soviet physics has secured commanding positions and a leading place on this advanced front of science.

The achievement of these results has been made possible thanks to the exceptional conditions for the development of research work on an unusual scale that have been created in our country by the Party and the Government, and thanks to the broad support and assistance that have invariably been given to us with a generous hand.