New Data on the Nature of Cosmic Rays

A. I. Alikhanov and A. I. Alikhanyan¹

The main tendency in the development of physics in recent decades has consisted in an ever greater departure from the study of macroscopic bodies and the phenomena associated with them, and in a deepening into the microscopic world. From phenomena occurring with solid, liquid, and gaseous bodies as a whole, physics passed through the kinetic theory of matter to the structure of atoms and molecules, and now at the center of its attention are phenomena occurring in a volume of \(10^{-39}\ \text{cm}^3\)—in the atomic nucleus.

The transition to the microworld was accompanied by a fundamental change in such physical concepts as causality, particle, and wave, and led to the creation of a new mechanics—micromechanics, known by the name quantum mechanics. At the same time, with the transition to the microworld, physics entered the domain of enormous velocities approaching the velocity of propagation of light, i.e., that region of velocities where the principles of Einstein’s mechanics reign undividedly. However, the energies of particles in the nucleus are not limiting, just as nuclei are not the limit of the microworld. We know well that atomic nuclei, despite their insignificant dimensions, are complex systems consisting of protons and neutrons. A moment has now arrived in physics when not only, through natural development, have questions of the properties and “structure” of elementary particles—protons, neutrons, mesons, etc.—come to the fore, but further progress in understanding the structure of the nucleus and the processes occurring in it is also blocked by ignorance of the properties and “structure” of these elementary particles.

The basic and most obvious method for studying the properties of particles consists in making them collide with one another and observing the result of the collision (the scattering of particles after impact, the portion of energy transferred by one particle to another, the form of this energy, the splitting of particles if they are complex, etc.). In this connection, the more robust the system being bombarded, i.e., the more strongly the particles in it are bound, the greater the energy with which we must drive the destructive particle into it.

Hence arises the striving to create instruments and installations that make it possible to impart large energies to charged particles, such as, for example, Lawrence’s cyclotron, Kerst’s electron accelerator, and others.

¹ Report at the September session of the Academy of Sciences of the USSR, 1943.

However, despite the fact that it has become possible to obtain particles with energies of several tens of millions and even up to hundreds of millions of volts, still no artificial sources can compare with the natural source of high-energy particles—the universe, which gives us cosmic rays, where the particles have, on the average, energies of the order of several billion volts. Moreover, phenomena are known to us in cosmic rays, the so-called “Auger showers,” which can be caused only by particles possessing energies of \(10^{14}—10^{18}\) V.

We now know very little about the origin of cosmic rays and cannot even imagine as yet as the result of what processes they acquire these fantastic energies. But when we elucidate this—and such a moment will undoubtedly come—then at the same time we shall, apparently, learn much that is new about the universe as well. Thus, by delving into the microscopic world, we shall in our own way arrive at the unification of the study of both worlds—the microscopic and the macroscopic.

What has already been said explains the extraordinary growth of interest in cosmic rays that is being observed at the present time. One cannot name another field of physics whose number of adherents has increased so strongly from day to day, despite the specific difficulties of work in this field. It attracts both beginning scientists and scientists renowned for their work in other fields of physics. And this is quite understandable, since thanks to cosmic rays we have the possibility of penetrating into the deepest secrets of nature—the constitution, properties, and structure of the elementary particles that make up the universe—which, in the absence of cosmic rays, would have remained hidden from us for a long time.

Unfortunately, the intensity of the cosmic-ray flux is insignificant, and therefore it is not so easy to carry out quantitative investigations with them. Observation of them is based on the fact that a fast charged particle, passing through matter, tears electrons away from atoms, i.e. creates ions, as a result of which an electric track remains after its passage. This track can be made visible if it has formed in a gas and if drops of water are condensed on each ion from the vapors contained in the gas. In the Wilson chamber, which operates precisely on this principle, D. V. Skobeltsyn was the first to see with his own eyes the paths of cosmic particles.

A simpler and more portable instrument (and this property is very important for the study of cosmic rays) is the Geiger counter, in which ions initially created by particles in a gas, when moving in an electric field, form a large number of new ions, so that eventually, in the gas gap between the electrodes, a process occurs similar to the breakdown of a gas. The entry of each particle into the counter, however many primary ions it may have created (only, of course, not fewer than one pair), will cause such a breakdown, which makes it possible to determine the number of particles that have passed through the counter. At sea level, one particle per minute passes through \(1\ \mathrm{cm}^2\).

An ionization chamber is just as simple. In it, under the action of an electric field, the ions from the track of a particle move apart toward two electrodes without any increase in number. In this case what is measured is the total number of ions produced in the gas by the flux of cosmic particles over a certain interval of time.

What properties of cosmic rays, above all, were investigated by these methods?

Whenever a new radiation was discovered in physics, the study of its properties began with measurement of its penetrating power, i.e. of the absorption of this radiation in matter. It was exactly the same with cosmic rays. The very fact that the intensity of cosmic rays increases with altitude, i.e. with a decrease in the thickness of the atmospheric layer that they have to overcome, shows that some of the rays are absorbed along the way and do not reach sea level. The presence of radiation at sea level and lower (beneath thick layers of earth—in the subway, in mines) shows that part of the radiation has enormous penetrating power.

According to the first works in this field, it appeared that the absorption is approximately determined only by the amount of matter traversed by the particle, independently of the kind of absorbing substance. However, a careful comparison of the absorption of cosmic rays in various heavy and light substances, i.e. substances with small (air, water) and large (lead) atomic number, made it possible to obtain new data.

The foremost French investigator of cosmic rays, Pierre Auger, drew attention to the fact that, in the absorption of cosmic rays, one can discern two fairly sharply different components. One part is absorbed very weakly and independently of the kind of substance, but in accordance with the amount of it encountered in the path of the rays, i.e., in short, only in accordance with the density of the absorbing substance. Thus, in absorption, 1 cm of lead will be approximately equivalent, for this radiation, to 11 cm of water. The other part is absorbed considerably more strongly, and the absorption at the same density also depends on the atomic number of the absorber: it is greater the larger the atomic number of the absorber. Accordingly, the difference in the properties of the two components becomes ever sharper as the atomic number of the absorber increases.

For the second component, 4 mm of lead are, in absorption, equivalent to 30 cm of water.

Auger called the first component hard, and the second soft. This seemingly purely descriptive distinction soon acquired profound physical content and led to one of the greatest discoveries in modern physics—the discovery of a new particle, called the meson. This discovery was reached thanks to the fact that the absorption experiments were supplemented by a whole series of direct observations on the properties of individual particles making up the hard and soft components.

Thus it turned out that the soft component consists of electrons and quanta of high energies (on the average about 100 million volts), and that, when they pass through matter, electrons are transformed into quanta, and conversely—quanta into electrons. The consequence of such mutual transformations is the appearance of so-called cosmic-ray showers, i.e., bundles of electrons issuing as it were from a single point.

This property—of creating showers of particles—is a characteristic feature of electrons and quanta of high energies and is an understandable consequence of all the experimental and theoretical data well known to us on the properties of these particles. It is the reason why the soft component is absorbed more strongly than the hard one, and, moreover, more strongly by heavy substances than by light ones.

As for the hard component, it turned out to consist of particles previously unknown—mesons—which have the same charge as the electron, but a mass 130–180 times greater than the mass of the electron, or, if compared with the mass of the lightest nucleus, 10–12 times smaller than the mass of the proton—the nucleus of hydrogen.

Like electrons, mesons possess the ability to ionize atoms along their path, losing in this act a certain amount of energy, so that the path length of a meson in matter depends on its store of kinetic energy. The greater the energy of a meson, the longer the path it can traverse in matter, and thus the attenuation of a beam of mesons in matter is explained simply by the fact that some mesons have greater energy and, consequently, a greater possibility of losing energy in the formation of ions upon collision with atoms, while others have less.

As will be seen below, one more refinement in measurements of the absorptive power of the hard component led to a new remarkable discovery. A comparison of the absorption of the hard component in dense matter, for example in water, with absorption in air, which has a density 700 times smaller, shows that, for layers equivalent in amount of matter (1 km of air is equivalent, approximately, to 1 m of water), air absorbs more strongly than water. Since the atomic numbers of the elements in air and in water are very close, practically the same, this can be due only to the sharp difference in density. The same difference in magnitude remains if we compare absorption in air with absorption in lead equivalent in amount of matter.

From this it follows with irrefutable obviousness that, if we could create a vacuum along the path of the mesons over a section 1 km long, the intensity of the hard component passing through it would noticeably weaken. Thus mesons spontaneously, without external action, disappear—“die”—on the way. The average duration of the “life” of slow mesons, as experiments show, is 0.000002 sec.

Of course, the energy, both the kinetic energy and the rest energy of the meson, cannot disappear, and, consequently, the “death” of the meson is accompanied by the “birth” of other particles. At the same time, from the property of mesons to decay it follows that they could not have come to the Earth from outside, from distant cosmic spaces, for even at a speed close to the speed of light (the maximum possible), their short “life” would not suffice. This means that mesons are produced somewhere in the atmosphere by primary cosmic rays.

It is precisely this stage in the development of our knowledge of cosmic rays that marks the beginning of our work at an altitude of 3250 m on Mount Alagez in Armenia.

The discovery of meson decay posed two questions: 1) into what does the meson transform upon decay, and 2) if it transforms into an electron and a neutrino, as might be thought from a number of general physical considerations, then are there in fact definite relations between the intensity of the soft and hard components (i.e., between the number of electrons and mesons), which should hold in the case where one component generates the other. It must be said that until the very last moment there existed on this question many contradictory observations, owing to erroneous or insufficiently pure experiments, as has now become clear, and to insufficiently careful analysis of the features of the measurement methods employed.

As a result of the first expedition in 1942, we showed that at the altitude of Alagez the ratio of the soft component to the hard one—what we call the ratio of the number of electrons to the number of mesons—depends on the instrument with which this quantity is measured.

If one uses an ionization chamber, the ratio is equal to 1.0; if it is measured with the aid of a Geiger counter, it is equal to 0.65. Meanwhile, if all electrons are generated by mesons, i.e. if the soft component is a derivative of the hard one, the ratio should be equal to 0.45.

At a lower point, in the city of Yerevan (altitude about 900 m), the differences are smoothed out.

What is the physical meaning of this difference in the readings of the two instruments—the Geiger counter and the ionization chamber?

It was said above that the counter records the number of particles irrespective of how many ions in the gas of the counter a particle has produced—one pair or any number more. The ionization chamber, on the contrary, records the number of ions created by the stream of particles in the volume of the chamber.

Thus, the differences in the value of the ratio of the soft component to the hard one are due to the fact that the former on Alagez consists not only of electrons and quanta, but also contains other particles that ionize the gas more strongly than fast electrons and mesons.

In this connection, two tasks arose before us: 1) to determine the origin of these particles and 2) to determine their nature. To determine the pro-

origin of particles—means establishing whether they are derivative, secondary products of the electron-photon or meson component (i.e., whether they are created in matter through the interaction of one of these two components) or arise independently of them. As an example of such a genetic connection one may cite the fact already mentioned above: decay electrons are generated by mesons. A second example may be furnished by collision electrons, i.e., electrons knocked out of the atom by mesons in collisions with them. These electrons, too, are genetically connected with mesons—the hard component.

Fig. 1.

Fig. 2.

To decide the question of the origin of strongly ionizing particles, we turned to a previously tested method of cosmic-ray analysis—the absorption method. As the absorbing medium we chose water, immersing the instruments in Lake Kara-Gel at an altitude of 3225 m above sea level.

In its mean atomic number, water is essentially no different from air, and therefore all processes of interaction of cosmic rays with matter will be identical both in water and in air. Only because the density of water considerably exceeds that of air will mesons, over paths equivalent in quantity of matter, not have time to decay and, consequently, will not give rise to decay electrons.

Figures 1 and 2 present the results of measurements obtained by us in the second expedition (in 19431), when the ionization chamber and counter were immersed in water without a lead filter (i.e., when

measured is the sum of the soft and hard components) and with a lead filter (when only the hard component is measured, the soft one being absorbed in the lead).

From these curves it is first of all evident that the character of the absorption of the hard component in water, when measured with the counter and with the chamber, does not change and corresponds to a very weak absorption \((10\% \text{ per } 1\ m\) of water).

The behavior of the soft component is much more complicated. First of all, attention is drawn to the ratio of the soft component to the hard one, which at a depth of about \(2\ m\) is only 10. Meanwhile, 2200 \(m\) lower (in the city of Erevan), which in amount of matter is approximately equivalent to immersion to a depth of \(2.2\ m\) in Lake Kara-Gël, it turns out that the ratio of the soft component to the hard one is 0.35.

The difference in the conditions of measurement consists only in the fact that the density of water is considerably greater than the density of air.

It follows from this that 25 out of 35% of the soft component in the city of Erevan are electrons from the decay of mesons that decayed in the air on the path from Alagez to the city of Erevan. In the corresponding place of the lake, at a depth of \(2.2\ m\), there is no decay of these electrons, since on the short path of \(2.2\ m\) the mesons did not have time to decay, while those electrons which resulted from the decay of mesons above the lake, owing to their small penetrating power (less than that of mesons), were absorbed in the layer of water of about \(2\ m\).

Thus, from the analysis of absorption we were able to establish the existence of decay electrons and their number for the level of the city of Erevan.

Knowing the number of these electrons in the city of Erevan, we can determine how many of them there should be at Alagez. As has already been stated, the ratio of the soft component to the hard one, if we take into account only that part which is born by mesons, should be 0.45 at Alagez. Continuing the analysis further, we singled out separately the absorption curve of the soft component both as the “counter” curve (obtained with the aid of the counter) and as the “chamber” curve.

Comparison of them shows that not only in absolute magnitude, as was established during the first expedition, but, more than that, in penetrating power they differ sharply from each other. In Fig. 3 a comparison of the absorption curves is given. In the same figure are given data from still another experiment, which was carried out on the same lake.

As has already been said, electrons and quanta of large energies produce showers, which distinguishes them from other particles. We immersed an instrument registering only showers in the lake and in this way established how electrons are absorbed in water. These data are shown in Fig. 3 in the form of asterisks. The triangles show the absorption data in the form of the soft component, obtained by immersing an ionization chamber in a lake situated at an elevation of only \(800\ m\) above sea level.

Finally, a cross indicates what the intensity of the electrons at a depth of 1 m of water should be, if their intensity above this layer is known from the theoretical calculations of L. D. Landau and I. E. Tamm. Comparison of these data leads us to very important conclusions.

1. The component to which the ionization chamber is more sensitive than the counter—let us call it, in distinction from the soft and hard components, the third—is absorbed in water more slowly than the electron component registered by the counter and by the shower apparatus, and, consequently, is not generated by electrons.

2. The third component consists neither of electrons (it does not produce showers) nor of mesons.

3. At small altitudes the third component is already almost absent.

4. The third component is not produced by mesons as a result of their action on matter, since in that case it would have to be absorbed in water in the same way as the hard component. Likewise, this component is not produced by mesons as a result of their decay, since in that case at small altitudes it would have to be present in the same ratio to the decay electrons as at the top, which in fact is not the case.

5. The third component is absorbed more strongly in lead than in water.

Fig. 3.

Thus, the third component is an independent, separate component, or, as it is customary to call it, a nonequilibrium component.

Its distinguishing feature is the great ionizing power either of it itself or, most probably, of the secondary particles produced by it.

If one assumes that the excess 15% of the soft component over the calculated ratio 45% for Alagez, which was also observed in the measurements with the counter, is due to the third component, then its ionizing power proves to be 3–4 times greater than that of a meson.

Thus, the main results of our two expeditions are as follows.

1. The existence has been proved and the number of electrons from meson decay has been determined.

2. The existence of a third, nonequilibrium component has been proved.

3. It has been shown that the properties of the third component differ from the properties of electrons and mesons.

Our task now is to answer the question of the nature of the particles of the third component itself and of the secondary particles it creates. It is quite possible that these secondary particles created by it are protons. This is not an easy task, but we may hope that the expedition of 1944 will bring us closer to solving it.

In conclusion, a few words about the conditions under which the expedition’s work took place. Naturally, they were very difficult, especially in the summer of 1942, during the most difficult times for the Caucasus. But thanks to the enormous assistance given to us by the Central Committee of the Communist Party (Bolsheviks) of Armenia, it proved possible to solve such difficult tasks as supplying the expedition with fuel and food, delivering loads to the foot of the mountain, and transferring almost 15 \(m\) of cargo by pack transport from the foot to the summit and back.

Great assistance was given to us by the rector of Yerevan University, Prof. G. Kh. Bunatyan, and by the staff of the Department of Physics of Yerevan University, as well as by the deputy chairman of the Armenian Branch of the Academy of Sciences, V. O. Gulkanyan. All this enabled us to put our instruments in order in Yerevan, test them before the ascent, and carry out observations on the summit.

The work was carried out partly in the premises of the Alagez meteorological station and partly in tents; the presence of the meteorological station on Alagez and the comradely attitude toward us on the part of its staff greatly facilitated our work.

1. K. M. Kacharyan (Yerevan University), I. F. Kvartskhava, and G. M. Mirionishvili (Georgian Academy of Sciences) also took part in these measurements. ↩