NEW APPROACHES TO STUDYING THE NATURE OF COSMIC RAYS

L. V. Mysovskii, Leningrad

In my previous review* on cosmic rays, the difficulties that confronted physicists studying the nature of cosmic rays were described in detail. In that same review it was noted that, of the two different types of apparatus—namely, apparatus with a Wilson chamber and apparatus with Geiger–Müller counters—preference should be given to apparatus with counters. Apparently, the majority of physicists working in this field held the same opinion. During the last two years an enormous number of works devoted to the investigation of the nature of cosmic rays have appeared, and almost all of them have been carried out with Geiger–Müller counters. The number of these works is so great that here we have no possibility of describing all of them even in the briefest outline. We shall therefore have to dwell only on those which, in our opinion, most deserve the reader’s attention. It should be stipulated, however, that the Geiger–Müller counters have by no means justified the hopes placed upon them. In what follows we shall see that the question of the nature of the primary cosmic rays, just as was the case two years ago, rests on three different hypotheses—the quantum hypothesis, the electron hypothesis, and the proton hypothesis. The difficulties connected with choosing one of these hypotheses became especially clear at the conference on cosmic rays held in Cambridge in the autumn of 1931. In the exchange of views at this conference there took part outstanding physicists working on cosmic rays and on radio-

* See the Bibliography, p. 648.

activity; however, no general point of view had been found, and the nature of cosmic radiation remained, as before, unexplained. Yet, as we shall see, the question of the composition of cosmic radiation has very recently moved from a dead point, but this shift has occurred thanks to work carried out in another field and by other methods.

Curves obtained by Regener and conclusions from them

Before proceeding to describe the work with Geiger–Müller counters, we shall report additional information on Regener’s work, who, as is known, traced the absorption of cosmic rays in Lake Constance down to a depth of 250 m. In essence, Regener’s apparatus and the principal results obtained by him have already been described by us in the preceding review[^1]. This description was made on the basis of a letter (admittedly, a fairly detailed one) published in Naturwissenschaften in 1929. A full description of the work was given by Regener in Z. f. Physik only in February 1932. Since Regener found the hardest component of cosmic rays, we consider it necessary to present some additional information from his latest article[^3]. Leaving aside the details of the apparatus, let us turn to the curves plotted by Regener on the basis of photographic registration of the readings of his electrometer and the subsequent processing of these readings by the method of least squares.

In Fig. 1 the curves of the ionization current at various depths are given. First of all, let us note the slight rise of the middle part of the curves with time. Regener writes that this rise can be observed only at comparatively great depths. At a depth of 32.4 m, where the ionization current is much stronger, no rise of the curve can be noticed. From this circumstance one may conclude that the increase of ionization with time is due to the residual current[^4] in the apparatus itself. Raising the entire apparatus to the surface for changing the photographic plates, winding the clock, and charging the electrometer again lowers the mean level of the curves to its initial position. Since

the aforementioned operations are accompanied by the opening of certain parts of the apparatus, and the air in them thereby changes; it is therefore natural to suppose that the rise of the curve is caused by the gradual accumulation of a barely measurable quantity

Fig. 1.

of emanation released from the inner walls of the apparatus. However, it was not possible to verify this supposition, because the effect is too small, and special and prolonged experiments would have had to be arranged in order to investigate it. In addition to the rise on the same curves (Fig. 1), we ob-

we can observe fluctuations of the ionization current, sometimes exceeding in magnitude the observational error by 50 times. These fluctuations cannot be explained by changes in atmospheric pressure[^5], since at a depth of 100 m and more, according to Regener’s observations, the magnitude of the atmospheric pressure no longer noticeably affects the intensity of the ionization current in the chamber. In Regener’s opinion, not only the rise of the curve, but also its fluctuations are likewise due, in their origin, chiefly to the residual current. Without entering into a criticism of this assertion, we shall only note that, in describing Steincke’s last work, we shall have to return once more to this kind of fluctuation and examine it in greater detail.

Fig. 2.

Let us now turn to Fig. 2, which shows the absorption curve obtained by Regener. Along the abscissa are plotted, in meters, the depths to which the instrument was lowered. Along the ordinate are plotted the values of the ionization-current strength in volts/hour. \(J_0\) is the straight line corresponding to the residual current. Since the lower part of the curve is so extended that it no longer gives an idea of the course of absorption, the segment of the curve beginning at 30 m is shown once more (the upper curve), the scale of ordinates being increased 10 times. Without denying the fact that cosmic rays come from all directions, and not as a parallel beam, and that, consequently, the law of absorption of cosmic rays must be represented by a function \(\Phi(\mu x)\)[^6], and not by a simple exponential factor \(e^{-\mu x}\), Regener nevertheless calculates the absorption coefficient from his curve, using the expression:

\[ J_x = J_0 + (J_1 - J_0)e^{-\mu x}. \]

(where \(J_1\) is the intensity of cosmic rays in world space, and \(J_0\) is the residual current caused by traces of radioactivity on the walls of the instrument). In his opinion, calculation by \(\Phi(\mu x)\), taken with allowance for the scattering of primary rays, leads to the same result as calculation by \(e^{-\mu x}\) without allowing for scattering. It is rather difficult to agree with this assertion of Regener’s. The point is that we still know too little about the scattering of cosmic rays to make such an assertion with confidence. Regener himself assumes that the scattering of the hardest rays proceeds according to the Klein–Nishina formula, and that the nuclear electrons participate in this process on an equal footing with the outer ones. Thus, in calculating \(\mu\) in water he takes 18 electrons for the \(\mathrm{H_2O}\) molecule (8 outer and 10 nuclear). If, in addition, one assumes that the most penetrating cosmic rays are produced when the mass of the helium atom is wholly transformed into a quantum of radiant energy, then the absorption coefficient obtained on the basis of the Klein–Nishina formula is equal to \(2.0 \cdot 10^{-4}\ \mathrm{cm}\ \mathrm{H_2O}\). The absorption coefficient found by Regener from the experimental curve, \(1.88 \cdot 10^{-4}\ \mathrm{cm}\ \mathrm{H_2O}\), indicates, in his opinion, the probability of such an assumption. As we see, Regener firmly adheres to the quantum hypothesis concerning the nature of cosmic rays. Unfortunately, however, all his final conclusions are based on a whole series of assumptions, and these assumptions themselves, as we shall ascertain below, must be regarded as at least incomplete and insufficient for a final solution of the question of the nature of cosmic rays.

An apparatus with counters and a Wilson chamber

Mott-Smith and Locher attempted to create an apparatus in which it would be possible simultaneously to use the advantages of the Wilson chamber and of Geiger counters[^7]. Initially the authors of this work set themselves the aim of showing clearly (with the aid of a Wilson chamber) precisely what distinguishes ...

is called a coincidence of the readings of Geiger counters. However, the number of coincidences in two counters is so small that it was difficult to detect them in a Wilson chamber. Therefore Mott-Smith and Locher abandoned coincidences in counters and studied only the simultaneous appearance of a pulse in the counter and a track in the Wilson chamber. To increase the number of pulses, two counters \(T_1\) and \(T_2\) were used (Fig. 3). The tracks were photographed with the aid of chamber \(C\). A mercury lamp was used to illuminate the chamber. The experiment itself consisted of the following. The counters operated continuously.

Fig. 3.

At certain arbitrarily chosen moments of time the piston of the Wilson chamber was lowered, and the tracks appearing at that moment were photographed. The photographs obtained in this way Mott-Smith and Locher divide into two groups. The first group, which they call \(C\), includes photographs taken simultaneously with a pulse in one of the counters. The second group, \(N\), includes photographs made at moments when there were no discharges in the counters. The negatives of group \(C\) differed from the negatives of group \(N\) by the gleam from the lamp, which, by means of a special circuit of connections, was ignited only when a pulse in the counter coincided with the lowering of the piston in the Wilson chamber. The arrangement of the device for igniting the lamp can be understood from Fig. 4. In this fig-

... in the drawing the letter \(H\) denotes a source of potential at 1500 V, \(T\)—Geiger–Müller counters, \(A\)—an amplifier, \(L\)—a lamp, \(R\)—a relay in the lamp circuit, and \(F\)—a key with crank \(M\), connected with the piston of the Wilson chamber \(E\). The lowering of the piston was carried out by rotating the axis in the direction indicated in the drawing by the arrow. The key \(F\) was designed so as to close the lamp circuit only for the interval of time necessary for the formation of tracks in the Wilson chamber (about 0.06 sec.). If, simultaneously with the appearance of tracks, there also occurred the closing of the relay caused by a discharge in one of the counters, then the lamp proved to be switched on and lit up (photograph of group \(C\)). But if the circuit closed only at one place, owing to the lowering of the piston or to the switching-on of the relay, then the lamp did not light up (photographs of group \(N\)).

Fig. 4.

To obtain quantitative results, photographs of both groups were required. From all the tracks in the photographs of group \(C\), it was first necessary to select only those tracks whose directions pass through one of the counters. However, some of these tracks could have an accidental origin. Therefore the number of tracks selected in this way still had to be reduced somewhat. The percentage of the reduction could be found by using photographs of group \(N\). It is evident that for this purpose it is sufficient to find in photographs \(N\) the ratio of the number of directed tracks to their total number. In all, Mott-Smith and Locher obtained 137 \(C\)-photographs and 1107 \(N\)-photographs. In the \(C\)-photographs the number of tracks directed toward the counters was 12, and in the \(N\)-photographs 23. Thus the probability of such tracks for \(C\) will be

\[ \frac{12}{137} = 0.088, \]

and for \(N = \dfrac{26}{1107} = 0.024\). Decreasing \(0.088\) by \(0.024\), we obtain \(0.064\). Considering the distribution of cosmic rays by solid angles relative to the vertical and taking into account the angle subtended by the counters and the chamber, Mott-Smith and Locher, on the basis of the work of other authors, obtain a value agreeing with their result, namely \(0.066\). Unfortunately, in their article the authors do not reproduce any of the photographs they obtained. On the basis of their photographs they themselves arrive at the conclusion that the traces in the Wilson chamber, isolated by them in the manner described above, can most readily be explained by ionization due to high-velocity electrons. Mott-Smith and Locher completely reject the supposition that such tracks may be obtained through ionization by photons.

In essence, the work we have just described, although interesting from the standpoint of experimental technique, gives almost nothing new with respect to cosmic rays. Apparently the authors of the paper came to the same conclusion, to judge from the fact that one of them, Locher, subjected the photographic images obtained to further processing and critical analysis.^8 First of all, Locher attempted to determine precisely the intensity of the ionization produced by a ray over a path segment of \(1\ \mathrm{cm}\). For this purpose, photographs of the tracks of rays that had passed through the counter were placed in the field of a microscope at small magnification and photographed again. In his article Locher reproduces several prints from such photographs. It must be said, however, that even in the original article the microphotographs of the track segments came out rather poorly and seem unconvincing. With further reprinting the images would become still more indistinct, and therefore we do not reproduce them here. Examining his microphotographs and counting the ions on them, Locher obtains that there are 32 pairs of ions per \(1\ \mathrm{cm}\) at a pressure of \(68\ \mathrm{cm}\) (the pressure in the chamber when the piston is lowered). For atmospheric pressure the number of double ions will be \(^{76}/_{68}\) times larger, i.e. 36. Thus, judging by

by the character of the ionization, it is easiest to assume that we are dealing with fast electrons.

The second question taken up by Locher concerned the simultaneous appearance of two or more tracks emerging from a single point. In 148 tracks, 20 groups were discovered. In one case 5 tracks were found on one photograph, three of them emerging from the wall of the chamber. In 5 cases groups of 3 tracks were discovered. The remaining 14 groups were pairs. Locher also gives photographs of the groups, but, since they are just as unclear as in the preceding case, we consider it unnecessary to reproduce them. As for the ionization, it still proved to be equal to 36 pairs of ions per 1 cm, at 76 cm of mercury. From this Locher concludes that, if each member of a group is an electron, then their velocity must be greater than 0.9 the velocity of light. In Locher’s opinion, the presence of groups makes it possible to conclude that primary cosmic rays consist of photons of high energy. If electrons were coming to us directly from outer space, as Bothe and Kolhörster suppose, it would be difficult to explain such frequent crossing of their paths at a single point. It is likewise difficult, in Locher’s opinion, to imagine that tracks diverging from a single point are produced by the Compton effect (the hypothesis of Auger and Skobeltsyn) ⁹. The thickness of the glass in the chamber (5 mm²) is too small for one to expect such frequent collisions of a photon with an electron. It remains to accept that groups of fast electrons arise in the nucleus of the atom. However, Locher does not succeed in explaining more or less satisfactorily the emission from an atomic nucleus of two or even three electrons simultaneously. Thus the experimentally established presence of groups does not yet make it possible to decide the question of the nature of primary cosmic rays. One cannot, however, fail to agree with Locher when he says that the data obtained with counters must be approached with caution. The presence of two, three, or even four electrons simultaneously piercing a counter is also noted by him,

both the passage of a single electron. As a result, the number of pulses in the counters is always smaller than the actual number of electrons that have passed through them. If, however, one bears in mind that groups of electrons in cosmic rays, judging from experiments with a Wilson chamber, occur quite often, then on the basis of data obtained with counters we must, in calculating the ionization, obtain too large a number of ion pairs per 1 cm of path. Such an overestimate, in Locher’s opinion, was made in the work of Kolhörster and Tuwim.

Passage of cosmic rays through a magnetic field

If Locher believed that more data could be obtained by relying on photographs of tracks in a Wilson chamber, Mott-Smith decided to take another path and set about improving the apparatus with Geiger counters. It had already been repeatedly suggested that the most accurate idea of the nature of cosmic rays could be obtained by attempting to deflect them in a suitable magnetic field. Such an attempt was made by Mott-Smith. His apparatus is shown schematically in Fig. 5.

Fig. 5.

To observe coincidences Mott-Smith used three counters instead of two (A, B, and C in Fig. 5). In the path of the rays from counter B to the third counter C, a bar of magnetizable iron was placed. The dimensions of the bar are shown to scale in Fig. 5. The winding of this magnet consisted of 2000 turns of wire, and a current of 2.2 A could circulate through it. With this current, a field $\mu H$ equal to 17,000 gauss was produced inside the iron bar. It was assumed that, after the current was switched on in the coil, a cosmic ray, having passed about

15 cm in a field of 17,000 gauss, will noticeably deviate from its rectilinear path, and in order to obtain the previous number of coincidences the counter \(C\) will have to be shifted by some distance \(d\) (Fig. 5). The counting of the coincidences themselves in the three counters was carried out with the aid of the circuit shown in Fig. 6. The pulses of the counters were transmitted through capacitors to one of the grids of the double-grid tubes \(T_1\), \(T_2\), and \(T_3\). If the discharges in the counters do not coincide with one another, then the pulses did not pass beyond \(T_1\), \(T_2\), and \(T_3\). When discharges coincided in counters

Fig. 6.

\(A\) and \(C\), the pulses passed only through \(T_2\) and \(T_3\). When the discharge coincided in counters \(A\) and \(B\), the circuit \(ABT_4\) was closed. In the presence of a coincidence in all three counters, not only the circuit \(ABT_4\) proved to be closed, but also the circuit \(ABCT_6\). From \(T_6\) the pulse was transmitted through a capacitor to an amplifier of two stages, \(T_7\) and \(T_8\). The pulse thus amplified was transmitted by means of relay \(R_1\) to an electric counter. In practice it turned out, however, that the duration of closure of relay \(R_1\) was so short that a quantity of energy sufficient for the operation of the electric counter did not have time to flow through it. Therefore, on the way to the counter it was necessary to introduce one more amplifying tube, \(T_9\). The values of the capacitances, resis-

resistances and potentials are given in the drawing itself. Some details of the connections are also visible there.

TABLE

With an increase, in the measuring apparatus, in the number of Geiger–Müller counters arranged in a straight line, the solid angle subtended by them decreases, and together with this the number of registered coincidences also decreases. In order to obtain a sufficient number of coincidences in three counters, Mott-Smith had to leave his apparatus in operation for weeks. Obviously, in this case it was extremely important throughout this entire period to keep the operating conditions of the counters as far as possible unchanged. Therefore special attention was paid to maintaining the constancy of the potential and of the gas pressure inside the counters. To keep the pressure unchanged, the electrodes of the counter were placed in a glass tube in the manner shown

in Fig. 7a. The arrangement of the inlets, as well as the branch for pumping out, are visible in the same figure and require no further explanation. The pressure in the counters was equal to 7 cm

Fig. 7a.

of mercury. The diameter of the central electrode (platinum wire) was 0.076 mm. The dimensions of the second electrode are given in Fig. 5. The final results obtained with this setup are shown in the form of the curve in Fig. 7b.

Fig. 7b.

In the graph:

• \( \times \) The magnet is not magnetized.
• \(+\) The field deflects the electrons to the north.
• \(\square\) The field deflects the electrons to the south.
• \(\circ\) Calculated intensity distribution.

Vertical axis: number of coincidences per hour.
Horizontal axis: displacement of the analyzer (cm northward).

Along the ordinate axis is plotted the number of coincidences per hour, and along the abscissa axis the displacement of counter \(C\) from its mean position, expressed in centimeters (see \(d\) in Fig. 5). The circles correspond to the absence of a magnetic field. The crosses (\(\times\)) and

\((+)\)—with the magnetic field in two mutually opposite directions. The small squares represent the calculated values under the assumption that the path of the cosmic rays is rectilinear and that all rays entering the counters produce discharges in them. In the calculation, the central point \((O)\) and the extreme point on the right were taken as given.

To explain the method of drawing the curve, Mott-Smith gives the table used for finding the point \((\times)\) \((0,\ 29.4)\). In the first column are placed the intervals of observation; in the second, the numbers of coincidences \(C\) for the given interval; in the third, the numbers of coincidences per hour \(C/h\); in the fourth, the deviations from the mean for the ratio \(C/h\). In the last column is given the calculation of the probable error by the usual formula:

\[ 0.67\sqrt{\frac{\sum D^2}{C}}=\frac{0.67\sqrt{\sum C D^2}}{C}. \]

On looking at the curve it is evident that the expected displacement of the maximum of coincidences was not obtained, and the only conclusion that can be drawn from this curve is that the rays are not deflected at all by the given magnetic field. The discrepancy between the points marked by circles and by crosses must be ascribed to the action of the magnetic field on secondary (and perhaps tertiary) rays of small energy. It remains, however, unclear why the action of the field does not depend on its direction (\(\times\) and \(+\) almost coincide). Since no deflection occurred, Mott-Smith, proceeding from the intensity and dimensions of the magnetic field, calculates that the energy of the electrons must be greater than \(2 \cdot 10^9\ \mathrm{V}\). If it is assumed that the coincidences in the counters are caused not by electrons but by protons, then their energy, on the basis of the same calculations, must be greater than \(10^9\ \mathrm{V}\). It must be borne in mind, however, that the existence of such fast protons or electrons is poorly compatible with the exponential law of absorption of cosmic rays, established, as is well known, by a whole series of observations.

Improvement of Apparatus for Counting Coincidences in Geiger–Müller Counters

The difficulties that have to be overcome in working with counters have led physicists to think about improving and perfecting apparatus with counters. From this point of view, replacing the counting of simple pulses by the counting of coincidences in two or three counters may already be regarded as an unquestionable advance. Here we shall touch upon two works devoted to the further study and simplification of the coincidence method. The first work belongs to Hummel in Göttingen. Hummel proposed a connection scheme for any number of counters, by means of which it is possible to count or automatically record only coincidences, while all other discharges, irrespective of their origin, remain aside and do not burden the tables or film as unnecessary ballast. This scheme, shown in Fig. 8, is very simple and requires for its implementation only an increase in the number of cells in the batteries. The number of cells increases directly in proportion to the number of counters.

Fig. 8.
Annotations in the figure: \(Z_1\), \(Z_2\), \(C\), \(R_1\), \(R_2\), \(C \sim 1000\ \mathrm{cm}\), \(R_1 \sim 10^8\ \Omega\), \(R_2 \sim 5 \cdot 10^9\ \Omega\).

A clear idea of the operation of the proposed apparatus can be formed on the basis of the scheme in Fig. 8. Counter \(Z_2\) operates independently of the first, and its peculiarity (if this can be called a peculiarity) consists only in the fact that its inner electrode is at a high potential, while the potential difference between the wire and the outer cylinder remains the same as in all other apparatus. As for counter \(Z_1\), a discharge of noticeable magni-

only in the case where the amount of electricity necessary for discharge reaches it from counter \(Z_2\). If there is no influx of electricity, then the inner electrode can be charged only to a very small potential, and the deflection on the electrometer \(E\) will be barely noticeable. By introducing the additional capacitance \(C\), this incipient deflection from \(Z_1\) can be made arbitrarily small. It is obvious that the same circuit can be assembled for three or more counters. If, instead of the electrometer \(E\), an amplifier connected to a relay is placed there, then the relay will close only from pulses of considerable strength, i.e., from coincidences, while the weak pulses from a single counter \(Z_1\) will remain unregistered. It is precisely such an arrangement that is now operating at the State Radium Institute in Leningrad, with the only difference that between the counters \(Z_1\) and \(Z_2\) there is also connected a string electrometer, which makes it possible continuously to monitor the operation of each counter separately. (It is obvious that the body of this electrometer is at the same potential as the inner electrode of \(Z_2\), i.e., 1000 V.) Although such an arrangement registers only coincidences, among the coincidences there is also a large number of random ones\(^1\). It is not difficult to determine the number of random coincidences per unit time. For this it is sufficient to place the counters at a comparatively large distance from one another. Then almost exclusively random coincidences will be registered. One may proceed otherwise and use, for determining the number of random coincidences, a formula analogous to that which was used for the same circuit by Bothe and Kolhörster\(^1\), namely:

\[ v = 2\tau N_1 N_2, \]

where \(N_1\) and \(N_2\) are the numbers of discharges in counters \(Z_1\) and \(Z_2\) per unit time, \(\tau\) is the duration of the discharge, and \(v\) is the sought number of random coincidences. Thanks to the Hummel arrangement it proved possible to dispense with counting discharges in the individual counters and to take up the counting of individual coincidences. The method proposed by Medicus\(^ {13}\) makes it possible to go still further and to get rid of random coincidences. The essen-

...ness of the method they proposed consists in the use, for observation and for counting coincidences, of a cathode oscilloscope. For connecting the counters, the usual circuit is used (Fig. 9). To each counter one of the deflecting plates of the oscilloscope is connected. Since the charges on the plates deflect the cathode beam in mutually perpendicular directions, in the case of a coincidence of discharges in the counters the spot of the oscilloscope moves along the diagonal. An elementary calculation, made according to the formula just cited by us, \(v = 2\tau N_1N_2\), shows that in this arrangement the number of accidental coincidences is several thousand times smaller than the true ones. Thus one may assume that, in practice, in an arrangement with a cathode oscilloscope accidental coincidences are absent. In order to test the suitability of his method for the study of cosmic rays, Medicus investi—

Fig. 9.

Fig. 10.

gave the distribution of the intensity of cosmic radiation by angles with the vertical. The general form of the extremely simple device is shown in the appended photograph (Fig. 10). Two Geiger–Müller counters are inserted into a frame, which can be inclined at any angle to the horizontal plane. Observing the number of pulses at different inclinations of the frame, Medicus obtained the curve presented in Fig. 11. Alongside his own curve Medicus gives, for comparison, the curve obtained by Mysovskii and Tuwim in Leningrad (dashed line). The agreement, as is clear from the drawing, must be considered quite good, especially if one takes into account the difference in methods and apparatuses.

Fig. 11.

Conclusion

Recently, more and more often in interpreting experimental results relating to cosmic rays, one has had to turn to the atomic nucleus. As we have already indicated, Regener, in calculating the absorption of cosmic rays, decided to take into account not only the external but also the internal electrons. But if, by some means (whether by the Compton effect or by the photoelectric effect), the nucleus, under the action of cosmic rays, loses its electrons, then a transformation of elements must take place. Physicists who have observed the tracks of cosmic rays in the Wilson chamber pay special attention to the formation of groups of tracks, and, in order to explain this phenomenon, they usually also consider the action of cosmic rays on the atomic nucleus. Mott-Smith sees a way out of those contradictions to which the data of various experimenters lead (for example, the contradiction between the presence of fast electrons and the exponential law of absorption) in the supposition that the absorption of cosmic

of rays occurs mainly through atomic nuclei. Finally, quite recently a paper by Steinke and Schindler^14 appeared, in which they report the splitting of lead atoms by cosmic rays. Although the calculations of Steinke and Schindler, made by them on the basis of the results of their work, raise doubts, their

Fig. 12.

Fig. 13.

work is of undeniable interest. The apparatus with which Steinke and Schindler worked is shown in Fig. 12. Without going into a detailed description of the apparatus, we shall indicate only that covering the chambers with lead 10 cm thick caused an increase in the number of sudden oscillations of the electrometer filament. The character of the deflections is shown in Fig. 13. The magnitude of the deflections of the electrometer filament leads Steinke and Schindler to the conclusion that a sudden increase in ionization takes place in the chamber, caused by \(H\)-particles emitted from lead. Of course, an apparatus with ionization chambers

are poorly suited for detecting such a subtle phenomenon, all the more so since it occurs rather rarely. As an example, let us note that over the course of 1345 hours, in the presence of the cover, only 102 impacts were observed in all, i.e. fewer than one \(H\)-particle over a time interval of 10 hours. Steinke and Schindler point out that, in all probability, the most suitable method for investigating phenomena of this kind would be the photographic method. It is possible that the most convenient will prove to be the method proposed by the author of the present review specifically for this purpose: photographing on plates with a thick emulsion layer[^15].

If we had to confine ourselves only to the conclusions set forth above, the picture of the present state of the problem of the nature of cosmic rays could hardly be called especially comforting. However, already in the introduction it was pointed out that help unexpectedly came to cosmic-ray researchers from another field of physics. As is known, in recent times, thanks to the work of Joliot and Irène Curie, a very penetrating radiation was discovered, produced when beryllium is bombarded by \(\alpha\)-particles from a polonium preparation. At first, just as had been the case with cosmic rays, no one doubted the quantum nature of the newly discovered radiation. In subsequent investigations, however, it was found that the rays emitted by the beryllium nucleus can, in turn, produce \(H\)-rays with a large range. Far more \(H\)-rays were observed than could have been expected if the quantum nature of the beryllium radiation were taken into account. In this review, devoted to cosmic rays, we cannot describe in detail the gradual change in views on the essence of the new penetrating rays. Let us note only that, as a result of various difficulties and contradictions, which in many respects recalled the difficulties encountered in the study of cosmic rays, Chadwick proposed and substantiated the hypothesis of neutrons. The mass of the neutron is equal to the mass of hydrogen, but it does not possess an electric charge; in this sense it is neutral. Of all the works devoted to neutrons, I shall cite only a few photographs obtained

D₁₆ by means of a Wilson chamber. Figure 14 shows a fast proton that has received acceleration from an impact with a neutron. This figure is given by us only in order to characterize one of the basic properties of the neutron—its speed. According to Chadwick, the maximum speed possessed by a neutron from beryllium is \(3.3 \cdot 10^{9}\) cm. Fig. 15

Fig. 14.

gives us a picture of the disintegration of a nitrogen atom. The track of the neutron that caused this disintegration, as was to be expected, is not visible. The long branch, with gradual thickening as the speed decreases and with a slight bend at the end, is the path of an \(\alpha\)-particle emitted from the nitrogen nucleus during

Fig. 15.

its disintegration. The short branch is the track of the recoil atom. Let us note here that \(\alpha\)-particles arising inside the Wilson chamber had to be observed both by the author of this review and, probably, by many other physicists who worked with the Wilson chamber. It was rather difficult to explain these phenomena by the presence of emanation, since they recurred with the same frequency and over a long period in a hermetically sealed chamber. Most likely, these

α-particles appeared as a result of the splitting of nitrogen by cosmic rays. Descriptions of such α-particles have also occurred in the literature.

For example, the author of the present review has presented[^17] a stereoscopic photograph with an α-particle obtained together with β-particles of radium. But if α-particles that can be attributed to cosmic rays are encountered comparatively rarely, then much more often it was possible to observe in the Wilson chamber short, thick tracks that could not be attributed to α-particles of any of the radioactive elements known to us.

Fig. 16.

Photographs with such tracks can be found in the literature in descriptions of various experiments with the Wilson chamber. In order not to be unsubstantiated, one may point, for example, to Orban’s work,[^18] in which one of the photographs contains a short track. It must be said that until now the appearance of these short tracks was considered merely as a circumstance interfering with the obtaining of good photographs. The situation is quite different after the appearance of works on the study of the action of neutrons. It turned out that α-particles with an anomalously small range are the track of a nitrogen atom that has collided with a neutron. At present there are already works specially devoted to the study of the collision of a neutron with a nitrogen atom. For illustration we shall present a very indicative photograph by Dee (Fig. 16). In this figure we simultaneously see a nitrogen track and the angular track of a slow electron. It is obvious that they differ sharply from one another, and therefore it is impossible, as was sometimes done earlier, to attribute the appearance of short tracks to the action of slow electrons. In conclusion we shall also present a photograph of two electron tracks emerging from one point (Fig. 17). On examining this photograph, a comparison with “groups” from cosmic rays suggests itself.

Can one, on the basis of all these data, with confidence...

can one say that cosmic rays consist of neutrons? To this question one has to give a negative answer. For the time being there are no quantitative data; the presence of short thick tracks and the appearance of $\alpha$-particles can be explained by “radioactive contamination.” If one wishes, one may even use the neutron hypothesis and say that tracks of radioactive substances cause the formation of neutrons, and that these latter, in turn, condition the phenomena observed in the Wilson chamber.

Fig. 17.

It should be recalled that in its time much effort had to be spent on proving the independence of the ionization caused by cosmic rays from the “radioactivity of the environment.” However, carefully arranged experiments, which yielded not only qualitative but also quantitative results, finally clarified this question. It is obvious that now too, in connection with the new hypothesis concerning neutrons, investigators will follow the very same path of quantitative analysis. In the near future we should expect the appearance of a number of very interesting experimental works on cosmic rays.

References

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