ARTIFICIAL TRANSFORMATION OF ELEMENTS

E. V. Shpolsky, Moscow

§ 1. In a note on the same subject, printed in nos. 2–3 of this journal for the current year, the results obtained in the Cavendish Laboratory in bombarding certain light elements with a stream of fast protons have already been reported. At present we have the opportunity to acquaint ourselves with the details of the experimental setup of this remarkable work and with its further results.*

We shall begin with a detailed description of the scheme of the high-voltage apparatus, which in itself is of great interest. In L. V. Mysovsky’s article*** the reasons are indicated why the impulse generator and the Tesla transformer, although they make it possible to obtain very high voltages, cannot be regarded as convenient sources of potential for the given purpose. In this respect, the method of Lawrence and Livingston has an enormous advantage on account of its exceptional ingenuity and elegance. By means of this method it has so far been possible to obtain currents of the order of \(10^{-9}\) A. Although even at these small currents the number of fast particles considerably exceeds what the strongest radioactive preparations can give, it is natural to attempt to obtain a more powerful stream of positive ions at a sufficiently constant accelerating potential. For this purpose one may use a combination of capacitors and

* Uspekhi Fizich. Nauk, 12, 357, 1932.
* J. Cockroft and E. Walton, Proc. Roy. Soc. A 136, 619, 1932; 137, 229, 1932.
* See p. 580 ff. in this issue of Uspekhi*.

rectifiers, making it possible to increase the main voltage severalfold. It was precisely along this path that the workers of the Cavendish Laboratory—Cockcroft and Walton—proceeded. Earlier, various authors (Greinacher, Schenkel) had indicated circuits by means of which, having \(n\) capacitors and \(n\) kenotrons, one can raise the main voltage \(n\) times. Cockcroft and Walton developed several original circuits possessing a considerable advantage over the earlier circuits.

Let us first consider the simplest circuit, illustrating the principle of the apparatus. Suppose there is a source of constant potential \(E\) and a system of three main capacitors \(K_1, K_2, K_3\), connected in series, as shown in the drawing. Let us also take a system of two auxiliary capacitors \(X_1, X_2\), connected with the capacitors \(K_1, K_2, K_3\) by means of some continuously acting (for example, rotating) commutator. Suppose that in the first half of the cycle the switch is arranged so that the connection of the system \(K_1, K_2, K_3\) with the system \(X_1, X_2\) is made along the dotted lines \(S_1, S_2, S_3\). This means that in the first half of the cycle the capacitors \(K_2\) and \(K_3\) will be connected with the capacitors \(X_2, X_1\), and the capacitor \(X_2\) will thereby be charged up to the potential \(E\). In the second half of the cycle the switch will take the position \(S'_1, S'_2, S'_3\), and the capacitors \(X_2, X_1\) will be connected with the capacitors \(K_3, K_1\).

Fig. 1.

Fig. 2.

As a result, capacitor \(X_2\) will charge capacitor \(K_2\) to a potential \(\dfrac{E}{2}\), if they both have the same capacitance.

In the next cycle, when \(X_1\) is connected to \(K_2\), the latter will share its charge with \(X_1\), and this, in turn, in the next phase will charge capacitor \(K_1\). Equilibrium will occur when all the capacitors are charged to the potential \(E\), and consequently at the ends of the system \(K_1, K_2, K_3\) there will be a potential difference \(3E\). It is clear that the system can be enlarged by adding capacitors, and at the same time the main potential can be raised—theoretically, by any amount.

The described circuit is connected with the use of mechanical commutators. In order to avoid the latter, one may indicate a number of other circuits similar to the one described, but operating with cathode tubes—diodes and triodes. In one of these circuits two switches, for example \(S_1\) and \(S_2\), are replaced by diodes \(D_1, D'_1\) and \(D_2, D'_2\), and the third by two triodes \(T_1\) and \(T_2\), whose grid potentials vary cyclically and in opposite phase, so that when one conducts, the other is cut off. Let triode \(T_2\) now conduct during the first half-period. Then capacitor \(X_2\) will be charged through diode \(D_2\) and triode \(T_2\). In the next half of the cycle, when \(T_1\) conducts, the potential of terminal \(t_1\) will rise from zero to \(E\), and the potential of \(t_2\) to \(2E\). At this time capacitor \(X_2\) charges capacitor \(K_2\) through diode \(D'_2\) and triode \(T_1\) (since at \(p_3\) the potential \(E\) is added here, the whole circuit \(X_2—K_2\) will be under this potential). Further, in the subsequent cycles, when triode \(T_2\) conducts, capacitor \(K_2\) charges capacitor \(X_1\) through diode \(D_1\) and triode \(T_2\). Finally, when triode \(T_1\) conducts, capacitor \(X_1\) charges capacitor \(K_1\). As we see, this circuit is completely analogous to the preceding one.

However, the implementation of this circuit encounters a certain difficulty, consisting in the construction of a triode withstanding a voltage of \(200—400\ kV\), which is necessary for obtaining potentials of the order of \(800\ kV\). It turns out, however, that in those cases when a relatively sma-

... power, one can make do with simpler means. Indeed, from consideration of the preceding circuit it follows that the purpose of the triodes is to apply alternately the potentials \(p_3\) and \(p_1\) to the terminal \(t_3\). The use of triodes can be avoided altogether by replacing them with two diodes, if one takes one more auxiliary capacitor \(X_3\) and connects it through the diode \(D'_3\) in series with the secondary winding of a transformer giving the potential \(\dfrac{E}{2}\).

Fig. 3.

It is seen from the circuit that when the transformer terminal connected to \(X_3\) becomes the negative pole, the capacitor \(X_3\) is charged to the potential \(\dfrac{E}{2}\). Further, it is clear that since \(X_3\) and the secondary winding of the transformer are connected in series, the potential \(t_3\) oscillates between zero and \(E\). When the potential \(t_3\) approaches \(E\), the capacitor \(K_3\) is charged through the diode \(D_3\). When, however, the potential \(t_3\) begins to fall, the potential \(t_2\) becomes less than the potential \(p_3\), and the capacitor \(K_2\) begins to charge the capacitor \(X_2\) through the diode \(D'_2\). In short, everything proceeds as in the preceding circuit, and equilibrium is established when the capacitors \(K_1\), \(K_2\), \(K_3\), \(X_1\), \(X_2\) prove to be charged approximately to the potential \(E\).

ARTIFICIAL TRANSMUTATION OF ELEMENTS

It is possible to calculate what the degree of constancy of the potential is in such a system. If the capacitors \(K_1,—K_2,—K_3\) are loaded by a resistance \(3R\), and if \(\tau\) is the duration of the entire cycle, \(C\) the capacitance of an individual capacitor, and \(n\) the number of capacitors, then a simple calculation shows that:

\[ \frac{\delta V}{V}=\frac{n+1}{2}\cdot\frac{\tau}{CR}. \]

If, for example, \(n=2,\ C=0.001\ \mu F,\ R=10^9,\ \tau=10^{-2}\) sec., then

\[ \frac{\delta V}{V}=1.5\%. \]

Of all three circuits described, the last is the simplest. Since, moreover, in this circuit each rectifier and each capacitor must withstand at most twice the transformer voltage, the problem of constructing suitable capacitors and rectifiers, for any number of stages of potential increase, is thereby simplified. This advantageously distinguishes the circuit described from the analogous circuit earlier proposed by Schenkel,* in which some links must withstand the full voltage of the entire system. Since the authors knew from previous experience that rectifiers capable of withstanding up to \(400\ kV\) could be constructed, they decided to make use of a circuit consisting of two stages, i.e., requiring 4 capacitors and 4 rectifiers and giving a fourfold increase of the transformer potential.

The construction of the rectifiers used by the authors is very simple and ingenious. Four glass cylinders, each 35 cm in diameter and about 1 m (3 feet) high, were placed one upon another in the form of a tower 4 m high. Between the individual cylinders sheets of iron \(A\) were laid, by means of which the electrodes \(B\) were fastened. The latter consisted of thin-walled steel tubes ending in thick rings \(C\) for pre-

* Schenkel, E. T. Z., 40, 333, 1919.

...to prevent autoelectronic discharge. The top and bottom of the tower were closed with solid metal plates, and all joints were made airtight by means of a special plastic compound, “plasticine” (“plasticene”). The properties of this compound, unfortunately, are completely unknown to us*; judging from the authors’ description, they are remarkable. Suffice it to say that so primitively constructed a rectifier, with an enormous number of “unreliable” places (the cylinders were simply placed on iron sheets without any soldering), maintained a high vacuum perfectly. At the same time, the simplicity of the construction afforded great convenience: at any moment the apparatus could be taken apart and reassembled.

Fig. 4.

The filament had a V-shaped form and projected slightly beyond the limits of the metal cathode tube. For heating, one end of the filament was connected to the tinplate sheet, while the other was led outside through an opening drilled in the glass wall of the rectifier and sealed with the same “plasticine.” The heating itself was produced by 6-volt storage batteries placed in special metal boxes, which were located above the capacitors (see the photograph of the installation). Evacuation was carried out by a three-stage diffusion—

* It is manufactured by the firm “Metropolitan Vickers Electrical Co.,”

with an oil* pump, placed, to save space, under the floor (Fig. 4).

The experimental tube was built on the same principle as the rectifier. Here two glass cylinders of the size indicated earlier were taken and overlaid with a steel plate \(A\) (Fig. 6). The use of tin sheets in this case proved inadvisable, since here the electrode \(B\), supported by the plate, must be centered very carefully, which cannot be done with thin sheet metal.

Fig. 5.

The purpose of the electrodes \(B\) is to focus the beam of protons traveling in the tube. The protons were obtained by a discharge in the tube, which was supplied by a special transformer at 60 kV, and, under the influence of the electric field, passed into the experimental tube through a slit, similarly to ions in Aston’s mass spectrograph. The unit consisting of the transformer for supplying the additional tube and the alternator sending current into the primary winding of the transformer was located on an iso-

* On this new type of pump see Deshman, Uspekhi Fizicheskikh Nauk, 11, 669, 1931.

accelerating column, visible in the photograph of the whole apparatus behind the experimental tube (Fig. 5). Evacuation of the tube, as well as of the rectifier, was carried out by a fast-acting diffusion oil pump.

With such a tube the authors were able to reach approximately potentials of about 700 kV at a current of the order of 10 microamperes. The authors intend to increase the length of the cylinders of the experimental tube and to proceed to increasing the potential to 800 kV. It may be expected that, by taking a larger

Fig. 6.

Fig. 7.

number of capacitors and rectifiers, it will be possible without particular difficulty to reach accelerating potentials of a million volts.

§ 2. In order to be able to experiment with the fast protons obtained by means of the tube described, the latter were released into a special apparatus shown in Fig. 7. Here \(A\) is a plate of the substance under investigation, placed at an angle of \(45^\circ\) to the direction of the proton beam; the aperture at \(B\) could be closed either with a thin mica or a ZnS screen. In the first experiments with lithium, a lithium disk was placed at \(A\), at \(B\) there was a ZnS screen turned with its sensitive side inward, and at \(C\) a mica plate was attached with an absorbing capacity of 1.4 cm of air. This thickness of mica with iz-

ARTIFICIAL TRANSFORMATION OF ELEMENTS

Experimental experience was sufficient to absorb all the protons reflected from screen \(A\), since preliminary experiments showed that protons accelerated at \(600\ \mathrm{kV}\) had a maximum range of the order of \(10\ \mathrm{mm}\) in air. As is clear from the drawing, the current to plate \(A\) could be measured by means of an electrode soldered into the tube; in the experiments of Cockcroft and Walton the currents reached \(5\) microamperes.

The very first experiments with lithium showed that at \(125\ \mathrm{kV}\) and at a current of about \(1\ \mu\mathrm{A}\), bright scintillations began to appear on the screen. Under the indicated conditions about 5 of them were observed per minute, while at \(600\ \mathrm{kV}\) and a current of \(0.3\ \mu\mathrm{A}\) the number of particles emitted by lithium reached 700 per minute (in the latter case the count was made not by observing scintillations, but from the photographic record of the oscillograph deflections; see below). No scintillations were observed when the current in the discharge tube with hydrogen was switched off or when the screen was shielded from the proton beam by a brass plate.

In order to determine the nature of the particles obtained, several series of additional experiments were carried out. Namely, since the scintillations, in their character and brightness, very much resembled the scintillations of \(\alpha\)-particles, experiments were first set up to determine the range of the particles obtained. For this purpose the aperture in \(B\) was covered with a sheet of mica with an absorbing power of \(2\ \mathrm{cm}\) of air; the screen was placed outside the tube, and between the tube and the screen it was possible to insert any number of mica sheets. In this way it was possible to find that the scintillations were caused by particles having a definite, well-pronounced range, equal to about \(8\ \mathrm{cm}\) of air, and this range changed almost not at all when the potential was increased from \(250\) to \(500\ \mathrm{kV}\).

In order to make it possible to decide more definitely the question of the nature of the particles, a series of experiments was performed in which the particles, emitted through the mica window, entered a Wilson chamber. It turned out that, as soon as a potential was applied to the tube, the familiar cloud tracks appeared in the chamber, com—

tion of the same type as in the passage of $\alpha$-particles. Determination of the range by measuring the length of these strips yielded the same results as the previously described experiments.

Fig. 8.

Finally, precise experiments were carried out with the aid of an ionization chamber connected through an amplifier to an oscillograph. In this way, by placing mica sheets in front of the ionization chamber, the curves of Fig. 8 were obtained, also showing that the particles have a sharply defined range. The exact value of the range, measured in this way, is equal to 8.4 cm of air.

It was then possible very simply to measure the ionization curve of an individual particle. Since the system consisting of the chamber, amplifier, and oscillograph is designed so that the deflections of the oscillograph are strictly proportional to the ionization current, the magnitude of the deflections recorded on the photographic tape made it possible to judge the ionizing power of the particles after passage through a definite layer of air. In this way the ionization curve shown in Fig. 9 was obtained. This curve, as

Fig. 9.

it is seen, coincides completely with the well-known Bragg curve for the ionizing power of $\alpha$-particles.

The totality of all the results enumerated leads to an unambiguous conclusion: the particles obtained in the destruction of a lithium nucleus by a stream of protons are $\alpha$-particles, i.e. helium nuclei. Thus the following picture of the process is obtained: fast protons are captured by the nucleus of the lithium isotope $\mathrm{Li}^7$ (which is present in large excess over the isotope $\mathrm{Li}^6$), and the resulting nucleus of mass 8 breaks up into two $\alpha$-particles. If in this process the law of conservation of momentum is obeyed, then the $\alpha$-particles produced must possess the same store of energy, and from the observed ranges of the particles we may conclude that the transformation process takes place with the release of energy in the amount of $17.2 \times 10^6\ \mathrm{V}$. That this is indeed possible is shown above all by calculations of the mass defect. According to Aston’s precise measurements, the mass of $\mathrm{Li}^7$ is $7.0104 \pm 0.003$; the decrease of mass in the transformation of the nucleus will be
$7.0104 + 1.0072 - 8.0022 = 0.0154 \pm 0.003$. This decrease of mass is equivalent to the release of energy of $(14.3 \pm 2.7) \times 10^6\ \mathrm{V}$—a number close to that found directly from the range of the $\alpha$-particles produced.

Finally, the last, very elegant experimental proof of the correctness of the hypothesis made is based on the simultaneous counting of scintillations on two screens. If, in the decay of the nucleus, the law of conservation of momentum is obeyed, then the particles produced must fly apart in exactly opposite directions. Thus, by placing a very thin and small lithium sheet between two screens at an angle of $45^\circ$ to the proton beam, we may expect that a significant number of scintillations on both screens will coincide in time. The corresponding experiments showed that such coincidences are in fact observed, and in a number corresponding to the geometry of the arrangement and to the “excitation function” of the fluorescent screen.

In addition to lithium, experiments were carried out with a whole series of elements: Be, B, C, O, F, Na, Al, K, Ca, Fe, Co, Ni, Cu, Ag;

Pb, U. In all cases, when observing with a fluorescent screen, one can clearly detect a certain number of bright scintillations. The relative orders of magnitude of the effect at 300 kV for various elements are presented graphically in Fig. 10. More careful experiments were carried out with some of these elements. In almost all cases the deviations of the oscillograph were of such a magnitude that, with a high degree of probability, it could be concluded that the transformation products are $\alpha$-particles.

Fig. 10. Atomic number

In beryllium two types of scintillations were observed: a small number of bright scintillations, very similar to the scintillations of $\alpha$-particles, and, in addition, at 500 kV a considerably larger number of weak scintillations appears, whose number rapidly increases with increasing potential. The nature of the particles causing these scintillations is not yet clear; it is certain only that they have a very short range. The element boron gives the largest number of scintillations after beryllium. Already at 115 kV the first, few flashes of the screen are observed; their number reaches 100 per minute at 375 kV. At present still

there are not enough data to decide by which of the two possible paths:

\[ \mathrm{B}^{11}+\mathrm{H}^{1}=\mathrm{Be}^{8}+\mathrm{He}^{4} \]

or

\[ \mathrm{B}^{11}+\mathrm{H}^{1}=3\mathrm{He}^{4} \]

the transformation actually proceeds. It is interesting to note that, of the four elements situated next to one another in the periodic system—Fe, Ni, Co, Cu—iron produces almost no effect, whereas the remaining elements produce a rather considerable effect.

Finally, perhaps the most striking results were obtained with uranium. In this case it turned out that, upon bombardment by strong proton currents at a potential of 600 kV, the number of scintillations from uranium increases fourfold in comparison with the number of natural scintillations, and the artificially induced particles have a longer range than the natural ones. There is as yet no complete certainty that these additional scintillations belong to the uranium itself and not to some contaminant. If, however, the fact of such a considerable enhancement of the radioactivity is confirmed, this will be a remarkable discovery. It must be pointed out here that understanding the mechanism of artificial transformation in the case of uranium is very difficult. Undoubtedly, at 600 kV a proton cannot approach the uranium nucleus closely enough to be captured by it. The only way to explain such capture is to assume a resonance process. The existence of resonance disintegration of the atomic nucleus when bombarded by α-particles has been definitely proved by the experiments of Pose and Chadwick with their collaborators. It is not impossible that here too we are dealing with a resonance effect of the same kind.

As a general conclusion from the results obtained so far, it may be noted that the most considerable effect is produced by the elements lithium, boron, and fluorine, i.e. elements having atomic weights of the type \(4n+3\). It is natural to suppose that the captured proton, together with the three existing-

forming in the nucleus creates a new $\alpha$-particle, and the entire system turns out to be unstable.

There is no doubt that the results obtained by Cockcroft and Walton, together with a number of other remarkable discoveries made this year in the physics of the atomic nucleus (see the preceding articles in this issue of Uspekhi), will give a powerful impetus to the development of the theory of the atomic nucleus. The first interesting attempts in this direction have already been made, and we have reason to expect that in this very field, in the near future, results will be obtained that are highly significant not only for the theory of the atomic nucleus, but also for all of quantum physics in general.*

* W. Heisenberg, Z. Physik, 77, 1 (1932).

** In the time that elapsed after this article was written, experiments on the artificial disintegration of lithium were repeated in two places: in our country, in the USSR, by Sinelnikov, Walter, and Latyshev at the Ukrainian Physico-Technical Institute (Kharkov), and in America at the University of California by Lawrence, Livingston, and White (Phys. Rev., October 1, 42, 150, 1932). In both cases positive results were obtained, confirming the experiments of Cockcroft and Walton. In Kharkov the experimental apparatus apparently was analogous to that of Cockcroft and Walton, and the observations were carried out by counting scintillations, whereas the American researchers used the method of obtaining fast protons developed by Lawrence and Livingston (see Mysovsky’s article, p. 601), and used a Geiger counter (with a point) to count the particles obtained. It is interesting to note that the numerical values of the probability of liberation of an $\alpha$-particle from the lithium nucleus under impact by fast protons, found from Lawrence’s experimental data, agree with those theoretically predicted on the basis of Gamow’s theory.