FROM CURRENT LITERATURE

PHYSICAL METHOD FOR THE SEPARATION OF NUCLEAR ISOMERS

It is known that the separation of isotopes is associated with considerable difficulties. As for the separation of isomers, i.e., atoms whose nuclei, with identical charges and masses, differ somewhat in energy, it proves to be still more difficult. Until recently there existed only a chemical method for separating isomers, developed in 1939[^1].

L. I. Rusinov and A. S. Karamyan have developed a physical method for the separation of nuclear isomers[^2][^3][^4]. In studying the transformations of isomeric bromine nuclei, they encountered the necessity of obtaining extremely thin-layer radioactive preparations, in order, as far as possible, to eliminate the absorption in them of electrons of low energies. At first they tried to obtain such preparations by the usual method[^5][^6]. Ethyl bromide (C₂H₅Br) was irradiated with slow neutrons, as a result of which radioactive bromine atoms were produced. As a result of recoil, they flew out of the ethyl bromide molecules; after irradiation, the free atoms of radioactive bromine were washed out with distilled water, which was then separated by specific gravity from the ethyl bromide and subjected to electrolysis; in this process the radioactive bromine was deposited on the silver anode. However, the minimum thickness of the preparations obtained in this way was too great, because during electrolysis on the silver foil there were deposited also nonradioactive bromine atoms, as well as foreign impurities present in the water. Therefore the authors applied a new method for separating radioactive bromine: the electrolysis was carried out directly in the ethyl bromide. The anode was a silver disk 15 mm in diameter with a carefully polished depositing surface; the opposite side of the anode was covered with cellophane; during the electrolysis the disk was rotated uniformly at a speed of about 80 rev/min, and the electrolysis itself lasted about 30 minutes. The radioactive preparations obtained in this way had a thickness amounting to fractions of a micron, and the coefficient of extraction proved to be several times greater than in electrolysis in water. All subsequent experiments were carried out with the aid of this method of separating radioactive preparations.

It is known that irradiation of bromine with neutrons leads to the formation of the radioactive isotopes Br⁸⁰ (half-life 18 min.) and Br⁸² (34 hours), and also of the isomer Br⁸⁰*, which passes from the metastable state to the ground state by internal electron conversion with a period equal to 4.4 hours. According to experimental data[^7][^8], obtained in irradiat—

of bromine with thermal neutrons, the ratio of the probabilities of formation of Br\(^{80}\) in the ground state (\(T = 18\) min.) and in the metastable state (\(T = 4.4\) hours) is \(\sim 2.5\).

Already in the first experiment on isolating the radioactive preparation by the new method it was found that two hours after intensive irradiation of ethyl bromide with slow neutrons, carried out for a time sufficient to saturate the 4.4-hour period, in addition to the expected electron activities, decaying with half-lives of 4.4 and 34 hours, there is also an electron activity with a half-life close to 18 min. It would have seemed that such a short-lived activity should have been practically exhausted in two hours after cessation of irradiation. However, extrapolation of the decay curves of radioactive bromine to the moment when irradiation ended gave for the ratio of the initial intensities decaying with half-lives of 18 min. and 4.4 hours an anomalously large value, \(\sim 100\). Moreover, a similar activity was observed four hours after the end of irradiation, and the ratio of the initial intensities proved to be greater than 1000.

The presence of 18-minute \(\beta\)-activity after 2- and 4-hour intervals, as well as its anomalously high intensity, could be explained only on the assumption that it arises in a secondary process—the decay of radioactive Br\(^{80}\) nuclei formed as a result of the transition of metastable Br\(^{80*}\) nuclei to the ground state; for, when the isomers Br\(^{80}\) and Br\(^{80*}\) are formed in the primary process—the discharge of the compound nucleus—the ratio of the initial intensities should be \(\sim 2.5\) independently of the time of measurement. Further experiments confirmed this conclusion. At the same time, the anomalously high intensity ratio indicates a considerable increase in the separation coefficient of the bromine isomers in such a method of isolating radioactive atoms. (Analogous results were obtained independently in work \(^{9}\).)

To clarify the mechanism of separation of the bromine isomers, many experiments were performed. First of all, the authors showed that separation also occurs in the absence of an electric field and therefore cannot be explained by different mobilities of the Br\(^{80}\) and Br\(^{80*}\) ions. For this purpose, radioactive atoms were isolated from 50 cm\(^3\) of C\(_2\)H\(_5\)Br 4.7 hours after irradiation directly onto an ethyl-silvered plate lowered into it, without passing current. It turned out that the radioactive preparations obtained in this way also contain the \(\beta\)-active isomer Br\(^{80}\), decaying with a half-life equal to 18 min. The next series of experiments showed that, under certain conditions, separation of the isomers also occurs in the original method of isolating activity by washing with water. Thus it turned out that separation of the nuclear isomers of bromine is not determined by the method of isolating the radioactivity, but takes place as a result of the disturbance of radioactive equilibrium in the ethyl bromide itself; the mechanism of separation coincides with the processes that disturb the radioactive equilibrium of the isomeric nuclei Br\(^{80*}\) and Br\(^{80}\), decaying according to the successive scheme.\(^{10}\)

To clarify these processes, the following experiments were carried out. All radioactive atoms that had appeared in the free state were removed from irradiated ethyl bromide by means of an electric field or water, as completely as possible. Then, after six hours, this operation was repeated by the same methods. The radioactive preparation obtained in the repeated separation in both cases has 18-minute \(\beta\)-activity of Br\(^{80}\), and its ratio to the activity of Br\(^{80*}\) is approximately 10 times greater than in preparations obtained from ethyl before the preliminary removal of the free radioactive atoms. The appearance of new radioactive atoms in the free state and the increase in the coefficient of separa-

From Current Literature

...are explained by the fact that part of the metastable nuclei \( \mathrm{Br}^{80*} \) is present in bound form in organic molecules and therefore is not removed after the end of irradiation.

Initially, practically all atoms of radioactive bromine formed upon neutron capture, owing to the large recoil energy, fly out of the \( \mathrm{C_2H_5Br} \) molecules. However, after this, part of the free atoms, including the isomers \( \mathrm{Br}^{80*} \), is again synthesized into organic molecules. During the discharge of the metastable state by internal electron conversion, the resulting \( \mathrm{Br}^{80} \) nuclei again fly out of the molecules and enrich ethyl bromide with the \( \mathrm{Br}^{80} \) isomers (\(T = 18\) min.) in the free state, disturbing the radioactive equilibrium between \( \mathrm{Br}^{80} \) and \( \mathrm{Br}^{80*} \).

Having studied various pathways by which free radioactive bromine atoms enter organic molecules, the authors showed that the most probable is the so-called “bromination,” i.e., replacement in \( \mathrm{C_2H_5Br} \) molecules of one hydrogen atom by a free bromine atom, leading to the formation of ethylene bromide \( \mathrm{C_2H_4Br_2} \). To show that bromination does indeed occur, they added to ethyl bromide irradiated with neutrons a different amount of non-irradiated ethylene bromide. After some time the two substances were again separated by distillation. In the ethylene bromide thus separated, radioactive atoms of all three kinds were found.

Further experiments showed that bound radioactive bromine atoms are also detected in ethyl bromide molecules. We shall describe here briefly only one series of such experiments. In them the irradiated ethyl bromide was divided into two equal parts, one of which was distilled at the boiling point of ethyl bromide (\(38^\circ\mathrm{C}\)). Then, from the distilled fraction, the residue, and the undistilled part, radioactive atoms were separated by an electric field and their activity was investigated. It turned out that the distilled fraction, consisting of pure \( \mathrm{C_2H_5Br} \), emits an anodic radioactivity with a half-period of 18 min., which could have arisen only upon discharge of metastable \( \mathrm{Br}^{80*} \) atoms captured in ethyl bromide molecules. Preparations isolated from the two other parts revealed all three radioactive periods characteristic of bromine. These conclusions were also confirmed when measuring the total activities of all three parts. Thus, it was established that radioactive bromine atoms exist in three states: free and bound in the molecules \( \mathrm{C_2H_5Br} \) and \( \mathrm{C_2H_4Br_2} \).

To find out whether the synthesis of free radioactive bromine atoms into organic molecules occurs continuously (in the process of thermal diffusion) or only at the moment of irradiation (during the slowing down of bromine atoms that have flown out of the molecules upon neutron capture), experiments were carried out to determine the dependence of the number of free and bound radioactive bromine atoms on the time elapsed after the end of irradiation. It turned out that the number of free radioactive atoms decreases according to an exponential law, with a half-period of about 20 hours, whereas the number of bound radioactive atoms grows according to the same law with a half-period of about 25 hours. The authors believe that a certain difference in the half-periods is due to the fact that part of the free \( \mathrm{Br}^{80} \) nuclei, arising during the discharge of \( \mathrm{Br}^{80*} \) nuclei bound in molecules, again attaches to organic molecules.

From these experiments it is clear that separation of bromine isomers occurs independently of the method of isolating radioactive preparations, as a result of disturbance of radioactive equilibrium between isomeric nuclei in the very system subjected to neutron irradiation. This disturbance is caused by the presence of metastable isomers of bromine bound in the molecules \( \mathrm{C_2H_5Br} \) and \( \mathrm{C_2H_4Br_2} \), and by the fact that the discharge energy of the meta-

of the stable state is sufficient to overcome the chemical bond and for the ejection of the resulting radioactive Br^80 nuclei from the molecules.

Thus, as a result of this work, not only was the physical separation of nuclear isomers of bromine discovered, but the mechanism by which this separation is accomplished was also clarified.

V. Leskovtsev

References Cited

1. E. Segre, R. Halford, G. Seaborg, Phys. Rev. 55, 321 (1939).
2. L. I. Rusinov and A. S. Karamyan, Doklady of the Academy of Sciences of the USSR, LV, No. 7, 603 (1947).
3. A. S. Karamyan and L. I. Rusinov, Doklady of the Academy of Sciences of the USSR, LVIII, No. 4, 573 (1947).
4. A. S. Karamyan, Doklady of the Academy of Sciences of the USSR, LXIV, No. 4, 491 (1949).
5. L. Szilard, T. Chalmers, Nature 134, 462 (1934).
6. L. Roussinow, A. Yousephovich, Journal of Physics 3, 281 (1940).
7. R. Fleischman, Zeitschrift fur Physik 107, 305 (1935).
8. L. Rusinov and A. Iosefovich, Doklady of the Academy of Sciences of the USSR, XX, 647 (1938).
9. R. S. Sargon, G. Stokkink and M. Meersche, Nature 157, 806 (1946).
10. L. I. Rusinov and A. A. Iosefovich, Doklady of the Academy of Sciences of the USSR, XXII, 580 (1939).