Recent Experimental Results in Nuclear Isomerism

B. Pontecorvo

Editor’s Note

The idea that two atomic nuclei might be identical in their physical composition and mass but nevertheless might differ in their radioactive properties goes back to 1917 and to Frederick Soddy, one of the pioneers of radioactive studies. It is equivalent to saying that isomeric nuclei are capable of decaying by more than one route. With the discovery of artificial radioactivity, pairs of isomeric nuclei became more common. Bruno Pontecorvo, who migrated from France to Britain in 1940, had made a special study of isomeric nuclei and published this account of their properties a few months before the Second World War began.ft　　中文

THE hypothesis that two atomic nuclei indistinguishable in respect of atomic and mass number could nevertheless have different radioactive properties (the hypothesis of nuclear isomerism) was put forward for the first time by Soddy1 in 1917. In 1921 uranium Z was discovered by Hahn2; by studying the chemical and radioactive properties of this element, Hahn deduced that uranium Z and uranium X2 are isomeric nuclei. The problem of uranium Z has been taken up recently by Feather and Bretscher (Proc. Roy. Soc., 165, 542; 1938). It should be noted that, for many years, uranium Z and uranium X2 were the only known example of an isomeric pair.ft　　中文

After the discovery of artificial radioactivity, the study of isomerism received considerable impetus on account of the experimental material assembled in the course of research on artificial radio-elements. The first certain example of an isomeric pair to which it has been possible to attribute a mass number (A = 80) in the domain of the artificial radio-elements was furnished3 by the study of the radioactivity produced in bromine by neutrons (slow and fast) and by γ-rays of great energy.ft　　中文

Then, as the experimental material on artificial radio-elements has increased, the number of pairs of nuclei which are undoubtedly isomeric has grown to such an extent that it is not possible to quote here all the investigations which have been published on the question. More than thirty such pairs are known and there is no doubt that the number still unknown is much greater. We can say, now, that nuclear isomerism is by no means an exceptional phenomenon.ft　　中文

It is natural to think that the physical difference between two isomeric nuclei is connected with two states of different excitation of the same nucleus (let us say ground state and first excited state). But in this case, how could the upper state be metastable, that is, how could it live for any length of time (greater than one day, in some cases)? By what mechanism would it be preserved from destruction in a very short time by the emission of an electromagnetic radiation? Weiszäcker has answered this question4.ft　　中文

According to Weiszäcker’s hypothesis, nuclear isomerism may be explained by assuming that the lowest excited state of the nucleus has an angular momentum differing by several units from that of the ground state. Selection rules may then be invoked to weaken considerably the probability per unit of time of the transition from the upper to the ground state of the nucleus. Of course, experiments which make it possible to test the truth of Weiszäcker’s hypothesis are of great interest.ft　　中文

One of the most important points is the study of the γ-radiation eventually emitted in the transition from one isomeric state to another: I say “eventually emitted” because, the nucleus being radioactive, the upper state corresponding to one of the isomeric forms may be destroyed by an ordinary β transition. The γ-ray–β-ray branching ratio will depend on the relative life-times for the two modes of decay. The first researches made to observe this γ-radiation failed. But it should be noted5 that the considerable difficulty in detecting this radiation might be due to the fact that transitions between isomeric nuclei can be strongly converted: in this case electrons of small energy would be emitted and not γ-rays.ft　　中文

The very complete theory of the internal conversion of radiations emitted in the transitions between isomeric states6, given by Hebb and Uhlenbeck, Dancoff and Morrison, has shown that these radiations must have internal conversion coefficients of approximately 1. Since these calculations are based on Weiszäcker’s hypothesis, it can be concluded that experiments which prove that the conversion coefficient in question is very high would indicate, to a certain extent, that Weiszäcker’s hypothesis is correct.ft　　中文

Indeed, in the case of the isomerism of radio-rhodium, Pontecorvo7 has observed a radiation of low-energy electrons, which he interpreted as an electron line emitted in the transition from the metastable state to the ground state of the nucleus.*ft　　中文

At the present time, after a number of recent experiments, there is no longer any doubt as to the fact that these transitions are generally strongly converted. In particular, in the cases of isomeric nuclei of radio-bromine8 and of element 439, strong lines of conversion electrons have been photographed in the Wilson chamber or in the magnetic spectrograph. Of course, the internal conversion is accompanied by emission of X-rays: as a rule, the analysis of these rays is an invaluable test in the interpretation of these phenomena9,10.ft　　中文

It is interesting to find possible genetic relations between isomeric states of the same nucleus (β-radioactive): in this direction an extremely brilliant method has been described by Segré, Halford and Seaborg11, who have succeeded in separating, one from the other, the two isomeric forms of radio-bromine. The principle of their method is as follows. Suppose the element, of which the isomeric states are being studied, can give compounds suitable for the application of the Szilard-Chalmers method of concentration. When the isomer in the upper state decays to the lower state, there is a γ-ray emission: corresponding recoil may be sufficient to knock the decayed atom out of the compound. The daughter activity can then be separated, as in the Szilard-Chalmers method.ft　　中文

This method, which has been successfully applied in several cases12, can then be used (a) to separate known isomers in some cases; (b) to discover the existence of isomeric pairs, still unknown, in the study of artificial radioactivity.ft　　中文

Moreover, it has given a striking new proof that the transitions between isomeric states are strongly converted: in effect, the recoil due to the γ-emission is not sufficient to knock the decayed atom out of the compound, while the recoil of a conversion electron can be sufficient.ft　　中文

So far we have discussed radioactive isomers: the isomerism in this case, implies a difference in the life-times of the isomers. It has been noticed by Pontecorvo5 that β-stable nuclei with a metastable excited state ought not to be very rare and should be revealed by the study of the radiation emitted by this metastable state. These nuclei are interesting for the understanding of nuclear isomerism, because the radiation corresponding to the transition from one isomeric state to the other is not troubled by the presence of β- or γ-rays. It should be possible to obtain a β-stable nucleus in a metastable state, after a nuclear transmutation or a radioactive disintegration.ft　　中文

Dodé and Pontecorvo13, by bombardment of cadmium with fast neutrons, have obtained an activity (T=50 min.) which chemical proofs have shown to be due to an isotope of cadmium. On the other hand, there is no question of a reaction of simple neutron capture or of an n,2n reaction. They interpreted the soft radiation emitted by cadmium (50 min.) as proceeding from a metastable state of an isotope of cadmium; the reaction of excitation without capture by fast neutrons (reaction n,n), having a considerable cross-section (some 10－24 cm.2), it is not impossible, indeed, that part of the nuclei so excited might fall into a metastable state.ft　　中文

Segré and Seaborg9 have observed a metastable state of element 43, decaying (only a line spectrum of electrons) with a 6-hour period into the ground state, which is stable or perhaps radioactive with a long life: the 6-hour activity is daughter of a β-radioactive molybdenum.ft　　中文

A very interesting case has been observed and studied thoroughly by Goldhaber, Szilard and Hill14. They have obtained by the n,n reaction already quoted, a metastable state (115In) of 115In, decaying with a period of 4.1 hours; moreover, the same state can be obtained after the disintegration of a radio-cadmium (T=2.5 days). The radiation emittedby 115In has not yet been sufficiently studied; its properties are of the greatest interest both for the understanding of the nuclear isomerism and for that of isobaric pairs. In effect, 115In and 115Sn are one of the rare cases of stable neighbouring isobaric nuclei: 115In* might then decay into 115Sn (β-emission) or into 115In, or into both together.ft　　中文

The same metastable state of 115In has been obtained also by irradiating indium with 5.8 Mev. protons (reaction p,p), by Barnes and Aradine15. Nevertheless, it is not yet clear whether the mechanism of nuclear excitation is that discussed by Weisskopf, that is, excitation by the action of the electric field of the proton16.ft　　中文

In all these cases and in others studied more recently17, the metastable states of stable isotopes are obtained from nuclear transmutations.ft　　中文

Lazard and Pontecorvo18 have tried a new method, by which it would be impossible to transmute the nucleus and, consequently, to obtain artificial radio-elements, the presence of which may interfere with the investigation.ft　　中文

This method consists of irradiating the target with a continuous spectrum of X-rays, the energy of which is less than the dissociation energy of the nuclei. Suppose the radiated nuclei have a metastable state; the X-rays may excite higher levels of the nuclei; a part of the nuclei thus excited can fall into the metastable state, and it is the radiation from this state which can be observed. The maximum energy of the continuous spectrum utilized was 1,850 kilovolts: indium gives an activity of approximately 4 hours period, which is obviously due to the same metastable state 115In*, of which we have already spoken. Similar results on the stable nuclear fluorescence of indium were obtained by Collins and others19.ft　　中文

There is no doubt that new isomers of β-stable nuclei will be discovered, in the course of research undertaken in different laboratories; systematic research on the radiations emitted by metastable states will certainly be very useful for the understanding of nuclear isomerism.ft　　中文

In conclusion, we may remark that it is very probable, on account of the great number of known isomers, that the radiative transitions of life-times between, say, 10－7 sec. and 1 sec. are much more frequent than is generally supposed. These transitions, on the other hand, are strongly converted6. We would expect20, consequently, that transitions of this kind, with conversion coefficients approximately 1, may be found frequently.ft　　中文

Indeed, in the radiation emitted in the capture of slow neutrons by gadolinium, a strong component of soft electrons has been observed by Amaldi and Rasetti21 (life-time less than 10－3 sec.). Soft electronic components have also been observed in the capture of slow neutrons by other nuclei22. On the other hand, these strongly converted transitions may play a considerable part in the interpretation of γ- and X-spectra emitted by certain natural radio-elements20.ft　　中文

(144, 212-213; 1939)

B. Pontecorvo: Laboratory of Nuclear Chemistry, College de France, Paris.

References:

  1. Soddy, Proc. Roy. Inst., 22, 117 (1917).

  2. Hahn, Ber. dtsch. Chem. Ges., B, 54, 1131 (1921).

  3. Kourtchatow, Myssowsky, Roussinow, C.R., 200, 1201 (1935). Amaldi, d’Agostino, Fermi, Pontecorvo, Segré, Ric. Scient., 6, 581 (1935). Amaldi and Fermi, Phys. Rev., 50, 899 (1936). Bothe and Gentner, Z. Phys., 106, 236 (1937).

  4. Weiszäcker, Naturwiss., 24, 813(1936).

  5. Pontecorvo, Congrès du Palais de la Decouverte, Paris, 1937, p. 118.

  6. Hebb and Uhlenbeck, Physica, 5, 605 (1938). Dancoff and Morrison, Phys. Rev., 55, 122 (1939).

  7. Pontecorvo, Phys. Rev., 54, 542 (1938).

  8. Valley and McCreary, Phys. Rev., 55, 666 (1939). Siday, Nature, 143, 681 (1939).

  9. Seaborg and Segré, Phys. Rev., 55, 808 (1939). Kalbfell, Phys. Rev., 54, 543 (1938).

  10. Alvarez, Phys. Rev., 54, 486 (1938). Roussinow, Yusephovitch, Phys. Rev., 55, 979 (1939). Siday, ref. 8. Walke, Williams and Evans, Proc. Roy. Soc., A, 171, 360 (1939).

  11. Segré, Halford and Seaborg, Phys. Rev., 55, 321 (1939).

  12. De Vault and Libby, Phys. Rev., 55, 322 (1939). Le Roux, Lu and Sugden, Nature, 143, 517 (1939). Seaborg and Kennedy, Phys. Rev., 55, 410 (1939).

  13. Dodé and Pontecorvo, C.R., 207, 287 (1938).

  14. Goldhaber, Hill and Szilard, Phys. Rev., 55, 46 (1939).

  15. Barnes and Aradine, Phys. Rev., 55, 50(1939).

  16. Weisskopf, Phys. Rev., 53, 1018 (1938).

  17. Delsasso, Ridenour, Sherr and White, Phys. Rev., 55, 113.

  18. Pontecorvo and Lazard, C.R., 208, 99(1939).

  19. Collins, Waldman, Stubblefield and Goldhaber, Phys. Rev., 55, 507 (1939).

  20. Pontecorvo, C.R., 207, 230 (1938).

  21. Amaldi and Rasetti, Ric. Scient., 10, 115 (1939).

  22. Hoffman and Bacher, Phys. Rev., 54, 644 (1938). Pontecorvo, C.R., 207, 856 (1938).

* Note added in proof. A similar conclusion was independently obtained by Roussinow and Yusephovitch [C. R. Acad. Sci. U.R.S.S., 20, 9 (1938)] who observed a soft electron radiation in the case of isomeric forms of radio-bromine.