From Current Literature

Direct Methods of Isotopic Identification in Nuclear Research

Many experimental results in nuclear physics contain an uncertainty due to the complexity of the isotopic composition of elements. For example, the result of measuring the resonance absorption of neutrons by some element is usually given in the form of a neutron-capture cross section calculated from the total number of all atoms of this element in the absorber. A more interesting figure would be the true capture cross section, i.e., the cross section referred to the atoms of only that isotope which is responsible for the given resonance absorption. However, in most cases no definite answer could be given to the question of exactly which isotope of the element possesses the resonance level under consideration, since the indirect methods of isotopic identification that had been used had limited applicability.

Another extensive group of experimental results that have remained indeterminate concerns the study of artificial radioactivity. In the table of artificially radioactive isotopes (see, for example, Seaborg’s table[^1]) there still remains a very large number of cases in which the activity found experimentally has not been assigned to any definite nucleus, since indirect methods did not make it possible to do this unambiguously.

Recently, questions of isotopic identification have been successfully solved by direct methods. Their application has become possible thanks to the new level of technical means in nuclear physics. The appearance of new powerful neutron sources in the form of chain-reaction piles has increased the neutron intensity available to the experimenter hundreds of times, and the development of isotope-separation techniques has made it possible to set up experiments with weighable quantities of material consisting wholly or almost wholly of a single isotope.

Several varieties of the direct method of isotopic identification may be indicated. To determine the mass number of an isotope responsible for strong neutron absorption, the following methods have been used.

1) The substance under investigation, with its natural isotopic composition, is subjected to prolonged irradiation by a powerful neutron flux. With a sufficiently large capture cross section and a sufficiently high flux intensity, a rather large change in isotopic composition occurs in the sample (as a result of the transformation of some isotopes into others upon neutron capture), so that it can readily be detected by the ordinary mass-spectrographic method. Analysis of the change that has occurred makes it possible to draw a conclusion as to exactly which isotopes of the given element strongly absorb neutrons.

This method was applied, in particular, to cadmium². For investigation in the mass spectrograph, only the surface layer of a cadmium plate facing the neutron source was taken, since it is only in this layer that strong absorption of neutrons occurs and, consequently, the change in the isotopic composition of the sample will be greatest here. In the mass spectrum of the irradiated sample there proved to be almost no visible line \(M = 113\), while the line \(M = 114\) was denser than in the spectrogram of the unirradiated cadmium sample. In other respects the mass spectra of the irradiated and unirradiated cadmium samples practically did not differ.

Photometric determination of the percentage content of the indicated isotopes in the irradiated and unirradiated cadmium samples gave the following figures: in unirradiated cadmium the content of \(\mathrm{Cd}^{113}\) is \(12.3\%\), \(\mathrm{Cd}^{114}\)—\(28\%\); in the irradiated sample \(\mathrm{Cd}^{113}\) is \(1.6 \pm 0.2\%\), \(\mathrm{Cd}^{114}\)—\(39.5 \pm 1.5\%\). Thus, within the accuracy of the measurements, the increase in the content of \(\mathrm{Cd}^{114}\) is equal to the decrease in the content of \(\mathrm{Cd}^{113}\). These measurements show unambiguously that the isotope responsible for the strong absorption of slow neutrons by cadmium is \(\mathrm{Cd}^{113}\) [reaction \(\mathrm{Cd}^{113}(n,\gamma)\mathrm{Cd}^{114}\)].

It should be noted that in this case the product of the reaction is a stable isotope of cadmium, and therefore identification by radioactive methods is impossible here.

By the same method³ it was established that the strong absorption of slow neutrons by samarium is due to the isotope \(\mathrm{Sm}^{149}\), and the absorption of neutrons by gadolinium to the isotopes \(\mathrm{Gd}^{155}\) and \(\mathrm{Gd}^{157}\), the capture cross section of slow neutrons by the nucleus \(\mathrm{Gd}^{157}\) being approximately 3.5 times greater than the cross section for the nucleus \(\mathrm{Gd}^{155}\).

2) With the aid of some installation for isotope separation, samples of the element are prepared with a very high content of one of its isotopes, different in different samples. Then a comparative measurement is made of the resonance neutron-capture cross section for several samples, of which one is normal and the others are enriched. The result makes it possible to indicate the mass number of the isotope responsible for strong absorption of neutrons of a definite energy.

This method was also applied to cadmium⁴. The enriched samples were obtained with the aid of a mass spectrograph. In all, seven enriched samples were prepared, of which one had a high percentage content of \(\mathrm{Cd}^{113}\) (of the order of 70–80%), the second—\(\mathrm{Cd}^{106}\), the third—\(\mathrm{Cd}^{108}\), etc. Measurement of the absorption of slow neutrons in these samples showed that, for the sample enriched in the isotope \(\mathrm{Cd}^{113}\), the absorption cross section was 6 times greater than for cadmium of normal isotopic composition, whereas for all the other enriched samples the cross sections proved to be much smaller than for normal cadmium. These results show that the large capture cross section of slow neutrons by cadmium should be attributed to the isotope \(\mathrm{Cd}^{113}\), and that all the other cadmium isotopes have incomparably smaller neutron-capture cross sections.

The following methods were used to determine the mass number of the carrier of one or another activity.

1) By means of intense neutron irradiation, it is possible to create in a sample a large number of artificial radioactive nuclei of a given kind. Then, in a mass spectrograph, the sample is separated into isotopes, and each line of the resulting mass spectrum is studied with the aid of a Geiger–Müller counter or by some other method. In this case, the half-life of the given artificial radioactive nucleus is used as the principal identifying parameter. In this way, the direct assignment of each half-life to a nucleus with a definite mass number is obtained.

This method is not new⁵, but only recently have possibilities appeared for its broad use.

One of the groups of physicists carrying out an extensive program of isotopic identification in the field of artificial radioactivity and in the field of uranium fission products uses a specially constructed high-aperture mass spectrograph.^6 On the recording plate of the instrument, up to \(1/40\) of all the ions formed in the ion source are collected. The design of the source itself is distinguished by the high ion yield achieved (for example, in the case of lanthanum about \(16\%\) of all atoms are ionized).

The position of the lines of radioactive isotopes on the spectrograph plate is often determined by means of the autoradiography method, which consists in the following. A second plate is placed on the photographic plate exposed in the mass spectrograph before it is developed, in such a way that the emulsion layers are in contact with each other; in this position the plates are kept for a certain time. After this the standard development process for both plates follows. On the first plate the entire mass spectrum present is obtained, while on the second only those lines are obtained which correspond to \(\beta\)-radioactive isotopes. In order to judge the half-life of each of the active isotopes, several autoradiographs are made with different exposure times, and after photographic development the darkening of the corresponding lines is compared. In this way a large number of assignments of radioactive periods to definite nuclei was made.^7 The isotopic assignment of periods to uranium fission products should be especially noted.^6 In this case indirect methods of isotopic identification (for example, the “cross-bombardment method”) are inapplicable, since, as a rule, the nuclei obtained in uranium fission in most cases cannot be produced in nuclear reactions of the usual type.

2) A series of samples is prepared, each of which is strongly enriched in one of the isotopes of the given element, different for the different samples. The isotopic composition of each sample is determined in the mass spectrograph. Then all the samples, as well as a non-enriched sample, are subjected to identical irradiation, after which the activities that have arisen in these samples are compared and studied. This makes it possible to draw a conclusion about the mass number of the isotope to which the given activity should be assigned.

As an example, let us consider the assignment of the mass number to the isotope Cd (6.7 hours). Indirect methods of assignment show that the carrier of this activity may be either \(\mathrm{Cd}^{107}\) or \(\mathrm{Cd}^{109}\). To obtain an unambiguous assignment, two samples^6 were prepared, each about 100 mg in weight, one of which was enriched in the isotope \(\mathrm{Cd}^{106}\), and the other in \(\mathrm{Cd}^{108}\). Mass-spectrographic analysis showed that the first contains \(58.3\%\) \(\mathrm{Cd}^{106}\) and \(0.6\%\) \(\mathrm{Cd}^{108}\), while the second contains \(0.7\%\) \(\mathrm{Cd}^{106}\) and \(45.4\%\) \(\mathrm{Cd}^{108}\). The relative content of the remaining isotopes approximately corresponded to the normal composition of cadmium.

The samples were subjected to irradiation with deuterons in a cyclotron for several hours. The sample enriched in \(\mathrm{Cd}^{106}\) gave a large activity with a period of 6.7 hours, whereas in the sample enriched in \(\mathrm{Cd}^{108}\) this period was not found. It follows from this that the activity with a period of 6.7 hours belongs to \(\mathrm{Cd}^{107}\), which is formed in this case in the reaction \(\mathrm{Cd}^{106}(d,p)\).

A second example is the assignment of the mass number to the isotope Co (1.75 hours). By means of a calutron (an electromagnetic installation for separating isotopes), two nickel samples^9 were prepared, enriched respectively in the isotopes \(\mathrm{Ni}^{61}\) and \(\mathrm{Ni}^{62}\). After irradiation of the samples with fast neutrons from a cyclotron it turned out that in the sample with \(\mathrm{Ni}^{61}\) the activity with a period of 1.75 hours was one of the strongest. Chemically it was shown that this activity belongs to cobalt. Thus, the isotope Co (1.75 hours) is \(\mathrm{Co}^{61}\), which in this case arises in the reaction \(\mathrm{Ni}^{61}(n,p)\). This conclusion was checked as follows. It was shown that, after irradiation of pure copper with fast neutrons, in the chemical separation ...

of cobalt, activity with a period of 1.75 hours is observed. Then the sample irradiated with fast neutrons was separated into isotopes by means of a calutron, and the activity with a period of 1.75 hours was found to be collected in the line corresponding to mass number 61. Consequently, Co (1.75 hours) is Co\(^{61}\), and here it is formed in the reaction Co\(^{65}\) \((n,\alpha n)\) Co\(^{61}\).

This example shows that the direct method of isotopic identification is also valuable in that it makes it possible to establish with certainty the type of reaction that has occurred in a given case, something that without this method might prove very difficult.

A. P. Grinberg

CITED LITERATURE

1. G. T. Seaborg, UFN, 28, 145 (1946).
2. A. J. Dempster, Phys. Rev., 71, 829 (1947).
3. R. E. Lapp, J. R. Van Horn and A. J. Dempster, Phys. Rev., 71, 745 (1947).
4. B. J. Moyer, B. Peters and F. N. Schmidt, Phys. Rev., 69, 666 (1946).
5. For the bibliography see, for example, A. P. Grinberg, Priroda 1, p. 59 (1943). See also W. M. Schwarz and M. L. Pool, Phys. Rev., 64, 43 (A6) (1943).
6. L. G. Lewis and R. J. Hayden, Phys. Rev. 70, 111 (R 3, R 4) (1946); W. Pauli, Phys. Rev. 70, 112 (R 8), (1946).
7. M. G. Inghram and R. J. Hayden, Phys. Rev., 70, 130 (1947); M. G. Inghram, R. J. Hayden and D. C. Hess, Phys. Rev., 71, 270 (1947), et al.
8. A. C. Helmholz, Phys. Rev., 70, 982 (1946).
9. T. J. Parmly and B. J. Moyer, Phys. Rev., 72, 82 (1947).