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$K$-Spectrum of Element No. 61
Since the discovery of Moseley’s law, X-ray spectra have played a major role in establishing the correct arrangement of the elements in Mendeleev’s periodic system. It is known, for example, that thanks to them it was possible in due time to establish the order of arrangement of the rare-earth elements, whose chemical properties differ extremely little from one another and whose atomic weights, because of their scarcity, long remained unreliable. The basic properties of most of the rare-earth elements were established comparatively long ago; however, among them there remained one element (No. 61) about which essentially nothing was known. Numerous researchers in the chemistry of the rare earths made many unsuccessful attempts to isolate this element, but all work carried out on material occurring in nature indicated, at best, only traces of its presence. There was a time when it seemed that element 61 had finally been discovered: its properties were reported and it was even named “illinium,” but it soon became clear that the facts presented were unreliable.
FROM CURRENT LITERATURE
In recent years, in connection with successes in the field of the “artificial” production of elements, it has proved possible not only to extend the limits of the periodic system to 96 elements, but also to fill all the “blank spots” in it; technetium (element No. 43), astatine (element No. 85), and francium (element No. 87) have been discovered; element 61 has also been discovered. It turned out that it has no stable isotopes, and it was obtained artificially by various nuclear reactions, chiefly by bombarding the neighboring element neodymium; in addition, it was found among the fission products of uranium and plutonium. At present the existence of three isotopes of element 61 with masses 147, 148, and 149 has been reliably established; there is also information on isotopes with masses 143 and 151; half-lives, energy spectra, and other radioactive properties have been measured.1
Recently Burkhart, Plym, and Spitscher2 reported that they had succeeded in obtaining photographs of the \(K\)-spectrum of element 61 and in determining the wavelengths of the \(K_{\alpha 1}\), \(K_{\alpha 2}\), \(K_{\beta 1}\), and \(K_{\beta 2}\) lines of this spectrum.
The work was carried out with \(1.5\ mg\) of the chloride salt of this element, obtained artificially at Oak Ridge. The spectrum was obtained with an X-ray spectrograph with a calcite crystal of dimensions \(15 \times 30 \times 2.5\ mm\). The authors faced two tasks: first, it was necessary to show that the element obtained artificially is indeed element 61, and that the lines obtained in the spectrum belong precisely to it, and not to some other element entering as possible impurities; and, second, to measure the wavelengths of the spectrum. At first an X-ray spectrum was obtained for the sample of the presumed element 61. The spectrum was taken at a voltage of \(70\ kv\), in order to reduce the probability of possible influence of second-order reflections from elements of higher atomic number. It turned out that it contains only four pairwise arranged lines of one element and does not reveal the presence of any impurities in the sample.
| Lines of the \(K\)-spectrum of element No. 61 | Wavelengths in X-units — measured | Wavelengths in X-units — calculated | Wavelengths in X-units — nearest lines of other elements |
|---|---|---|---|
| \(\alpha_2\) | 323.68 | 324.27 | La\(\beta_1\) 327.26 |
| \(\alpha_1\) | 319.02 | 319.60 | La\(\beta_2\) 319.06 |
| \(\beta_1\) | 282.00 | 282.99 | Nd\(\beta_2\) 285.73 Tb\(\alpha_2\) 282.94 Tb\(\alpha_1\) 278.19 |
| \(\beta_2\) | 275.03 | 275.33 | Dy\(\alpha_2\) 275.64 Sm\(\beta_1\) 272.50 |
To demonstrate that this spectrum is indeed the \(K\)-spectrum of element 61, after it had been obtained neodymium and samarium were added to the sample under investigation. These substances have atomic numbers 60 and 62 and are the neighbors of element 61 in the periodic system. Then the \(K\)-spectrum of the three elements was obtained. If the four lines of the first spectrum are indeed the \(K_{\alpha 1}\), \(K_{\alpha 2}\), \(K_{\beta 1}\), \(K_{\beta 2}\) lines of element 61, then, according to Moseley’s law, they should respectively be located in the intervals between the \(K_{\alpha}\)- and \(K_{\beta}\)-lines of neodymium and samarium. Indeed, in the spectrum of the three elements presented in the work, it is clearly seen that both \(K_{\alpha}\) lines belonging to the sample are located between the \(K_{\alpha}\)-lines of neodymium and samarium; the lines are also located
\(K_{\beta}\). This confirms that the lines obtained in the spectrum of the pure sample are the \(K_{\alpha 1}\), \(K_{\alpha 2}\), \(K_{\beta 1}\), \(K_{\beta 2}\)-lines of element 61.
Measurement of the wavelengths gave the values listed in the second column of Table B. In the third column are given the wavelengths calculated as follows: screening constants were calculated for elements 58, 59, 60, 62, 63, and 64, and the value of the screening constant for element 61 was determined graphically; then the wavelengths were determined. These values are in good agreement with the results of the measurements. In the last column are given the wavelengths of lines of other elements nearest to the lines of element 61, in order to show that they differ by amounts considerably exceeding the possible errors in determining the wavelengths, and thus none of the lines of the \(K\)-spectrum of element 61 can be a satellite of any line of a possible impurity of other rare-earth elements.
V. Leshkovtsev.
CITED LITERATURE
- G. T. Seaborg and I. Perlman, Rev. Mod. Phys. 20, No. 4 (1948).
- L. E. Burkhart, W. F. Peed, E. J. Spitzer, Phys. Rev. 75, 86 (1949).