From Current Literature

Radioactivity of Rubidium

It has long been known that potassium and rubidium are radioactive elements emitting β-rays. For potassium, a number of experiments, carried out mainly in recent times, established that its radioactivity must be ascribed to the isotope \(^{40}\mathrm{K}\), the percentage content of which in ordinary potassium is very small. For rubidium, until recently there were no experiments that would make it possible to establish to which isotope of the element its radioactivity should be attributed. Ordinary rubidium consists of two isotopes, \(^{85}\mathrm{Rb}\) (75%) and \(^{87}\mathrm{Rb}\) (25%). Some authors considered \(^{87}\mathrm{Rb}\) to be the radioactive isotope, others—an unknown isotope \(^{86}\mathrm{Rb}\). For an unambiguous solution of the question as to which of these isotopes is radioactive, it would have been desirable to have a mineral with a high rubidium content. Then it would be possible, by mass-spectroscopic analysis, to determine which isotope of strontium arises in the β-decay of rubidium, thereby determining the radioactive isotope of rubidium. In such a mineral, strontium should be present in negligible amounts, so that it could be regarded essentially as the product of the radioactive decay of rubidium. This condition is necessary in order that, as a result of the decay, the enrichment of the mass spectrum of strontium in one or another isotope should prove appreciable. However, minerals rich in rubidium do not exist in nature. Therefore, to solve the problem posed, it was necessary to make use of minerals containing only a few percent of rubidium. Recently Hahn* obtained from Northern Canada samples of mica containing 2–3% rubidium and very little (hundredths of a percent) alkaline-earth metals. Thus these samples satisfy the conditions noted above.

The Canadian mica was subjected to chemical treatment by Strassmann and Willing in Mattauch’s laboratory. They separated from it strontium, which was then examined mass-spectroscopically by Mattauch himself. He found that the strontium obtained in this way consists mainly of \(^{87}\mathrm{Sr}\), and only with very long exposures do traces of \(^{88}\mathrm{Sr}\) appear in the mass spectrograph. His estimate showed that \(^{88}\mathrm{Sr}\) is present here in amounts less than 0.3% of \(^{87}\mathrm{Sr}\). From this it follows that the strontium obtained from the Canadian mica consists practically of a single isotope, \(^{87}\mathrm{Sr}\), whereas in ordinary strontium we have three principal isotopes—\(^{86}\mathrm{Sr}\) (\(\sim 10\%\)), \(^{87}\mathrm{Sr}\) (\(\sim 7\%\)), and \(^{88}\mathrm{Sr}\) (\(\sim 83\%\)). Therefore it may be considered established that the strontium in Canadian mica arises through the radioactive decay of the rubidium contained in it; in this case the radioactive isotope is \(^{87}\mathrm{Rb}\), which after β-decay gives \(^{87}\mathrm{Sr}\). Here we have a case analogous to lead. As is known, ordinary lead consists of many isotopes. However, lead isolated from some thorium and uranium minerals consists practically of a single isotope, which is the end product of the decay of the thorium or uranium radioactive series. For thorium minerals this isotope is \(^{208}\mathrm{Pb}\), and for uranium minerals—\(^{206}\mathrm{Pb}\). Thus, in the case

* O. Hahn, F. Strassmann, F. Willing, Naturwiss., 25, 189, 1937; I. Mattauch, Naturwiss., 25, 189, 1937.

of lead and strontium, we have rare examples of how, under natural conditions, pure isotopes of complex elements are sometimes formed.

It may not be without interest to note that the isobars \(^{87}\mathrm{Rb}\)—\(^{87}\mathrm{Sr}\) constituted one of the exceptions to Mattauch’s empirically established rule. This rule says that stable isobars with odd mass numbers cannot exist. After the establishment of the radioactivity of \(^{87}\mathrm{Rb}\), this exception disappears.

L. Groshev, Moscow