NATURAL RADIOACTIVITY OF LANTHANUM

It is known that if there are two isobars with neighboring atomic numbers \(Z^A\) and \((Z-1)^A\), then the isobar with the larger atomic mass must be \(\beta\)-radioactive. Therefore such isobars usually do not occur in nature, and the existence of the few examples of isobars with neighboring atomic numbers is explained by the fact that the unstable isobar has an exceptionally long half-life as a consequence of some selection rules.\(^1\) Confirmation of this is provided by the half-lives of \(\beta\)-decay for \(K^{40}\) (\(1.4 \cdot 10^9\) years), \(Re^{187}\) (\(4 \cdot 10^{12}\) years), \(Lu^{176}\) (\(2.4 \cdot 10^{10}\) years), and \(Rb\) (\(6.3 \cdot 10^{10}\) years). There exist, however, pairs \((Sb^{123}, Te^{123})\), \((In^{115}, Sn^{115})\), \((Cd^{113}, In^{113})\), and also triplets (i.e., double pairs) \((Ba^{138}, La^{138}, Ce^{138})\) and \((Ti^{50}, V^{50}, Cr^{50})\), for which activity has not been established, probably because of an exceptionally long period or an extremely small decay energy, or, finally, because of the difficulty of detecting \(K\)-capture if the specific activity is small.

Recently, in connection with the development of scintillation \(\beta\)-spectrometers of exceptionally high sensitivity, a method has appeared for detecting weak activity and analyzing it in order to distinguish the activity of the principal substance from the activity of possible contaminants (Th, U, K). Using this method, the authors of paper \(^{2}\) subjected 39 g of lanthanum trioxide to study and found a component of \(\gamma\)-radiation with an energy of \((1.05 \pm 0.05)\) MeV. The number of \(\gamma\)-quanta of this energy proved to be equal to \((0.7 \pm 0.1)\) quanta/sec per gram of ordinary lanthanum. Taking into account the percentage content of \(\mathrm{La}^{138}\) (0.086%) in the mixture of lanthanum isotopes, the authors obtained for the period of \(\mathrm{La}^{138}\) the value \(1.2 \cdot 10^{11}\) years. The \(\gamma\)-radiation with the energy found, 1.05 MeV, is attributed by the authors to the process of \(K\)-capture with the formation of \(\mathrm{Ba}^{138}\) according to the scheme:

\[ \begin{array}{ccc} \mathrm{Ba}^{138} & \mathrm{La}^{138} & \mathrm{Ce}^{138} \\ & \swarrow\; K\text{-capture} & \searrow\; \beta^{-}? \\ \gamma\; 1.05\ \mathrm{MeV} & ?\,\beta^{+},\ K\text{-capture} & \end{array} \]

E. Sh.

References Cited

1. E. V. Shpol’skii, UFN 40, 142 (1950).
2. R. W. Pringle, S. Standil and Roulston, Phys. Rev. 78, 303 (1950).