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On the Fourth Radioactive Family with Atomic Weights \(4n+1\)
As is known, all existing naturally radioactive elements decay into three families: the thorium family, the uranium–radium family, and the uranium–actinium family; moreover, the atomic weights of all elements of the thorium family are expressed as \(4n\), of the uranium–radium family as \(4n+2\), and of the uranium–actinium family as \(4n+3\).
Until now not a single naturally radioactive element belonging to a fourth radioactive family with atomic weights \(4n+1\) had been discovered. Recently, however, a number of works have shown that although the elements of this family are not yet found in nature, nevertheless some of them can be obtained artificially by bombarding thorium with neutrons.
Irradiating thorium with neutrons, Fermi and collaborators[^1] established that in this case two radioactive elements with \(\beta\)-decay and periods of 1 and 24 min arise. According to these authors, the intensity of the artificial radioactivity of thorium does not depend on the presence of hydrogen-containing substances. In checking these experiments on carefully purified thorium, Meitner and Hahn[^2] found that when thorium is irradiated with neutrons two different processes take place, one of which does not depend on the presence of a hydrogen-containing substance, while the other depends on it to a high degree. As a result of the first process a radioactive element with a decay period of 1 min is formed, which after \(\beta\)-decay is transformed into another radioactive element with a period of 11–12 min. As a result of the second process a radioactive element with a decay period of about 30 min arises. Later work by Curie, Halban, and Preiswerk[^3] established that in the case of neutron irradiation of thorium five different radioactive elements arise, with periods: 1, 2.5, 15 min (according to Meitner and Hahn 11–12 min), 25 min (Meitner and Hahn—30 min), 3.5 hours. As in the work of Meitner and Hahn, so also in the work of Curie, Halban, and Preiswerk, it was shown that the element with the 25-minute decay period is an isotope of thorium, formed, obviously, by the simple capture of a neutron by the thorium nucleus, which is in good agreement with the fact established by Fermi that in the presence of a hydrogen-containing substance the intensity of those reactions increases which occur when a neutron is captured without emission of heavy particles.
Chemical precipitation from a solution of the 2.5-minute element showed that it is, in all probability, an isotope of protactinium. To establish the genetic connection of this element with the others, Curie, Halban, and Preiswerk performed the following experiment. From a solution of irradiated thorium (irradiation time—1 hour) they precipitated thorium peroxide in the presence of Ba and La. The precipitate obtained was dissolved in 20% hydrochloric acid. Then Zr was added to the solution and precipitation with phosphoric acid was carried out. The precipitate possessed an activity decreasing with a period of 2.5 min. Separation of the precipitate was carried out 1.5 min after the end of irradiation of thorium by neutrons; therefore the 2.5-minute element could not have formed directly from thorium. It must be a prod-
decay of a radioactive element with a noticeably longer period. The authors believe that it arises in the β-decay of a 25-minute element. The results obtained may be represented by the following scheme:
\[ {}^{232}_{90}\mathrm{Th}+{}^{1}_{0}\mathrm{n} \longrightarrow {}^{233}_{90}\mathrm{A} \xrightarrow[\beta]{25\ \mathrm{min}} {}^{233}_{91}\mathrm{A} \xrightarrow[\beta]{2.5\ \mathrm{min}} {}^{233}_{92}\mathrm{A} \xrightarrow{\alpha} \]
By precipitating barium sulfate from a neutron-irradiated solution of thorium, Curie, Halban, and Preiswerk succeeded in showing that the element with a decay period of 1 min. was precipitated together with this precipitate. It follows from this that this element is an isotope of radium, as was also assumed by Meitner and Hahn. Other experiments by the same authors established that radioactive elements with decay periods of 15 min. and 3.5 hours have chemical properties different from those of radium, thorium, protactinium, and uranium. They behave similarly to lanthanum; therefore there are grounds to regard both these elements as isotopes of actinium.
The transformation scheme for these elements, proposed by Meitner and Hahn, has the following form:
\[ {}^{232}_{90}\mathrm{Th}+{}^{1}_{0}\mathrm{n} \longrightarrow {}^{229}_{88}\mathrm{A}+{}^{4}_{2}\mathrm{He} \]
\[ {}^{229}_{88}\mathrm{A} \xrightarrow[\beta]{1\ \mathrm{min}} {}^{229}_{89}\mathrm{A} \xrightarrow[\beta]{15\ \mathrm{min}} {}^{229}_{90}\mathrm{A} \]
In accordance with the emission of a heavy particle upon neutron capture, the probability of the initial reaction of this series does not depend on the presence of a substance containing hydrogen.
Considering both series of transformations, one may note that they consist of elements with atomic weights \(4n+1\), characteristic of the fourth radioactive family, which has hitherto been absent. How this family is constructed, and whether some of its members exist in nature, is still difficult to say. However, one must hope that further investigations in this field will be able to clarify many questions concerning this family, and in particular to establish the connection between the first and second series of transformations.
L. Groshev
LITERATURE
- Amaldi, D’Agostino, Fermi, Pontecorvo, Rasetti, Segrò, Proc. Roy. Soc. 149, 522, 1935.
- Meitner u. O. Hahn, Naturwiss. 23, 320, 1935.
- I. Curie, H. Halban, P. Preiswerk, C. R. 200, 1841, 1935; 200, 2079, 1935.