The parent of actinium

Abstract

It has been recognised for more than a decade that actinium cannot be a primary radio-element, but hitherto all attempts to obtain evidence of its continuous production, in the same way as has been established for polonium and for radium, have met with no success. As our knowledge of radioactive change has become precise, the conceivable modes in which actinium could originate have been narrowed down and, one by one, experimentally eliminated without the parent of actinium having been discovered. In the present paper an account is given of experiments which have been successful in separating from uranium minerals preparations initially free from actinium but producing that element in the course of years. In 1913, a minute production of actinium, almost too small to be confident of, was recorded by one of us in the old preparations of uranium-X separated from 50 kgrm. of uranyl nitrate in 1909. The subsequent history of these preparations has confirmed this minute growth, and placed it beyond all doubt, although even now, after eight years, it is still very small in comparison with the growth in the later preparations separated from uranium minerals. Before dealing with the newer experiments, the opportunity will be taken of giving a fairly full account of the long and involved history of the problem, and of correlating and bringing up to date the evidence that can be derived from these old uranium-X preparations. The experiments were undertaken when the course of the disintegration of uranium and its connection with radium was quite obscure, but the experiments afford valuable data, which has never been properly discussed in the light of recent discoveries, on the possible modes of origin of actinium. With regard to the new work, in the absence of one of us on military service since 1915, the experiments were continued for a time by Miss Ada Hitchens, B. Sc., Carnegie Research Scholar, until she also left to engage in war duties. Her valuable assistance has contributed very materially to the definiteness of the conclusions that it has been possible to arrive at.