Transformations of Actinium.—Actinium of period about 20 years is believed to emit weak (3 -rays changing into radio actinium, an a-ray product of period 19 days, first separated by Hahn. This changes into actinium X, an a-ray product of period I I days, discovered by Godlewski. Then follows the actinium emanation of period 3.9 sec., which gives rise to four further products named actinium A, B, C, C". Actinium A has the short est life of any product whose rate of transformation has been directly determined. Its period, as determined by Geiger and Moseley and Fa jans, is .002 second. After emitting an a-particle, A changes into B, a product of period 36 minutes emitting weak (3- and y-rays, analogous to thorium B. Actinium C of period 2.16 minutes undergoes a complex transformation, giving rise to two distinct groups of a-particles. The main branch gives rise to actinium C" of period 4.8 minutes, which is readily isolated by the recoil method. Actinium C", which emits (3- and y-rays, is analogous in all respects to thorium C".
In the above discussion on branch products it has been shown that the parent of actinium, called protoactinium, has been re cently isolated by Hahn and Soddy. This substance emits a-rays and has an estimated period of 20,000 years. We have seen that the actinium series is believed to have its origin in a dual trans formation of uranium The first branch product, representing about 3% of the total, is believed to be uranium Y, a /3-ray product of period one day. This is directly transformed into protoactinium.
Assuming a branching ratio of 3%, 1,000 kilograms of uranium contains 130 milligrams of protoactinium. Hahn and Walling have developed methods of purification with a view of determin ing its atomic weight. This constant should prove of great im portance in fixing the atomic weights of the actinium series of products and in throwing light on the origin of the actinium series.
End-products of the Transformations.—Af ter the radio active transformations have come to an end, each of the elements uranium, thorium and actinium should give rise to an end or final product, which may be a known element or an unknown element of very slow period of transformation. Since the expulsion of an a-particle lowers the mass of the atom by four units, and there are eight a-ray products, the atomic weight of the end atom should be 238 —8 X 4 = 206. The atomic weight of radium by this rule should be 238-3 X4= 2 26, a result in good accord with ex periment. The atomic weight of the end-product of uranium is close to that of lead, viz., 207, and Boltwood early suggested that lead was the end-product of radium. Since in old minerals the transformations have been in progress for intervals measured by millions of years, the end-product should collect and be an in variable companion of the radio-element. Boltwood showed that
lead is always present in old radioactive minerals, and in amount to be expected from their uranium content and geologic age.
This problem has been definitely attacked in the light of the chemical generalisation already given. It was clear from this that the end-products of uranium, thorium and actinium should all be isotopes of lead but with atomic weights 206, 208 and 206 respec tively. In other words, uranium-lead if uncontaminated with ordinary lead should show a smaller atomic weight than ordinary lead (207), while thorium-lead should give a higher value. By the work of Richards, Soddy and Honigschmid, these conclusions have been definitely confirmed. The lowest value for uranium lead is 206, and the highest for thorium-lead 207.7.
Since any admixture with ordinary lead tends to give a value nearer 207, these results may be considered as a definite proof of the nature and atomic weight of the end-products. In minerals containing both uranium and thorium the atomic weight of the mixture of the isotopes will depend on the relative amounts of these two elements and their relative rates of transformation. In unaltered minerals the determination of the amount of lead coupled with its average atomic weight allows us to determine the amount of uranium-lead even if some ordinary lead be present. In this way it should be possible to make a reliable estimate of the age of selected minerals and thus indirectly the age of the geologic strata (see GEOLOGY). The amount of helium in the mineral gives a minimum estimate of its age, for, except in the most compact minerals, some of the helium must undoubtedly escape.
Production of Helium.—In 1902 Rutherford and Soddy sug gested that the helium which is invariably found in radioactive minerals was derived from the disintegration of radioactive matter. In 1903 Ramsay and Soddy definitely showed that helium was produced by radium and also by its emanation. From the ob served mass of the a-particle, it seemed probable from the first that the a-particle was an atom of helium. This conclusion was confirmed by the work of Rutherford and Geiger, who showed that the a-particle was an atom of helium carrying two unit charges of electricity. In order to prove definitely this relation, it was necessary to show that the a-particles, quite independently of the active matter from which they were expelled, gave rise to helium. This was done by Rutherford and Royds, who allowed the a-particles from a large quantity of emanation to be fired through the very thin glass walls of the containing tube. The collected particle gave the spectrum of helium, showing, without doubt, that the a-particle must be a helium atom.