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Origin of Actinium.
1) Otto Hahn and Lise Meitner. Die Muttersubstanz des Actiniums, ein neues radioaktives Element von langer Lebensdauer. Phys. ZS. 19, 208, 1918.
2) Otto Hahn and Lise Meitner. Über das Protactinium und die Lebensdauer des Actiniums. Phys. ZS. 20, 127, 1919.
3) Otto Hahn and Lise Meitner. Der Ursprung des Actiniums. Phys. ZS. 20, 529, 1919.
4) Otto Hahn. Protactinium, seine Lebensdauer und sein Gehalt in Uranmineralien. [Lectures of the 86th Congress of Natural Scientists in Nauheim, September 19–25, 1920. Phys. ZS. 21, No. 21/22, 1920].
As the investigations of Boltwood1 and Fussler2 have shown, the ratio of the content of actinium to uranium in minerals is approximately constant. From this one may draw the conclusion that the actinium series is in genetic connection with the uranium series. However, it is impossible to place $Ac$ in the main uranium series, since the ratio of the activities (by $\alpha$-rays) of $Ac$ and $U$ in minerals, as Boltwood1 showed, is considerably smaller than it would have had to be in the case of a direct genetic connection. Therefore the supposition was put forward that actinium is a side product in the uranium series, i.e. that one of the members of this series disintegrates in such a way that the greater part of its atoms is transformed into the subsequent elements of the main series, while the remainder gives rise to the beginning of the actinium series.
The most natural supposition is that $Ac$ originates from $UY$, which is undoubtedly a side product in the uranium series, and whose connection with uranium was proved by direct observations. However, $UY$ cannot be the immediate ancestor of $Ac$, since between uranium and actinium there must be an intermediate element of long life, for the formation of actinium in uranium salts could not be proved directly.
The first of the papers reviewed is devoted to the search for this intermediate link between uranium and actinium.
Actinium is placed in the 3rd group of the Mendeleev system and, according to the displacement law3, may be a decay product of an element of the 2nd group that emits $\beta$-rays, or of an element of the 5th group that emits $\alpha$-rays4.
The probable connection of actinium with \(UY\) makes it necessary to search for the ancestor of actinium in the 5th group (since \(UY\) is in the 4th group and emits \(\beta\)-rays, the product of its disintegration must be an element of the 5th group).
Since, in its chemical properties, the ancestor of actinium should be similar to tantalum, Hahn and Meitner attempted to isolate the new element from the residues left after dissolving uranium ore in nitric acid, consisting chiefly of silicic acid. To such a residue they added a small quantity of the potassium salt of tantalo-hydrofluoric acid, and from the mixture, by ordinary chemical operations, \(Ta\) and elements akin to it were separated. The precipitate obtained as a result, consisting chiefly of a mixture of tantalic and niobic acids, did indeed contain a new radioelement emitting \(\alpha\)-particles, to which the name “protactinium” was given (chemical symbol \(Pa\)).
By processing a large quantity of residues from the factory treatment of uranium ore, 73 mgr of a substance rich in \(Pa\) was isolated.
Using a strong preparation, the authors of the works reported measured, by the method of Kleeman and Bragg, the range of the \(\alpha\)-rays of \(Pa\). This quantity proved to be 3.34 cm (at \(0^\circ\) and 760 mm pressure). The accuracy of the determination is estimated by the authors at 1–2%.
Knowledge of the range of the \(\alpha\)-particles makes it possible to calculate approximately the lifetime of protactinium.
Between the decay constant \(\lambda\) and the range of the \(\alpha\)-rays \(R\) of radioelements there exists, as Geiger and Nuttall\({}^{1}\) have shown, the following relation:
\[ \log \lambda = A + B \log R, \]
where \(A\) and \(B\) are constants for each radioactive series. The values of the constants \(A\) and \(B\) for the actinium series are not precisely known to us. If the extreme values obtained by Meyer and Paneth\({}^{2}\) are adopted, then the half-life of protactinium will be equal to 1,200 years or 180,000 years.
The formation of actinium from protactinium was demonstrated directly in two ways.
1) An increase with time was observed in the amount of actinium emanation carried by a current of air from a preparation of protactinium. The amount of emanation increased according to the law corresponding to the course of decay in the actinium series.
2) The change with time in the quantity of active deposit precipitated on a negatively charged metal plate, placed for a known time near a preparation of protactinium, was investigated. The deposit obtained possessed all the properties of the active deposit of actinium, and the amount of it deposited on the plate increased according to the same law as did the amount of emanation.
The newly discovered element \(Pa\) is an isotope of \(UX_2\), the former sole representative of the corresponding “pleiad,” and for this reason has received the special name brevium. Since it is customary to regard the longest-lived isotope as typical for a given pleiad, protactinium must occupy the place \(UX_2\) (atomic number 91).
The second and third of the works reported are devoted to determining the lifetime of actinium and the relation of actinium and uranium in minerals.
The determination of the lifetime of actinium was carried out by two methods. Owing to the accumulation of actinium and of the products of its decay, which emit \(\alpha\)-rays, the initial activity of the \(Pa\) preparation increases with time. From the growth curve one can find the half-life of actinium.
\({}^{1}\) Phil. Mag. 22, 613, 1911; 23, 439, 1912.
\({}^{2}\) Wien. Ber. 123, 1453, 1914.
The decrease with time in the activity of the actinium preparation likewise makes it possible to determine the half-life. Both methods gave concordant results, and the half-life of actinium was determined to be 20 years (with an accuracy of up to 10%). Soddy and Cranston1, independently having isolated Pa, called by them eka-tantalum (UZ), found 3460 years for the half-life of actinium. Hahn and Meitner explain the discrepancy obtained by the imperfection of the separation method used by Soddy and Cranston.
In their latest work Hahn and Meitner determine the ratio Pa/U in Austrian uraninite ore. Since protactinium transforms directly into actinium, the ratio found at the same time expresses the ratio Ac/U. Determination with the aid of Pa gives the following advantages:
1) The separation of Pa from the mineral can be carried out more completely than the separation of Ac.
2) The separated Pa can immediately be compared, by α-rays, with uranium, whereas in the case of actinium it is necessary to wait several months for the attainment of radioactive equilibrium, since actinium itself gives no α-rays.
Protactinium was separated from the ore by three different methods2 and was compared with uranium by α-rays, all necessary corrections being taken into account. The numbers obtained for the different preparations are in complete agreement with one another (deviations less than 10%).
As a result Hahn and Meitner found that in the uranium series not 8%, as had hitherto been assumed, but only 3% branches off into actinium. The value measured by Hahn and Meitner is undoubtedly closer to the truth, in view of the advantages of the method mentioned. The value obtained is in good agreement with the value 2.1% accepted for UY, characterizing the ratio in which branching occurs in the uranium series.
The material obtained by the authors of the papers reviewed makes it possible to represent the transformations in the uranium series in the following manner.
A priori two schemes are equally probable:
\[ \begin{array}{l} \text{(I)}\quad UI \xrightarrow{\alpha} UX_1 \xrightarrow{\beta} UX_2 \xrightarrow{\beta} UII \xrightarrow{\alpha} Io \xrightarrow{\alpha} Ra \longrightarrow \\ \qquad\ \ VI \qquad\quad IV \qquad\quad V \qquad\quad VI \qquad\quad IV \qquad\quad II \\[4pt] \qquad\quad \searrow^{\alpha}\quad UY \xrightarrow{\beta} Pa \xrightarrow{\alpha} Ac \longrightarrow \\ \qquad\qquad\qquad\quad IV \qquad\quad V \qquad\quad III \end{array} \]
\[ \begin{array}{l} \text{(II)}\quad UI \xrightarrow{\alpha} UX_1 \xrightarrow{\beta} UX_2 \xrightarrow{\beta} UII \xrightarrow{\alpha} Io \xrightarrow{\alpha} Ra \longrightarrow \\ \qquad\ \ VI \qquad\quad IV \qquad\quad V \qquad\quad VI \qquad\quad IV \qquad\quad II \\[4pt] \qquad\qquad\qquad\qquad\qquad\searrow^{\alpha}\quad UY \xrightarrow{\beta} Pa \xrightarrow{\alpha} Ac \longrightarrow \\ \qquad\qquad\qquad\qquad\qquad\qquad\quad IV \qquad\quad V \qquad\quad III \end{array} \]
The following considerations speak in favor of scheme (II).
If one graphically represents the relation between range and life-duration for the radium and actinium series, then, as Meyer and Paneth have shown (loc. cit.), the straight line characterizing the actinium series intersects the straight line of the uranium series near UII.
Fajans’s rule, which connects the atomic weight of isotopes with the life-duration3, shows that the members of the actinium series are best arranged—
fit into the periodic system if the atomic weight of \(Ac\) is taken to be 227. In that case branching must occur at \(UI\), since the emission of an \(\alpha\)-particle lowers the atomic weight of the element by 4 units, and the atomic weight \(UI = 238.2\).
At the congress of natural scientists in Nauheim, September 19–25, 1920, O. Hahn reported work that experimentally proved the formation of \(Ac\) from \(U\). The amount of \(Pa\) in old uranium salts was determined. Uranium salts prepared by ordinary methods are initially free of \(Pa\). Since the ratio of the activities of \(U\) and \(Pa\) at equilibrium is known, the decay period of \(Pa\) can be determined indirectly from the magnitude of the activity of \(Pa\) separated from a uranium salt of known age.
Three uranium salts aged from 20 to 60 years were investigated. The results were in full agreement with one another, and the mean lifetime of \(Pa\) was found on average to be 12,000 years. From this it is easy to calculate the ratio of the amounts of \(Pa\) and \(U\) at equilibrium:
\[ \frac{0.03 \cdot 12 \cdot 10^9}{5 \cdot 10^9} = 7.2 \cdot 10^{-8}. \]
One ton of \(U\) contains 72 \(mgr\) of \(Pa\) (the corresponding amount of \(Ra = 330\ mgr\)). During the technical processing of uranium ore, a large amount of \(Pa\) remains in the waste. The author of the paper being reviewed is conducting investigations on the extraction of \(Pa\) from such waste, and he expresses the hope that in time \(Pa\) will be obtained as a new chemical element.
A direct determination of the atomic weight of \(Pa\) will make it possible finally to establish the connection of actinium with the uranium series.
V. Baranov.
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Fajans’s rule: within a pleiad the life-duration of isotopes emitting α-particles decreases with decreasing atomic weight; while the life-duration of isotopes emitting β-particles increases with decreasing atomic weight. ↩↩
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Not one of the known radioelements of the uranium series can be directly transformed into actinium. Of the elements of the 2nd group, the uranium series contains only $Ra$, which gives both $\alpha$- and $\beta$-rays. But the careful experiments of Fajans, Soddy, and other investigators have proved that actinium is not formed from $Ra$ or from any of the subsequent members of the series. The only known element of the 5th group (at the beginning of the uranium series), brevium $(UX_2)$, likewise cannot be the immediate ancestor of actinium, since it has a short lifetime and does not emit $\alpha$-rays. ↩