Proof of the Formation of Active Isotopes of Barium from Uranium and Thorium Irradiated with Neutrons; Proof of the Existence of More Active Fragments Produced by Uranium Fission

[Nachweis der Entstehung activer Bariumisotope aus Uran und Thorium durch Neutronenbestrahlung; Nachweis weiterer aktiver Bruchtucke bei der Uranspaltung.]

Otto Hahn and Fritz Strassmann (Berlin-Dahlem)
Die Naturwissenschaften, Volume 27, No. 6, pp. 89-95 (10 February 1939)
[Translation from Journal of Chemical Education, May 1989, p. 363-363]

A. Final Proof of the Origin of Barium from Uranium

In a report that appeared recently in the journal, we stated that the new radioactive substances produced by irradiating uranium with neutrons, which originally had been thought to be isotopes of radium, have the chemical characteristics of barium and are evidently not radium but isotopes of barium. In the following we wish to prove this assertion and to extend it also to thorium. In the scheme of radioactive decay in our last report we listed four "radium" isotopes, of which three had been proved to exist and half-lives had been established for them. The fourth "radium" isotope, labeled Ra-I, was assumed to have a short half-life, and its existence had been deducted from another substance which was taken to be its decay product.

As regards the three alkaline earth metals the existence of which had been proved, they could be only radium or barium; strontium and calcium had been eliminated. Since fractionating crystallization with barium salts did not produce the results to be expected from radium, we made a number of indicator experiments with radium isotopes known to be such, so that we could ascertain the chemical behavior of the alkaline earth metals produced by uranium.

The indicator experiments are described and numerically evaluated in the following. They were carried out, on the one hand, with "radium-III" with a half-life of 86 min and "radium-IV" with a half-life of 300 h, and, on the other, with the long-known radium isotopes mesothorium-1 (a beta-ray emitter) and thorium-X (an alpha-ray emitter). From now on we shall use Ba-III and Ba-IV Instead of Ra-III and Ra-IV.

The following combinations were investigated.

(1) Ba-III + Mesothorium-1, fractionated with barium bromide.
(2) Ba-III + Thorium-X, fractionated with barium chromate.
(3) Ba-IV + Thorium-X, fractionated with barium chromate.
(4) A "circular process" with a series of crystallizing barium salts was carried out.

In experiment (1) a quantity of about 15 g of purified uranium was irradiated for 12 h with radium-beryllium neutrons. After stopping the irradiation we waited for 2 1/2 h so that the Ba-II with its short half-life of 14 min could decay. Thus we obtained mainly the 86-min isotope Ba-III with a small quantity of the 300-h Ba-IV. Then 2 g of barium in the form of its chloride were precipitated from the solution containing the irradiated uranium and afterwards the barium in the form of its carbonate. The two carbonates were dissolved together in bromic acid and the bromides then crystallized in the manner that is used for obtaining radium. During this fractionating crystallization the decay products of the active barium isotopes were steadily removed, as was the barium and radium salts. During the decay of the Ba-III (86-min half-life) the figures for the activity of this substance have to be calculated for a common zero time (the beginning of the first crystallization); the formation of mesothorium-2 from the constantly active mesothorium-1 has to be calculated for each crystallization. Figure I -1 shows the results for 500 mg (each) of anhydrous barium bromide. Curves I, II, and III in the left-hand section of the diagram show the intensities as measured for a period of more than 70 h, up to the radiation balance between mesothorium-1 and mesothorium-2. The ratio of the mesothorium content of the fractions I, II, and III is 67.6, 25, and 11; six times as much in the first fraction than in the third.

The center section of the diagram shows the beginnings of the activity curves for the first 500 min on a larger scale. The three broken lines show the increase of activity due to mesothorium-2. The right-hand section of the diagram shows the decrease of Ba-III by the differences between curves I, II, and III and the mesothorium curves.

The inclination of the lines in the right-hand section corresponds to the 86-day half-life of Ba-III. The line representing the second fraction is a little lower than the lines for the first and the third fractions, but the difference is certainly within the limits of experimental error. The Ba-III lines show a very faint flattening because the 12-h irradiation of the uranium also produced some Ba-IV which has a 300-h half-life. If the curves are calculated for zero time, we find the values of 81, 72, and 81 for the activities of the three fractions; a difference of about 10%.

Comparing the results with the 6:1 "enrichment" of the radium isotope mesothorium-2, we see that there has been no "enrichment" for the Ba-III: proof that it differs from radium and a strong hint of its identity with barium.

[The following excerpt is from the conclusion section.]

As chemists we really ought to revise the decay scheme given above and insert the symbols Ba, La, Ce, in place of Ra, Ac, Th. However, as "nuclear chemists," working very close to the field of physics, we cannot bring ourselves yet to take such a drastic step which goes against all previous experience in nuclear physics. There could perhaps be a series of unusual coincidences that has given us false indications.

[N.B. The following excerpt's translation is from "The Discovery of Nuclear Fission. (1971) Van Nostrand Reinhold. p. 48.]

. . . Summary:

1. The creation of barium isotopes from uranium was conclusively demonstrated.

2. For thorium, the formation of barium isotopes was also established.

3. Some suggestions are made regarding the atomic weights of the barium isotopes.

4. Evidently, some of the barium isotopes produced from thorium and uranium are identical.

5. It is our belief that the "transuranic elements" still retain their placement without change, as previously described.

6. A second group of fission fragments, Strontium [element 38] and Yttrium [element 39], was determined.

7. By an appropriate experimental arrangement, the formation of a noble gas was established, which in turn decays into an alkali metal. It has not been possible yet to show if the substances in question are xenon-cesium or krypton-rubidium.

In a rather short time it has been possible to identify numerous new reaction products described above -- with considerable certainty, we believe -- only because of the previous experience we had gathered, in association with L. Meitner, from the systematic study of uranium and thorium reaction products.