New Products of the Fission of the Thorium Nucleus

L. Meitner

Editor’s Note

In late 1938, German physicists Otto Hahn and Fritz Strassmann in Berlin discovered nuclear fission: the splitting of large atomic nuclei such as uranium into smaller fragments, with an immense release of energy. Production of neutrons during uranium fission meant that the process could be self-sustaining: in reactors and in a bomb. Hahn and Strassmann had been working with the Austrian physicist Lise Meitner, but as a Jew she had been forced to flee from the Nazi regime that summer. Here Meitner, now in Stockholm, extends that work by showing that the element thorium can also undergo fission, and that both uranium and thorium fission lead to similar end products. Hahn was later awarded the Nobel prize for the discovery of fission, but it is now acknowledged that Meitner’s contributions were vital to understanding the process.　　中文

IN a preceding communication1 it has been shown that the “transuranium” elements are found among the fission products of uranium under neutron bombardment. Consequently they must be lower elements, probably partly somewhere near tellurium2, partly (the complementary fission fragments) in the region of ruthenium.　　中文

It was then natural to carry out similar experiments with thorium in order to see whether the fission of thorium gave rise to elements with chemical properties similar to those of the “transuranium” elements, that is, elements which can be precipitated with hydrogen sulphide from a strong hydrochloric acid solution. No search for such elements had previously been carried out with thorium, since the formation of elements beyond uranium, from thorium, was not to be expected. So far, chemical analysis of the radioactive bodies produced in thorium by neutron bombardment has revealed (apart from a thorium and a protactinium isotope resulting from pure neutron capture) only products which, on the basis of their chemical properties, had originally to be assigned to radium and actinium isotopes3 and which have more recently been identified with barium and lanthanum isotopes4.　　中文

A “thick” layer of thorium oxide on a glass plate was irradiated by neutrons obtained by bombarding a lithium target with deuterons of 800 kv. from the high-tension equipment of the Institute of Theoretical Physics, Copenhagen. In order to collect the recoil nuclei, a water surface was used in the way previously described1. After irradiation (of about one hour) a fraction (2 c.c.) of the water was evaporated without chemical separation; with the rest (9 c.c.) the usual hydrogen sulphide precipitation was carried out in exactly the same way as in the uranium recoil experiments1.　　中文

The precipitate showed a clearly measurable activity, the decay of which was distinctly different from the decay of the analogous uranium products. After an initial decrease with a half-value period of about 40 minutes, an activity remained which was followed for almost two days and showed a single decay period of 14–15 hours. The evaporated sample showed first a much faster decay which then gradually became slower; after two days, the activity had vanished.　　中文

A second experiment with 2.5 hours’ irradiation gave the same result, with correspondingly greater intensity, especially of the longer period. In this experiment, after the sulphide precipitation, the filtrate was neutralized and the barium and lanthanum1 were precipitated as carbonates. The carbonate precipitate decayed first with about 20 minutes and then with 4 hours half-value period, in agreement with the periods already known3,4. Analysis of the decay of the sulphide precipitate showed again the presence of a substance of half-period about 40 minutes and of one of 14.5 hours, which was followed for nearly three days and found to decay to zero. The initial intensities of the two periods were about equal, in spite of the short duration of bombardment. The possible existence of very short or very long additional periods can, of course, not be excluded on the basis of this experiment. The two periods observed are quite different from those of the chemically analogous uranium products.　　中文

Several check experiments without neutrons showed that there was no contamination of the water by thorium B or C, which might have resulted from radioactive recoil or traces of emanating thoron. As a further protective measure, the thorium layer was sealed, in the second experiment, by a celluloid membrane of 0.3 mm. stopping power.　　中文

From the evidence given above, one can conclude that some of the fission products of thorium show a chemical behaviour similar to that of the “transuranium” elements. This is a further indication that essentially the same chemical elements are produced in the fission of uranium and thorium.　　中文

In conclusion, I wish to express my thanks to Dr. T. Bjerge, Dr. J. Koch, and K. J. Brostrøm for kind help in the irradiations with the high-tension tube. I am especially grateful to Prof. N. Bohr for the opportunity to carry out these experiments and for the facilities kindly put at my disposal at the Institute of Theoretical Physics, Copenhagen.　　中文

(143, 637; 1939)

Lise Meitner: Physical Institute, Academy of Sciences, Stockholm, March 26.

References:

1. Meitner, L., and Frisch, O. R. Nature, 143, 471 (1939).

2. Abelson, P., Phys. Rev., 55, 418 (1939). Feather, N., and Bretscher. E., Nature, 143, 516 (1939).

3. Meitner, L., Hahn, O., and Strassmann, F., Phys., Z. 109, 538 (1938).

4. Hahn, O., and Strassmann, F., Naturwiss., 27, 89 (1939).