Barium from uranium bombardment

In Germany in 1938, Otto Hahn and Fritz Strassmann, skeptical of claims by Enrico Fermi and Irene Johot-Curie that bombardment of uranium by neutrons produced new so-called transuranic elements (elements beyond uranium), repeated these experiments and chemically isolated a radioactive isotope of barium. Unable to interpret these findings, Hahn asked Lise Meitner, a physicist and former colleague, to propose an explanation for his observations. Meitner and her nephew, Otto Frisch, showed that it was possible for the uranium nucleus to be spfit into two smaller nuclei by the neutrons, a process that they termed fission. The discovery of nuclear fission eventually led to the development of nuclear weapons and, after World War II, the advent of nuclear power to generate electricity. Nuclear chemists were involved in the chemical purification of plutonium obtained from uranium targets that had been irradiated in reactors. They also developed chemical separation techniques to isolate radioactive isotopes for industrial and medical uses from the fission products wastes associated with plutonium production for weapons. Today, many of these same chemical separation techniques are being used by nuclear chemists to clean up radioactive wastes resulting from the fifty-year production of nuclear weapons and to treat wastes derived from the production of nuclear power. 

In 1938 Niels Bohr had brought the astounding news from Europe that the radiochemists Otto Hahn and Fritz Strassmann in Berlin had conclusively demonstrated that one of the products of the bom-bardmeiit of uranium by neutrons was barium, with atomic number 56, in the middle of the periodic table of elements. He also announced that in Stockholm Lise Meitner and her nephew Otto Frisch had proposed a theory to explain what they called nuclear fission, the splitting of a uranium nucleus under neutron bombardment into two pieces, each with a mass roughly equal to half the mass of the uranium nucleus. The products of Fermi s neutron bombardment of uranium back in Rome had therefore not been transuranic elements, but radioactive isotopes of known elements from the middle of the periodic table. 

The evidence for this supposed process came from chemical analysis. Hahn and Strassmann published a scientific paper showing that small amounts of barium (element 56) were produced when uranium (element 92) was bombarded with neutrons. It was very puzzling to them how a single neutron could transform element 92 into element 56. 

When Fermi s group analyzed the products of the neutron bombardment, it appeared to them that radium had been produced, especially since they had no reason to even suspect that barium could be a product. Since radium is the daughter element formed by two successive alpha decays of a uranium atom, they decided their quest for a transuranium element was unsuccessful. Subsequently, Otto Hahn (1879-1968), Fritz Strassmann (1902-80), and Lise Meitner (1878-1968), all from Germany, reinterpreted the results to show that it was not radium atoms that had been formed, but barium atoms instead from the nuclear fission of uranium. Thus, Fermi and his group just missed discovering fission. 

Among those joining the investigation were Hahn and Meitner, the discoverers of protactinium twenty years before (page 215). They treated the bombarded uranium with barium, which carried down in precipitation a certain fraction of strongly radioactive material. This reaction made them suspect that one of the products of the bombardment was radium. Radium is very similar, chemically, to barium and would be expected to accompany barium in any chemical manipulations. However, no radium could be obtained from those barium-containing fractions. 

By 1938, Hahn began to wonder if it were not a radioactive isotope of barium itself that had been formed from the uranium in the course of neutron bombardment. Such radioactive barium would merge with ordinary barium and the two could not then be separated by ordinary chemical techniques. Such a combination seemed impossible, however. All nuclear reactions known up to 1938 had involved changes in elements of only 1 or 2 units in atomic number. To change uranium to barium meant a decrease, in atomic number, of 36 It was as though the uranium atom had broken more or less in half uranium fission). Hahn hesitated even to speculate on such a possibility—at least, not in public. 

Lise Meitner at 59 in 1937. At Christmastime 1938 in Stockholm she heard from Otto Hahn of his stunning discovery with Fritz Strass-mann that slow neutrons bombarding uranium made barium—the first evidence that the uranium atom split. 

O. Hahn and F. Strassmann by co-precipitating with barium a solution of the product of the bombardment of uranium with neutrons, obtained what they thought were isotopes of radium, and after j8-ray decay, products of these were precipitated with lanthanum, and hence regarded as actinium isotopes. Mme. Joliot-Curie and P. Savitch found that the product concentrated with lanthanum rather than with actinium, but thought it should be separable from lanthanum. Hahn and Strassman, early in 1939, found that their supposed radium was barium, and that the chemical evidence showed that their actinium and thorium were lanthanum and cerium. Such results, they said, would contradict accepted views in nuclear physics. 

During the mid-1930s, scientists bombarded uranium (Z = 92) with neutrons in an attempt to synthesize transuranium elements. Many of the unstable nucUdes produced were tentatively identified as having Z > 92, and eventually, one was shown to be an isotope of barium (Z = 56). The Austrian physicist Lise Meitner and her nephew, Otto Frisch, proposed that barium resulted from the splitting of the uranium nucleus into smaller nuclei, a process that they called fission as an analogy to cell division in biology. Element 109 was named meitnerium in honor of this extraordinary physicist. 

Among the products of the neutron bombardment of uranium, a radioactive element was found that behaved chemically like the elements of Group 2R of the periodic system (strontium, barium, and radium). Among these, only radium had been known to be radioactive, and therefore the new radioactive element was assumed to be radium. However, because its radioactivity differed from that of natural radium, it was thought to be some other isotope of radium, In 1939, Otto Hahn and Fritz Strassman attempted to isolate this "radium by the same chemical methods used in the original discovery of radium by the... 

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