Of course, Fermi never presented the Queen of Italy with a test–tube containing a salt of the first transuranium element. It is no more than a typical newspapermen’s copy. But it is true that Fermi had in his hands element 93 though it could not be proved at the time. In his experiments the uranium target consisted of two isotopes, namely, uranium–238 and uranium–235. The latter underwent fission under the effect of slow neutrons giving rise to fragments which were the nuclei of the elements belonging to the central part of the periodic system. They greatly complicated the chemical situation but this was understood only when fission was discovered.
But uranium–238 absorbed neutrons converting into uranium–239, a new isotope of uranium. This beta–active isotope gave rise to an isotope of the first transuranium element with an atomic number of 93. This was just what Fermi and his group thought. But the future neptunium was hard to distinguish among the multitude of fragments. This is why the experiments in mid–thirties yielded no results. The discovery of Hahn and strassmann decisively stimulated actual synthesis of transuranium elements. To start, a reliable technique was needed for detection of the atoms of element 93 in a mass of fission fragments. As the masses of these fragments were comparatively small they had to travel longer distances (had longer paths) than the atoms of element 93 with a large mass.
Thus went the argument of E. McMillan, an American physicist from the University of California. Back in the spring of 1939 he started to analyse the distribution of uranium fission fragments along their paths. He managed to obtain a sample of fragments whose path was very short and in this sample he found traces of a radioactive substance with a half–life of 2.3 days and a high radiation intensity. Other parts of the fission fragments did not exhibit such activity. McMillan demonstrated that this unknown substance was a fission product of a uranium isotope which was also found in the short–path fragments. Thus, the reaction sequence first suggested by Fermi was written as
Now the search was no longer conducted in darkness. Chemical analysis had then to be the final step in verification of the new element. On summer vacations McMillan invited his friend, the chemist P. Abelson, and this visit played a crucial part in the discovery of element 93. Together they established the chemical nature of the new element with a half–life of 2.3 days. The element could be chemically separated from thorium and uranium though in some aspects it was similar to them. But the new element was in no way similar to rhenium. This finally refuted the hypothesis that element 93 had to be eka–rhenium.
At the beginning of 1940 the Physical Review journal reported the real discovery of element 93. It was named neptunium after the planet that is beyond Uranus in the solar system (there is some analogy to the periodic system where neptunium follows uranium).
Synthesis of neptunium exhibited a significant feature which was to prove typical for syntheses of all transuranium elements (and other synthesized elements, too). First, one isotope with a certain mass number was synthesized. For neptunium this was neptunium–239. From that time it became a rule to data a discovery of a new transuranium element by the time of reliable synthesis of its first isotope. But sometimes this isotope proved to be so short–lived that it was difficult to subject it to physical and chemical analyses let alone find a useful application for it. A study of a new element would best be conducted with its longest–lived isotope. In the case of neptunium this was neptunium–237 synthesized in 1942 in the following reaction:
This isotope has a half–life of 2.2 × 106 years. However, its synthesis involves great technical difficulties. Therefore, all the initial studies of the properties of neptunium were performed with its third isotope, neptunium–238, synthesized in the nuclear reaction (d, 2n) Np. Therefore, the history of transuranium elements notes also the date of synthesis of the isotope that is most convenient for analysis but which is by no means always the longest–lived one. Starting from neptunium the American scientists for a long time played a leading part in discoveries of transuranium elements. This can easily be explained by the fact that the USA hardly experienced the hardships of the World War II. It should be noted, however, that in 1942 element 93 was independently synthesized by the German physicist K. Starke.
In 1944 a weighable amount (a few micrograms) of neptunium was synthesized. Now it is produced in tens of kilograms in nuclear reactors. Thirteen neptunium isotopes are currently known. One of them (neptunium–237) was found in 1952 in nature. This is another example when a previously synthesized element was found in nature and for which two discovery dates can be given (as for technetium, promethium, astatine, and francium).