The isotope neptunium–239 was beta–active and had to convert regularly into an isotope of the next element (No. 94). McMillan and Abelson, of course, hoped to discover this element, too, but their dream did not come true. As found later, the isotope of element 94 with a mass number of 239 has a long half–life and its activity is low. The discoverers of neptunium only detected alpha particles of an unknown origin (later found to be emitted precisely by element 94) and discontinued their work.
The work on the synthesis of element 94 was headed by the famous American scientist G. Seaborg whose group discovered many transuranium elements. During the winter of 1940–1941 they studied the nuclear reaction 238U (d, 2n) which gave rise to the isotope neptunium–238. An alpha–active substance accumulated with time in the reaction product. The scientists extracted this substance and found that it was an isotope of element 94 with a mass number of 238 and a half–life of 50 years. The new element was named plutonium after the respective planet of the Solar system.
But once more this isotope was not the longest–lived one. The longest–lived isotope with a mass number of 244 and a half–life of 8.3 × 107 years was found only in 1952. The decisive progress in the study of plutonium was due to the isotope plutonium–239 synthesized in spring of 1941. First, it was long–lived (a half–life of 24 360 years) and, second, the intensity of its fission under the effect of slow neutrons was much higher than that of uranium–235. This was the decisive factor for its use in nuclear weapons. Therefore, an especially careful study was made of the physical and chemical properties of this element. As a result, plutonium became one of the best–studied elements of the periodic table. Moreover, plutonium–239 could be used as a target for syntheses of next transuranium elements. All this became widely known only at the end of the forties when much of the work on nuclear energy was declassified. This was an unusual feature for the history of elements that discoveries of new elements were kept secret for some time.
The efforts invested into the work on plutonium were so intense that as early as August 1942 weighable amounts of it were prepared (the fastest work in the history of synthesized elements). In our days plutonium is produced in quantities that are much greater than those of many stable elements found on Earth. A total of 17 isotopes of plutonium are currently known. As in the case of neptunium, the plutonium–239 isotope was found in uranium minerals, of course, in symbolic amounts. It is produced in uranium under the effect of natural neutrons. Thus, plutonium serves as a kind of the natural upper boundary of the periodic system and we can speak about two dates of its discovery.