Discovery of Transuranium elements

Transuranium elements

 Transuranium elements are all the elements whose numbers are higher than 92, that is, the elements that directly follow uranium. Now 15 such elements are known. How many more transuranium elements can be found? The answer is still unknown. This is one of the fascinating mysteries of science.

Though the first transuranium element, neptunium, (No. 93) was born not so long ago, in 1940, the question about possible existence of such elements was raised much earlier. Mendeleev did not ignore it either. He believed that even if the transuranium elements would be found on Earth their number will be limited. This was his opinion in 1870. For more than 25 years the problem remained open. Every year saw several erroneous reports on discoveries of new elements but not once the element in question had an atomic mass greater than that of uranium. It seemed axiomatic that uranium was the last element in the periodic system though nobody could say why.

But when radioactivity was discovered thorium and uranium, that is the heaviest elements in the Mendeleev table, were found to possess this property. It would logically seem that the transuranium elements had existed in nature in past but, being highly unstable, had decayed to other, known elements. This simple explanation had a hidden trap, namely, the possible half–lives of even the nearest right–hand neighbours of uranium were quite unknown. Nobody could state with certainty that these hypothetical elements were less stable than uranium and thorium. Thus, it would be reasonable to look for natural transuranium elements.

Years passed and occasionally allegedly successful discoveries of the first transuranium element were reported in scientific journals. As theoretical physics developed it repeatedly attempted to explain the break–off of the periodic system at uranium. Many of these explanations were fascinating but none convincing. In other words, in the twenties of this century the question of transuranium elements looked as vague as in the last quarter of the 19th century.

One amazing hypothesis appeared, however, against this dismal background though at first scientists treated it with suspicion. Only 40 years later the hypothesis found a new meaning. It was put forward in 1925 by the German scientist R. Swinne who looked for the transuranium elements in a peculiar material–a dust of space origin collected on the icefields of Greenland. A sample of the dark powder was given to the Stockholm museum by the well–known polar explorer E. Nordenskjöld in the eighties of the last century. Swinne hoped to find in this powder traces of transuranium elements with the numbers 106–110 and in one of his reports he even mentioned that he had recorded an X–ray spectrum containing lines that, in his opinion, corresponded to element 108. But nobody believed him and he himself discontinued his work.

Swinne made a theoretical study of the variation of various properties of radioelements and, in particular, half–lives. He came to the conclusion that the elements directly following uranium had to have short half–lives. But the elements with the numbers in the ranges between 98 and 102 and between 108 and 110 could be expected to have sufficiently long half–lives. Where to look for them? Swinne suggested that the best bet would be not terrestrial minerals but space objects. This is why he studied the dust of space origin collected in Greenland. All this was quite fascinating but not substantiated and therefore looked like doomed for oblivion.

Now we come to the point in time when the words transuranium elements started to be linked with the word synthesis.

Paradoxical as it seems the attempts to synthesize new elements (namely, trasuranium elements) had started a few years before technetium was produced. The stimulus for this work was the discovery of neutron. The scientists regarded this chargeless elementary particle as possessing infinite penetrating capacity and being capable of producing a wide variety of transformations of all kinds of elements. Thus, all laboratories that had neutrons sources started to bombard with neutrons targets made of various materials including uranium. Especially active in this work was the Italian physicist E. Fermi who was the leader of a group of young enthusiasts at the University of Rome.

They detected some new activity in irradiated uranium. As they irradiated uranium–238 it absorbed neutrons converting into an unknown uranium isotope with a mass number of 239. Since the isotope had an excess of neutrons it exhibited a definite tendency to beta decay. If the left–hand side of the reaction equation is 239U– then the right–hand side is necessarily 23993.

Fermi and his young coworkers argued in approximately this way (though not very clearly as many concepts of nuclear physics at the time were not sufficiently developed). Now chemical verification was needed to prove the synthesis of the first transuranium element. It had to be demonstrated that the activity induced by neutrons in uranium did not belong to any of the preceding elements. This was established within the limits of the capacity of radiochemistry. Thus, Fermi and his group had in their hands a new element, a transuranium one, and one that was the first to be discovered owing to the nuclear synthesis (all this happened in 1934). Fermi and his group, however, were not completely sure of their results. Meanwhile the news about the new element leaked to the press and the discovery was embellished with non–existent details such as Fermi presenting the Queen of Italy with a test–tube containing a dissolved salt of element 93. A lot of such false sensations were published in press while the group continued to assess the results obtained after irradiation of uranium with neutrons.

They extracted several beta–active substances from the uranium target. Two of them were chemically peculiar as they could be precipitated with manganese (IV) oxide easier than the elements preceding uranium. This observation suggested that element 93 was eka–rhenium–a manganese analogue. It was named auzonium (Ao). Being beta–active it could convert into the next element with Z = 94 known as hesperium (Hs). Fermi described this series of nuclear transformations in the following way:

\[_{92}^{238}U+n\xrightarrow{{}}_{92}^{239}U\xrightarrow{{{\beta }^{-}}}_{93}^{239}Ao\xrightarrow{{{\beta }^{-}}}_{94}^{239}Hs\

This series was continued further by the German scientists O. Hahn, L. Meitner, and F. Strassmann, highly experienced radiochemists, particularly O. Hahn who had made his name having had discovered several radioelements. As a result of careful studies the number of new transuranium elements increased by three (including element 97):

\[_{94}Hs{{\xrightarrow{{{\beta }^{-}}}}_{\,\,95}}EkaIr\xrightarrow{{{\beta }^{-}}}\,{{\,}_{96}}EkaPt\xrightarrow{{{\beta }^{-}}}\,{{\,}_{97}}EkaAu...\                       

The prefix Eka means that the respective transuranium elements were considered to be analogues of iridium, platinum, and gold from the sixth period of the periodic system. But it was precisely here that a serious mistake was made, which took quite a time to be found. The properties of the nearest transuranium elements were, in fact, quite different.

The history of science knows of many marvelous insights which seemed at first quite unsubstantiated. One of them was the idea put forward by I. Noddack back in 1934 that when uranium was bombarded with neutrons the uranium nuclei did not convert into new elements at all, rather, they were split into fragments which were the nuclei of lighter, known elements. Her colleagues made light of Noddack’s idea and Hahn’s comments were especially ironic. But his irony turned to be the irony of fate.

Meanwhile other scientists tried to ascertain what happened to uranium under neutron bombardment. I. Joliot–Curie and her coworker, the Serbian physicist P. Savich, particularly carefully analysed and irradiated uranium target. Among the resulting activities they detected traces of a chemical element whose properties were very similar to those of actinium, that is, an element preceding uranium, rather than following it in the periodic table. Soon it was found to have more in common with lanthanum than with actinium. Thus, one of the products obtained after bombardment of uranium with slow neutrons was similar to lanthanum.

If I. Joliot–Curie and Savich had not drawn a line at cautiously stating that the unknown element was similar to lanthanum but had definitely proven that it was lanthanum they would have become the authors (or, at least, coauthors) of one of the greatest discoveries of the 20th century. (it would be in order here to recall that lanthanum has the number 57 and uranium the number 92 and to recall the idea of I. Noddack, too). This seemed more than improbable. But facts remained facts. The results of I. Joliot–Curie and Savich looked so convincing that O. Hahn took it upon himself to verify them, the very same O. Hahn who was an ardent opponent of these results. This meant that he started to question his former opinions.

Hahn, together with his coworker Strassmann, reproduced the experiments of the French scientists whom he so recently had regarded as his opponents. Almost all the results were confirmed. The uranium target contained isotopes of lanthanum and its preceding neighbour in the periodic system, barium. As a chemist, Hahn could not doubt this. As a physicist, he was baffled by the fact.

The fact was that under neutron bombardment uranium nuclei seemed to split into two fragments and these fragments were the nuclei of the isotopes of elements belonging to the centre of the periodic system. Nuclear physics never encountered such a phenomenon. But facts had to be faced and the German scientists concluded that uranium nuclei were capable of breaking down under neutron bombardment.

This happened on December 23, 1938. The scientists immediately reported their discovery. Later Hahn reminisced that after posting the report it all had seemed so improbable to him that he had wished that he could take the letter back from the post box. The improbable proved to be right. A few days later a letter from Hahn was received by L. Meitner who had worked with him for many years. She, together with her nephew, the physicist O. Frisch, attempted theoretical treatment of this phenomenon.

To a certain extent, nuclei can be likened to drops of liquid and scientists have repeatedly tried to draw an analogy between the properties of a nucleus and those of a drop of liquid. If we transfer a sufficient energy to a drop and make it move it can break down to smaller drops. If a nucleus is excited (by a neutron, say) then it can also split into smaller fragments. Gradually, a uranium nucleus is deformed, it elongates, narrowing appears in it, and, finally, it splits into two parts. This is how Meitner and Frisch described the process of splitting of the uranium nuclei. They wrote that the process was remarkably similar to division of bacterial cells by which they propagate and suggested naming the effect “nuclei fission”.

A uranium nucleus splits into two fragments liberating an enormous amount of energy in the process. Other products of fission were free neutrons. They could hit other uranium nuclei leading to their fission and so on. Under favourable conditions a chain fission reaction could occur in a uranium lump producing a nuclear explosion of immense power. As early as 1940 the Soviet scientists Ya. Zel’ dovich and Yu. Khariton developed a rigorous theory of the chain fission reaction. Man mastered a process which, apparently, was unknown in nature. This was the most comprehensive process of transformation of elements man had ever encountered. The fragments of uranium fission were found to contain isotopes of 34 elements, from zinc (number 30) to gadolinium (number 64). The fission reaction proved to be a veritable factory of radioactive isotopes.

Uranium fission caused by neutrons was forced or artificial. Not each uranium nucleus could be split and not each neutron could produce fission. When scientists had studied the fission mechanism in more detail they understood that the intensity of fission was higher under the effect of slow neutrons and if the uranium isotope with a mass number of 235 was used. The other uranium isotope, uranium–238, experienced fission only when bombarded by fast neutrons. Can there be a natural process similar to artificial uranium fission? N. Bohr thought about that and put forward a hypothesis about possible spontaneous uranium fission (without external energy being transferred to the nuclei).

The Soviet scientists G. Flerov and K. Petrzhak attempted an experimental verification of this hypothesis. But how to establish that fission of the uranium nuclei was really spontaneous? Random neutrons of cosmic rays getting into the laboratory could distort the results of experiments. This is why one autumn midnight of 1940 Flerov and Petrzhak went down to one of the deepest stations of Moscow underground railway. There, tens of metres under the surface of earth, the harmful effect of cosmic rays could be escaped. The same night they obtained the final proof of the existence of a new type of radioactive transformations, namely, spontaneous fission of nuclei (they worked only with uranium–238). Later many isotopes of heavy elements (thorium and, particularly, transuranium elements) were found to exhibit this mechanism of radioactive decay. At present science knows of about a hundred nuclei of various elements capable of spontaneous fission. The mechanism of spontaneous fission is similar to that of fission under neutron bombardment.

We now know enough to embark of the tale of the discoveries of individual transuranium elements since it is just in this range of elements that spontaneous fission plays a very significant part. The history of transuranium elements covers forty years and during this, by modern standards, fairly long period scientists managed to take fifteen steps beyond uranium up to element 107. If we take a frame of reference and plot the numbers of elements from 1 to 92 along the horizontal axis and the years of their discovery along the vertical axis the resulting plot will look like a seismogram of a catastrophic earthquake. A similar plot for the transuranium elements is a comparatively smoothly rising line exhibiting distinct peaks. Each new synthesis of a transuranium element meant an increase in the atomic number by one (with a single exception).

The history of syntheses saw its periods of breakthroughs and slack periods. The first breakthrough period was from 1940 to 1945 when four transuranium elements were synthesized, namely, neptunium (Z = 93), plutonium (Z = 94), americium (Z = 95), and curium (Z = 96). The period till 1949 was a slack time and no new elements were discovered. In the next breakthrough period from 1949 to 1952 four more transuranium elements were added to the periodic system, namely berkelium (Z = 97), californium (Z = 98), einsteinium (Z = 99), and fermium (Z = 100). In 1955, fifteen years after the synthesis of the first tranuranium element, one more element, mendelevium (Z = 101), was synthesized. The next 25 years saw much less syntheses and only six new elements appeared in the periodic system. Here scientists encountered an entirely new situation and many former criteria for evaluating discoveries of elements proved inapplicable.

This changing pattern is by no means random, all the breakthroughs and failures had their quite objective causes. They will be apparent when we discuss syntheses of transuranium elements one by one starting from the first one, neptunium.

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