Super-heavy elements

Two experiments succeed in creating super-heavy elements, improving our understanding of the nuclear structure

Published: Saturday 04 July 2015

the first attempt to prepare a transuranic element (which is heavier than uranium) was made in 1934 in Rome, where a team of Italian physicists headed by the legendary Enrico Fermi bombarded uranium nuclei with free neutrons. The first such element produced was neptunium with an atomic number 93. (Atomic number is the number of protons in the nucleus, and uranium has 92). Since then, many new elements have been produced in the laboratory and their properties studied. Recently, scientists have discovered three new elements, of atomic numbers 114, 116 and 118. The elements are very significant as they could lead us to a much better understanding of nuclear structure.

It is an amazing paradox that though scientists can be reasonably confident of their understanding of elementary particles like electrons, neutrinos and quarks, their understanding of a much larger structure, the atomic nucleus is still rudimentary. Though many theories have explained several observed properties, a completely reliable theory is still missing. For instance, an outstanding challenge in nuclear physics is to find out the maximum number of protons that a nucleus can hold and still be relatively stable.

In nature, there are many elements that are stable and have a very long lifespan. On the other hand, scientists have synthesised many new elements that are unstable, that is, they form other elements through radioactive decay. For instance, neptunium has a half-life (a length of time in which the quantity of a substance is reduced to half) of about two million years. Most other artificially produced, super-heavy elements have a lifetime of a few milliseconds.

One of the predictions of the nuclear theory is the existence of a so-called island of stability in the hierarchy of elements. As we look at heavier elements, we first encounter unstable elements (like those with atomic numbers from 92 to 112) and then somewhere around atomic number 114, there are several relatively stable elements. It is this unknown region which is now being explored by the new experiments.

The way to create new super-heavy elements is to literally bang together two elements, one light and one heavy with great force and let the nuclei fuse. For instance, in 1996, element 112 was produced by colliding a lead nuclei (atomic number 82) with a suitable light nuclei. But scientists at the Joint Institute of Nuclear Research in Dubna, Russia, have adopted a different technique to produce the new elements. They have used plutonium nuclei instead of lead to get the desired reactions ( Nature , Vol 400, p242).

At Dubna, the researchers used a plutonium target with calcium projectiles to produce an element with atomic number 114. This is difficult, as the particular kind of calcium used is extremely rare in nature and to get it involves a long process of separation of the relevant atoms from the other, more abundant types. The real experimental ingenuity comes in separating the element from the debris generated by the collisions. This is done with the help of a gas-filled mass separator, a device that separates atoms of different masses. The element 114 that they obtained had a lifetime of 30 seconds, which is very long compared to the millisecond lifetimes of other super-heavy elements.

Another experiment at the Lawrence Berkeley National Laboratory ( lbnl ), usa , uses a lead target with Krypton projectiles. In this way, they have been able to detect the presence of element 118 and in its decay process, element 116. Though the time of decay in these cases are also long compared to other super-heavy elements, the elements are short-lived compared to the element 114 produced at Dubna.

The presence of these relatively long-life elements has led to a lot of excitement in the field of nuclear physics. For the first time, any experiment is close to exploring the predicted island of stability. If at all it exists, it would be a pathbreaking achievement for nuclear theoretical models and we may be closer to understanding the structure of the atomic nucleus.

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