March 1, 2024

The search for the limit of the periodic table

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The TASCA recoil separator at GSI/FAIR in Darmstadt used for the production and isolation of super heavy elements. Credit: G. Otto, GSI/FAIR

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The TASCA recoil separator at GSI/FAIR in Darmstadt used for the production and isolation of super heavy elements. Credit: G. Otto, GSI/FAIR

Since the turn of the century, six new chemical elements have been discovered and subsequently added to the periodic table of elements, the very icon of chemistry. These new elements have high atomic numbers, up to 118, and are significantly heavier than uranium, the element with the highest atomic number (92) found in the greatest quantities on Earth.

This raises the following questions: how many more of these super-heavy species are waiting to be discovered? Where – if anywhere – is a fundamental limit on the creation of these elements? And what are the characteristics of the so-called island of greater stability?

In a recent review, experts in the theoretical and experimental chemistry and physics of the heaviest elements and their nuclei summarize key challenges and offer new insight into new superheavy elements and the edge of the periodic table.

One of them is Professor Christoph Düllmann from the GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt, the Johannes Gutenberg University in Mainz and the Helmholtz Institute Mainz (HIM). In its February issue, Nature Review Physics presents the theme as a cover story.

Visualizing an island of stability of superheavy nuclei

Already in the first half of the last century, researchers realized that the mass of atomic nuclei is smaller than the total mass of their constituent protons and neutrons. This difference in mass is responsible for the binding energy of the nuclei. Certain numbers of neutrons and protons lead to a stronger bond and are called “magic”.

In fact, scientists observed early on that protons and neutrons move in individual shells similar to electronic shells, with lead metal nuclei being the heaviest, with completely filled shells containing 82 protons and 126 neutrons – a doubly magical nucleus.

Early theoretical predictions suggested that the extra stability of the next “magic” numbers, far from the nuclei known at that time, could lead to lifespans comparable to the age of Earth. This has led to the notion of a so-called stability island of superheavy nuclei separated from uranium and its neighbors by a sea of ​​instability.

There are numerous graphic representations of the island of stability, portraying it as a distant island. Many decades have passed since this image emerged, so it’s time to take another look at the stability of superheavy nuclei and see where the journey to the limits of mass and charge might take us.

In their recent article titled “The search for superheavy elements and the limit of the periodic table”, the authors describe the current state of knowledge and the most important challenges in the field of these superheavy elements. They also present important considerations for future development.

Elements up to oganesson (element 118) have been produced in experiments, named and included in the periodic table of elements at accelerator facilities around the world, such as at GSI in Darmstadt and in the future at FAIR, the international accelerator center being built at GSI . These new elements are highly unstable, with the heaviest ones disintegrating within seconds at most.

A closer look reveals that its lifetime increases toward the magic neutron number 184. In the case of copernicium (element 112), for example, which was discovered at GSI, the lifetime increases by less than a thousandth of second to 30 seconds. However, the number of neutrons 184 is still a long way from being achieved, so the 30 seconds are just a step on the way.

As the theoretical description is still subject to great uncertainties, there is no consensus on where the longest lifetimes will occur and how long they will last. However, there is general agreement that truly stable superheavy nuclei are no longer expected.

Reviewing the superheavy element map

This leads to a review of the superheavy landscape in two important ways. On the one hand, we actually reached the coasts of the region of greater stability and experimentally confirmed the concept of an island of greater stability. On the other hand, we still don’t know how big this region is – so we can keep the image. How long will the maximum lifespans be, with the height of the mountains on the island typically representing stability, and where will the longest lifespans occur?

O Nature Review Physics The article discusses several aspects of the relevant theory of nuclear and electronic structure, including the synthesis and detection of superheavy nuclei and atoms in the laboratory or at astrophysical events, their structure and stability, and the location of current and predicted superheavy elements in the periodic table.

Detailed investigation of superheavy elements continues to be an important pillar of the GSI Darmstadt research program, supported by the infrastructure and expertise of HIM and Johannes Gutenberg University Mainz, forming a unique setting for such studies.

Over the past decade, several groundbreaking results have been obtained, including detailed studies of its production, which have led to the confirmation of element 117 and the discovery of the comparatively long-lived isotope lawrence-266, of its nuclear structure by a variety of experimental techniques. , the structure of their atomic shells, as well as their chemical properties, where flerovium (element 114) represents the heaviest element for which chemical data exist.

Calculations on production in the cosmos, especially during the merger of two neutron stars, observed experimentally for the first time in 2017, complete the research portfolio. In the future, the investigation of superheavy elements could be even more efficient thanks to the new HELIAC linear accelerator, for which the first module was recently assembled at HIM and then successfully tested in Darmstadt, so that even more, even more exotic and , therefore, presumably longer living nuclei will also be experimentally achievable.

An overview of element discoveries and early chemical studies at the GSI can be found in the article “Five decades of superheavy element discoveries and chemical investigation at the GSI”, published in May 2022 in Acta Radiochemistry.

More information:
Odile R. Smits et al, The search for superheavy elements and the limit of the periodic table, Nature Review Physics (2023). DOI: 10.1038/s42254-023-00668-y

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