As is well known, the series of chemical elements of natural origin ends with uranium and the extremely rare naturally occurring plutonium, whose atomic nuclei consist of 92 and 94 protons.

But if you look at the periodic table, you see other entries beyond the heaviest naturally occurring element.

These elements are not of natural origin, but artificially produced in reactors or particle accelerators.

It is thanks to physicists like Sigurd Hofmann that the periodic table today contains 26 such heavy exotic species.

In the 1990s, the researcher and his colleagues at the Society for Heavy Ion Research discovered six heavy elements - most recently in the 1990s the elements with the atomic numbers 110, 111 and 112.

Manfred Lindinger

Editor in the department "Nature and Science".

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Already at the beginning of the 1940s it was recognized that the periodic table could be extended artificially.

First, uranium was irradiated with neutrons in a reactor, thereby creating the first transuranic elements plutonium and neptunium with the atomic numbers 93 and 94. Here, use was made of the fact that the neutron absorbed by the uranium nucleus changes into a proton through beta decay and in this way a new element of the next higher atomic number is formed.

By irradiating the elements that had already been obtained with neutrons, deuterons and helium nuclei, ever heavier transuranic elements up to the element fermium (atomic number 100) could be produced in the following years.

However, atomic nuclei with more than a hundred protons could only be obtained by fusing two atomic nuclei.

In this field, the Lawrence Berkeley National Laboratory in California and the Institute for Nuclear Physics Research in Dubna near Moscow initially proved to be leaders.

There, researchers used particle accelerators to irradiate some of the transuranic elements with light and heavy ions.

In 1974, this led to the element with the atomic number 106, rutherfordium.

However, nuclear fusion placed high demands on the experimental art of the physicists.

First, the electrostatic repulsion between the positively charged atomic nuclei had to be overcome.

This was achieved by shooting the projectile ions at fixed targets with high energy.

However, the energy of the projectile must not be too great.

Otherwise, the resulting nucleus would be excited too much and would immediately split into two fragments as a result of spontaneous fission.

Despite everything, it was becoming increasingly difficult for researchers to create new elements.

The half-lives of the nuclei became shorter and shorter with increasing atomic number and the effective cross-sections became smaller and smaller.

Great success despite low yield

In the early 1980s, the scientists working with Sigurd Hofmann in Darmstadt pursued a new strategy to produce elements beyond rutherfordium with “cold” – soft – fusion.

In this process, the speed of the projectile ions must not be too high so that the fusion produces an atomic nucleus that is as “cold” as possible.

Instead of fission, these individual neutrons evaporate.

With the soft fusion, the Darmstadt researchers have created six heavy elements.

The last of these, Element 112, was credited to the work group led by Peter Armbruster and Sigurd Hofmann in 1996.

However, only two atoms were detected within three weeks.

With each new chemical building block, the hope of reaching the so-called island of stability increased.

Nuclear physicists understand this to be an area on the nuclide chart in which long-lived superheavy elements are suspected, which do not decay within a very short time like the previously known ones.

The Stability Island has not yet been spotted.

Nevertheless, other heavy elements were produced with the "cold" fusion.

Even if the team around Sigurd Hofmanns, who was born on February 15, 1944 in Böhmisch-Kamnitz, came to Groß-Umstadt shortly after the end of the war, went to school there and later studied physics at the TU Darmstadt and also received his doctorate there, no more entries in the periodic table, Hofmann closely followed the activities in Berkeley, but above all in Dubna.

The heaviest element to date, element 118, was detected there in 2006.

His colleagues valued his modesty, perseverance and reliability.

He was always in the office or at the experiment, even late in the evening and on weekends, so that you could ask him at any time and always get detailed answers and competent advice.

There was virtually nothing in nuclear physics and at GSI that he didn't know, writes GSI in a press release.

As has just become known, Sigurd Hofmann died on June 17 at the age of 78.