Peter Armbruster is a scientist emeritus in physics at GSI, Darmstadt, Germany. He built up and headed the Nuclear Chemistry Group at GSI that discovered the six elements between Z = 107 and Z = 112 from 1972 to 1996. He won the ACS Award for Nuclear Chemistry in 1997

Nearly a century ago, Ernest Rutherford showed that the nucleus of an atom has identical proton and atomic numbers (Z). The stability of the nucleus and the number of possible elements then became a question of nuclear physics. The ratio of two forces--the short-range nuclear force and the long-range electromagnetic force--determines the stability of nuclei. The nuclear force keeps nucleons together, whereas electric repulsion between protons tries to break the nucleus into smaller pieces. Small changes in the ratio of the two forces decide whether a chemical element can or cannot exist.

As Z increases, the electric repulsion between protons rises in proportion to the square of their number, whereas the attractive nuclear forces grow less than linearly with the total number of nucleons. An energy barrier protects the atomic nucleus against fission. This barrier becomes smaller and smaller as Z increases. Danish physicist Niels Bohr predicted in 1939 that, assuming the nucleus to be a droplet of nuclear matter, the number of elements should be limited to about a hundred.

However, the quantum mechanical order of atomic electrons--the essence of chemistry--has an equivalent in atomic nuclei. Nuclear structure, as the order in the chaotic soup of nucleons is called, gives additional binding energy compared to a structureless nuclear droplet, and an increase in the number of possible chemical elements was predicted in the 1960s. This idea of new superheavy elements in the range up to Z = 120 stabilized by nuclear structure inspired nuclear researchers and, in Germany, led to different initiatives for entering the element-hunting race.

In December 1969, the Gesellschaft für Schwerionenforschung (GSI) was founded at Darmstadt in order to build a heavy-ion accelerator and start research on the physics and chemistry of superheavy elements. This decision led to the synthesis of six new elements between 1981 (Z = 107) and 1996 (Z = 112). Their atomic nuclei are strongly stabilized by the quantum-mechanical order of their constituents, and they have a barrel-like shape. These elements are the first superheavy elements. Their nuclei are protected against spontaneous fission decay by a high fission barrier built up by the nuclear structure of the system.

Our surprising success was a consequence of long-term planning combined with fortuitous circumstances. At the start of the project, we had a unique technological base in Germany. Christoph Schmelzer had started work in the late 1950s on acceleration of heavy ions, and Heinz Ewald and I developed and built recoil separators for fission fragments at nuclear reactors. These were essential provisions for an accelerator (UNILAC) and a recoil separator for fusion products (SHIP) to be available by 1975. Both of these--viewed somewhat skeptically by the outside community--were genuine innovations. UNILAC was built by the GSI team, and SHIP was designed and built in collaboration with the University of Giessen by a team headed by Gottfried Münzenberg. Besides the recoil-separator technique, new target technology and position-sensitive silicon-detector techniques were decisive. The successful team of Münzenberg, Sigurd Hofmann, Fritz Peter Hessberger, Willibrord Reisdorf and Karl-Heinz Schmidt synthesized elements Z = 107­109 between 1976 and 1989.

Vousatý vedoucí skupiny Peter Armbruster stojí uprostřed, tato skupina našla či pomohla najít šest prvků - transuranů.

In addition to the technological base, new scientific findings in the 1970s played a major role. Experiments on reaction mechanisms pursued in all the heavy-ion laboratories of the time showed that, with increasing system mass, nuclei fuse more and more rarely. Fusion leading to superheavy nuclei will be an ever more elusive reaction channel: Only highly sensitive methods will help progress. In 1973, Yuri Oganessian and Alexander Demin at Dubna, Russia, discovered a new way of producing heavy elements--the fusion of lead and bismuth nuclei with medium-weight ions in the mass range of 40 to 54. This method avoids the use of reactor-bred actinide targets and gives independence from access and availability of these isotopes. Moreover, the new reaction type--soft fusion--produces less heated nuclear systems that cool down by the emission of only one or two neutrons, whereas the actinide-based reactions--hot fusion--liberate about four to five neutrons. To survive fission in the de-excitation of the primary system, soft fusion is highly advantageous.

The element bohrium (Bh) was first identified on Feb. 24, 1981, in Darmstadt. A chain of correlated -decays was registered, allowing for an unambiguous reconstruction of the isotope ²⁶²Bh. The isotope was produced by fusion of ²⁰⁹Bi and ⁵⁴Cr into an excited compound nucleus, which cooled down by prompt emission of one neutron to ²⁶²Bh. Bohrium transmutes by a chain of time-correlated -decays within milliseconds to known isotopes of the elements dubnium, lawrencium, mendelevium, and fermium. So far, about 70 atoms of ²⁶²Bh have been observed and identified. In 1997, the International Union of Pure & Applied Chemistry (IUPAC) accepted the proposal to name the new element with Z = 107 after Bohr.

"Our surprising success was a consequence of long-term planning combined with fortuitous circumstances."

Today, we know five isotopes of bohrium (with mass numbers 261, 262, 264, 266, and 267); the two heaviest were discov-ered in 2000 at LBL. All of the isotopes are -emitters, and their half-lives increase from 12 milliseconds for ²⁶¹Bh to 17 seconds for ²⁶⁷Bh. All bohrium isotopes were identified by time correlations to known isotopes of lighter elements using single-event detection. For the odd atomic number element, no spontaneous fission decays were registered.

Computer simulation of the fusion reaction of two nuclei for the creation of new elements.

At the Paul Scherrer Institute in Switzerland, it was shown that bohrium is a group 7 element, the big brother of rhenium, technetium, and manganese. The volatility of oxychlorides of short-lived isotopes of group 7 elements could be measured and compared by gas chromatography. As expected from relativistic calculations of molecular properties and following the trend in the periodic table for group 7 elements, bohrium shows the lowest volatility of its oxychloride compound compared with the lighter homologs in group 7. Its place in the periodic table is below rhenium.

Replacing the ⁵⁴Cr projectiles with ⁵⁸Fe projectiles opened the way to element 109, meitnerium (Mt). On Aug. 29, 1982, an 11.1-MeV -particle correlated within 5 milliseconds to the previously discovered ²⁶²Bh-chain gave evidence for the first atom of ²⁶⁶Mt. Today, we know two isotopes of meitnerium (atomic masses of 266 and 268). They are millisecond -emitters produced with a few picobarns. The classification of meitnerium in the periodic table is still open.

The element hassium (Hs) was identified first on March 14, 1984, in Darmstadt. A chain of correlated -decays was registered, allowing for an unambiguous reconstruction of the isotope ²⁶⁵Hs. The isotope was produced by fusion of ²⁰⁸Pb and ⁵⁸Fe into an excited compound nucleus, which cooled down by prompt emission of one neutron to ²⁶⁵Hs. Hassium transmutes to known isotopes of seaborgium, rutherfordium, nobelium, and fermium. So far, about 40 atoms of ²⁶⁵Hs have been observed and identified. IUPAC accorded the major credit concerning discovery to our group and, in 1997, accepted the proposal to name the new element with Z = 108 hassium after the state of Hessen (Hassia in Latin). Darmstadt was the former capital of Hessen, and our institute wanted to acknowledge the people and the state that host the institute and help to continuously finance our costly budgets and the GSI laboratory.

Today, we know six isotopes of hassium (with mass numbers of 264-267, 269, and 270). All isotopes are -emitters. Their half-lives increase from 0.5 milliseconds for ²⁶⁴Hs to 21 seconds for ²⁷⁰Hs. All hassium isotopes were identified by time correlations to known isotopes of lighter elements using single-event detection. Only the lightest even-even isotope, ²⁶⁴Hs, has a spontaneous fission-decay branch. ²⁶⁷Hs was discovered by fusion of ³⁴S and ²³⁸U in 1994 at Dubna. In 2001, nuclear chemistry groups at Darmstadt synthesized the isotopes ^{269, 270}Hs by fusion of ²⁶Mg and ²⁴⁸Cm.

Like osmium, hassium is expected to form a very volatile tetraoxide. HsO₄ has a deposition temperature on a thermochromatography column that is higher than its homolog, OsO₄. HsO₄ behaves as a group 8 element, and it is slightly less volatile than the osmium compound. Its place in the periodic table is below osmium in group 8.

The decay chains of ²⁶⁹Hs were seen before in the decay of ²⁷⁷112. The agreement of the thermochromatography experiment with this earlier experiment indirectly corroborates the discovery of element 112. The isotope ²⁷⁰Hs has a half-life of 4 seconds for -decay, allowing for the application of a radiochemical separation method. ²⁷⁰Hs is the center of a shell-stabilized region of deformed superheavy nuclei having barrel-like shapes.

The unexpected disappearance of spontaneous fission decay beyond rutherfordium and increasing -half-lives approaching 162-neutron isotopes in the 10-second range are prerequisites for chemical investigations. Nuclear-structure physics has allowed the elements up to hassium to enter the periodic table. The strongest nuclear-shell corrections ever seen until now for a deformed nucleus are found in ²⁷⁰Hs. The order of its nucleons in the barrel-like shape of the nucleus gives -half-lives that just meet the limits of today's fast chemical methods applicable to single-atom detection techniques.

* isotopes discovered at GSI (status: November 2004)