Chemical & Chemical Engineering News (80th Anniversary Issue), Vol. 81, No. 36, 2003, Sept. Edited by X. Lu Introduction

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.

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.

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.

BOHRIUM & HASSIUM AT A GLANCE

Name: Bohrium is named after Danish physicist Niels Bohr, who proposed the modern concept of the atom; hassium comes from the Latin Hassia, Germany.

Atomic mass: Bh: (264); Hs: (270).

History: Russian scientists at the Joint Institute for Nuclear Research first reported producing bohrium in 1976. This was confirmed by German physicists Peter Armbruster and Gottfried Münzenberg at the Gesellschaft für Schwerionenforschung in 1981. Hassium was discovered in 1984 by a team of physicists led by Armbruster and Münzenberg in Germany.

Occurrence: Artificially produced.

Appearance: Solids of unknown color.

Behavior: Highly radioactive.

Uses: No commercial uses.