List of Nuclear Isotope Discoveries Shows U.S. Contributions in Decline

List of discoveries shows US contributions have declined, but Japanese, Russian and European work is on the rise.

By Eugenie Samuel Reich of Nature magazine

When it comes to discovering nuclear isotopes, retired physicist Gottfried Münzenberg is top, with 222. His colleague Hans Geissel, from the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, Germany, is next with 211. Then comes Francis Aston, the British chemist who won the 1922 Nobel Prize in Chemistry, with 207.

These provisional rankings have been assembled by Michael Thoennessen, a physicist at the National Superconducting Cyclotron Laboratory at Michigan State University in East Lansing. With the help of several undergraduate students, he has assembled a database of the almost 3,100 known nuclear isotopes--atoms that share the same number of protons but have varying numbers of neutrons. He lists the discovery of each by researcher, the lab at which they worked and the method by which they produced the isotope.

The vast majority of these isotopes only exist naturally for fleeting moments inside stars, formed during the production of heavy elements. They never exist on Earth at all, except for a few nanoseconds in elite nuclear-physics labs.

The International Union of Pure and Applied Chemistry, headquartered in Zurich, Switzerland, assigns the official credit for discovering elements, but Thoennessen is now doing the same for all the isotopes of those elements. "I don't think it matters so much who gets the credit, but historically it's important to identify which countries and labs are driving the field," he says.

Isotope hotspots

The database lets Thoennessen rank labs as well as people, and to track how different countries' rising and falling investments in nuclear technology have affected where and how isotopes are discovered (Click here for an animated chart of known isotopes that shows how many were found in quick succession with each new wave of technology).

Top of the lab list is Lawrence Berkeley National Laboratory in Berkeley, California, with 634 isotopes. GSI Darmstadt is second with 368; the University of Cambridge, UK, is third with 222, its discoveries made mostly in the days of Aston; and the Joint Institute for Nuclear Research in Dubna, Russia, is fourth with 215.

Berkeley may reign supreme in terms of sheer numbers, but the trend over time tells a different story. In the 1980s and 1990s the lab failed to upgrade its equipment and so fell behind other labs. It also lost some credibility in 2002, when allegations surfaced that physicist Victor Ninov had faked the discovery of elements 116 and 118 there.

These days, most new finds come from GSI Darmstadt, Dubna and other locations, such as the Radioactive Isotope Beam Factory(RIBF) in Wako, part of Japan's RIKEN national network of labs.

Although the decline of Berkeley means that the United States is losing ground, in the longer term Thoennessen expects the country to maintain its supremacy with the Facility for Rare Isotope Beams (FRIB), construction of which is expected to begin in 2012 at Michigan State University.

Innovation or investigation

Not everyone agrees that this ranking is the best way to capture the science of the field. Krzysztof Rykaczewski, a nuclear physicist at Oak Ridge National Laboratory in Tennessee, who according to Thoennessen's list has discovered 60 isotopes, would like to see a ranking include not only discoveries, but also the first person to study the properties and nuclear structure of the atoms. "Identifying is only the first step," he says.

But Patrick Regan, a physicist at the University of Surrey in Guildford, UK, who has 20 isotope discoveries to his name, thinks that the discovery itself does reflect something important. "I'm proud of each one of those 20," he says, "each of them is like a child to science."

Thoennessen set the bar high when deciding what counted as a discovery, in the hope of improving standards in the field. He considers an isotope "discovered" only if its mass and charge have been identified in a peer-reviewed paper.

As he dug into the literature, he found that some generally accepted isotopes in fact appeared only in less rigorously vetted sources, such as PhD theses, conference proceedings or unpublished 'private communications'. He discounted such discoveries. "There are numbers people take for granted where one really shouldn't," says Thoennessen.

One person who is pleasantly surprised by the ranking is Münzenberg. He was aware that he had made a large contribution, including discovering elements 107-112 and some of their isotopes, but he hadn't realized that he would be top in the world. "I didn't know how many we'd made," he says. GSI Darmstadt is expected to announce a further 60 isotopes in the next year, and as equipment Münzenberg designed is still in use he may well remain ahead for the foreseeable future.

This article is reproduced with permission from the magazine Nature. The article was first published on October 4, 2011.

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