Paris

Unstable issue: there is disagreement over whether the Vivitron's stable-beam technology remains useful.

European nuclear physicists are up in arms over a decision to shut one of the world's largest van de Graaff electrostatic accelerators, the two-million-volt Vivitron in Strasbourg, France.

The researchers say that the move is premature, as the field's next generation of machines is still on the drawing board. They fear that the measure will drive some Europeans who study 'exotic' nuclei out of the field for good.

The study of exotic nuclei — which have an unstable balance of neutrons and protons, and so are prone to rapid radioactive decay — is one of the hottest areas of contemporary nuclear physics. The research is expected to yield clues as to how elements form in stars and supernovae. It could help to refine the standard model, and may even give rise to better medical isotopes.

The Vivitron is one of three facilities in Europe that generate beams of stable ions, alongside machines at Legnaro in Italy and Jyväskylä in Finland. But a more recent technology uses beams of radioactive ions, and there is a consensus in the United States, Europe and Japan that building next-generation machines that use this approach is the top priority for studying exotic nuclei.

The plan to close the Vivitron was announced by the National Institute of Nuclear and Particle Physics, part of France's national research agency, the CNRS. Daniel Guerreau, the institute's deputy scientific director, says that the decision was a simple budgetary one. “Radioactive beams are considered the priority,” he says.

But some researchers dispute the idea that existing stable beams are obsolete. They accept that, for most work on exotic nuclei, stable beams will eventually be unable to compete with radioactive-beam technology. But the first radioactive-beam machines, such as the recently opened SPIRAL in France (see Nature 416, 114; 2002), generate beams of limited intensity, and will not fulfil the requirements of some experiments.

Diverse experimental needs mean that even if the next generation of high-intensity machines is built, researchers say that a mixture of machines will still be needed. “Sometimes you have a question where you need a radioactive beam, whereas sometimes you need a stable beam,” says Herbert Hübel, a physicist at the University of Bonn.

Moreover, although planned facilities such as the Rare Isotope Accelerator proposed at the Argonne National Laboratory in Illinois, the Eurisol project, and another machine at RIKEN in Japan, would combine many of the best features of existing accelerators, they will not come online for at least eight years.

But Daniel Huss, director of the Institute of Subatomic Research, which houses the Vivitron, is unmoved by such protests. “I entirely agree with the decision,” he says. “The physics we do on Vivitron is no longer a priority.”

But the move has come under fire from Juha Äystö, chairman of the Nuclear Physics European Collaboration Committee (NuPECC), which meets under the auspices of the European Science Foundation to produce five-year plans for nuclear physics, similar to those created by the US Nuclear Science Advisory Committee (see next article).

Äystö says that researchers cannot simply move elsewhere, as beam time in Europe is already oversubscribed. “There is still very important physics to do on stable beams,” he says. NuPECC's next plan is due to be published at the end of this year, and Äystö says that he is “disappointed” that the CNRS has taken its decision without waiting for it.

Several hundred Vivitron users from outside France are now planning to challenge the closure, which they claim will hit students particularly hard. “I have eight PhD students,” one of them complains. “Where are they going to get the data to complete their theses?”