British physicists plan to build an advanced machine to create beams of radioactive nuclei that could shine new light on astrophysics and revolutionise radiotherapy.
Sirius would be the first British machine to create radioactive beams rather than beams of stable, naturally occurring nuclei. If approved, the Pounds 65 million facility could be operating in four years.
"Sirius will be of enormous benefit to fundamental studies of nuclear physics and nuclear astrophysics and to applications in materials, biomedical science and environmental science," says Bill Gelletly of the University of Surrey. "The most valuable and exciting discoveries to emerge from it will undoubtedly be the least expected. The prospects are limited only by our imagination." Professor Gelletly recently chaired an Engineering and Physical Sciences Research Council committee to look at the possibilities for a new machine. Its report is due out later this month.
To create a beam of radioactive nuclei, the report recommends using two accelerators. The first would produce a broad range of unstable nuclei such as radioactive oxygen and uranium. The second would accelerate the nuclei up to an energy of 10 mega-electron volts per nucleon, corresponding to 200 MeV for oxygen and 2,420 MeV for uranium. When nuclei collide at such high energies, certain nuclear reactions occur that are impossible to achieve at lower energies.
The best way of producing radioactive nuclei, says the report, is to bombard various target materials with high-energy protons. A source of such protons already exists at the Rutherford Appleton Laboratory near Oxford. The plan is to build Sirius alongside it.
Sirius could help to answer some fundamental questions about nuclear physics. The different elements are defined by the number of protons in their atomic nuclei. Scientists have long been able to add neutrons to create various different isotopes of the elements. But they do not have a precise formula for the number they can add before a neutron pops out each time a new one is added.
"It is amazing that, after all this time, we still don't know what the limits of nuclear existence are," Professor Gelletly says. "Sirius will allow basic theoretical predictions of, for example, how many neutrons or protons a nucleus can hold, to be tested experimentally over a much broader range than before."
Sirius could also be used to create new elements. "It seems very esoteric at the moment but it may be that if you can make them, their properties will change and you can use them for something," says Professor Gelletly. For example, if the elements are long-lived enough, they could be used for nuclear weapons, although Professor Gelletly hopes his work will not lead to this ultimate use.
Nuclear astrophysicists want to use Sirius to understand why stars collapse and then explode as supernovae, X-ray bursters and gamma-ray bursters. "Very, very rapid nuclear transformations take place inside collapsing stars," says Alan Shotter of the University of Edinburgh. "Because the collapse is so rapid, it can build up nuclei that are very, very unstable because the elements do not have time to decay." Professor Shotter wants to recreate and study these nuclei using Sirius.
Radioactive beams produced by Sirius could ultimately have medical uses as well. "The use of radio-isotopes in radiotherapy is of real value," says Dewi Lewis of Nycomed Amersham. "The question is whether there will be better radio-isotopes in future. This sort of facility will be fundamental in providing that knowledge."
The next step is to raise the money to pay for Sirius. Professor Gelletly estimates that the total cost would be about Pounds 65 million. The price could be cut by recycling shielding and magnets from existing facilities. Assuming high-energy protons are provided by the existing machine at the Rutherford Appleton Laboratory, Sirius could be run on Pounds 5 million a year."The UK is in a unique position to provide the leading facility of this type in Europe and, indeed, the world," says Professor Gelletly.