Radioactive Nuclear Beams

Abstract

In his News Focus article (1 Oct., p. [28][1]), Andrew Watson describes the current effort to create facilities that will produce slow nuclear beams by postacceleration of rare isotopes. The article, however, may leave the impression that the other main technique for exploring nuclear structure, based on fast direct beams from heavy-ion fragmentation, will be superseded because of “poor beam intensity.” This is definitely not the case. The greatest scientific perspective in research with radioactive beams lies in the exploration of exotic nuclear species at the limits of stability of the nucleus. Experience from the last couple of years suggests that, for this purpose, the high-energy beams from the fragmentation of heavy ions are superior to beams produced from isotope separation online (ISOL) combined with passage of sorted radioactive nuclei through a small accelerator. As a rough measure, in a study based on nuclear reactions, the same amount of information can be obtained with a fragmentation beam that is a factor of 100,000 weaker than an ISOL beam of the same isotope. Because the production rates are comparable, this implies that direct fragmentation beams will reach several mass numbers farther away from stability, to rare species produced with very small probabilities. This pioneering exploration is already in progress at existing fragment separators at major heavy-ion laboratories: the Large National Accelerator for Heavy Ions (GANIL) in Caen, France; the Center for Heavy-Ion Research (GSI) in Darmstadt, Germany; the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University; and the Institute of Physical and Chemical Research (RIKEN) in Tokyo, Japan. An upgrade now in progress here at the NSCL will by 2001 increase our beam intensity by about a factor of 1000 over the current value. Any plan for the future must have even more powerful fragmentation beams as a major, if not the main, component. For a future heavy-ion machine representing the ultimate in modern technology, the “second-generation” radioactive-beam facility referred to in Watson's article, the fragmentation beams are expected to have at least the same intensity as those based on the ISOL technique. [1]: /lookup/doi/10.1126/science.286.5437.28