Research using accelerated beams of short-lived nuclei has grown into a major activity. Karsten Riisager reports from a recent symposium in Copenhagen that looked at the status and future prospects of the field.
If you have access to a high-energy accelerator or a nuclear reactor it is relatively easy to produce a lot of radioactive nuclei. It is a much harder job to include selectivity so that just one specific isotope among those produced reaches detectors. This is particularly true for short-lived nuclei with half-lives of a second or less, or for nuclei with very low abundances. Spatial separation of production and detection sites is necessary, and the practical solution is to bring nuclei of interest from one site to the other in the form of a radioactive beam. Following a pioneering experiment carried out in 1951 at the Niels Bohr Institute (NBI) in Copenhagen, several ways have been developed to do this. A symposium celebrating the NBI experiment, and taking the opportunity to assess current state-of-the-art techniques, was held at the Royal Danish Academy in Copenhagen in November 2001.
Otto Kofoed-Hansen and Karl Ove Nielsen were the authors of NBI’s first experiment. Their basic intent was to measure the recoil momentum resulting from the emission of a neutrino in beta decay. The best way to do this is with noble gas atoms, so Kofoed-Hansen and Nielsen set out to collect neutron-rich krypton isotopes produced in the fission of uranium. Their technique was similar in principle to the converter technique currently being suggested for use in the next generation of radioactive ion-beam facilities. Deuterons from the NBI cyclotron generated neutrons in an internal target. These bombarded an external uranium oxide target in which baking powder was mixed to create a gas flow out of the target. The krypton atoms produced flowed towards a nearby isotope separator. They were ionized, mass separated and then collected, allowing the decay measurements to be done.
Soon after, the NBI cyclotron was moved to a new area and the experiment closed. The idea was taken up again a decade later and finally led to the creation of the ISOLDE facility at CERN where experiments began in 1967. Nielsen was again actively involved in the start-up phase. Both he and Kofoed-Hansen, who continued in weak interaction physics, were active at CERN for many years.
Radioactive beam techniques
One of the two major techniques used for producing radioactive beams today, isotope separation online (ISOL), is a direct descendant of the NBI experiment. At the symposium, Juha Äystö of CERN and the University of Jyväskylä discussed the widespread use of ISOL around the world today. In an ISOL system, the nuclei produced are thermalized and extracted through an ion guide if they are still in an ionized state, or through an ion source if they are not. Target and ion source technology is a key element of ISOL systems, as Helge Ravn of CERN pointed out. Current developments are leading towards ever more selective systems and targets capable of coping with very high power. The hope is to go from present generation targets, which can take up to 30 kW, to targets that are able to withstand up to or above 1 MW. The converter method with neutrons offers one possible approach. An alternative was discussed by Alex Mueller of Orsay, who suggested using an electron beam converted into bremsstrahlung photons to induce photofission. Conventional fission in a high-flux reactor is also being explored in the Munich Accelerator for Fission Fragments (MAFF) project presented by Dietrich Habs of Garching.
The other major technique for production of a radioactive beam is separation in flight. At low energies, fusion (and transfer) reactions dominate – this is the way superheavy elements are made – but at energies above 100 MeV per nucleon, projectile fragmentation and fission are the important reaction mechanisms. The rather thick production targets currently in use lead to very efficient use of the primary beam, explained Brad Sherrill of Michigan State University, but the radioactive beam produced is quite extended in phase space.
Many experiments can still be done, but stopping in a gas cell and reacceleration to achieve better beam quality is being investigated at several laboratories, as outlined by Piet van Duppen of Leuven. This is one of the key ingredients in the US Rare Isotope Accelerator (RIA) project discussed by Jerry Nolen of Argonne (see Climbing out of the nuclear valley). Two major projects to upgrade existing fragmentation facilities were discussed. Isao Tanihata of the Japanese institute of physical and chemical research, RIKEN, presented his laboratory’s radioactive beam factory upgrade, and Walter Henning of GSI in Darmstadt presented his laboratory’s upgrade project.