Rare isotope science may get a boost from design for new radiation-resistant magnet

With research facilities around the world investing in their capabilities to create never-before-seen nuclei, next-generation nuclear science promises to be a golden age of rare isotope research. Yet getting from here to there means solving a host of technical challenges associated with building new, more powerful accelerators. Now, one of the more daunting obstacles – designing a magnet that can handle high-intensity environments – appears to have been cleared away by NSCL researchers.

One way to produce rare isotopes is to aim a beam of accelerated atomic nuclei at a thin target. Many of these speeding nuclei pass around or through the target. Among those nuclei that pass through, a scant few gain or lose protons and neutrons, thus creating new rare isotopes. This so-called fast-beam method is especially useful in creating and studying extremely short-lived isotopes that help to define the limits of nuclear stability. The alternative method – a seconds-long process of extracting at-rest isotopes from a thick, irradiated target – is impractical for measuring extremely rare isotopes, many of which have lifetimes measured milliseconds.

The fast-beam method requires use of powerful magnets to aim and focus the beam at the target. Magnets also are used to thin out the beam downstream from the target. More specifically, by tuning the magnetic field, researchers are able to sift the few novel, sought-after nuclei from among a riot of other particles.

Cross-section of the Cable-in-Conduit-Conduction technology at the core of NSCL's prototype superconducting magnet. The technology resists heat and radiation and scales well to larger dimensions. Courtesy Jonathan DeLauter. more

In new accelerator facilities, the magnets will have to be able to operate in intense radiation environments marked by massive fluxes of neutrons severe enough to wear out even the best-insulated conventional superconducting magnets in just months. Even though scientists have known for years that such magnets might be properly designed and shielded to handle high-radiation environments, a precise engineering blueprint has remained elusive.

Working with several collaborators in the laboratory, Jonathan DeLauter, NSCL research and development physicist, and Al Zeller, head of the NSCL research facilities department, developed a prototype superconducting magnet robust enough to be used in new, high-beam-intensity facilities. The magnet, described in the June issue of IEEE Transactions on Applied Superconductivity, contains two new features. First, the magnet sufficiently insulates the superconducting coils from the damaging effects of radiation. Second, the conductors pass through a conduit capable of removing excess heat. This keeps them at or near extra-cold temperatures similar to those in outer space, which is required of all superconducting systems.

Roughly the size of a large shoebox, the prototype magnet that performed so well in preliminary tests is only a fraction of the size of the magnets that will eventually be needed in new facilities. However, the researchers took care in selecting materials and making design choices such that, in the future, the dimensions could be increased dramatically without affecting the magnet’s overall performance.

“Everything in the magnet fabrication process can be scaled up to meter-long magnets, which is not possible for other technologies of equivalent radiation resistance,” writes DeLauter, who worked on the magnet project during his graduate studies at NSCL.

Given the required magnetic field strengths and ambitious scientific reach of next-generation isotope science facilities “superconducting magnets are the only option,” wrote Zeller in a 2006 paper that introduced the concept for the new magnet.

At present, two variations of superconductors – one of which operates at a slightly higher temperature than the other – are leading candidates for use in new facilities. “In either case, the design presented here will be useful,” wrote Zeller, who also served as conference chair of the 2006 Applied Superconductivity Conference.

The U.S. National Science Foundation and the heavy-ion research facility Gesellschaft für Schwerionenforschung in Darmstadt, Germany provided funding for the research. Brookhaven National Laboratory, the Plasma Science and Fusion Center at the Massachusetts Institute of Technology, and Tyco Thermal Controls made important technical contributions.

NSCL is a world-leading laboratory for rare isotope research and nuclear science education.

- Geoff Koch, May 10, 2007