Rare isotope research is accelerating

Argonne physicists have figured out a way to turn the beam of ions produced by the proposed Rare Isotope Accelerator from a trickle to a torrent. Their technique quintuples beam intensity at a minimal increase in cost — and they are looking at ways to double the intensity yet again.

The Rare Isotope Accelerator, or RIA, will enable physicists to explore the nature of nuclei — the clusters of particles that occupy the center of every atom. RIA will produce intense beams of short-lived nuclei 10,000 times more intense than any now available. Physicists will use these beams to study the origin of the elements and test current models of physics. RIA also holds promise for important applications to medicine, industry and other applied physics research.

The Argonne-developed concept has been approved by a U.S. Department of Energy advisory committee. Michigan State University and others are involved with Argonne in the design and prototyping work.

By raising the intensity of RIA’s ion beam, scientists can conduct experiments more quickly and get higher-quality data from them. Argonne physicists have found a way to increase beam intensity by accelerating more than one "charge state" of ions at a time.

In conventional machines, a sample of an element, uranium for example, is heated until some of the atoms’ electrons get excited enough to leave the atom. This gives the atom a positive charge and a new name: ion. Magnetic fields are used to accelerate the ions into a "stripper," a thin carbon foil, which removes even more electrons. The stripping produces a range of charge states: uranium atoms with positive charges of +25, +26 and so on.

In most accelerators, only ions of one charge state are accelerated to the experiment areas. Other charge states are considered "parasitic" since they are not needed at the beam intensities required for conventional experiments, and there is no practical application for them.

"Up until now, there was no need for such a concentrated beam," said Argonne physicist Peter Ostroumov, who developed the technique with colleagues Jerry Nolen and Ken Shepard. "And at any rate, it wasn’t thought it could be done at a reasonable cost." But to conduct experiments at the cutting edge of nuclear physics, where ions last only fractions of a second before they decay, RIA must produce extremely intense beams — ions cannot be wasted.

STRIPPING ELECTRONS
In recent experiments at the Argonne Tandem-Linac Accelerator System, or ATLAS, researchers started with atoms of uranium-238, which pack 92 electrons. Twenty-six electrons were removed from the atoms in the source; the resulting atoms, with a +26 charge state, were accelerated in ATLAS to 286 million electron volts (MeV), then sent through a stripper.

Normally, ions of just one charge state would be selected from the range of charge states emerging from the stripper. By carefully tuning the accelerator’s magnetic fields, Argonne physicists were able to select ions with several charge states near +38 and further accelerate them in the ATLAS "booster" linear accelerator, or linac. RIA’s driver linacs will require picosecond precision to ensure the beam continues from section to section over the machine’s length of 100 meters. (A picosecond is a trillionth of a second.)

MULTI-CHARGE BEAM INCREASES FINAL BEAM INTENSITY
The multi-charged uranium beam was stripped again at the exit of the booster linac. Magnetic fields were again used, this time to select only uranium atoms with a charge of +51, which were accelerated up to 1.4 billion electron volts (GeV) and used for an unrelated, scheduled experiment. The use of a multi-charge uranium beam increased the intensity of the final beam five-fold.

Argonne physicists are also conducting tests that can double beam intensity once again by accelerating two charge states right from the ion source. Design and computer modeling confirmed that the technique is possible. Experiments are under way to confirm the findings.

BUT WHERE?
Argonne is well-positioned to be the host site for RIA, based on the laboratory’s pathbreaking and enduring expertise in advanced accelerator technology. Argonne has a long history in groundbreaking accelerator design: its ATLAS accelerator was the world’s first superconducting heavy-ion accelerator for physics research. Much of ATLAS can be incorporated into the new, larger accelerator, reducing the facility’s cost by nearly $100 million.

Argonne scientists are also hard at work developing other new technologies needed for RIA, including a novel helium "catcher" cell to capture short-lived reactive ions and a liquid-lithium experimental target that will be able to withstand the high-energy-density RIA beam.

For more information please contact Dave Jacqué