Physicists Probe Supernova Reactions; Proposed Facility Would Extend Research

Research at Argonne will help astronomers zero in on supernova remnants, the plumes of gas that mark the death agonies of giant stars. This study only scratches the surface of what will be possible with a proposed new Argonne facility called the Rare Isotope Accelerator.

Astronomers suspect many supernovae remnants lie unseen in the center of our Milky Way galaxy, where only gamma rays can penetrate intervening clouds of dust and gas and reveal their presence. But they only know of two. In an attempt to find more, an international effort is sending the European Space Agency’s “Integral” orbiting gamma-ray telescope aloft in 2001. How far into our galaxy it will be able to detect supernova remnants hinges on an exotic material called titanium-44 and recent research conducted at Argonne.

SUPERNOVA INFERNO
A supernova occurs when a large star uses up all its fuel and suddenly blows off most of its outer layers, radiating more power than a billion suns for a short time. In the supernova’s inferno, lighter elements “melt” into alpha particles, clusters of two protons and two neutrons. As the explosion expands and cools, these alpha particles begin forming heavier elements, including titanium-44. This form of titanium has 22 neutrons, as opposed to the 24 in the nuclei of the nearest stable titanium isotope. Titanium-44 decays gradually into calcium-44, which emits gamma rays, a high-frequency form of light, with exactly 1.157 million electron volts (MeV) of energy.

“That energy is a fingerprint of this particular isotope,” said Ernst Rehm of Argonne’s Physics Division, who led the team conducting the experiment. “If you find a source of gamma rays in the sky at 1.157 MeV, you’re looking at a young supernova remnant.”

The amount of titanium-44 produced in a supernova is governed by a subtle interplay between the reactions that produce it and those that destroy it, Rehm said. He and his team studied a reaction predicted to be the biggest destroyer of titanium-44. In this reaction, a nucleus of titanium-44 absorbs an alpha particle and releases a proton, to form vanadium-47. It was the first time this reaction was ever experimentally measured.

TEAM EFFORT
The titanium-44 was produced at Argonne’s Intense Pulsed Neutron Source and processed by Argonne’s Analytical Chemistry Group. The sample was loaded into the Argonne Tandem Linac Accelerator System (ATLAS) ion source, and a beam of titanium-44 ions was fired at ATLAS’s gas cell target. Behind the target lay the Fragment Mass Analyzer, a sensitive instrument that captured the reaction products and detected the vanadium-47 the reactions produced.

RESULTS
The experiment showed the alpha-capture process agreed with predictions at higher beam energies. But at the critical lower energies, the titanium-destroying process was more efficient than expected. Supernovae, it turns out, probably produce about 25 percent less titanium-44 than previously thought.

Supernova remnants aren’t as bright in gamma-ray light. The Integral satellite may see fewer supernova remnants than had been hoped for, and it will miss older objects in which the titanium-44 has decayed beyond detectability. However, this study can aid design of future spaceborne gamma-ray telescopes.

RARE ISOTOPE ACCELERATOR
This is just one type of experiment that can be accomplished with a facility capable of producing and accelerating intense beams of unstable, or “exotic” nuclei. Since the early 1990s, U.S. nuclear physicists have discussed constructing a world-class facility to produce and accelerate beams of short-lived nuclei. They proposed building a facility called the Rare Isotope Accelerator, or RIA.

RIA’s design would allow scientists to create and immediately accelerate heavy and short-lived nuclei—those with half-lives of fractions of a second. Currently physicists are limited to species like titanium-44, which exist long enough to be transported from the facility where they are made to the experiment site. With RIA, beams of ions of all nuclear species will be available at 10,000 times more intensity than is currently available and with a wide range of energies.

With intense beams of short-lived nuclei from RIA, physicists could explore:

• The nature of nucleonic matter – Detailed looks at highly unstable nuclei with large excesses of protons or neutrons will provide new insight into the nature of interacting nuclear systems and the forces in the nuclear environment.
  
  
• The origin of the elements – Nuclear reactions in stars play a critical role in the history of the universe. All of the heavy elements, which form the basis for the diversity of life, are produced in these stellar cauldrons. The energy released in these reactions is crucial for the origin and sustenance of life on Earth.
  
  
• Tests of the Standard Model – Physicists’ understanding of fundamental subatomic particles and the forces that act on them can be tested with new precision with RIA.

RIA may also play a role in developing new nuclear medicines and techniques. A RIA facility could create more isotopes with shorter half-lives, enabling doctors to apply existing nuclear medicine techniques to other conditions and diseases.

NEW TECHNOLOGIES
Argonne is one of several potential sites for RIA. Physicists and engineers are developing the technologies needed to make the machine a reality.

The Rare Isotope Accelerator will be based on the powerful superconducting linear ATLAS accelerator, which uses superconductors — materials that conduct electricity without resistance — for increased beam power with lower energy costs. RIA could be built as an ATLAS extension, saving millions in construction costs. Combined with ATLAS, the new facility could accelerate both light and heavy ions to produce an unprecedented variety of isotopes. Argonne physicists developed a “fast gas catcher” to supply high-quality exotic beams of any element in the periodic table. The catcher magnetically separates energetic exotic ions produced in thin targets and brings them to rest in a “catcher cell” filled with pressurized helium. Argonne scientists are developing new target technologies, such as a liquid lithium target based on Argonne fusion power research. These new techniques will be needed to deal with the intense beam energies RIA will provide.

For more information please contact Dave Jacque at 630-252-5582

Next: Neutrons Reveal Secrets of How Water Forms Cages around Gas Molecules