April 2000

Cosmic bottleneck

Holifield’s fluorine-17 beam helps experimenters understand stellar processes

It’s in the stars, and Physics Division researchers are using the Holifield Radioactive Ion Beam Facility to understand exactly what is happening in those stars. Beams generated at the unique facility are recreating the processes that happen only under violent stellar conditions—and they are helping researchers understand them .

Principal investigator Jeff Blackmon, Group Leader Michael Smith, postdoctoral researcher Dan Bardayan and university collaborators just completed an experiment that uses a beam of fluorine-17, which has a nucleus that lives for only one minute. Many thought it was too difficult to create in sufficient quantities for experiments.
The reaction products oxygen-14 and helium-4 have a unique energy relationship that allows them to be easily distinguished from other reaction products. Particles from nuclear reactions are detected in a large array of silicon semiconductor detectors; the energy, trajectory and time of arrival of each particle are then measured. When two particles arrive within 100 nanoseconds of each other, the energy of one particle is plotted versus the energy of the other particle, and in the case of 0-14 and He-4, you get a distinctive streak.

Holifield’s fluorine-17 beam, says Smith, is “fantastic, a very high-quality beam that is equal in quality to widely available beams of stable nuclei. It allows us to do precision measurements with a unique, very exotic radioactive nucleus.”

Blackmon describes the experiment as an attempt to understand how elements are created in X-ray bursts and nova explosions, which are more frequent events than the headline-grabbing supernovae. “Nova explosions occur in our galaxy every year, but we would be fortunate to see even one supernova occur in our galaxy in our lifetime,” says Blackmon.

“Novae and X-ray bursts occur in binary star systems when gas is transferred from one star to another and reacts violently. The explosions occur on the surface of the star and can recur, as opposed to a supernova, in which a massive star is obliterated from the inside. The star that explodes in an X-ray burst is a neutron star, and in a nova it’s a white dwarf.”

Reactions between hydrogen and other light elements, such as carbon, start the explosion. Gradually nuclear reactions produce heavier and heavier elements. “It’s this synthesis of the elements, and the prodigious generation of energy, that we’d like to understand,” Blackmon says. “The reaction of carbon and hydrogen creates oxygen-14, a radioactive oxygen nucleus with low neutron content. Oxygen-14 is special because it can’t react with the abundant hydrogen. This suppresses the explosion for a time. For the explosion to continue, the oxygen-14 must react with helium, which is very hard to do. This is called a bottleneck at O-14. Understanding how oxygen-14 reacts with helium is very important to understanding these explosions and the elements they produce.”

Fluorine-17 and hydrogen are the two products of the reaction of oxygen-14 with helium. The chances of such a reaction occurring hinge upon the relative velocities of the particles.

“There are certain ‘magic’ velocities where, if particles collide, nuclear reactions occur much more frequently,” Blackmon says. “Certain energies make things happen. We try to find and study these magic velocities, called resonances.”

Blackmon and collaborators, along with the HRIBF operations staff, have essentially reverse-engineered the stellar process—creating the fluorine-17 beam and smashing it into a hydrogen target, a polyethylene film similar to food wrap. Those collisions create oxygen-14 and helium, which they detect in a complex array of silicon semiconductor detectors.

“Less than one in a trillion atoms will undergo this reaction,” Blackmon says. “Our beam has up to two million atoms per second, which is fantastic, and it is a very high-quality beam. We can vary the velocities of the beam and look for the magic velocities that lead to significantly enhanced probabilities for reactions.”

What Blackmon has seen is that reactions at lower energies, while less common than those at higher energies, could be more prevalent than previously believed. And that’s important because those lower energies play a more important role in X-ray bursts and novae—they are more representative of the energies of particles on the surfaces of neutron stars or white dwarfs.

“This intense fluorine-17 beam allows us to look at the lower energies,” he says.

Michael Smith says this experiment helps scientists place their models of stellar explosions on a firm empirical foundation. The facility’s success with the fluorine-17 beam is that much sweeter because most doubted is was feasible.

“In the last year the staff has increased the beam’s intensity by a factor of 100,” Smith says. “The high intensity at low energies allowed us to pull this experiment off.”

“Hits,” or reactions, at low energies occurred only a few times a day during the 24-hour-a-day run of the experiment, which began in January and closed on February 11.

Smith says the reliability of the beam and the success of the experiment is a triumph in particular for the Holifield Facility’s radioactive beam development and operations staff.

“This was a difficult experiment operationally because we had to change the beam’s energy frequently,” he says. “It was hard work for the operations staff, who were also instrumental in the years of preparation of the facility.”

Blackmon is now analyzing the data to make a final determination of the yield of this reaction as a function of energy, and Smith will put the rate into a computer model of the exploding stars to determine the astrophysical impact of their measurements.

“The beam has run almost continuously since December. It’s logged well over 900 hours of fluorine-17 beam in that time. This has been a great payoff for the years of development work.”—B.C.

The reliability of the Holifield Facility’s beam, which many thought was too difficult to create, and the success of the experiment is a triumph in particular for the facility’s development and operations staff.