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By using new experimental results from the Holifield Radioactive Ion Beam Facility in an astrophysics computer model, ORNL researchers have more accurately predicted the amounts of 87 different isotopes produced by exploding stars.

How Much Stuff is Made in Stellar Explosions? ORNL's Answer

Imagine a large explosion on the surface of a star. Trillions and trillions of swirling hydrogen and fluorine nuclei race past each other, propelled by the exploding star's soaring temperatures. In maybe one in a million close encounters, an unstable, radioactive fluorine-17 (17F) nucleus will collide with and scatter off a hydrogen nucleus, like a cue ball glancing off a smaller billiard ball. In one in a trillion close encounters, an 17F nucleus will capture a hydrogen nucleus (proton), and the fused particles will form a neon-18 (18Ne) nucleus.

Such energy-releasing nuclear reactions involving radioactive isotopes in stars are thought to be crucial to the production and dissemination of elements that sustain life on the earth. In our terrestrial environment, unstable 17F spontaneously decays within a couple of minutes to oxygen-17 (17O). However, at the high temperatures and densities in stellar explosions, there is a small probability that a nucleus of 17F can capture an additional proton before it decays, forming 18Ne. A series of such fusion reactions in stellar explosions can synthesize heavier isotopes, such as the iron that circulates in our blood.

Expanding gas ring (jpeg, 12K)
The expanding gas ring was ejected into space by a giant stellar explosion on the surface of a white dwarf star (bright spot in the center) in the galactic nova system QU Vulpeculae. The gas in the ring is glowing in the light of hydrogen atoms excited by ultraviolet radiation. This image of Nova QU Vul was obtained by an infrared camera on the Hubble Space Telescope. (Photograph courtesy of the University of Wyoming and the Space Telescope Science Institute.)

The probability that this fusion reaction will occur varies greatly with the relative velocities of the 17F nucleus and the proton. Theorists have predicted that certain relative velocities will correspond to an excited quantum state in 18Ne, where the fusion rate will be dramatically enhanced. They have also predicted the energy and lifetime of this 18Ne quantum state, but the results of nine experiments using stable nuclear beams did not support this prediction. However, in 1999, the first published data from ORNL's Holifield Radioactive Ion Beam Facility (HRIBF) showed that the Nuclear Astrophysics Research Group, led by Michael S. Smith, in ORNL's Physics Division, was the first to confirm the existence of and determine the properties of this 18Ne quantum state.

Bombarding a hydrogen-containing polypropylene target with a high-quality 17F beam from HRIBF, the team measured the number of 17F ions scattered off the target at different angles for different beam energies. The number of protons scattered at each beam energy was counted in a second detector. When they shifted from one beam energy to another, the researchers noticed a significant change in the proton scattering rate. This beam energy can be correlated with a star temperature range that may be critical for the synthesis and expulsion of particular isotopes. Postdoctoral research associate Dan Bardayan, the principal investigator on this experiment, was then able to calculate the probability that 17F ions will fuse with protons to produce 18Ne ions at various stellar temperatures.

"We determined that the properties of the neon-18 quantum state do significantly enhance the rate at which fluorine-17 nuclei fuse with hydrogen under some conditions occurring in stellar explosions," Smith says. "We recently put this reaction rate into an astrophysical model that runs on a supercomputer. It calculates the amounts of 87 different isotopes produced in the stellar explosion. When we examine the prediction using our new rate, our preliminary results indicate that a number of isotopes are produced in significantly different quantities than previously thought. The new prediction and older predictions differ in some cases by a factor of 1000."

The model of the synthesis of isotopes in nova explosions was written by postdoctoral research associate W. Raphael Hix, Smith, ORNL theoretical astrophysicist Anthony Mezzacappa, and two outside collaborators. It predicts that roughly 800 times more oxygen-18, 40 times more carbon-13, 30 times more carbon-14, 70 times more nitrogen-15, and 3 times more nitrogen-14 were produced than previously predicted. Many isotopes were produced in far smaller amounts than previously predicted, including many which are radioactive isotopes that later decay to stable isotopes. The team will continue their investigation by varying the temperatures and densities describing the explosion in their model.

It took the HRIBF staff a number of years to achieve the very difficult feat of generating a high-quality 17F beam for the 1999 experiment. The beam current was 8000 particles per second and the beam energy was 10 to 12 million electron volts. Since that experiment, they have increased the beam current by a factor of 100. ORNL physicist Jeff Blackmon led an experiment in January and February 2000 in which the more intense beam was used to study another important reaction in astrophysics.

"Our measurements are having important astrophysical implications," Smith says. "It is exciting that we can make measurements in the laboratory that help us better understand the details of what happens when stars explode."

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