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.
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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.)
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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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