Protons may waltz off nuclear dance floor

In an atomic nucleus, protons and neutrons presumably dance around in twosomes, like couples at a grand ball. This hypothetical pairing, which is supported by experimental evidence, enables physicists to explain numerous traits of nuclei, including why some hold together more firmly than others.

For decades, experimenters have tried to find a nuclear decay in which a proton pair sashays right out of the nuclear ballroom, still entwined. Such decays could shed light on the strengths of particle liaisons and the distribution of those strengths within the nucleus. Such details could affect theories about how elements form in stellar explosions.

Jorge Gómez del Campo of Oak Ridge (Tenn.) National Laboratory (ORNL) and his colleagues now report detecting simultaneous nuclear emissions of two protons. The events occurred under conditions in which there is no possibility that the protons could have emerged one by one.

Using a particle accelerator that can propel radioactive particles (SN: 8/7/99, p. 95:, the experimenters slammed unstable fluorine-17 ions into a polypropylene target. Collisions between fluorine and hydrogen atoms in the plastic produced energetic neon-18 ions.

Most often, the neon disintegrated back into fluorine-17 by emitting a single proton, which hit a detector. However, two protons sometimes tripped the detector at nearly the same moment. Because fluorine-17 nuclei, as formed from the neon decays, can’t eject single protons, the scientists concluded they were seeing the simultaneous emission of two protons from the neon.

The ORNL finding is “the first breakthrough,” comments Michael Thoennessen of Michigan State University in East Lansing. Previous accelerator experiments have always left open the possibility that the protons under study popped out one at a time.

In the Oak Ridge experiment, was it a random event that two protons appeared at the same time, or were they partners in the nucleus? Thoennessen notes that the recent work can’t answer that question, but planned experiments may.

Protons that were partners in a nucleus ought to exit in a similar direction, researchers say. However, the new work relied on a sensor that was too narrow to detect cases in which protons might have diverged widely, Thoennessen says. If many of the protons exit the nucleus at widely different angles, the double emissions recorded are probably chance events, like two wallflowers abandoning a disappointing dance at the same instant.

The international team hopes to clear up the situation this winter by repeating the experiment using a wider-angle sensor.

The group, including researchers from the Universidad Nacional Autónoma de México in Mexico City, presented the results at the American Physical Society’s Division of Nuclear Physics meeting in Williamsburg, Va., and also announced the unpublished findings on Oct. 12.

Emissions of partnered protons, or diprotons, were predicted 40 years ago. Means to measure their energies would provide scientists with a powerful beacon to shine into the nucleus, says ORNL team member James R. Beene.

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