AFTER a 40-year search, physicists have finally caught a glimpse of a rare
form of radioactivity in which a nucleus spits out two protons at the same
time.
The researchers believe the protons emerge stuck together as a pair—a
unique configuration that will give a an important insight into the strong
nuclear force that glues the particles in nuclei together. “This might provide
us with a new way of looking at how nucleons interact,” says Philip Woods, a
nuclear physicist at the University of Edinburgh.
There are only a few ways in which unstable nuclei can transform themselves.
They can either split apart or chuck out one of a small repertoire of particles,
such as a neutron or a helium-4 nucleus—two protons and two neutrons.
Decades ago, nuclear theorists predicted that this list should include di-proton
decay, where two protons fly away stuck together as a helium-2 nucleus. But
researchers could never be sure they were seeing such decays.
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Physicists hope that by studying diproton decay they will learn about
the arrangement of protons inside the nucleus and how they escape as a pair. “It
could tell us something about the strength of the pairing interaction in the
strong nuclear force,” says Bertram Blank of the Centre for Nuclear Studies in
Bordeaux-Gradignan, France. “Right now we know very little about this.”
Researchers have known since the 1980s that decaying radioactive nuclei emit
two protons. But they could not confirm whether they came out at the same time,
or just very quickly one after the other. Now a team using the Holifield
Radioactive Ion Beam Facility at Oak Ridge National Laboratory in Tennessee has
found a way to make sequential emission impossible.
They did this by firing a beam of fluorine-17 atoms at a thin plastic surface
rich in hydrogen atoms. The fluorine grabs hydrogen from the plastic and is
converted into neon-18, which then decays to oxygen-16. If the neon-18 is in a
particular excited state, it is energetically impossible for it to decay by
emitting only one proton. “That’s the clever part,” says team member Alfredo
Galindo-Uribarri. “There’s no intermediate step of the ladder.” So the atoms are
forced to emit two protons at once to become oxygen-16.
There is still a possibility that the protons could be leaving simultaneously
but separately in a process called democratic emission
(see diagram). Since these
protons probably weren’t “living together” inside the nucleus, says
Galindo-Uribarri, they would tell us little about the strong nuclear force. But
his colleague Jorge Gomez del Campo has few doubts. “I’m convinced we’re seeing
helium-2 emission.”
Galindo-Uribarri says they’ll need a bigger detector—due to be up and
running early next year—to decide for sure. The team has submitted a paper
to Physical Review Letters.FIG-mg22632101.JPG