Based on preliminary
experiments at ORNL's Holifield Radioactive Ion Beam Facility (HRIBF),
nuclear physicists have identified a new form of radioactivity—simultaneous
emission of two protons from the decaying nucleus of an atom. The discovery
of the protons, which may have been initially bound together in an ephemeral
helium-2 nucleus as they were emitted from a neon-18 nucleus, is significant.
It will allow physicists to better understand the strong nuclear force
that holds protons and neutrons together in the nucleus, countering
the repulsive Coulomb force that drives protons apart because of their
like charges. Information about the energy levels and other properties
of the emitted pair of protons will help scientists determine how protons
are bound together in the nucleus and how they interact with each other
and with neutrons.
"The nucleus is
sending us a message about how it is put together," says Jim Beene,
director of HRIBF, the only two-accelerator facility for producing radioactive
ion beams in the United States.
"This is the first
time that two-proton emission has been observed," says Witold Nazarewicz,
deputy director of science at HRIBF, a leading theorist in nuclear structure
physics, and a professor of nuclear physics at the University of Tennessee
at Knoxville (UTK). "Experimenters at the Holifield facility have already
discovered five radionuclides that emit single protons through decay."
Nazarewicz makes calculations to describe the decay of one-proton emitters
and is developing theory to describe two-proton emitters.
Led by George
Gomez del Campo of ORNL's Physics Division, a group of ORNL and UTK
physicists discovered two-proton emission from the decay of neon-18
nuclei formed in an experiment at HRIBF. Fluorine-17 ions in an intense,
difficult-to-produce HRIBF beam bombarded hydrogen atoms (protons) in
a polypropylene target. For the most part, protons were scattered from
the bombarded target. Once in a billion encounters, a fluorine ion captured
a proton in the target, forming neon-18.
One particular
quantum state of neon-18, with an energy just over 6 million electron
volts (MeV), was found to decay about one time out of 3000 by emitting
two protons simultaneously to form oxygen-16. The remaining 2999 times
it decayed by emitting a single proton to form fluorine-17.
 |
|
Future
experiments at the HRIBF could determine whether the neon-18 nucleus
can decay by forming an oxygen-16 nucleus and helium-2 nucleus
that breaks apart instantly into two protons, or whether the neon-18
undergoes a direct, three-body breakup into oxygen-16 and two
protons, sometimes called democratic decay.
|
"We still have
to answer a key question," Nazarewicz says. "Were the two protons leaving
the neon-18 nucleus closely coupled together to form helium-2, or were
they emitted almost independently in a direct three-body breakup into
oxygen-16 and two protons, sometimes called 'democratic' decay? Even
if the protons were emitted as a helium-2 nucleus, they would fly apart
almost instantly. Our data favor the helium-2 emission, but a further
experiment will be required to definitively distinguish between the
two possibilities."
Two-proton (helium-2)
decay was predicted in 1960 by the Russian theorist V. I. Goldanski.
In the succeeding 40 years many efforts have been made to identify this
elusive process definitively, but none have succeeded. These searches
have invariably found sequential emission of single protons, through
an intermediate state, instead of simultaneous two-proton emission (either
as helium-2 or "democratically").
"For states of neon-18
up to 6.4 MeV," Nazarewicz says, "two protons can be emitted along with
oxygen-16 only if they are emitted simultaneously. The sequential one-proton
emission process is not possible, because no appropriate intermediate
state exists."
The identification
of two-proton emissions is important, but it will be especially significant
if the emitted protons come out bound together and then separate, providing
information on how they are entangled in their original state inside
the neon-18 nucleus.
At HRIBF five single-proton
emitters have been discovered by researchers led by ORNL's Krzysztof
Rykaczewski. The proton-rich nuclides are holmium-140 (140Ho),
holmium-141m (141mHo), thulium-145 (145Tm),
lutetium-150m (150mLu), and lutetium-151m
(151mLu).
"Of the single-proton
emitters, we found that thulium-145 has the shortest half-life yet measured
for proton radioactivity," Rykaczewski says. "It decays in 3.5 microseconds
to form another radionuclide, erbium-144."
Nuclear physicists
at HRIBF also discovered that thulium-146 breaks down into erbium-145
(145Er), releasing protons of different energies. In this
case, Rykaczewski says, the observed proton fine structure offers a
tool for studying neutron states in exotic nuclei.
Rykaczewski is
looking forward this year to HRIBF's first radioactive ion beams for
nuclear structure physics research—a proton-rich, nickel-56 beam,
as well as neutron-rich beams. These beams will help nuclear physicists
explore uncharted territory as these scientists create and discover
some of the 3000 neutron-rich and proton-rich radioactive nuclides believed
to exist when conditions are right. Recently commissioned digital signal
processing electronics should help researchers reach this goal.
Beginning
of Article
Related Web
sites
ORNL's
Holifield Radioactive Ion Beam Facility
