Physicists count subatomic particles released by the sun
COLLEGE STATION, -The sun not only radiates light all over the place, but
it also emits millions of tiny invisible particles called neutrinos. A
team of Texas A&M University physicists has reported in the journal
Physical Review C one of the most precise results about the number of
solar neutrinos by using an original approach starting a new
sub-discipline within nuclear astrophysics.
“The main puzzle about solar neutrinos,” says Carl A. Gagliardi, a
Texas A&M professor of physics and a leader of the team, “is that their
measured number has always been lower than expected. Though the main
explanation is now that neutrinos have mass (which was not originally
predicted), details about how they are produced in the sun still need to
be investigated.”
Gagliardi and his collaborators have been investigating for about six years
how neutrinos are produced in the sun. Current models predict that neutrinos
are produced in the sun through various nuclear reactions. So Gagliardi and
his collaborators studied in the laboratory the reaction for which most of
the solar neutrinos are produced, called “proton capture reaction,” by
measuring a related process, called the “proton transfer reaction.”
In the laboratory, the scientists used a device called the Momentum Achromat
Recoil Spectrometer (MARS) in which a beam of beryllium 7 nuclei coming from
Texas A&M’s cyclotron are projected onto a target of nitrogen 14 nuclei.
After the collision, a proton escapes from a nitrogen 14 nucleus and binds
to a beryllium 7 nucleus to make a boron 8 nucleus.
In the sun, when a proton and a beryllium 7 nucleus collide, they can make a
boron 8 nucleus, which in turn decays into a beryllium 8 and a neutrino.
Though the proton capture reaction is the predominant source of solar
neutrinos, it mostly occurs at the very high temperatures inside the sun
while its probability is strongly reduced in man-made nuclear reactions.
“As the probability of the capture reaction is quite small,” Gagliardi says,
“to observe it, you need large numbers of radioactive nuclei and protons,
you collide them together, you perform the reaction, and then you count the
few times that you make boron 8.”
To determine the probability of producing boron – and in turn the number of
neutrinos produced in the sun – the physicists determine the probability of
the reaction between the proton and beryllium 7 nucleus as a function of the
distance between them. This probability depends on the distribution of
protons inside boron 8, which is high for small distances and becomes
smaller at large distances.
“The distribution has the shape of a hill,” Gagliardi says, “with the
hilltop at small distances and the hillside at larger distances. Previous
experiments looked at the distribution at small distances, focusing on the
bulk of the hill. Instead I have been interested in the large distances,
using the proton transfer reaction to focus on the hillside and the bottom
of the hill.”
The probabilities at large distances are much more sensitive than those at
small distances. Indeed small changes in the probability at the center of
the distribution are barely noticed, but the same small changes can
dramatically affect the shape and size of the tail of the distribution,
Gagliardi says.
Because both approaches are not similarly sensitive to changes due to
experimental uncertainties, the new approach offers an independent insight
on the number of expected solar neutrinos.
Gagliardi and his collaborators determined the probability of the tails of
the distributions – called “Asymptotic Normalization Coefficients” – with a
10.5 percent uncertainty. Their results, published in the May issue of the
journal Physical Review C, matches most previous results and exclude values
published in 1966 by Peter D. Parker, professor of physics and astronomy at
Yale University and in 1969 by Ralph W. Kavanagh, professor emeritus of
physics at California Institute of Technology in Pasadena.
“The consensus is now that less boron 8 is produced from the sun, which in
turn reduces the number of solar neutrinos by around 20 percent compared to
previous calculations,” Gagliardi says.
These new numbers should allow scientists to improve the current model
describing how the sun produces neutrinos, and ultimately determine
neutrino’s mass with a better precision.
“A big ambiguity has been resolved,” Gagliardi says. “This new result sets
our calculations of stellar physics on a much firmer foundation, which makes
the interpretation of the results in terms of neutrino oscillations
considerably more reliable than it was before.”
By studying the proton transfer reaction of protons with beryllium nuclei,
Gagliardi has developed an approach that is now being used to study other
nuclear reactions occurring inside stars, supernova explosions and various
astrophysical environments involving radioactive nuclei.
“Our goal was to achieve around 10 percent uncertainty and we achieved 10.5
percent uncertainty,” Gagliardi says. “But at the same time we have started
a new sub-discipline within nuclear astrophysics, which was not our goal. It
is particularly rewarding to see other people pick up what you have been
doing and emulate it.”
