New TRIUMF research from the Saint Mary's University-led IRIS group has
unveiled an unexpected shape deformation in the nucleus of helium-8 (He8),
providing further insight into the unique dynamics of how neutron-rich
nuclei take shape and maintain stability.
Published on November 10, 2021 in Physics Letters B, Proton inelastic
scattering reveals deformation in helium-8 combines high-precision and
high-statistics experimental data gathered with the IRIS spectroscopy
station and ab initio theoretical calculations undertaken by TRIUMF's Theory
Department to demonstrate a significant deformation in the arrangement of
outer neutrons in He8.
In the nuclear shell model, protons and neutrons are arranged in the nucleus
based on energy levels, or shells. Determining the precise rules of the
shell model, especially for nuclei that are rich in neutrons, helps us
better understand how elements are created in the Universe, and in what
proportions.One key characteristic is shape. With a magic number of protons
and a presumed neutron subshell, He8 has been predicted to be double
closed-shell; since all double closed-shell nuclei have been shown to have
spherical nuclei, one way to assess the neutron shell arrangement in He8 is
to ask: is it spherical?
Led by IRIS post-doctoral research fellow Matthias Holl and Principal
Investigator Rituparna Kanungo (also Professor of Physics at Saint Mary's
University, TRIUMF Affiliate Scientist, and IRIS lead), the experimental
team used the IRIS spectroscopy station to study He8 via proton inelastic
scattering, an experimental technique that involves smashing a stationary
target (made of small nuclei, like the proton in hydrogen) with a
fast-moving, energized stream of heavier nuclei.
The scatter pattern of the reaction products provides an energy
fingerprint—a complex, highly distinctive indicator of the nuclear energy
levels and arrangement of neutrons and protons. In this experiment, the team
used a beam of +2 charged He8 at 8.25A MeV, produced using TRIUMF's 520 MeV
cyclotron and impinged on a novel solid hydrogen target cooled to 4 degrees
Kelvin.
In their results, the team recorded a first excited energy state for He8 at
3.54(6) MeV—a large energy gap that supports the notion of a closed subshell
at 6 neutrons. Its probability, however, signals it to have unexpectedly
large deformation. The high-precision IRIS data were also found to be in
agreement with first principles predictions that were pursued independently
by Petr Navratil, TRIUMF Theory Department Head and acting Associate
Laboratory Director, Physical Sciences Division. Theoretical predictions
from collaborators from Oak Ridge National Laboratory, Lawrence Livermore
National Laboratory, the University of Seville and the University of
Tennessee reinforced the agreement with the data.
The theoretical predictions and the data consistency represent precise
signatures of a shape deformation in He8, pointing to a rugby ball-shaped
shell of neutrons surrounding a sphere of protons. The results indicate
deeper nuclear interactions at play and open a new paradigm for future
investigations.
"Here at TRIUMF, we can leverage the laboratory's world-leading capacity to
produce a variety of intense rare isotope beams, including He8," said
Kanungo. "But we also have the tremendous advantage of having access to a
community of highly-qualified experimental and theoretical physicists,
engineers, technicians, and others, and the unique infrastructure of IRIS,
which allows us to investigate short-lived, exotic nuclei with high
precision."
"These results are an important building-block for our understanding of
nuclear shell dynamics and the nature of matter," said TRIUMF Deputy
Director, Research Reiner Kruecken. "Further, the synergy between theory and
experiment achieved here is a prime example of how TRIUMF brings together
world-leading infrastructure, talent, and a network of academic and
experimental collaborators to produce excellent science."
Reference:
M. Holl et al, Proton inelastic scattering reveals deformation in He8,
Physics Letters B (2021).
DOI: 10.1016/j.physletb.2021.136710
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Physics