Physicists have measured the “skin” of an atom for the first time and,
perhaps unsurprisingly, it is extremely thin. The measurement may help us
understand the properties of neutron stars.
Lead-208, an isotope that contains 82 protons and 126 neutrons, has a type
of nucleus that physicists refer to as “doubly magic” because both the
protons and the neutrons are arranged neatly into shells inside the nucleus.
These shells keep the atom relatively stable and make it simpler to
experiment on, so when the PREX collaboration at the Thomas Jefferson
National Accelerator Facility in Virginia set out to measure neutron skin,
they opted to experiment on lead-208.
Because lead-208 has so many more neutrons than protons, the neutrons and
protons are only mixed together in the centre of the nucleus, with some
neutrons making up a layer around the edge. We already know the density of
protons inside the nucleus from previous experiments. As the neutron skin is
created by the interior of the nucleus being so dense it squeezes some
neutrons to the outside, measuring the thickness of this neutron layer reveals
the density of the nucleus as a whole.
“It tells us something fundamental about how nuclei are put together, and
that piece of information really tells us how difficult it is to push
neutrons into matter when there are already a lot of neutrons there, how
hard it is to make matter more dense,” says Kent Paschke at the University
of Virginia, a spokesperson for the PREX group.
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The researchers measured the thickness of the neutron skin by sandwiching a
sample of lead-208 between two diamonds and bombarding it with a powerful
beam of electrons. The way the electrons bounced off the lead revealed where
in the nucleus the neutrons were located. The researchers found that the
neutron skin is about 0.28 femtometres – 0.28 trillionths of a millmetre –
across, very slightly thicker than physicists had predicted.
Understanding this fundamental fact about nuclei could help us understand
the pressure inside neutron stars, which are mostly made of neutrons, which
may help set a limit on their size. “The physics that is responsible for the
skin of lead-208 is also responsible for the size of a neutron star,” says
Jorge Piekarewicz at Florida State University. “Gravity wants to crunch the
neutron star and make it a black hole, and something is stopping it from
collapsing – that something is the same thing that makes the neutron skin.”
Reference:
Brendan T. Reed, F. J. Fattoyev, C. J. Horowitz, and J.
Piekarewicz Implications of PREX-2 on the Equation of State of
Neutron-Rich Matter
Physical Review Letters, DOI:
10.1103/PhysRevLett.126.172503