About 20 years ago, Michigan State University's B. Alex Brown had an idea to
reveal insights about a fundamental but enigmatic force at work in some of
the most extreme environments in the universe.
These environments include an atom's nucleus and celestial bodies known as
neutron stars, both of which are among the densest objects known to
humanity. For comparison, matching the density of a neutron star would
require squeezing all the Earth's mass into a space about the size of
Spartan Stadium.
Brown's theory laid the blueprints for connecting the properties of nuclei
to neutron stars, but building that bridge with experiments would be
challenging. It would take years and the unique capabilities of the Thomas
Jefferson National Accelerator Facility. The facility, also known as
Jefferson Lab, is a U.S. Department of Energy Office of Science, or DOE-SC,
national laboratory in Virginia. So experimentalists got to work on a
decades-long series of studies and Brown largely returned to his other
projects.
That is, until 2017. That's when he said he started thinking about the
beautiful precision experiments run by his colleague Kei Minamisono's group
at the National Superconducting Cyclotron Laboratory, or NSCL, and in the
near-future at the Facility for Rare Isotope Beams, or FRIB. FRIB is a
DOE-SC user facility at MSU that will start scientific user operation in
early 2022.
"It's amazing how new ideas come to you," said Brown, a professor of physics
at FRIB and in MSU's Department of Physics and Astronomy.
The goal of this new idea was the same as his earlier theory, but it could
be tested using what are known as "mirror nuclei" to provide a faster and
simpler path to that destination.
In fact, on Oct. 29, the team published a paper in the journal Physical
Review Letters based on data from an experiment that took a few days to run.
This comes on the heels of new data from the Jefferson Lab experiments that
took years to acquire.
"It's quite incredible," Brown said. "You can do experiments that take a few
years to run and experiments that take a few days and get results that are
very similar."
To be clear, the experiments in Michigan and Virginia are not competing.
Rather, Krishna Kumar, a member and past chair of the Jefferson Lab Users
Organization, called the experiments "wonderfully complementary."
"A detailed comparison of these measurements will allow us to test our
assumptions and increase the robustness of connecting the physics of the
very small—nuclei—to the physics of the very large—neutron stars," said
Kumar, who is also the Gluckstern Professor of Physics at the University of
Massachusetts Amherst. "The progress made in both experiment and theory on
this broad topic underscores the importance and uniqueness of the
capabilities of Jefferson Lab and NSCL, and the future will bring more such
examples as new measurements are carried out at FRIB."
These projects also underscore the importance of theorists and
experimentalists working together, especially when tackling fundamental
mysteries of the universe. It was this type of collaboration that kicked off
the Jefferson Lab's experiments 20 years ago, and it's this type of
collaboration that will power future discoveries at FRIB.
A mirror to examine the neutron skin
One of the ironies here is that Brown hasn't spent a lot of his time working
on the two theories central to this story. Brown has published more than 800
scientific papers during his career, and the ones that inspired the
experiments at NSCL and Jefferson Lab are distinct from his other work.
"I work on many things and these are very isolated papers," Brown said.
Despite that, Brown shared them quickly. "I wrote both papers in a couple
months."
When Brown completed the draft of his 2017 theory, he immediately shared it
with Minamisono.
"I remember I was at a conference when I got the email from Alex," said
Minamisono, a senior physicist at FRIB. "I was so excited when I read that
paper."
The excitement came from Minamisono's knowledge that his team could lead the
experiments to test the paper's ideas and from the theory's implications for
the cosmos.
"This connects to neutron stars and that is so exciting as an
experimentalist," Minamisono said.
Neutron stars are more massive than our sun, yet they're only about as big
as Manhattan Island. Researchers can make accurate measurements for the mass
of neutron stars, but getting exact numbers for their diameters is
challenging.
A better understanding of the push and pull of forces inside neutron stars
would improve these size estimates, which is where nuclear physics comes in.
A neutron star is born when a very large star becomes a supernova and
explodes, leaving behind a core that is still more massive than our sun. The
gravity of this massive leftover causes it to collapse on itself. As it
collapses, the star also begins converting its matter—the stuff that makes
it up—into neutrons. Hence, "neutron star."
There's a force between the neutrons, known as the strong interaction, that
works against gravity and helps puts the brakes on the collapse. This force
is also in action in atomic nuclei, which are made up of neutrons and
particles known as protons.
"We know gravity, of course. There's no issue there," Brown said. "But we're
not so sure about what the strong interaction is for pure neutrons. There's
no laboratory on the Earth that has pure neutrons, so we make inferences
from things we see in nuclei that have both protons and neutrons."
In atomic nuclei, the neutrons stick out a teensy bit, forming a thin,
neutron-only layer that extends beyond the protons. This is called the
neutron skin. Measuring the neutron skin enables researchers to learn about
the strong force and, by extension, neutron stars.
In the Jefferson Lab experiments, researchers sent electrons hurtling at
lead and calcium nuclei. Based on how the electrons scatter or deflect from
the nuclei, scientists could calculate upper and lower limits for the size
of the neutron skin.
For the NSCL experiments, the team needed to measure how much room the
protons take up in a specific nickel nucleus. This is called the charge
radius. In particular, the team examined the charge radius for nickel-54, a
nickel nuclei or isotope with 26 neutrons. (All nickel isotopes have 28
protons, and those with 26 neutrons are called nickel-54 because the two
numbers add up to 54.)
What's special about nickel-54 is that scientists already know the charge
radius of its mirror nucleus, iron-54, an iron nucleus with 26 protons and
28 protons.
"One nucleus has 28 protons and 26 neutrons. For the other, it's flipped,"
said Skyy Pineda, a lead author on the new research paper and a graduate
student researcher on Minamisono's team. By subtracting the charge radii,
the researchers effectively remove the protons and are left with that thin
neutron layer.
"If you take the difference of the charge radii of the two nuclei, the
result is the neutron skin," Pineda said.
To measure the charge radius of nickel-54, the team turned to its Beam
Cooler and Laser Spectroscopy facility, abbreviated BECOLA. Using BECOLA,
experimentalists overlap a beam of nickel-54 isotopes with a beam of laser
light. Based on how the light interacts with the isotope beam, the Spartans
can measure the nickel's charge radius, Pineda said.
Using Brown's earlier theory, Jefferson Lab scientists needed on the order
of a sextillion electrons for a measurement, or a trillion billion
particles. Using the new theory, researchers instead need thousands, maybe
millions of nuclei. That means that measurements that once required years
can be replaced with experiments that take days.
A future of discovery built on a history of teamwork
This new research feels like the passing of a baton in a couple ways. For
one, the Jefferson Lab experiments are entering their final phase, while
FRIB stands poised to continue the exploration.
FRIB itself represents another leg of the relay. BECOLA started running at
NSCL and will continue operating at FRIB.
Each leg builds on the last and on the collective work the runners have put
in together.
Again, that formula is nothing new. It's what enabled a theorist at NSCL to
inspire and inform experiments at a world-class lab in Virginia. What stands
out about NSCL and FRIB, however, is that the user facilities are connected
to a university, letting veterans and the next generation of leaders
interact and share ideas that much sooner.
"MSU is unique in having had NSCL and now FRIB. In most cases, labs like
these aren't integrated into a university campus," said Kristian Koenig, a
postdoctoral researcher on Minamisono's team and a co-lead author on the new
paper. "It gives everyone here a great opportunity."
Joining the MSU team on the Physical Review Letters publication were
researchers from Florida State University along with the Technical
University of Darmstadt and the GSI Helmholtz Center for Heavy Ion Research
in Germany.
Reference:
Skyy V. Pineda et al, Charge Radius of Neutron-Deficient Ni54 and Symmetry
Energy Constraints Using the Difference in Mirror Pair Charge Radii,
Physical Review Letters (2021).
DOI: 10.1103/PhysRevLett.127.182503
Tags:
Physics