CHART: FRIB; NUCLEI: T. TIBBITTS22 SCIENCE NEWS | November 20, 2021T. TIBBITTSFEATURE | IN SEARCH OF EXTREME NUCLEIUp to speed FRIB’s accelerator is bent into a paper clip shape to fit the full 450-meter
length of the apparatus in the tunnel where it is housed. Forty-six cryomodules (green
boxes) contain superconducting cavities that accelerate particles. Once the ions are
accelerated, they are slammed into a target to create new isotopes. Farther down
the line, magnets separate out the specific isotopes that scientists want to study.stability, and its properties tend to get stranger.
Such exotic specimens test the limits of scien-
tists’ theories of the atomic nucleus. While a given
theory might correctly explain nuclei that are
near stability, it may fail for more unusual nuclei.
But physicists want a theory that can explain the
most unusual to the most banal.
“We would like to understand how the atomic
nucleus is built, how it works,” says theoretical
nuclear physicist Witold Nazarewicz, FRIB’s
chief scientist.A fast clip
Accelerating beams of ions in FRIB is like
herding cats.
In the beginning, “it’s just a gaggle of cats,” says
Thomas Glasmacher, FRIB’s laboratory direc-
tor. The cats meander this way or that, but if
you can nudge the unruly bunch in a particular
direction — maybe you open a can of cat food —
then the cats start moving together, despite their
natural tendency to wander. “Pretty soon, it’s a
stream of cats,” he says.
In FRIB’s case, the cats are ions — atoms with
some or all of their electrons stripped off. And
rather than cat food, electromagnetic forces get
them moving en masse.
The journey starts in one of FRIB’s two ion
sources, where elements are vaporized and ion-
ized. After some initial acceleration to get the ions
moving, the beam enters the linear accelerator,
which is what sets the particles really cruising.
The linear accelerator looks like a scaled-down
freight train — a line of
46 boxes the color of pista-
chio ice cream, each about
2.5 meters tall, of varying
lengths. But the accelera-
tor sends the beam movingmuch faster than a cargo-filled train — up to about
half the speed of light.
Within the green boxes, called cryomodules,
superconducting cavities are cooled to just a few
kelvins, a smidge above absolute zero. At those
temperatures, the cavities can accelerate the ions
using rapidly oscillating electromagnetic fields.
The chain of pistachio modules wends around the
facility in the shape of a paper clip, a contortion
necessary so that the approximately 450-meter-
long accelerator fits in the 150-meter-long tunnel
that houses it.
When the beam is fully accelerated, it’s
slammed into a graphite target. That hard hit
knocks protons and neutrons off the nuclei of the
incoming ions, forming new, rarer isotopes. Then,
the specific one that a scientist wants to study
is separated from the riffraff by magnets that
re direct particles based on their mass and elec-
tric charge. The particles of interest are then sent
to the experimental area, where scientists can use
various detectors to study how the particles decay,
measure their properties or determine what reac-
tions they undergo.
The energy of FRIB’s beam is carefully selected
for producing rare isotopes. Too much energy
would blow the nuclei apart when they collide
with the target. So FRIB is designed to reach less
than a hundredth the energy of the Large Hadron
Collider at CERN near Geneva, the world’s most
energetic accelerator.
Instead, the new accelerator’s potential rests on
its juiced-up intensity: Essentially, it has lots and
lots of particles in its beam. For example, FRIB
will be able to slam 50 trillion uranium ions per
second into its target. As a result, it will produce
more intense streams of rare isotopes than its
predecessors could.
For isotopes that are relatively easily produced,Ions enter hereLinear acceleratorNew isotopes
created
when beam
hits targetMagnets
select
desired
isotopesTo experimentsrare-isotope.indd 22rare-isotope.indd 22 11/3/21 9:26 AM11/3/21 9:26 AM