How do the chemical elements, the building blocks of our universe, get
built? This question has been at the core of nuclear physics for the better
part of a century.
At the beginning of the 20th century, scientists discovered that elements
have a central core or nucleus. These nuclei consist of various numbers of
protons and neutrons.
Now, scientists at Michigan State University's Facility for Rare Isotope
Beams (FRIB) have built and tested a device that will allow pivotal insights
into heavy elements, or elements with very large numbers of protons and
neutrons. Ben Kay, physicist at the U.S. Department of Energy's (DOE)
Argonne National Laboratory, led this effort. FRIB is a DOE Office of
Science User Facility.
Kay and his team have completed their first experiment using the device,
called SOLARIS, which stands for Solenoid Spectrometer Apparatus for
Reaction Studies. Planned experiments will reveal information about nuclear
reactions that create some of the heaviest elements in our world, ranging
from iron to uranium.
Also planned are experiments with exotic isotopes. Isotopes are elements
that share the same number of protons but have different numbers of
neutrons. Scientists refer to certain isotopes as exotic because their
ratios of protons to neutrons differ from those of typically stable or
long-lived isotopes that occur naturally on Earth. Some of these unstable
isotopes play an essential role in astronomical events.
"Exploding stars, the merger of giant collapsed stars, we are now learning
details about the nuclear reactions at the heart of these events," said Kay.
"With SOLARIS, we are able to recreate those reactions here, on Earth, to
see them for ourselves."
The new device follows in the footsteps of HELIOS, the Helical Orbit
Spectrometer, at Argonne. Both use similarly repurposed superconducting
magnets from a magnetic resonance imaging (MRI) machine like that found in
hospitals. In both, a beam of particles is shot at a target material inside
of a vacuum chamber. When the particles collide with the target, transfer
reactions occur. In such reactions, neutrons or protons are either removed
or added from nuclei, depending on the particles, and their energies, used
in the collision.
"By recording the energy and angle of the various particles that are
released or deflected from the collisions, we are able to gather information
about the structure of the nuclei in these isotopes," said Kay. "The
innovative SOLARIS design provides the necessary resolution to enhance our
understanding of these exotic nuclei."
What makes SOLARIS truly unique is it can function as a dual-mode
spectrometer, meaning it can make measurements with either high or very low
intensity beams. "SOLARIS can operate in these two modes," explained Kay.
"One uses a traditional silicon detector array in a vacuum. The other uses
the novel gas-filled target of the Active-Target Time-Projection Chamber at
Michigan State, led by SOLARIS team member and FRIB senior physicist Daniel
Bazin. This first experiment tested the AT-TPC." The AT-TPC enables
scientists to use weaker beams and still collect results with the needed
high accuracy.
The AT-TPC is essentially a large chamber filled with a gas that serves as
both the target for the beam and the detector medium. This differs from the
traditional vacuum chamber that uses a silicon detector array and a
separate, thin, solid target.
"By filling the chamber with gas, you are ensuring that the fewer, larger
particles from the low-intensity beam will make contact with the target
material," said Kay. In that way, the scientists can then study the products
from those collisions.
The team's first experiment, led by research associate Clementine Santamaria
of FRIB, examined the decay of oxygen-16 (the most common isotope of oxygen
on our planet) into much smaller alpha particles. In particular, the eight
protons and eight neutrons in oxygen-16 nuclei break up into a total of four
alpha particles, each consisting of two protons and two neutrons.
"By determining how oxygen-16 decays like this, comparisons can be made to
that of the 'Hoyle state,' an excited state of a carbon isotope that we
believe plays a key role in the production of carbon in stars," explained
Kay.
Kay and his team recorded over two million reaction events during this
experiment and observed several instances of the decay of oxygen-16 into
alpha particles.
The dual functionality of SOLARIS will allow for an even broader range of
nuclear reaction experiments than before, and give scientists new insights
into some of the greatest mysteries of the cosmos.
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