In collaboration with an international team of researchers, Michigan State
University has helped create the world’s lightest version, or isotope, of
magnesium to date.
Forged at the National Superconducting Cyclotron Laboratory at MSU, or NSCL,
this isotope is so unstable, it falls apart before scientists can measure it
directly. Yet this isotope that isn’t keen on existing can help researchers
better understand how the atoms that define our existence are made.
Led by researchers from Peking University in China, the team included
scientists from Washington University in St. Louis, MSU and other
institutions.
“One of the big questions I’m interested in is where do the universe’s
elements come from,” said Kyle Brown, an assistant professor of chemistry at
the Facility for Rare Isotope Beams, or FRIB. Brown was one of the leaders
of the new study, published online Dec. 22 by the journal Physical Review
Letters.
“How are these elements made? How do these processes happen?” asked Brown.
The new isotope won’t answer those questions by itself, but it can help
refine the theories and models scientists develop to account for such
mysteries.
Earth is full of natural magnesium, forged long ago in the stars, that has
since become a key component of our diets and minerals in the planet’s
crust. But this magnesium is stable. Its atomic core, or nucleus, doesn’t
fall apart.
The new magnesium isotope, however, is far too unstable to be found in
nature. But by using particle accelerators to make increasingly exotic
isotopes like this one, scientists can push the limits of models that help
explain how all nuclei are built and stay together.
This, in turn, helps predict what happens in extreme cosmic environments
that we may never be able to directly mimic on or measure from Earth.
“By testing these models and making them better and better, we can
extrapolate out to how things work where we can’t measure them,” Brown said.
“We’re measuring the things we can measure to predict the things we can’t.”
NSCL has been helping scientists worldwide further humanity’s understanding
of the universe since 1982. FRIB will continue that tradition when
experiments begin in 2022. FRIB is a U.S. Department of Energy Office of
Science, or DOE-SC, user facility, supporting the mission of the DOE-SC
Office of Nuclear Physics.
“FRIB is going to measure a lot of things we haven’t been able to measure in
the past,” Brown said. “We actually have an approved experiment set to run
at FRIB. And, we should be able to create another nucleus that hasn’t been
made before.”
Heading into that future experiment, Brown has been involved with four
different projects that have made new isotopes. That includes the newest,
which is known as magnesium-18.
All magnesium atoms have 12 protons inside their nuclei. Previously, the
lightest version of magnesium had 7 neutrons, giving it a total of 19
protons and neutrons — hence its designation as magnesium-19.
An illustration of magnesium-18 with 12 red orbs representing protons and
six blue orbs representing neutrons.
To make magnesium-18, which is lighter by one neutron, the team started with
a stable version of magnesium, magnesium-24. The cyclotron at NSCL
accelerated a beam of magnesium-24 nuclei to about half the speed of light
and sent that beam barreling into a target, which is a metal foil made from
the element beryllium. And that was just the first step.
“That collision gives you a bunch of different isotopes lighter than
magnesium-24,” Brown said. “But from that soup, we can select out the
isotope we want.”
In this case, that isotope is magnesium-20. This version is unstable,
meaning it decays, usually within tenths of a second. So the team is on a
clock to get that magnesium-20 to collide with another beryllium target
about 30 meters, or 100 feet, away.
“But it’s traveling at half the speed of light,” Brown said. “It gets there
pretty quickly.”
It’s that next collision that creates magnesium-18, which has a lifetime
somewhere in the ballpark of a sextillionth of a second. That’s such a short
time that magnesium-18 doesn’t cloak itself with electrons to become a
full-fledged atom before falling apart. It exists only as a naked nucleus.
In fact, it’s such a short time that magnesium-18 never leaves the beryllium
target. The new isotope decays inside the target.
This means scientists can’t examine the isotope directly, but they can
characterize tell-tale signs of its decay. Magnesium-18 first ejects two
protons from its nucleus to become neon-16, which then ejects two more
protons to become oxygen-14. By analyzing the protons and oxygen that do
escape the target, the team can deduce properties of magnesium-18.
“This was a team effort. Everyone worked really hard on this project,” Brown
said. “It’s pretty exciting. It’s not every day people discover a new
isotope.”
That said, scientists are adding new entries every year to the list of known
isotopes, which number in the thousands.
“We’re adding drops to a bucket, but they’re important drops,” Brown said.
“We can put our names on this one, the whole team can. And I can tell my
parents that I helped discover this nucleus that nobody else has seen
before.”
Reference:
Jin Y, Niu CY, Brown KW, et al. First observation of the four-proton unbound
nucleus 18Mg. Phys Rev Lett. 2021;127(26):262502.
DOI: 10.1103/PhysRevLett.127.262502