Radioactive molecules are sensitive to subtle nuclear phenomena and might help physicists probe the violation of the most fundamental symmetries of nature.
By Jennifer Chu, MIT News Office
July
7, 2021 -- Imagine a dust particle in a storm cloud, and you can get an idea of
a neutron’s insignificance compared to the magnitude of the molecule it
inhabits.
But just as a dust mote might affect a
cloud’s track, a neutron can influence the energy of its molecule despite being
less than one-millionth its size. And now physicists at MIT and elsewhere have
successfully measured a neutron’s tiny effect in a radioactive molecule.
The team has developed a new technique
to produce and study short-lived radioactive molecules with neutron numbers
they can precisely control. They hand-picked several isotopes of the same
molecule, each with one more neutron than the next. When they measured each
molecule’s energy, they were able to detect small, nearly imperceptible changes
of the nuclear size, due to the effect of a single neutron.
The fact that they were able to see such
small nuclear effects suggests that scientists now have a chance to search such
radioactive molecules for even subtler effects, caused by dark matter, for
example, or by the effects of new sources of symmetry violations related to
some of the current mysteries of the universe.
“If the laws of physics are symmetrical
as we think they are, then the Big Bang should have created matter and
antimatter in the same amount. The fact that most of what we see is matter, and
there is only about one part per billon of antimatter, means there is a
violation of the most fundamental symmetries of physics, in a way that we can’t
explain with all that we know,” says Ronald Fernando Garcia Ruiz, assistant
professor of physics at MIT.
“Now we have a chance to measure these
symmetry violations, using these heavy radioactive molecules, which have
extreme sensitivity to nuclear phenomena that we cannot see in other molecules
in nature,” he says. “That could provide answers to one of the main mysteries
of how the universe was created.”
Ruiz and his colleagues have published
their results today in Physical Review Letters.
A special asymmetry
Most atoms in nature host a symmetrical,
spherical nucleus, with neutrons and protons evenly distributed throughout. But
in certain radioactive elements like radium, atomic nuclei are weirdly
pear-shaped, with an uneven distribution of neutrons and protons within.
Physicists hypothesize that this shape distortion can enhance the violation of
symmetries that gave origin to the matter in the universe.
“Radioactive nuclei could allow us to
easily see these symmetry-violating effects,” says study lead author Silviu-Marian
Udrescu, a graduate student in MIT’s Department of Physics. “The disadvantage
is, they’re very unstable and live for a very short amount of time, so we need
sensitive methods to produce and detect them, fast.”
Rather than attempt to pin down radioactive
nuclei on their own, the team placed them in a molecule that futher amplifies
the sensitivity to symmetry violations. Radioactive molecules consist of at
least one radioactive atom, bound to one or more other atoms. Each atom is
surrounded by a cloud of electrons that together generate an extremely high
electric field in the molecule that physicists believe could amplify subtle
nuclear effects, such as effects of symmetry violation.
However, aside from certain
astrophysical processes, such as merging neutron stars, and stellar explosions,
the radioactive molecules of interest do not exist in nature and therefore must
be created artificially. Garcia Ruiz and his colleagues have been refining
techniques to create radioactive molecules in the lab and precisely study their
properties. Last year, they reported on a
method to produce molecules of radium monofluoride, or RaF, a radioactive
molecule that contains one unstable radium atom and a fluoride atom.
In their new study, the team used
similar techniques to produce RaF isotopes, or versions of the radioactive
molecule with varying numbers of neutrons. As they did in their previous
experiment, the researchers utilized the Isotope mass Separator On-Line, or
ISOLDE, facility at CERN, in Geneva, Switzerland, to produce small quantities
of RaF isotopes.
The facility houses a low-energy proton
beam, which the team directed toward a target — a half-dollar-sized disc of
uranium-carbide, onto which they also injected a carbon fluoride gas. The
ensuing chemical reactions produced a zoo of molecules, including RaF, which
the team separated using a precise system of lasers, electromagnetic fields,
and ion traps.
The researchers measured each molecule’s
mass to estimate of the number of neutrons in a molecule’s radium nucleus. They
then sorted the molecules by isotopes, according to their neutron numbers.
In the end, they sorted out bunches of
five different isotopes of RaF, each bearing more neutrons than the next. With
a separate system of lasers, the team measured the quantum levels of each
molecule.
“Imagine a molecule vibrating like two
balls on a spring, with a certain amount of energy,” explains Udrescu, who is a
graduate student of MIT’s Laboratory for Nuclear Science. “If you change the
number of neutrons in one of these balls, the amount of energy could change.
But one neutron is 10 million times smaller than a molecule, and with our
current precision we didn’t expect that changing one would create an energy
difference, but it did. And we were able to clearly see this effect.”
Udrescu compares the sensitivity of the
measurements to being able to see how Mount Everest, placed on the surface of
the sun, could, however minutely, change the sun’s radius. By comparison,
seeing certain effects of symmetry violation would be like seeing how the width
of a single human hair would alter the sun’s radius.
The results demonstrate that radioactive
molecules such as RaF are ultrasensitive to nuclear effects and that their
sensitivity may likely reveal more subtle, never-before-seen effects, such as
tiny symmetry-violating nuclear properties, that could help to explain the
universe’s matter-antimmater asymmetry.
“These very heavy radioactive molecules
are special and have sensitivity to nuclear phenomena that we cannot see in
other molecules in nature,” Udrescu says. “This shows that, when we start to
search for symmetry-violating effects, we have a high chance of seeing them in
these molecules.”
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