Radioactive molecules are sensitive to subtle nuclear phenomena and might
help physicists probe the violation of the most fundamental symmetries of
nature.
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 (July 7, 2021) 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-antimatter 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.”
Reference:
S.M. Udrescu, A.J. Brinson, R.F. Garcia Ruiz, K. Gaul, R. Berger, J. Billowes,
C.L. Binnersley, M.L. Bissell, A.A. Breier, K. Chrysalidis, T.E. Cocolios,
B.S. Cooper, K.T. Flanagan, T.F. Giesen, R.P. de Groote, S. Franchoo, F.P.
Gustafsson, T.A. Isaev, A. Koszorus, G. Neyens, H.A. Perrett, C.M. Ricketts,
S. Rothe, A.R. Vernon, K.D.A. Wendt, F. Wienholtz, S.G. Wilkins, X.F. Yang.
Isotope Shifts of Radium Monofluoride Molecules. arXiv.org, May 21, 2021; [abstract]
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