A 40-year-old mystery of why mercury nuclei change shape dramatically has been solved by combining data from experiments done at the ISOLDE isotope separator at CERN with supercomputer calculations done in Japan.
Removing just one proton or neutron from an atomic nucleus can sometimes transform it from a sphere to a prolate spheroid that resembles a rugby ball. While such abrupt transitions are rare, they offer physicists the opportunity to study how changes in neutron and proton numbers drive shape changes in nuclei.
The isotopes of mercury are one example where the presence or absence of just one neutron affect the nuclear shape a lot. More than 40 years ago, in one of the first experiments at ISOLDE, physicists discovered that mercury isotopes with 101, 103 and 105 neutrons have strongly prolate spheroidal shapes, while most other isotopes in the 96-136 neutron range are spherical.
Exactly why this occurs has remained unclear for decades because it is extremely difficult to create and study the isotopes. Limits on available computing power also meant that theorists struggled to calculate the properties of these nuclei.
Laser ionization
Now, ISOLDE researchers have looked at mercury isotopes with up to 181 neutrons to gain a better understanding of why the shape-shifting occurs. Using laser ionization spectroscopy, mass spectrometry and nuclear spectroscopy techniques, they found the point at which adding extra neutrons does not result in dramatic changes in shape.
Nuclear physics goes pear-shaped
Using these new insights into shape-shifting, theoretical physicists in Japan used supercomputers to do computationally-intensive calculations of the nuclear shell model that provide important insights into the phenomenon. The computations suggest that the shape shifting is related to the excitation of four protons to higher energies than had been considered in previous studies. The four protons combine with eight neutrons to create the rugby-ball shape – which is the lowest energy state for isotopes with 101, 103 and 105 neutrons.
“It is only now, with new developments of ISOLDE’s Resonance Ionisation Laser Ion Source, and by joining forces with other ISOLDE teams, that we have been able to examine the nuclear structure of these isotopes,” says CERN’s Bruce Marsh who is lead author of a paper in Nature Physics that describes the work.
