Using an unprecedented combination of experimental nuclear physics and theoretical and computational modelling techniques, scientists working at the Isotope mass Separator On-Line facility (ISOLDE) nuclear physics experimental facility at CERN, the Geneva-based European Organisation for Particle Physics, have revealed the true nature of the alternating odd-even shapes of exotic mercury isotopes. The result has been published in the latest issue of “Nature Physics”.
The experiment has demonstrated and explained the unique behaviour of mercury isotopes, where the shapes of the nuclei keep shifting between that of a football and a rugby ball.
Isotopes are forms of an element that contain the same number of protons in their nuclei but different numbers of neutrons. The properties of different isotopes can be exploited in a variety of ways, including archaeological and historical dating and radiation medicine. Stable isotopes have an optimal neutron-proton ratio. However, as the number of neutrons decreases or increases, the isotope nucleus becomes unstable and its shape must change for stability. This means it will spontaneously transform itself towards a stable isotope of another element through radioactive decay. Isotopes with extreme neutron-proton ratios are typically short-lived, making them difficult to produce and study in the laboratory. ISOLDE at CERN is the only facility in the world where a wide range of exotic isotopes can be studied.
In one of the earliest experiments conducted in ISOLDE about 40 years ago, the dramatic nuclear shape staggering in the chain of mercury isotopes was seen for the first time. That result showed that although most of the isotopes with neutron numbers between 96 and 136 have spherical nuclei, those with 101, 103 and 105 neutrons have strongly elongated nuclei, the shape of rugby balls. This dramatic result was, however, difficult to believe.
This new experiment used laser ionisation spectroscopy, mass spectrometry and nuclear spectroscopy techniques to take a closer look at how, why and when these quantum phase transitions take place. The researchers not only reproduced the results of the historic experiment (observing isotopes up to mercury 181), they also produced and studied four additional exotic isotopes (atomic mass 177-180); they also discovered the point at which the shape staggering ceases and mercury isotopes return to normal isotope behaviour. Several theories have been advanced to describe what was happening, but none has been able to explain the behaviour convincingly.
Bruce Marsh of CERN explained: “Due to the extreme difficulty in producing such exotic nuclei, as well as the computational challenge of modelling such a complex system, the reasons for this shape staggering phenomenon remained unclear.” Only now, with the development of new ISOLDE’s Resonance Ionisation Laser Ion Source have scientists been able to examine the nuclear structure of these isotopes.
Using one of the world’s most powerful supercomputers, theorists in Japan performed the most ambitious nuclear shell model calculations to date and have been able to explain the shape staggering effect theoretically.
The calculations show that both nuclear shapes are possible for each mercury isotope, depending on whether it is in the ground or excited state, but most have a football-shaped nucleus in their ground state. The surprise is that the elongated rugby ball shape as the ground state for three of the isotopes.
