A lopsided atomic nucleus may help to refine nuclear theory. The stubby pear shape, described today in Nature, may also be pointing towards new tests of particle physics that could reveal why matter became more common than antimatter in the early moments of the Universe.
Nuclei are held together by the strong nuclear force, which acts against the electrostatic repulsion that pushes protons apart. But calculating the interplay of these forces from first principles is complex, and theorists have instead devised several competing models to describe the structure of nuclei, based on empirical data and simplifying assumptions. Most nuclei are roughly spherical or rugby-ball shaped, but the models suggest that some sport a permanent bump, like a pear (and some may even be shaped like bananas or pyramids). However, the models do not quite agree about which nuclei are most likely to be pear-shaped.
Until now, only one pear-shaped nucleus had been found experimentally: radium 226, whose shape was sketched out back in 1993. That isotope was relatively easy to work with because it is long-lived. Other putatively pear-shaped peers are highly unstable and difficult to handle.
To look for more pears, Peter Butler, a physicist at the University of Liverpool, UK, and his colleagues fired a high-energy proton beam at a piece of uranium carbide in the ISOLDE isotope mass separator facility at CERN, Europe's particle-physics laboratory near Geneva, Switzerland. “A whole cauldron of isotopes is made when you splat protons onto the target,” says Butler. The team isolated two species, radium 224 and radon 220, which they then channeled into a beam and aimed at a second target. When one of those nuclei had a close encounter with a nucleus in the target, it could get excited and start spinning with extra energy that it then lost as a γ-ray.
The shape of the nucleus affects how easily it becomes excited in the close encounter. Data from γ-ray detectors showed that radon 220 vibrates between a rough sphere and a lopsided shape, but radium 224 is a genuine, fixed pear. Not an elongated conference pear, more like a short-necked comice or Anjou (see video below).
Nuclear fruit basket
With two known pear-shaped nuclei, physicists can now start to tease apart the theoretical models. The cluster model, for example, treats pears like helium nuclei stuck onto the sides of plain spheres, and predicts that the lighter isotopes of radium should be more strongly pear shaped than the heavier ones. In fact, the latest results show that radium 224 is less lopsided than radium 226, calling the cluster model into doubt. Another approach, called the mean field model, fits the observed data more closely, although not perfectly. Such nuclear models can't be tested definitively yet, but Butler and his team hope to do that when a higher energy and intensity facility called HIE-ISOLDE opens at CERN in 2015.
A second study, out this week in Physical Review Letters, highlights how different mathematical models are needed to grapple with different kinds of nuclei. At ISOLDE, a team led by Deyan Yordanov, a physicist at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, analyzed the spectrum of ultraviolet light emitted by cadmium ions, which is subtly influenced by the shape of the nucleus.
Cadmium nuclei are nearly spherical, but the team found that slight deformations can be predicted accurately by a model that describes nuclei as built up from a series of shells. Butler says that type of simple description does not work for his pear-shaped radium.
Even more enticingly, the experiments could probe basic physics. The standard model of particle physics, which describes the strong and weak nuclear forces and the electromagnetic force, leaves several basic questions unanswered. For example, it cannot fully explain why there is more matter than antimatter in the Universe. If matter and antimatter behaved in the same way, they would have almost entirely annihilated one another during the first few seconds of the Big Bang, leaving little but radiation behind.
Various ideas proposed to supplant the standard model can account for the matter bias. They also predict that some nuclei should generate a weak electric dipole field, similar to the magnetic field of a bar magnet. If that is so, pear-shaped nuclei should have the strongest electric dipoles, and measuring these could help researchers to choose between the various models. The latest result confirms that radium isotopes should be a good place to look for electric dipoles, and that some isotopes of thorium and uranium might be even better.
“I believe that this will eventually lead to results of much broader impact than this experiment alone, with the possibility of placing constraints on the standard model,” says nuclear physicist Gavin Smith of the University of Manchester, UK, who is not a member of Butler's team.