9 May 2013

The researchers’ findings, which contribute to our understanding of nuclear structure and the underlying fundamental interactions, are presented in the journal Nature today.

Most nuclei that exist naturally are not spherical, but are the shape of a rugby ball. While the latest theories are able to predict this, the same theories have predicted that for some particular combinations of protons and neutrons, nuclei can also assume very asymmetric shapes, like a pear, where there is more mass at one end of the nucleus than the other.

Until now, it has been difficult to observe pear-shaped nuclei experimentally. However, a technique pioneered at the ISOLDE facility at CERN, the European laboratory for nuclear physics research in Geneva, has been used successfully to study the shape of the short-lived isotopes Radon 220 and Radium 224.

The data show that while Radium 224 is pear-shaped, Radon 220 does not assume the fixed shape of a pear but rather vibrates about this shape. 

Nuclear physicists from the University of York played an important supporting role in the research through their experimental work using radon beams.

Co-author Dr David Jenkins, from York’s Department of Physics, said: “This is one of the highlights of research led by UK nuclear physicists in the last decade. It represents a technological triumph as the nuclei studied are the heaviest radioactive nuclei ever accelerated and only the ISOLDE facility at CERN can produce these. It also demonstrates the important role that nuclear physics can play in testing the fundamental structure of the physical world.”

The experimental observation of nuclear pear shapes is important for understanding the theory of nuclear structure and for helping with experimental searches for electric dipole moments (EDM) in atoms.

The EDM is a measure of the separation of positive and negative electrical charges in a system of charges – i.e. a measure of the charge system’s overall polarity.

The Standard Model of particle physics predicts that the value of the EDM is so small that it lies well below the current observational limit. However, many theories that try to refine this model predict EDMs should be measurable.

In order to test these theories the EDM searches have to be improved and the most sensitive method is to use exotic (highly unusual) atoms whose nucleus is pear-shaped. Quantifying this shape will therefore help with experimental programmes searching for atomic EDMs.

The research team, which included scientists from the UK, Germany, the USA, Switzerland, France, Belgium, Finland, Sweden, Poland and Spain, was led by Professor Peter Butler, from the University of Liverpool’s Department of Physics.

Professor Butler said: “Our findings contradict some nuclear theories and will help refine others. The measurements will also help direct the searches for atomic EDMs currently being carried out in North America and Europe, where new techniques are being developed to exploit the special properties of radon and radium isotopes.

“Our expectation is that the data from our nuclear physics experiments can be combined with the results from atomic trapping experiments measuring EDMs to make the most stringent tests of the Standard Model, the best theory we have for understanding the building blocks of the universe.”

At the ISOLDE facility at CERN, beams of very heavy, radioactive nuclei can be produced in high-energy proton collisions with a uranium carbide target. They are then selectively extracted using their chemical and physical properties before being accelerated to eight per cent of the speed of light and strike a target foil of isotopically pure nickel, cadmium or tin.

When this happens, the relative motion of the heavy accelerated nucleus and the target nucleus creates an electromagnetic impulse that excites the nuclei. By studying the details of this excitation process, it is possible to understand nuclear shape.

The research was supported by the Science and Technology Facilities Council (STFC).