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The nucleus inside an atom is a clump of protons and neutrons held together by the nuclear force. Most nuclei are spherical in shape, but in 1993, scientists found that one combination of protons and neutrons could exist in pear-shaped clumps, too. This nucleus was unstable, and would quickly decay into a more symmetrical sphere or rugby-ball shape.
The pear shape meant the nucleus was bulging on one side. This happened because the nuclear force was acting in different amounts in different directions, a characteristic called anisotropy. As a result, many of the nucleus’ original properties would also become stretched and physically amplified, making it easier to study previously hidden things.
A team of physicists at CERN, France, have discovered two more combinations of protons and neutrons whose nuclei are pear-shaped. They were able to use a special facility at the lab called ISOLDE to preserve the nuclei for longer, recording their properties before they broke down into simpler shapes.
Their discovery provides a boost for nuclear theory, which is still incomplete when it comes to understanding how protons and neutrons are held together. Tim Chupp, professor of physics and biomedical engineering, University of Michigan, said, “The nuclear force on neutrons and protons has the important effect that it is not completely central and thus pushes protons and neutrons into unusual places.”
The finding is also promising for the role it could play in explaining why the universe has more matter than antimatter, a strange and unique natural bias.
Dr. Chupp and his colleagues used ISOLDE ( > I sotope S eparator O n- l ine D etector ) to first create a lot of isotopes – combinations of protons and neutrons – and then separate two of interest for further study. These two corresponded to types of nuclei of the elements radium (220) and radon (224).
When they were accelerated and smashed on a target, Chupp and co. found that they would get excited to some higher energy, and then lose that energy as gamma rays. Analysing the pattern of gamma rays streaming from the target betrayed to Chupp that the nuclei they were studying were pear-shaped.
“From the nuclear physics perspective, these are nuclei that were predicted to have strong pear-shaped effects,” Dr. Chupp said.
For the first time, physicists have evidence of pear-shaped nuclei as well as more data about them thanks to Dr. Chupp and his team. This time, they can include an anisotropic nuclear force in their models for nuclear shapes. They also hope that some unanswered problems from basic physics, such as the antimatter problem, can be explained on the way.
Antimatter and matter were created in equal amounts 13.82 billion years ago. When they meet each other, they are annihilated in a flash of energy, so all the matter should have been vanquished all those years ago – but that isn’t the case.
One explanation of this anomaly requires that the nuclei of atoms have a weak electric field, detectable by a force it exerts called the electric dipole moment (EDM). Because the anisotropic nuclear force stretches the nucleus, it will make the EDM more visible and easier to be detected.
Dr. Chupp and his colleagues hope to have an answer in 2015, when ISOLDE will be upgraded and more sensitive to smaller violation of physicists’ expectations when it comes to nuclei shapes.
