It turns out atomic nuclei – the tiny soccer balls at the heart of the atoms that make us up – sometimes turn into footballs. With a small injection of energy they can become elongated shapes, as much as twice as long as they are wide.
But new measurements of a special kind of calcium nucleus, the isotope calcium-40, show that, even though the energy jump is small, nuclei don’t easily switch from one shape to another. Which surprised the scientists – even though humans tend to stick with one code of football.
Unlike the mystery of human loyalty, nuclei’s persistence with one ball-shape has now been explained: quantum interference prevents the switch.
“No one has ever seen this transition before,” said Associate Professor Tibor Kibèdi, the ANU lead on the international team, who have published their work in Physical Review Letters.
“Everybody expected that it would be a strong transition, but it is very weak.”
Nuclei are far too small for their shape to be observed directly, but subtle shifts in their energy levels can signify shape differences, a phenomenon first observed 40 years ago.
To observe a nucleus switching from one shape to another, rearranging its component protons and neutrons, is very difficult due to a quirk of quantum physics: the quantum spin of both nuclear shapes is zero. This type of shape change is known as an E0 transition: energy cannot come out as electromagnetic radiation (in contrast with the gamma rays emitted in most nuclear transitions). Instead, E0 transitions need to find less visible ways of offloading energy, for example by emitting pairs of opposite particles, an electron and a positron.
To detect these pairs, the team turned to the NCRIS-funded Heavy Ion Accelerator Facility at ANU, whose Super-E spectrometer is designed to detect this distinctive event.
“We can look at the world through electron-positron glasses,” said Associate Professor Kibèdi.
The team bombarded a target of calcium-40 with protons to give it the energy needed to change its shape, and then observed the electron-positron pairs emitted as it changed back. They found there were in fact three states – spherical, moderately elongated and extremely elongated, twice as long as wide.
Surprisingly, the transition from the most elongated state to spherical (3 to 1) was found to be very much weaker than either transition to the moderately elongated state (3 to 2 or 2 to 1).
Collaborators from Japan, led by Professor Eiji Ideguchi from Osaka University, were able to model the process and describe the experiment perfectly, finding the transition was suppressed by destructive quantum interference between nuclear states of very different deformation that coexist at similar energies.
The findings are significant because calcium-40 is a particularly stable nucleus, one of a series of nuclei called alpha conjugates that contain equal number of protons and neutrons, in multiples of 4 – that is, multiples of alpha particles (or helium nuclei) stuck together. Other alpha-conjugates include carbon-12, oxygen-16, neon-20, magnesium-24, silicon-28, sulphur-32 and argon-36. In all cases, strong E0 transitions are observed between the moderately elongated and the spherical ground state, signalling shape coexistence.
“The theory predicts all these nuclei should have super-deformed states, but nobody has seen the E0 transitions: that’s the fingerprint we are looking for,” Associate Professor Kibèdi says.
“In the helium burning cycle in the centre of the stars, alpha particles fuse together to form primarily alpha-conjugate nuclei. Therefore, our finding could have major implication the alpha-capture reactions for nuclear astrophysics.”
