New measurements of the surface of atomic nuclei have thrown up a puzzle about how they behave.

A set of reactions used to measure properties of neutrons in similar forms of carbon and nitrogen nuclei, performed at ‘gentle’ energies, show a nearly 50 percent discrepancy with knockout experiments that are like a break in snooker that causes a single ball to shoot out of the pack. 

Despite using beams travelling at around 10 percent of the speed of light, these so-called transfer reactions are termed gentle because the particles in the beam only graze by the target particles, allowing the study of reactions on the surface of nuclei.

“It’s the first time we’ve been able to reliably make a direct comparison between transfer reactions and knockout reactions in systems that are weakly held together.”

“This discrepancy was not expected and shows that there are important questions relating to nuclear structure and reactions that we don’t know the answers to.” said Dr AJ Mitchell, ANU nuclear physicist and co-author of the paper in Physical Review Letters.

Understanding the ways that nucleons – protons and neutrons – interact inside nuclei is key to explaining many nuclear processes, such as radioactive decay, how stars create the chemical elements and the properties of neutron stars.

However, the interactions between up to 300 protons and neutrons inside a nucleus form a quantum many-body problem that is extremely difficult to solve, even with the current best super-computers.

Instead of calculations, the international team, led by Dr Benjamin Kay from Argonne National Laboratory in the US, used experiments with the HELIOS spectrometer of Argonne’s ATLAS User Facility to probe nuclei, thanks to a unique particle beam made of a cocktail of elements.

Initially the team planned to use a carbon-14 beam to make and study carbon-15, a radioactive isotope. However their technical staff pointed out that there would be impurities of nitrogen-14 in the carbon-14 beam, so nitrogen-15 would be created as well as carbon-15.

Realising the reaction species were similar but had important differences – especially that carbon-15 is weakly bound and radioactive, where nitrogen-15 is not – they seized the serendipity and went for a 50:50 mixture of the two nuclei.

This beam was collided with a target of polyethylene enriched with deuterium, a heavy isotope of hydrogen, with which the incident nuclei reacted to become carbon-15 and nitrogen-15.

“It’s a clever technique,” said Dr Kay. “It effectively removes many of the systematic errors associated with this kind of measurement.”

Yet, instead of narrowing down the numbers, the results are significantly at odds with previous measurements of the energy needed to remove a particle from a nucleus.

Within a nucleus, collisions between the particles alter the energies and orbits of nucleons, changing their energy compared with how a single particle would behave. This reduction, or quenching, is represented by the parameter R.

Based on existing knock-out reaction data, the team expected the quenching to differ significantly between the stable nitrogen-15 and the loosely bound carbon-15. Instead they measured almost identical values (R ~ 0.6) for the two species with their gentle transfer reactions.

Knockout reactions had previously found a much higher value for loosely bound nuclei (R ~ 1), but had good agreement with transfer reactions for stable nuclei.

“It was thought that, despite the large different in energies involved, the two reaction mechanisms probed similar aspects of nuclei,” Dr Mitchell said. 

“Clearly, there are essential features of either the reaction mechanisms or underlying physics that are being overlooked.

“These exciting features of nuclei that continue to emerge give us glimpses of ideas of what's going on, but we’re really only scratching the surface"