How Neutrons Soften Nuclear Shells express
Imagine a multi-story building where protons and neutrons live on each floor. The floors are energy levels, and usually there's a stairwell of a certain height between the sixth floor and the one above — the so-called gap. In the oxygen-20 nucleus, this gap acts as a barrier that particles find hard to cross.
But when extra neutrons move into the building, they subtly start to change the layout. Using the ACTAR TPC setup at the GANIL laboratory, physicists carefully knocked a proton out of the oxygen-20 nucleus to peek inside its structure. They saw that the gap between the sixth and seventh floors had become lower — about 5.30 MeV. That's less than you'd expect if the neutrons weren't interfering.
The measurements showed that the proton levels with l=1 are 86% and 72% occupied, respectively. It's as if some rooms on the floors are vacant while others are occupied. This picture suggests that the neutrons added to the nucleus weaken the proton framework — an effect predicted by the SFO-tls model, where tensor interactions play a key role. It acts like neighbors above and below can affect the stiffness of the floors in a building.
In the end, the building turns out to be not as monolithic as previously thought. This discovery helps us understand how nuclei with an excess of neutrons behave — for example, in the interiors of neutron stars or during supernova explosions.
Original abstract
as written by the authors arXiv:2604.24591
We present the first direct measurement of the Z=6 shell gap in the neutron-rich 20O nucleus. The one-proton removal transfer reaction 2H(20O,3He)19N has been studied using the ACTAR TPC setup at GANIL. The use of ACTAR TPC enabled the measurement of low-cross section proton-removal reactions while preserving resolution. Eight p-hole states with l=1 were identified in 19N accounting for total strengths of 86% and 72% of the 0p3/2 and 0p1/2 single-particle orbitals, respectively. The energies and spectroscopic factors of the measured states allowed to determine the proton spin-orbit splitting 0p3/2 - 0p1/2 in 20O. The Z=6 shell gap has been established to be 5.30(14) MeV. These findings indicate a reduction of the Z=6 shell gap while adding neutrons to the sd-valence orbitals, consistent with the effects of the tensor force predicted by state-of-the-art shell model interaction SFO-tls while at variance with the emergence of a large Z=6 gap observed in other studies.
Oxygen-20 lives for only 13.5 seconds, but in that time physicists manage to study its internal structure.
In Peter Watts's novel Blindsight, alien spacecraft use exotic nuclear structures — possibly with unusual shell effects.