Physicists found something surprising after peering into light nuclei

An atomic nucleus is a busy place.


When protons and neutrons (nucleons) are bound into atomic nuclei, they are close enough to feel significant attraction or repulsion. Strong interactions within them lead to hard collisions between nucleons.

While studying these energetic collisions in light nuclei via a new technique, physicists found something surprising: protons collide with their fellow protons and neutrons with their fellow neutrons more often than expected.

In earlier research, scientists examined energetic two-nucleon collisions in a small number of nuclei, ranging from lead (12 nucleons) to carbon (12 nucleons) (with 208). Consistent findings showed that proton-neutron collisions accounted for over 95% of all collisions, with proton-proton and neutron-neutron collisions making up the remaining 5%.

In a new experiment, physicists studied collisions in two “mirror nuclei” with three nucleons each. They found that proton-proton and neutron-neutron collisions were responsible for a much larger share of the total – roughly 20%.

An international team discovered scientists, including researchers from the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab). For the study, they used the Continuous Electron Beam Accelerator Facility at DOE’s Thomas Jefferson National Accelerator Facility (Jefferson Lab) in Virginia.

In most atomic nuclei, nucleons spend about 20% of their lives in high-momentum excited states resulting from two-nucleon collisions. Studying these collisions requires zapping nuclei with high-energy electron beams. Then, by measuring a scattered electron’s energy and recoil angle, scientists inferred the speed at which the nucleon it hit must have been moving.

John Arrington, a Berkeley Lab scientist, is one of four spokespersons for the collaboration, said, “This enables them to pick out events in which an electron scattered off a high-momentum proton that recently collided with another nucleon.”

These electron-proton collisions have an incoming electron with sufficient energy to completely remove the excited proton from the nucleus. The second nucleon also escapes the nucleus because this disrupts the rubber band-like interaction that usually holds the exciting nucleon pair in place.

Prior research on two-body collisions concentrated on scattering events where the rebounding electron and both expelled nucleons were observed. By tagging all the particles, they could determine the relative number of proton-proton pairs and proton-neutron pairs. However, as these “triple coincidence” events are exceedingly uncommon, careful consideration of any additional interactions between nucleons that can affect the count was necessary for the analysis.

Mirror nuclei boost precision

In the new study, physicists demonstrated a way to establish the relative number of proton-proton and proton-neutron pairs without detecting the ejected nucleons. Measurement of scattering from two “mirror nuclei” with the same number of nucleons—tritium, a rare hydrogen isotope with one proton and two neutrons, and helium-3, which has two protons and one neutron—was the trick. Helium-3 looks just like tritium with protons and neutrons swapped, and this symmetry enabled physicists to distinguish collisions involving protons from neutrons by comparing their two data sets.

Physicists started working on mirror nuclei after planning to develop a tritium gas cell for electron scattering experiments. This is the first use of this rare and temperamental isotope in decades.

Diagram showing a high-energy electron scattering from a correlated nucleon in the mirror nuclei tritium (left) and helium-3 (right). The electron exchanges a virtual photon with one of the two correlated nucleons, knocking it out of the nucleus and allowing its energetic partner to escape. Both nuclei have neutron-proton pairs, while tritium has an additional neutron pair and helium-3 has an additional proton pair. (Credit: Jenny Nuss/Berkeley Lab)

Through this experiment, scientists collected more data than in previous experiments. Hence, they could improve the precision of previous measurements by a factor of ten.

They didn’t have reason to expect two-nucleon collisions would work differently in tritium and helium-3 than in heavier nuclei, so the results were quite surprising.

Arrington said“Its clear helium-3 is different from the handful of heavy nuclei measured. We want to push for more precise measurements on other light nuclei to yield a definitive answer.”

Journal Reference:

  1. Li, S., Cruz-Torres, R., Santiesteban, N. et al. Revealing the short-range structure of the mirror nuclei 3H and 3He. Nature 609, 41–45 (2022). DOI: 10.1038/s41586-022-05007-2
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