An experiment that scattered high-energy electrons from helium-3 and tritium nuclei has provided the first evidence for three-nucleon short-range correlations. The data were taken in 2018 at Jefferson Lab in the US and further studies of these correlations could improve our understanding of both atomic nuclei and neutron stars.
Atomic nuclei contain nucleons (protons and neutrons) that are bound together by the strong force. These nucleons are not static and they can move rapidly about the nucleus. While nucleons can move independently, they can also move as correlated pairs, trios and larger groupings. Studying this correlated motion can provide important insights into interactions between nucleons – interactions that define the structures of tiny nuclei and huge neutron stars.
The momenta of nucleons can be measured by scattering a beam of high-energy electrons from nuclei. This is because the de Broglie wavelength of these electrons is smaller that the size of the nucleons – allowing individual nucleons to be isolated. During the scattering process, momentum is exchanged between a nucleon and an electron, and how this occurs provides important insights into the correlations between nucleons.
Electron scattering has already revealed that most of the momentum in nuclei is associated with single nucleons, with some also assigned to correlated pairs. These experiments also suggested that nuclei have additional momenta that had not been accounted for.
Small but important
“We know that the three-nucleon interaction is important in the description of nuclear properties, even though it’s a very small contribution,” explains John Arrington at the Lawrence Berkeley National Laboratory in the US. “Until now, there’s never really been any indication that we’d observed them at all. This work provides a first glimpse at them.”
In 2018, Arrington and others did a series of electron-scattering experiments at Jefferson Lab with helium-3 and tritium targets. Now Arrington and an international team of physicists has scoured this scattering data for evidence of short-range, three-nucleon correlations.
Studying these correlations in nuclei with just three nucleons is advantageous because there are no correlations between four or more nucleons. These correlations would make it more difficult to isolate three-nucleon effects in the scattering data.
A further benefit of looking at tritium and helium-3 is that they are “mirror nuclei”. Tritium comprises one proton and two neutrons, while helium-3 comprises two protons and a neutron. The strong force that binds nucleons together acts equally on protons and neutrons. However, there are subtle differences in how protons and neutrons interact with each other – and these differences can be studied by comparing tritium and helium-3 electron scattering experiments.
A clean picture
“We’re trying to show that it’s possible to study three-nucleon correlations at Jefferson Lab even though we can’t get the energies necessary to do these studies in heavy nuclei,” says principle investigator Shujie Li, at Lawrence Berkeley. “These light systems give us a clean picture — that’s the reason we put in the effort of getting a radioactive target material.”
Both helium-3 and tritium are rare isotopes of their respective elements. Helium-3 is produced from the radioactive decay of tritium, which itself is produced in nuclear reactors. Tritium is a difficult isotope to work with because it is used to make nuclear weapons; has a half–life of about 12 years; and is toxic when ingested or inhaled. To succeed, the team had to create a special cryogenic chamber to contain their target of tritium gas.
Analysis of the scattering experiments revealed tantalizing hints of three-nucleon short-range correlations. Further investigation is need to determine exactly how the correlations occur. Three nucleons could become correlated simultaneously, for example, or an existing correlated pair could become correlated to a third nucleon.
Three-nucleon interactions are believed to play an important role in the properties of neutron stars, so further investigation into some of the smallest of nuclei could shed light on the inner workings of much more massive objects. “It’s much easier to study a three-nucleon correlation in the lab than in a neutron star,” says Arrington.
The research is described in Physics Letters B.
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