The protons and neutrons that build the nucleus of the atom frequently pair up in fleeting partnerships called short-range correlations. Correlations can form between a proton and a neutron, between two protons, or between two neutrons. Scientists recently conducted an experiment that compared the prevalence of each type of pairing in helium-3 and in tritium, an isotope of hydrogen. Helium-3 and tritium have small, light nuclei. In small nuclei, protons and neutrons are further apart than they are in larger nuclei, which have more protons and neutrons. In larger nuclei, protons and neutrons pair with the other type of particle 30 to 40 times for every one time they pair with the same type. Conversely, this experiment found that in smaller nuclei, neutrons pair up with a proton only four times for every one time that neutrons pair with another neutron.
This surprising result comes from data that are an order of magnitude more precise than previous studies. These data have revealed new details about the nuances of the short-distance interactions between protons and neutrons in nuclei. Nuclear physicists think that this shows how these interactions may vary over small distances within nuclei. In addition, short-distance interactions may also affect results from experiments seeking to tease out further details of nuclear structure at this scale.
This unique experiment was carried out at Thomas Jefferson National Laboratory's (Jefferson Lab) Continuous Electron Beam Accelerator Facility, a Department of Energy (DOE) user facility. It compared the prevalence of each type of short-range correlation in helium-3 and in tritium, an isotope of hydrogen. These nuclei are considered "mirror nuclei" because each one's proton content mirrors the other's neutron content. For instance, helium-3 has two protons and one neutron, while tritium has one proton and two neutrons. The data revealed four neutron-proton pairs for every proton-proton or neutron-neutron pair. This result is vastly different from earlier studies. Those early studies found 30 to 40 proton-neutron pairs for every same-type pair in heavier nuclei, such as carbon, iron, and lead.
One explanation for this difference is that the interactions between protons and neutrons are modified somewhat by the distance between them. This interaction has a longer-range "tensor" piece, which generates neutron-proton pairs. A shorter-range "core" piece can generate proton-proton or neutron-neutron pairs. When the protons and neutrons are farther apart in light nuclei, researchers will measure a different balance between these interactions than when they are closer together in heavier nuclei. Further research will help test this idea.
This work was supported by the Department of Energy Office of Science, Nuclear Physics and by the National Science Foundation.