Physicists from the Jefferson Lab Hall A Tritium Collaboration have compared the nuclei of helium and hydrogen isotopes to each other to get a clearer view of how the components of the nuclei are arranged and found that there’s still more to learn about the heart of matter.
“We want to study nuclear structure, which is basically how protons and neutrons behave inside a nucleus,” said Dr. Reynier Cruz-Torres, a postdoctoral researcher at Lawrence Berkeley National Lab.
“To do that, we can measure any nucleus that we want. But to do a high-precision test of nuclear theory, we are limited to light nuclei that have precision calculations. Measuring these light nuclei is a benchmark for understanding nuclear structure in general.”
Dr. Cruz-Torres and colleagues focused on two of the simplest and lightest nuclei in the Universe: helium and hydrogen.
They focused on the light, stable isotope of helium called helium-3 and a rare and radioactive isotope of hydrogen called tritium.
Helium-3 contains two protons and one neutron, while tritium contains one proton and two neutrons.
“They are mirror nuclei,” said Dr. Florian Hauenstein, a postdoctoral researcher at Old Dominion University and MIT.
“So, you can assume that the protons in helium-3 are basically the same as the neutrons in tritium and vice versa.”
By comparing these relatively simple nuclei, the physicists can get a window into the strong nuclear interactions that can’t be duplicated elsewhere. These nuclei are excellent examples for comparing with the state-of-the-art theories that describe the basic structures of different nuclei.
“The theory calculations have been there for a while, but we didn’t know how good they are,” said Dr. Dien Nguyen, a postdoctoral researcher at MIT and Jefferson Lab.
“So, with this research, we are able to quantitatively say how good the calculation is. I think that is a really important step.”
To make the comparison, the physicists measured both nuclei in high-precision experiments in the Continuous Electronic Beam Accelerator Facility at Jefferson Lab.
Electrons from CEBAF were aimed at the nuclei of tritium and helium-3, where some interacted with the nuclei’s protons. The struck protons and the interacting electrons were then captured and measured in spectrometers.
“We use the spectrometers to study the properties of those final-state particles and look back to the nucleus and try to understand what was happening inside the nucleus before the reaction took place,” Dr. Cruz-Torres said.
The scientists then compared the full range of data from the experiments to theory calculations on the structures of the nuclei of helium-3 and tritium.
They found that the data generally matched theory well for both nuclei to the precision allowed by experiment. However, differences were also observed relative to some of the calculations, indicating that further refinements in the theoretical treatments are required.
“We expected that the helium-3 calculations at the end would easily match the data, but it actually turned out that the tritium cross section fit very well the theory calculation, and the helium-3 not so much through the whole range. So, we need to go back and look at helium-3,” Dr. Hauenstein said.
“Before, we thought we had a very good understanding of the calculations,” Dr. Nguyen said.
“But now, the result is what is driving us to do an even more detailed measurement, because we want to make sure that we have a good agreement with the theory.”
The results were published in the journal Physical Review Letters.
R. Cruz-Torres et al (Jefferson Lab Hall A Tritium Collaboration). 2020. Probing Few-Body Nuclear Dynamics via 3H and 3He (e,e′p)pn Cross-Section Measurements. Phys. Rev. Lett 124 (21): 212501; doi: 10.1103/PhysRevLett.124.212501
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