Conducted at the Mainz Microtron (MAMI) accelerator, the experiment opens a new path for probing extremely neutron-rich light nuclei and challenges established nuclear interaction models. Hydrogen-6, or 6H, contains one proton and five neutrons, making it one of the most neutron-heavy hydrogen isotopes known.
"This measurement could only be carried out thanks to the unique combination of the excellent quality of the MAMI electron beam and the three high-resolution spectrometers of the A1 Collaboration," said Professor Josef Pochodzalla from JGU. The experiment was led by doctoral candidate Tianhao Shao and included scientists from Fudan University (China), Tohoku University, and the University of Tokyo (Japan).
The team targeted a longstanding nuclear physics question: how many neutrons can be stably bound to a nucleus with just a single proton. Isotopes such as 6H and 7H, which possess the highest neutron-to-proton ratios observed, provide key insights into this problem. However, previous data on these isotopes has been limited and inconclusive, particularly concerning the ground-state energy of 6H.
To produce 6H, the researchers directed an 855 MeV electron beam onto a lithium-7 target. In a two-step process, a lithium proton is first excited and decays into a neutron and a charged pion. This neutron then interacts with another proton, forming 6H, while the pion and scattered particles are detected in coincidence by three magnetic spectrometers. This detection required a specially configured lithium target plate and an unusually long beam interaction path, achievable only with MAMI's focused, stable beam.
Over a four-week campaign, researchers observed an average of one 6H event per day, consistent with predictions. The successful simultaneous operation of all three high-resolution spectrometers in coincidence mode allowed for precise multi-particle detection and very low background noise.
Crucially, the experiment confirmed a low ground-state energy for hydrogen-6, suggesting stronger-than-expected interactions among its neutrons. This challenges prevailing theoretical frameworks and adds valuable insight into the structure of neutron-rich atomic nuclei.
Research Report:Measurement of 6H Ground State Energy in an Electron Scattering Experiment at MAMI-A1
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