For the first time, electron scattering experiments were used to generate and measure the highly neutron rich hydrogen isotope 6H, revealing unexpected strong interactions between neutrons within atomic nuclei.
Researchers from the A1 collaboration group of the Institute of Nuclear Physics at Johannes Gutenberg University (JGU) in Mainz, in collaboration with scientists from China and Japan, have successfully prepared hydrogen-6, one of the isotopes with the highest neutron content, for the first time using electron scattering. The experiment was conducted on the spectrometer device of the Mainz Microcelerator (MAMI), providing a new method for studying light, neutron rich atomic nuclei.
These findings provide new insights and pose significant challenges to existing models of multi nuclear interactions.
The completion of this measurement is attributed to the excellent quality of MAMI electron beams and the unique combination of three high-resolution spectrometers from the A1 collaborative project, "emphasized Professor Josef Pochodzala from the Institute of Nuclear Physics at Guggenheim University in Japan. Researchers from China and Japan participated in this experiment.

1、 The nuclear structural limit of highly neutron rich systems
One of the most fundamental questions in nuclear physics is how many neutrons can be bound to an atomic nucleus and what is the number of protons. For the basic isotope hydrogen containing only one proton, in addition to the well-known deuterium and tritium nuclei, some isotopes rich in neutrons have also been observed, ranging from 4H to 7H.
The extremely heavy hydrogen isotopes 6H (composed of one proton and five neutrons) and 7H (an additional neutron) have the highest known neutron to proton ratio to date. They are unique systems for answering this question. However, experimental data on these peculiar atomic nuclei is scarce, and the results are still controversial. Especially regarding whether the "H ground state energy" is high or low, there has always been controversy.


2、 A new method for generating hydrogen-6 in A1 collaborative experiment
The A1 collaboration group has collaborated with scientists from China and Japan to develop a new method for generating 6H. In this method, an electron beam with an energy of 855 megaelectronvolts (MeV) collides with a 7Li target, producing 6H through a two-step process: firstly, protons in the lithium nucleus undergo resonance excitation due to their interaction with electrons, and rapidly decay into neutrons and positively charged π mesons.
If the neutron subsequently transfers energy to another proton inside the nucleus, it can form the neutron rich hydrogen isotope 6H with the residual nucleus, while the π meson and proton will leave the nucleus and be detected simultaneously by three magnetic spectrometers along with scattered electrons. In order to achieve sufficient output for this rare process, electrons pass through a 45 millimeter long and 0.75 millimeter thick lithium plate along one side of the 45 millimeter long plate. This is very rare because electron scattering experiments typically use very thin target materials along the beam axis, causing the electron beam to collide with a wide surface perpendicular to its propagation direction.
This special device benefits from MAMI's excellent beam quality, especially the highly focused and stable electron beam. Another challenge is dealing with lithium itself, as lithium materials have extremely strong chemical reactivity, high mechanical brittleness, and are sensitive to temperature.
During the four week measurement activity, as expected, approximately one event was observed per day. This is a rare experiment in MAMI, where three high-resolution spectrometers in the A1 experimental hall operate simultaneously in coincidence mode, allowing for the simultaneous detection of three particles. This complex device achieves unprecedented accuracy while maintaining extremely low background noise.
The new measurement results provide a clear signal of 6H with extremely low ground state energy, indicating that the interaction between neutrons in 6H is stronger than expected by recent theoretical calculations. This result challenges our understanding of multi nuclear interactions in systems with extremely high neutron abundance.