Production and measurement of the extremely neutron-rich hydrogen isotope ⁶H achieved for the first time in an electron scattering experiment / Result shows stronger than expected interaction between neutrons within the nucleus
30 April 2025
The A1 collaboration at the Institute for Nuclear Physics at Johannes Gutenberg University Mainz, together with scientists from China and Japan, has for the first time successfully produced one of the most neutron-rich isotopes, hydrogen-6, in an electron scattering experiment. The experiment at the spectrometer facility at the Mainz Microtron (MAMI) particle accelerator presents a new method for investigating light, neutron-rich nuclei and challenges our current understanding of multi-nucleon interactions. “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,” comments Prof. Dr. Josef Pochodzalla from the Institute for Nuclear Physics. Researchers from Fudan University in Shanghai (China), Tohoku University Sendai, and the University of Tokyo (both Japan) were involved in the experiment. The experimental work was led by doctoral student Tianhao Shao and has been published in the renowned journal Physical Review Letters.
Limits of nuclear structure in extremely neutron-rich systems
One of the most fundamental questions in nuclear physics is how many neutrons can be bound in an atomic nucleus with a given number of protons. For the fundamental isotope hydrogen, which contains only a single proton, several very neutron-rich isotopes from ⁴H to ⁷H have been observed beyond the well-known deuteron and triton. The extremely heavy hydrogen isotopes ⁶H (consisting of one proton and 5 neutrons) and ⁷H (another neutron), which have the highest neutron-to-proton ratio known so far, are unique systems to address this question. However, experimental data on these exotic nuclei are scarce, and the results remain controversial. In particular, there is a long-standing debate about whether the ground-state energy of ⁶H is low or high.
New method for generating hydrogen-6 in the A1 collaboration experiment
Together with the Chinese and Japanese scientists, the A1 collaboration developed a new approach for producing ⁶H. An electron beam with an energy of 855 megaelectronvolts (MeV) impinged on a ⁷Li target, producing ⁶H via the reaction ⁷Li(e, e'pπ⁺)⁶H, in which first a proton of the lithium nucleus is resonantly excited by the interaction with the electron and promptly decays into a neutron and a charged pion. If this neutron transfers its energy to a proton within the nucleus, it can form the neutron-rich hydrogen ⁶H together with the residual nucleus, while the pion and the proton leave the nucleus and can be detected simultaneously together with the scattered electron using three magnetic spectrometers. In order to achieve a sufficient production rate for this rare process, a 45 mm long and 0.75 mm thick lithium plate was traversed by the electron beam along the 45 mm long side. This is highly unusual, as electron scattering experiments typically use very thin targets along the beam axis, with the beam striking a broad surface perpendicular to its direction of propagation. This special setup was made possible by the excellent beam quality of MAMI – in particular by the extremely focused and stable electron beam. An additional challenge was handling the lithium itself, as the material is highly chemically reactive, mechanically fragile, and sensitive to temperature.
During a four-week measurement campaign, approximately one event per day was observed, as previously estimated. It was one of the rare experiments at MAMI in which all three high-resolution spectrometers in the A1 experimental hall were operated simultaneously in coincidence mode, so that three particles could be detected at the same time. This complex setup enabled a level of precision that had not been achieved before, while maintaining an extremely low background.
The new measurement provided a clear signal of ⁶H with a ground-state energy only about 2 MeV above the ³H+n+n+n threshold. These 2 MeV indicate how much more energy the ⁶H state has compared to a nucleus that would be stable against decay into triton (³H) and three neutrons. Such a low ground state energy indicates a stronger interaction between the neutrons in 6H than was expected according to the latest theoretical calculations – and thus challenges our understanding of multi-nucleon interactions in very neutron-rich systems.
The experiment was funded by the German Research Foundation (DFG) as part of the National Key Research and Development Program of China and by the European Union's Horizon 2020 research and innovation program. Further support was provided by the National Natural Science Foundation of China and the Japan Society for the Promotion of Science (JSPS), Japan.