Hydrogen-6 (⁶H), an exotic and highly neutron-rich isotope of hydrogen, consists of a single proton and five neutrons. Its study provides crucial insights into nuclear stability and the interactions between nucleons in extreme neutron-to-proton ratios.
Groundbreaking Production via Electron Scattering
Recently, in a significant advancement in experimental nuclear physics, an international team of scientists, led by the A1 Collaboration at the Institute of Nuclear Physics, Johannes Gutenberg University Mainz (JGU), successfully produced ⁶H for the first time using an electron scattering experiment. This research, conducted in partnership with researchers from China and Japan at the Mainz Microtron (MAMI) particle accelerator, employed a novel two-step process.
The method involves an 855 megaelectronvolts (MeV) electron beam impinging on a lithium-7 (⁷Li) target.
- Step 1: The interaction resonantly excites a proton within the lithium nucleus. This excited proton promptly decays into a neutron and a positively charged pion.
- Step 2: If this neutron then transfers its energy to another proton within the same lithium nucleus, ⁶H is formed along with a residual nucleus.
To detect the fleeting ⁶H, the emitted pion, the proton, and the scattered electron are measured simultaneously using three magnetic spectrometers. This setup, requiring the simultaneous operation of all three spectrometers in coincidence mode—a rare configuration at MAMI—was crucial for enhancing the experiment's resolution, suppressing background noise, and successfully identifying the ⁶H signal. The experiment required a 45-millimeter-long and 0.75-millimeter-thick lithium plate, with the electron beam traversing its longer side to achieve a sufficient production rate for this rare process. During a four-week campaign, researchers observed an average of one ⁶H event per day.
Properties and Implications
Fleeting Existence: ⁶H is an extremely unstable isotope. Older data suggests it decays through triple neutron emission with a half-life of approximately 294 yoctoseconds (2.94 x 10⁻²² seconds). Ground-State Energy: A key finding from the recent MAMI experiment is that the ground-state energy of ⁶H is significantly lower than many theoretical predictions. This low binding energy suggests unexpectedly strong interactions between neutrons under these extreme neutron-rich conditions. Challenging Nuclear Models: This result poses a considerable challenge to existing nuclear models, which often underestimate the strength of multi-nucleon forces in such isotopes. The findings call for refined theoretical frameworks that can accurately account for these nuanced interaction dynamics. High Neutron-to-Proton Ratio: With one proton and five neutrons, ⁶H has one of the highest neutron-to-proton ratios known. Studying such isotopes helps address fundamental questions about how many neutrons can be bound in an atomic nucleus with a given number of protons, pushing the limits of our understanding of nuclear stability. Astrophysical Relevance: Insights from studies on exotic nuclei like ⁶H can have implications for astrophysics, particularly in understanding neutron stars and nucleosynthesis processes. The strong neutron correlations observed in ⁶H could inform models of matter under extreme densities and improve simulations of stellar environments where such exotic nuclei might transiently form.Future Directions
This successful production and measurement of ⁶H opens new avenues for investigating the structure and decay properties of other light, neutron-rich nuclei. It will help in delineating the "neutron drip line," the boundary beyond which nuclei cannot bind additional neutrons. The refined methodologies developed in this experiment could also be adapted to explore other isotopic chains, further enriching our comprehension of the nuclear landscape under extreme conditions.
In summary, the recent production of Hydrogen-6 represents a significant milestone in nuclear physics, offering new experimental data that challenges and refines our understanding of nuclear forces and the structure of matter at its most fundamental level.