Atomic nuclei are intricate quantum many-body systems composed of protons and neutrons bound by nuclear forces. The synthesis and study of rare and exotic isotopes open doors to unraveling aspects of nuclear structure and dynamics that remain largely unexplored. The nuclear chart theoretically encompasses around 7,000 nuclides, but experimental evidence exists for only about 3,300, leaving a significant domain uncharted. Creating neutron-deficient isotopes, especially in the heavy actinide region, is exceptionally challenging due to their fleeting existence and the minuscule likelihood of production, often quantified by extremely low cross-section values.

The newly synthesized isotope protactinium-210 lies deep within the proton drip line, an area of the nuclear chart where nuclei have so many protons relative to neutrons that they are prone to spontaneous proton emission or alpha decay. Producing such isotopes requires precision and innovation. At the China Accelerator Facility for Superheavy Elements (CAFE2), researchers accelerated a calcium-40 ion beam to bombard a lutetium-175 target. The resulting fusion-evaporation reaction led to the creation of protactinium-210 nuclei. Despite the extremely low production cross-section of approximately seven picobarns—equating to a probability of only a few events among trillions of reactions—researchers observed 23 distinct decay events, a testament to the facility’s sensitivity and the experiment’s meticulous design.

Central to the success was the use of the gas-filled recoil separator known as the Spectrometer for Heavy Atoms and Nuclear Structure-2 (SHANS2). This state-of-the-art instrument allows for efficient separation and identification of the desired heavy isotopes from a plethora of reaction byproducts. The detection of alpha decay signals enabled precise characterization of protactinium-210’s decay properties, extending existing systematics within this proton-rich region. The experimental results showed remarkable alignment with theoretical nuclear models, especially shell model calculations that predict nuclear behavior near and beyond the proton drip line, underscoring the robustness of modern nuclear theory.

Alpha decay, an essential mode of radioactive decay for heavy proton-rich nuclei, involves the emission of an alpha particle (two protons and two neutrons) from the parent nucleus, transforming it into a different element. Measuring the half-life and decay energies of protactinium-210 provides critical benchmarks for nuclear models and helps refine our understanding of nuclear forces under extreme proton-to-neutron ratios. The extremely short half-lives on the order of milliseconds to microseconds further highlight the experimental challenges faced by the team and the necessity for advanced detection and data acquisition systems.

The ramifications of synthesizing protactinium-210 extend beyond the identification of a new isotope. This milestone demonstrates the capability of CAFE2 to explore the landscape of heavy and superheavy nuclei, paving the way for future experiments aiming to discover new elements with even higher proton numbers. The delicate balance between nuclear binding energy and repulsive forces governs the limits of nuclear existence, and pushing these boundaries informs both nuclear physics and astrophysical phenomena such as nucleosynthesis in explosive stellar environments.

This research also reflects the continuous evolution and globalization of nuclear physics, with collaborative efforts crossing institutional and geographical boundaries. In addition to IMP, partners from the University of Chinese Academy of Sciences, Advanced Energy Science and Technology Guangdong Laboratory, Shandong University, and other contributing institutions played critical roles. Such joint ventures enhance the pooling of expertise, resources, and technologies necessary for high-stakes experimental undertakings.

Given the extraordinarily low production cross-sections and ephemeral existence of isotopes like protactinium-210, each observed decay event represents an invaluable data point. The statistical accumulation of 23 decay events was achieved through persistent experimentation and demonstrates the precision of experimental apparatus and methodology. This precision is vital for establishing reliable decay chains and confirms the isotope’s identity beyond reasonable doubt, distinguishing protactinium-210 from neighboring or contaminant nuclei.

From a broader perspective, studies of rare isotopes in the neutron-deficient actinide region yield insights into the nuclear shell effects and shape coexistence phenomena at the limits of nuclear stability. Such knowledge enriches theoretical frameworks and enhances predictive capabilities about nuclei far from stability, which are often inaccessible via other experimental means. This information is instrumental for applications ranging from nuclear medicine to understanding fundamental interaction forces within matter.

The fusion-evaporation technique employed here exemplifies the sophisticated experimental approaches required in modern nuclear synthesis. Accelerating medium-mass ion beams—such as calcium-40—and bombarding heavier targets can occasionally create compound nuclei that subsequently evaporate neutrons and protons to form new isotopes. Fine-tuning beam energies, target thicknesses, and detector sensitivities is essential for maximizing yields and isolating rare reaction channels that generate exotic isotopes like protactinium-210.

Looking ahead, the capability to synthesize and study such proton-rich isotopes suggests promising avenues for charting the unknown territories of the nuclear landscape. By extending alpha-decay systematics, researchers can validate nuclear models at extreme proton-to-neutron ratios, which has implications for understanding forces within the nucleus and predicting properties of yet-undiscovered elements. These explorations contribute fundamentally to our understanding of matter and the overarching principles that govern nuclear stability and transformation.

In summary, the landmark discovery of protactinium-210 represents a significant leap in nuclear science, achieved through advanced experimental ingenuity and international collaboration. It highlights the ongoing quest to map uncharted nuclides, challenges theoretical nuclear physics to account for extreme cases, and solidifies the prowess of cutting-edge research facilities like CAFE2. As humanity continues to probe the atomic nucleus, milestones such as these pave the way to unravel deeper cosmic and subatomic mysteries.

Subject of Research: Not applicable

Article Title: Discovery of the α-emitting isotope 210Pa

News Publication Date: 29-May-2025

Web References:
https://doi.org/10.1038/s41467-025-60047-2

References:
Nature Communications, DOI: 10.1038/s41467-025-60047-2, May 29, 2025.

Image Credits: IMP

Keywords

Particle physics, Nuclear physics, Particle accelerators, Particle theory

Hydrogen isotopes have long captivated nuclear physicists, particularly those with extreme neutron richness such as hydrogen-6 (⁶H) and hydrogen-7 (⁷H). While ordinary hydrogen consists of a single proton without neutrons, isotopes like ⁶H stretch the limits of nuclear stability, containing one proton bound with five neutrons. These isotopes occupy an uncharted territory where the conventional nuclear forces and nucleon interactions are put to the test. The ability to experimentally produce and measure such nuclei provides crucial insight into the fundamental question of how many neutrons an atomic nucleus can accommodate alongside a given number of protons.

One formidable challenge in studying ⁶H arises from its fleeting existence and the scarcity of empirical data. Conflicting experimental results have left the scientific community divided over its ground-state energy, a critical parameter that reveals the strength of the binding forces within the nucleus. Addressing this, the A1 Collaboration developed an innovative experimental methodology, leveraging MAMI’s exceptional electron beam and the precision detection capabilities of three high-resolution magnetic spectrometers positioned in the A1 experimental hall.

The experiment utilized a target made from lithium-7 (⁷Li), upon which a highly focused, 855 MeV electron beam was directed. Unlike traditional electron scattering experiments that rely on ultra-thin targets intersecting a broad electron beam, this setup involved the electron beam traversing a narrow yet long lithium plate. This unconventional configuration was necessitated to maximize the probability of the rare two-step reaction essential for ⁶H formation. In the first step, the electron beam’s interaction resonantly excites a proton within the lithium nucleus, which promptly decays into a neutron and a positive pion. Subsequently, if this neutron transfers its energy to another proton in the nucleus, the reaction culminates in the creation of hydrogen-6 alongside the residual nucleus. Both the emitted pion and the proton escape the nucleus, where their detection in tandem with the scattered electron provides unmistakable experimental signatures.

A key aspect facilitating this complex experiment was MAMI’s exceptional beam quality. The electron beam’s stability and precise focus permitted the prolonged traversal of the lithium target without compromising its integrity or experimental conditions. Handling the lithium target posed additional hurdles due to its chemical reactivity, fragility, and sensitivity to temperature fluctuations. Overcoming these technical challenges was vital for sustaining a continuous measurement campaign that extended over four weeks, affirming the experiment’s meticulous design and execution.

Remarkably, the rate of ⁶H production was estimated and observed to be about one event per day, reflecting the intricate nature and rarity of the nuclear reactions involved. The simultaneous operation of all three spectrometers in coincidence mode—a rare configuration at MAMI—enabled the detection of three particles produced during the reaction, greatly enhancing the experiment’s resolution and background suppression. This precision allowed the researchers to discern a clean and robust signal corresponding to ⁶H.

The resulting data revealed a ground-state energy for hydrogen-6 that was significantly lower than many theoretical predictions. Such a low binding energy suggests unexpectedly strong interactions between neutrons in these extreme neutron-rich conditions. This finding poses a formidable challenge to prevailing nuclear models that typically underestimate the strength of multinucleon forces in such isotopes. As a result, the study not only advances experimental nuclear physics but also calls for refined theoretical frameworks capable of accommodating these nuanced interaction dynamics.

Beyond its fundamental scientific implications, this experiment highlights the importance of international collaboration and cutting-edge technology. Scientists from Fudan University in Shanghai, Tohoku University in Sendai, and the University of Tokyo contributed critical expertise, showcasing the global nature of contemporary nuclear research. The multidisciplinary efforts underscore how the synthesis of advanced accelerator facilities, innovative detector technologies, and international scientific cooperation can push the boundaries of observable nuclear phenomena.

Funding from the German Research Foundation (DFG), the European Union’s Horizon 2020 program, the National Key Research and Development Program of China, the National Natural Science Foundation of China, and the Japan Society for the Promotion of Science (JSPS) played an essential role in enabling this ambitious research. The success of the experiment underlines the vital necessity of sustained investment in scientific infrastructure and international partnerships to unravel the complexities of the atomic nucleus.

Looking forward, the ability to produce hydrogen-6 with precise control heralds new experimental possibilities. Further investigations can probe the structure and decay properties of other neutron-rich isotopes, shedding light on the neutron drip line—the boundary beyond which nuclei cannot bind additional neutrons. The refined methodologies developed here could also be adapted to explore other isotopic chains, thereby enriching our comprehension of the nuclear landscape under extreme isospin asymmetries.

In addition to expanding fundamental nuclear physics knowledge, insights derived from such studies may resonate in astrophysical contexts, particularly in understanding neutron stars and nucleosynthesis processes. The strong neutron correlations revealed in hydrogen-6 could inform models of matter under extreme densities and enrich simulations of stellar environments where such exotic nuclei transiently form.

This landmark experiment, published in the prestigious journal Physical Review Letters, represents a milestone in the quest to delineate the limits of nuclear existence and the forces that govern atomic nuclei. The collaboration’s innovative approach, meticulous execution, and consequential findings epitomize the synergy between experimental prowess and theoretical challenge—propelling the frontier of nuclear science into new and exciting realms.

Subject of Research: Hydrogen-6 isotope production and ground-state energy measurement in an electron scattering experiment.

Article Title: Measurement of 6H Ground State Energy in an Electron Scattering Experiment at MAMI-A1

News Publication Date: 22-Apr-2025

Web References: http://dx.doi.org/10.1103/PhysRevLett.134.162501

Image Credits: Ryoko Kino, Josef Pochodzalla; Tianhao Shao

Keywords

Hydrogen-6, neutron-rich isotopes, electron scattering, Mainz Microtron, MAMI, nuclear structure, multinucleon interactions, high-resolution spectrometers, lithium-7 target, nuclear physics, neutron drip line, isotope production