🔬 The Groundbreaking Synthesis of Berkelium-235 and Americium-231
In a significant advancement for nuclear physics, researchers at the Institute of Modern Physics (IMP) of the Chinese Academy of Sciences (CAS) have successfully synthesized two previously unobserved isotopes: berkelium-235 (235Bk) and its alpha-decay daughter americium-231 (231Am). This achievement, detailed in a recent publication in Physics Letters B, marks a pivotal expansion of the nuclear chart in the neutron-deficient actinide region. These isotopes, with atomic numbers Z=97 for berkelium and Z=95 for americium, fill critical gaps where experimental data has been scarce due to production challenges.
The neutron-deficient side of the actinide chart, characterized by fewer neutrons relative to protons, presents unique difficulties. Low fission barriers lead to rapid disintegration of nuclei, and production cross-sections are minuscule—often in the picobarn range. Yet, understanding these nuclei is essential for probing the limits of nuclear stability and refining theoretical models that predict behavior near the 'island of stability' for superheavy elements.
IMP's team, led by J.G. Wang and collaborators including international experts like A.N. Andreyev, observed three correlated alpha-decay chains, confirming the identities unequivocally. This atom-at-a-time detection technique exemplifies the precision required in modern heavy-ion research.
Experimental Methodology: Precision at HIRFL
The experiment was conducted at the Heavy Ion Research Facility in Lanzhou (HIRFL), utilizing the newly operational China Accelerator Facility for Superheavy Elements (CAFE2). Researchers bombarded a 197Au target with a high-intensity 40Ar beam, triggering the fusion-evaporation reaction 197Au(40Ar,2n)235Bk. The recoil products were then separated using the state-of-the-art gas-filled recoil separator SHANS2 (Spectrometer for Heavy Atoms and Nuclear Structure 2).
SHANS2's design optimizes transmission efficiency for heavy recoils, employing helium gas to separate fusion products from the intense primary beam based on their electronic configuration and velocity. Implanted into a position-sensitive silicon strip detector array, the isotopes were identified through sequential alpha decays: 235Bk → 231Am + α, followed by 231Am → 227Np + α, and further decays into known isotopes.
- Beam energy: Optimized for maximum cross-section in the 2n evaporation channel.
- Target thickness: Tailored to balance production yield and energy degradation.
- Detection: High-resolution alpha spectroscopy with low background.
This setup allowed measurement despite the fleeting existence of these isotopes, with 231Am exhibiting a half-life of 75+90-25 seconds.
Measured Decay Properties and Key Findings
The alpha-particle energy for 235Bk was precisely measured at 7632 ± 17 keV, while for 231Am it was 7109 ± 18 keV. These values enable plotting on alpha-decay systematics, revealing trends in Qα (alpha decay energy) versus neutron number.
Significantly, the alpha-decay branching ratio for 231Am was estimated at 17%, indicating competition with spontaneous fission or beta decay—common in this region. No evidence of exotic decay modes like cluster decay was observed, aligning with expectations for odd-A nuclei.
Systematic comparisons with theoretical mass models such as WS4+RBF show discrepancies: predicted Qα values exceed experimental by up to several hundred keV, particularly for berkelium isotopes. This highlights the need for improved models incorporating shell effects and pairing correlations.
Challenges Overcome in Neutron-Deficient Actinide Synthesis
Synthesizing isotopes like 235Bk requires overcoming minuscule production cross-sections (estimated <1 pb), compounded by low fission barriers (~5-7 MeV) that favor fission over evaporation residue survival. Neutron-deficient actinides (N ≈ 138) lie beyond the neutron drip line for lighter elements, pushing nuclear structure limits.
IMP's success stems from CAFE2's high beam intensities (up to 1013 particles/s) and SHANS2's >30% transmission efficiency for heavy recoils. Previous attempts globally yielded no observations, underscoring China's leadership in this niche.
Photo by Markus Winkler on Unsplash
- Beam purity: Critical to minimize background.
- Gas pressure optimization: Balances separation and stopping.
- Decay chain correlation: Ensures unambiguous assignment.
Implications for Theoretical Nuclear Physics
These new data constrain mass models, vital for extrapolating to uncharted superheavy regions (Z>118). Discrepancies challenge macroscopic-microscopic approaches, suggesting enhanced microscopic calculations with beyond-mean-field effects.
In the quest for the island of stability (predicted around N=184, Z=114-126), neutron-deficient actinides provide benchmarks for deformation and shell quenching. For more details, see the original publication: Physics Letters B paper.
Alpha systematics reveal odd-even staggering, indicative of pairing gaps, aiding r-process nucleosynthesis models in astrophysics.
IMP/CAS: Pioneers in Heavy Ion Research
Established in 1957, IMP has been at the forefront since HIRFL's commissioning in 1988. Over 40 new isotopes discovered, including recent Pu-227, Ac-204, and Pa-210. The Superheavy Nuclide and Nuclear Structure group excels in neutron-deficient actinides.
CAFE2, operational since 2024, boosts superheavy production by 100x over predecessors. Collaborations with JINR (Russia), JYFL (Finland) enhance global impact. Read more on IMP's achievements: CAS news release.
Advanced Facilities Driving Discoveries
HIRFL accelerates ions up to uranium at 1.7 GeV/nucleon. SHANS2, commissioned 2023, features improved dipoles and quadrupoles for better mass/charge separation.
These tools enable routine access to pb cross-sections, positioning IMP as a global leader alongside GSI (Germany) and RIKEN (Japan).
Global Context and IMP's Contributions
While superheavy hunts focus on hot fusion (e.g., SHE Factory at JINR), IMP excels in cold fusion-like reactions for neutron-deficient sides. This complements efforts toward element 120+.
China's investment in nuclear physics infrastructure supports 'Double First-Class' initiatives, fostering talent at universities like Lanzhou University partnered with IMP.
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Future Prospects and Research Directions
Upcoming: Higher-intensity beams at CAFE2 for rarer isotopes, multi-nucleon transfer reactions for deeper neutron-deficient regions. Theoretical refinements will guide targets.
This discovery inspires young physicists, with opportunities in heavy-ion research amid China's nuclear science boom.
Careers in Nuclear Physics: China's Thriving Landscape
IMP and affiliated universities offer postdoc, faculty positions in experimental nuclear physics. Skills in accelerator ops, detectors, data analysis highly valued. Explore research jobs or China academic opportunities.
- PhD programs at IMP/LZU.
- International fellowships via CAS.
- Growing demand for superheavy expertise.