Nobelium Nucleosynthesis: Cold and Hot Fusion Pathways
The synthesis of Nobelium (No), a heavy transactinide element, represents a significant achievement in nuclear physics. Scientists utilize two primary methods of nucleosynthesis—cold fusion and hot fusion—to create various isotopes of this element. By bombarding heavy targets with specific ion beams, researchers can study the stability, decay characteristics, and isomeric states of nobelium, contributing to our broader understanding of the island of stability in the periodic table.
Cold Fusion Reactions
Cold fusion in this context refers to reactions with lower excitation energy, typically using lead (Pb) targets. A primary reaction used is Pb(Ca,xn)No, where x represents the number of neutrons emitted (1 to 4). This method has been extensively studied at the Flerov Laboratory of Nuclear Reactions (FLNR), the GSI Helmholtz Centre for Heavy Ion Research, and the Lawrence Berkeley National Laboratory (LBNL).
Research at the FLNR began as early as 1979, with subsequent measurements in 1988 and 1989 focusing on electron capture (EC) and spontaneous fission (SF)—a process where a heavy nucleus splits into two smaller nuclei—branching in nobelium isomers. In 2001, Yuri Oganessian and his team reported the 2n excitation function, which describes the probability of the reaction occurring at different energies.
Further advancements occurred at Jyväskylän Yliopisto Fysiikan Laitos (JYFL) using the RITU set-up to investigate K-isomerism (nuclear states with high angular momentum that hinder decay). Scientists identified two K-isomers with half-lives of 275 ms and 198 s, assigned to 8 and 16 K-isomeric levels, respectively.
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Specific Cold Fusion Findings
- No-252 and No-254: Studies at FLNR in 2003 and 2004–05 focused on the spectroscopy of these isotopes, confirming an isomeric level in No-254 with a half-life of 43.5 s.
- No-250: Research at GSI detected a K-isomer with spin and parity 8 and a half-life of 110 ms.
- No-252: In 2006, Argonne National Laboratory (ANL) detected activities with half-lives of 3.7 μs and 43 μs, the latter associated with a K-isomer. In 2020, FLNR identified a new isotope of nobelium via a 9.1-MeV alpha particle activity correlated to Fermium (Fm) and Californium (Cf).
Hot Fusion Reactions
Hot fusion involves reactions with higher excitation energies, often utilizing targets like Thorium (Th), Uranium (U), Plutonium (Pu), Americium (Am), Curium (Cm), and Californium (Cf). These reactions typically result in the emission of more neutrons (xn).
Uranium and Thorium Targets
The reaction U(Ne,xn)No was first explored in 1964 at FLNR. Early results identified decays from Fm-254 and Fm-256. A 10-second SF activity was tentatively assigned to No-252, and by 1966, a parent activity with a half-life of ~50 s was correctly assigned to No-252. In 1969, these reactions helped determine that nobelium is the heavier homologue of ytterbium.
Plutonium and Americium Targets
Early attempts using Pu(O,xn)No in 1958 measured alpha particles with a 30 s half-life, though later repeats suggested these results may have been due to background effects. Similarly, Am(N,xn)No reactions in 1966 detected Fm isotopes, and later work at Oak Ridge in 1977 calculated a 2.3 s half-life for No-254 with a 27% SF branching.
Curium and Californium Targets
The LBNL conducted seminal studies using Cm(C,xn)No. In 1967, isotopes No-252, No-254, and No-255 were detected. Earlier 1958 experiments using a curium target initially reported a 3 s activity assigned to No-254, which was later reassigned to No-252. In 1971, the Oak Ridge Laboratory used Cf(C,αxn)No to measure coincident Z=100 K X-rays, providing definitive confirmation of the element's discovery.
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Key Facts
- Primary Synthesis Methods: Cold fusion (Pb targets) and Hot fusion (Th, U, Pu, Am, Cm, Cf targets).
- Key Research Institutions: Flerov Laboratory (FLNR), Lawrence Berkeley National Laboratory (LBNL), GSI, and JYFL.
- Confirmation: The discovery of nobelium was confirmed via Z=100 K X-ray measurements in 1971.
- Isomeric States: K-isomers have been identified with half-lives ranging from 110 ms to 198 s.
- Decay Modes: Nobelium isotopes primarily undergo alpha decay and spontaneous fission (SF).
Summary of Nobelium Synthesis and Decay
| Reaction Type | Target/Beam | Key Isotopes/Findings | Notable Institutions |
|---|---|---|---|
| Cold Fusion | Pb + Ca | No-250, 252, 254; K-isomerism | FLNR, GSI, LBNL, JYFL |
| Hot Fusion | U + Ne | No-252; Chemical properties | FLNR |
| Hot Fusion | Cm + C | No-252, 254, 255 | LBNL |
| Hot Fusion | Cf + C | X-ray confirmation of Z=100 | Oak Ridge |
Nobelium as Decay Products
Beyond direct synthesis, nobelium isotopes appear as decay products of even heavier elements. The following observations have been recorded:
- Lawrencium (Lr): Decays into No-256 and No-259.
- Hassium (Hs), Seaborgium (Sg), Rutherfordium (Rf): Decays into No-252, No-254, and No-256.
- Rutherfordium (Rf): Decays into No-256.
Frequently Asked Questions
What is the difference between cold and hot fusion in nobelium synthesis?
Cold fusion uses lead (Pb) targets and results in lower excitation energy, while hot fusion uses heavier targets like Curium or Uranium, resulting in higher excitation energy and the emission of more neutrons.
What is a K-isomer in the context of nobelium?
A K-isomer is a long-lived nuclear excited state where the decay is slowed down due to a large difference in the projection of the total angular momentum (K) between the excited state and the lower energy states.
How was the discovery of nobelium finally confirmed?
The discovery was confirmed in 1971 at the Oak Ridge Laboratory by measuring coincident Z=100 K X-rays from No-254, which provided definitive proof of the element's atomic number.
Which isotopes of nobelium have been synthesized?
Various isotopes have been studied, including No-250, No-252, No-254, No-255, No-256, and No-259, through different fusion reactions and as decay products of heavier elements.
What is spontaneous fission (SF) branching?
SF branching refers to the probability that a nucleus will decay via spontaneous fission rather than alpha decay or other modes of radioactive decay.