Entry distribution, fission barrier, and formation mechanism of 254102No

Masses and Beta-decay Studies of Neutron-rich Nuclei using the X-array and Gammasphere

Properties of neutron-rich nuclei in the A˜160 region are important for achieving a better understanding of the nuclear structure in this region where little is known owing to diffculties in the production of these nuclei at the present nuclear physics facilities. These properties are essential ingredients in the interpretation of the rareearth peak at A˜160 in the r process abundance distribution, since theoretical models are sensitive to nuclear structure input. Predicated on these ideas, we have initiated a new experimental program at Argonne National Laboratory. During the first experiment, beams from the Californium Rare Isotope Breeder Upgrade radioactive beam facility were used in conjunction with the SATURN decay station and the X-array. We focused initially on several odd-odd nuclei, where β decays of both the ground state and an excited isomer were investigated. Because of the spin difference, a variety of structures in the daughter nuclei were selectively populated and characterized based on their decay properties. Mass measurements using the Canadian Penning Trap aimed at establishing the excitation energy of the β-decaying isomers were also carried out. Evidence was found for a change in the single-particle structure, which in turn results in the formation of a sizable N=98 sub-shell gap at large deformation. Results from the first experimental campaign using the newly-commissioned β-decay station at Gammasphere are also presented.

Fission barrier of superheavy nuclei and persistence of shell effects at high spin: cases of 254No and 220Th

We report on the first measurement of the fission barrier height in a heavy shell-stabilized nucleus. The fission barrier height of 254No is measured to be Bf=6.0±0.5  MeV at spin 15ℏ and, by extrapolation, Bf=6.6±0.9  MeV at spin 0ℏ. This information is deduced from the measured distribution of entry points in the excitation energy versus spin plane. The same measurement is performed for 220Th and only a lower limit of the fission barrier height can be determined: Bf(I)>8  MeV. Comparisons with theoretical fission barriers test theories that predict properties of superheavy elements.

Spectroscopy of actinide and transactinide nuclei

The role of isomers for nuclei with Z ≥ 100 is discussed. Recent advances in experimental instrumentation leading to combined in-beam gamma ray and conversion electron spectroscopy are discussed. The rotational spectra of nuclei with Z ≥ 94 and their moments of inertia are discussed. Examples for in-beam spectroscopy leading to the discovery and identification of isomers are given in 248,250Fm. Here some attention is given to the assignment of nuclear configurations from g-factors measured via the branching ratios of coupled bands built on the isomers. A full list of the longest lived isomers in nuclei with Z ≥ 82 is given.

Synthesis of superheavy elements by cold fusion

The new elements from Z = 107 to 112 were synthesized in cold fusion reactions based on targets of lead and bismuth. The principle physical concepts are presented which led to the application of this reaction type in search experiments for new elements. Described are the technical developments from early mechanical devices to experiments with recoil separators. An overview is given of present experiments which use cold fusion for systematic studies and synthesis of new isotopes. Perspectives are also presented for the application of cold fusion reactions in synthesis of elements beyond element 113, the so far heaviest element produced in a cold fusion reaction. Further, the transition of hot fusion to cold fusion is pointed out, which occurs in reactions for synthesis of elements near Z = 126 using actinide targets and beams of neutron rich isotopes of elements from iron to germanium.

K isomers in 254No: probing single-particle energies and pairing strengths in the heaviest nuclei

We have identified two isomers in 254No, built on two- and four-quasiparticle excitations, with quantum numbers K pi = 8- and (14+), as well as a low-energy 2-quasiparticle Kpi = 3+ state. The occurrence of isomers establishes that K is a good quantum number and therefore that the nucleus has an axial prolate shape. The 2-quasiparticle states probe the energies of the proton levels that govern the stability of superheavy nuclei, test 2-quasiparticle energies from theory, and thereby check their predictions of magic gaps.

Structure of the odd-A, shell-stabilized nucleus 253/102No

In-beam gamma-ray spectroscopic measurements have been made on 253/102No. A single rotational band was identified up to a probable spin of 39/2planck, which is assigned to the 7/2(+)[624] Nilsson configuration. The bandhead energy and the moment of inertia provide discriminating tests of contemporary models of the heaviest nuclei. Novel methods were required to interpret the sparse data set associated with cross sections of around 50 nb. These methods included comparisons of experimental and simulated spectra, as well as testing for evidence of a rotational band in the gammagamma matrix.

Conversion electron cascades in 254(102)No

The spectrum of prompt conversion electrons emitted by excited 254No nuclei has been measured, revealing discrete lines arising from transitions within the ground state band. A striking feature is a broad distribution that peaks near 100 keV and comprises high multiplicity electron cascades, probably originating from M1 transitions within rotational bands built on high K states.

Fission barriers at high angular momentum and the ground state rotational band of the nucleus 254No

We study the superheavy nucleus 254No in the framework of the Hartree-Fock-Bogoliubov approximation with the finite-range density-dependent Gogny force, at zero and high angular momentum. The properties of the ground state rotational band and the fission barriers are discussed as a function of angular momentum. We found a two-humped barrier up to spin values of (30-40)Planck's over 2pi and a one-humped barrier for higher spins. We reproduce fairly well with the binding energy, the ground state deformation, the gamma-ray energies, and the bound on the fission barrier height measured at high spin.