31 research outputs found
Probing the subshell closure: factor of the Mg(2) state
The first-excited state ~factor of Mg has been measured relative to
the factor of the Mg() state using the high-velocity
transient-field technique, giving . This new measurement is in
strong disagreement with the currently adopted value, but in agreement with the
-shell model using the USDB interaction. The newly measured factor,
along with and systematics, signal the closure of the subshell at . The possibility that precise -factor
measurements may indicate the onset of neutron admixtures in first-excited
state even-even magnesium isotopes below Mg is discussed and the
importance of precise excited-state -factor measurements on ~shell
nuclei with to test shell-model wavefunctions is noted.Comment: 8 pages, 5 figure
Three-quasiparticle isomers and possible deformation in the transitional nuclide, 195Au
Deep-inelastic reactions and γ-ray spectroscopy have been used to study excited states in 195Au. A three-quasiparticle isomer with a mean-life of 18.6(3) μs has been assigned at 2461+Δ keV, with decays into newly identified structures. Possible configurations for the isomer are discussed including a Jπ =31/2- intrinsic state produced by coupling the 11/2-[505] proton hole to the 10+ state obtained from the 9/2+[624],11/2+[615] two-neutron holes, expected from configuration-constrained potential-energy-surface calculations. Residual interactions are evaluated within a semi-empirical shell model basis. New lifetime information is also obtained for the 21/2+ and 25/2 + states at 1813 and 1980 keV. The relationship between these and other newly identified states and the negative-parity states in the even-even neighbors is discussed
Isomers and alignments in 191Ir and 192Os
Deep-Inelastic reactions have been used to populate high-spin states in the even-even osmium isotopes and in the iridium neighbors. New isomers have been identified in 190Os, 192Os, 194Os, 191Ir and 193Ir. These include a 2 ns 12+ state at 2865 keV and a 295 ns, 20+ state at 4580 keV in 192Os. Although a number of multi-quasiparticle states arising from prolate and triaxial deformations are expected in these nuclei, the main structures in 192Os can be interpreted as a two-stage alignment of i13/2 neutrons at oblate deformation, in close analogy with similar structures in the isotones 194Pt and 196Hg. The isomers are attributed to low-energy E2 transitions at the point of the alignment gains. The isomer observed in 191Ir is long-lived (τm ∼8s) and probably arises from coupling of the h11/2 proton to the 10 -ν/9/2- [505]11/2+ [615] prolate configuration that gives rise to long-lived isomers in 190Os and 192Os, although potential-energy-surface calculations indicate that the resultant three-quasiparticle state will be triaxial
Impact of triaxiality on the rotational structure of neutron-rich rhenium isotopes
A number of 3-quasiparticle isomers have been found and characterised in the odd-mass, neutron-rich, 187Re, 189Re and 191Re nuclei, the latter being four neutrons beyond stability. The decay of the isomers populates states in the rotational bands built upon the 9/2-[514] Nilsson orbital. These bands exhibit a degree of signature splitting that increases with neutron number. This splitting taken together with measurements of the M1/E2 mixing ratios and with the changes observed in the energy of the gamma-vibrational band coupled to the 9/2-[514] state, suggests an increase in triaxiality, with γ values of 5°, 18° and 25° deduced in the framework of a particle-rotor model
Decay of a three-quasiparticle isomer in the neutron-rich nucleus 183Ta
Excited states in neutron-rich tantalum isotopes have been studied with deep-inelastic reactions using 136Xe ions incident on a 186W target. New transitions observed below the τ=1.3 μs isomer in 183Ta have enabled the establishment of its energy and put limits on the spin and parity. On the basis of the reduced hindrances for the depopulating transitions, a 3-quasiparticle configuration of v1/ 2-[510]11/2+[615] ⊗ π9/2-[514] is suggested
Identification of Significant \u3cem\u3eE\u3c/em\u3e0 Strength in the 2\u3csub\u3e2\u3c/sub\u3e\u3csup\u3e+\u3c/sup\u3e → 2\u3csub\u3e1\u3c/sub\u3e\u3csup\u3e+\u3c/sup\u3e Transitions of \u3csup\u3e58,60,62\u3c/sup\u3eNi
The E0 transition strength in the 22+ → 21+ transitions of 58,60,62Ni have been determined for the first time following a series of measurements at the Australian National University (ANU) and the University of Kentucky (UK). The CAESAR Compton-suppressed HPGe array and the Super-e solenoid at ANU were used to measure the δ(E2/M1) mixing ratio and internal conversion coefficient of each transition following inelastic proton scattering. Level half-lives, δ(E2/M1) mixing ratios and γ-ray branching ratios were measured at UK following inelastic neutron scattering. The new spectroscopic information was used to determine the E0 strengths. These are the first 2+ → 2+ E0 transition strengths measured in nuclei with spherical ground states and the E0 component is found to be unexpectedly large; in fact, these are amongst the largest E0 transition strengths in medium and heavy nuclei reported to date
Long-lived three-quasiparticle isomers in 191Ir and 193Ir with triaxial deformation
Deep-inelastic reactions have been used to populate high-spin states in the iridium isotopes. New results include the identification of particularly long-lived three-quasiparticle isomers in 191Ir and 193Ir, with mean-lives of 8.2(7) s and 180(3) μs respectively, decaying into newly identified states of the h 11/2 proton bands and into other structures. Spins and parities of J π=31/2 + are suggested for both, consistent with coupling of the 11/2 -[505] proton to the 10 - two-neutron excitations in the cores. These and other configurations are discussed in the context of configuration constrained potential-energy-surface calculations. All calculated intrinsic states are expected to be associated with triaxial shapes and the extreme isomerism observed is attributed to spin-trapping rather than K-hindrance
Improved precision on the experimental E0 decay branching ratio of the Hoyle state
Stellar carbon synthesis occurs exclusively via the process, in
which three particles fuse to form C in the excited Hoyle
state, followed by electromagnetic decay to the ground state. The Hoyle state
is above the threshold, and the rate of stellar carbon production
depends on the radiative width of this state. The radiative width cannot be
measured directly, and must instead be deduced by combining three separately
measured quantities. One of these quantities is the decay branching ratio
of the Hoyle state, and the current \% uncertainty on the radiative width
stems mainly from the uncertainty on this ratio. The branching ratio was
deduced from a series of pair conversion measurements of the and
transitions depopulating the Hoyle state and state in C,
respectively. The excited states were populated by the C
reaction at 10.5 MeV beam energy, and the pairs were detected with the
electron-positron pair spectrometer, Super-e, at the Australian National
University. The deduced branching ratio required knowledge of the proton
population of the two states, as well as the alignment of the state in
the reaction. For this purpose, proton scattering and -ray angular
distribution experiments were also performed. An branching ratio of
was deduced in the current work,
and an adopted value of is
recommended based on a weighted average of previous literature values and the
new result. The new recommended value for the branching ratio is about 14%
larger than the previous adopted value of
, while the uncertainty has been
reduced from 9% to 5%.Comment: Accepted for publication as a Regular Article in Phys. Rev. C on July
29 202
Identification of significant strength in the transitions of Ni
The transition strength in the transitions of
Ni have been determined for the first time following a series of
measurements at the Australian National University (ANU) and the University of
Kentucky (UK). The CAESAR Compton-suppressed HPGe array and the Super-e
solenoid at ANU were used to measure the mixing ratio and
internal conversion coefficient of each transition following inelastic proton
scattering. Level half-lives, mixing ratios and -ray
branching ratios were measured at UK following inelastic neutron scattering.
The new spectroscopic information was used to determine the strengths.
These are the first transition strengths measured in
nuclei with spherical ground states and the component is found to be
unexpectedly large; in fact, these are amongst the largest transition
strengths in medium and heavy nuclei reported to date