61 research outputs found
Structure of 55Sc and development of the N=34 subshell closure
The low-lying structure of Sc has been investigated using in-beam
-ray spectroscopy with the Be(Ti,Sc+)
one-proton removal and Be(Sc,Sc+)
inelastic-scattering reactions at the RIKEN Radioactive Isotope Beam Factory.
Transitions with energies of 572(4), 695(5), 1539(10), 1730(20), 1854(27),
2091(19), 2452(26), and 3241(39) keV are reported, and a level scheme has been
constructed using coincidence relationships and -ray
relative intensities. The results are compared to large-scale shell-model
calculations in the - model space, which account for positive-parity
states from proton-hole cross-shell excitations, and to it ab initio
shell-model calculations from the in-medium similarity renormalization group
that includes three-nucleon forces explicitly. The results of proton-removal
reaction theory with the eikonal model approach were adopted to aid
identification of positive-parity states in the level scheme; experimental
counterparts of theoretical and states are
suggested from measured decay patterns. The energy of the first
state, which is sensitive to the neutron shell gap at the Fermi surface, was
determined. The result indicates a rapid weakening of the subshell
closure in -shell nuclei at , even when only a single proton occupies
the orbital
Testing microscopically derived descriptions of nuclear collectivity : Coulomb excitation of 22Mg
Many-body nuclear theory utilizing microscopic or chiral potentials has developed to the point that collectivity might be studied within a microscopic or ab initio framework without the use of effective charges; for example with the proper evolution of the E2 operator, or alternatively, through the use of an appropriate and manageable subset of particle–hole excitations. We present a precise determination of E2 strength in 22Mg and its mirror 22Ne by Coulomb excitation, allowing for rigorous comparisons with theory. No-core symplectic shell-model calculations were performed and agree with the new B(E2) values while in-medium similarity-renormalization-group calculations consistently underpredict the absolute strength, with the missing strength found to have both isoscalar and isovector components. The discrepancy between two microscopic models demonstrates the sensitivity of E2 strength to the choice of many-body approximation employed
Particle-γ Spectroscopy of the (p,d-γ)\u3csup\u3e155\u3c/sup\u3eGd Reaction: Neutron Single-quasiparticle States at N=91
A segmented Si telescope and HPGe array is used to study the 156Gd(p,d-γ)155Gd direct reaction by d-γ and d-γ-γ coincidence measurements using 25-MeV protons. The present investigation is the first time that this N = 91 nucleus and the N = 90 region—which is known for a rapid change from vibrational to rotational character, several low-lying 0+ states in the even-even nuclei, and large Coriolis (ΔΩ = 1) plus ΔN = 2 mixing in the even-odd nuclei—have been studied by particle-γ coincidence following a direct reaction with light ions. Gamma-ray energies and branches, excitation energies, angular distributions, and cross sections are measured for states directly populated in the (p,d) reaction. A new low-energy doublet state at 592.46 keV (previously associated with the K = 0⊗3−/2 [521] bandhead) and several new γ-ray transitions (particularly for states with excitation energies \u3e1 MeV) are presented. Most notably, the previous v 7+/2[404] systematics at and around the N = 90 transition region are brought into question and reassigned as ν 5+/2[402]. This reassignment makes the ν 1+/2[400], ν 3+/2[402], and ν 5+/2[402] orbitals, which originate from the 3s1/2, 2d3/2, and 2d5/2 spherical states, respectively, responsible for the three largest cross sections to positive-parity states in the (p,d)155Gd direct reaction. These three steeply upsloping orbitals undergo ΔN = 2 mixing with their N = 6 orbital partners, which are oppositely sloped with respect to deformation. The presence of these steeply sloped and crossing orbitals near the Fermi surface could weaken the monopole pairing strength and increase the quadrupole pairing strength of neighboring even-even nuclei, which would bring ν 2p-2h 0+ states below 2Δ. Indeed, this could account for a large number of the low-lying 0+ states populated in the (p,t)154Gd direct reaction
Coulomb excitation of the mirror pair
Background: Electric-quadrupole () strengths relate to the underlying
quadrupole deformation of a nucleus and present a challenge for many nuclear
theories. Mirror nuclei in the vicinity of the line of represent a
convenient laboratory for testing deficiencies in such models, making use of
the isospin-symmetry of the systems. Purpose: Uncertainties associated with
literature strengths in \textsuperscript{23}Mg are some of the largest in
nuclei in the -shell. The purpose of the
present work is to improve the precision with which these values are known, to
enable better comparison with theoretical models. Methods: Coulomb-excitation
measurements of Mg and Na were performed at the TRIUMF-ISAC
facility using the TIGRESS spectrometer. They were used to determine the
matrix elements of mixed / transitions. Results: Reduced
transition strengths, , were extracted for \textsuperscript{23}Mg and
\textsuperscript{23}Na. Their precision was improved by factors of
approximately six for both isotopes, while agreeing within uncertainties with
previous measurements. Conclusions: A comparison was made with both shell-model
and {\it ab initio} valence-space in-medium similarity renormalization group
calculations. Valence-space in-medium similarity-renormalization-group
calculations were found to underpredict the absolute strength - in
agreement with previous studies
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
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
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