Yrast structure of 206Bi: Isomeric states and one-proton-particle, three-neutron-hole excitations
N/A
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Engineering topics
Lister, C J
Publications and source records attributed to Lister, C J.
Nuclear structure studies along the proton dripline at ATLAS
Nuclei far from the line of stability are a focal point of contemporary nuclear physics. Nuclear structure studies along the proton dripline remain an important component of the scientific program at the ATLAS facility. The Gammasphere gamma-ray array and the Fragment Mass Analyzer offer the unprecedented sensitivity and selectivity required to study nuclei at and beyond the proton dripline. Recent results in proton decay studies, in-beam {gamma}-ray spectroscopy around {sup 100}Sn and studies of proton resonances in light nuclei involved in the astrophysical hydrogen burning are presented. Future prospects in the context of planned and on-going upgrades of ATLAS and experimental apparatus are also briefly discussed.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Spectroscopy of Neutron-rich Pu Nuclei
Spectroscopic studies of nuclei in the A{approx}250, Z{approx}100 region provide critical input to theoretical models that attempt to describe the structure and stability of the heaviest elements. We report here on new spectroscopic studies in the N = 150,151 nuclei {sup 244,245}Pu. (Z = 94). Excitations in these nuclei on the neutron-rich side of the valley of stability, accessed via inelastic and transfer reactions, complement fusion-evaporation studies of Z{>=}100 nuclei. States in {sup 244,245}Pu were populated using {sup 47}Ti and {sup 208}Pb beams incident on a {sup 244}Pu target, with delayed and prompt gamma rays detected by the Gammasphere array. The new results are discussed in the context of emerging systematics of one- and two-quasiparticle excitations in N{>=}150 nuclei.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Report on the workshop "Decay spectroscopy at CARIBU: advanced fuel cycle applications, nuclear structure and astrophysics". 14-16 April 2011, Argonne National Laboratory, USA.
A workshop on 'Decay Spectroscopy at CARIBU: Advanced Fuel Cycle Applications, Nuclear Structure and Astrophysics' will be held at Argonne National Laboratory on April 14-16, 2011. The aim of the workshop is to discuss opportunities for decay studies at the Californium Rare Isotope Breeder Upgrade (CARIBU) of the ATLAS facility with emphasis on advanced fuel cycle (AFC) applications, nuclear structure and astrophysics research. The workshop will consist of review and contributed talks. Presentations by members of the local groups, outlining the status of relevant in-house projects and availabile equipment, will also be organized. time will also be set aside to discuss and develop working collaborations for future decay studies at CARIBU. Topics of interest include: (1) Decay data of relevance to AFC applications with emphasis on reactor decay heat; (2) Discrete high-resolution gamma-ray spectroscopy following radioactive decya and related topics; (3) Calorimetric studies of neutron-rich fission framgents using Total ABsorption Gamma-Ray Spectrometry (TAGS) technique; (4) Beta-delayed neutron emissions and related topics; and (5) Decay data needs for nuclear astrophysics.
21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗
Single-particle and collective structures in {sup 55}Cr and {sup 55}V
Excited states in {sup 55}V and {sup 55}Cr have been populated via pn and 2n evaporation channels, respectively, following the fusion of a {sup 48}Ca beam at 172 MeV with a {sup 9}Be target. Level schemes have been deduced for the two nuclides to excitation energies of 7467 ({sup 55}V) and 12226 keV ({sup 55}Cr), with spins of 27/2{sup +} and 33/2{sup +}, respectively. Negative-parity states are compared with shell-model calculations using three different effective interactions in the full fp model space. Negative-parity levels of {sup 55}Cr are explained in terms of single-particle fp-shell configurations outside N=28 and N=32 cores. Positive-parity states in both isotopes show evidence for the involvement of neutron g{sub 9/2} configurations. In the case of {sup 55}Cr, a quasirotational structure based on the 1/2{sup +}[440] Nilsson orbital is observed up to the terminating state. In {sup 55}V, positive-parity states do not exhibit well-developed collective features, and the observation of octupole decays is an indication of their importance in transitions from neutron g{sub 9/2} configurations to the fp shell. Experimental results are compared with the predictions of a traditional shell model, the projected shell model, and total-Routhian-surface calculations.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Search for a 2-quasiparticle high-K isomer in {sup 256}Rf
The energies of 2-quasiparticle (2-qp) states in heavy shell-stabilized nuclei provide information on the single-particle states that are responsible for the stability of superheavy nuclei. We have calculated the energies of 2-qp states in {sup 256}Rf, which suggest that a long-lived, low-energy 8{sup -} isomer should exist. A search was conducted for this isomer through a calorimetric conversion electron signal, sandwiched in time between implantation of a {sup 256}Rf nucleus and its fission decay, all within the same pixel of a double-sided Si strip detector. A 17(5)-{mu}s isomer was identified. However, its low population, {approx}5(2)% that of the ground state instead of the expected {approx}30%, suggests that it is more likely a 4-qp isomer. Possible reasons for the absence of an electromagnetic signature of a 2-qp isomer decay are discussed. These include the favored possibility that the isomer decays by fission, with a half-life indistinguishably close to that of the ground state. Another possibility, that there is no 2-qp isomer at all, would imply an abrupt termination of axially symmetric deformed shapes at Z=104, which describes nuclei with Z=92-103 very well.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
O(6)-symmetry breaking in the {gamma}-soft nucleus {sup 126}Xe and its evolution in the light stable xenon isotopes
Low-lying collective states in {sup 126}Xe have been investigated via the {sup 12}C({sup 126}Xe,{sup 126}Xe{sup *}) projectile Coulomb excitation reaction at 399 MeV. The {gamma} decays were detected with the Gammasphere array. Coulomb excitation cross sections relative to the 2{sub 1}{sup +} state were obtained. Twenty-two absolute E2 transition strengths have been deduced. An sd- interacting boson model (IBM-1) fit agrees well with the new experimental data. This makes a quantitative test of O(6)-symmetry breaking in {sup 126}Xe possible. The measured absolute B(E2) values indicate a preservation of O(5) symmetry, while the O(6) symmetry is broken. The evolution of O(6)-symmetry breaking and of O(5)-symmetry conservation in the {sup 124,126,128}Xe isotopic chain is discussed.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Shape coexistence in 72 Se investigated following the β decay of 72 Br
Not Available
McCutchan, E. A.↗
Publisher's Note: High-spin structures in the neutron-rich isotopes {sup 57-60}Mn [Phys. Rev. C 81, 014305 (2010)]
No abstract prepared.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Cross-shell excitations in {sup 30}Al and {sup 30}Si at high spin.
Yrast and near-yrast states in {sup 30}Al and {sup 30}Si have been populated to high spin with the {sup 18}O + {sup 14}C fusion-evaporation reaction in inverse kinematics. The level schemes for these two isobars have been extended up to J {approx} 9 {h_bar} at 9.4 and 15.5 MeV, respectively. Their decay schemes indicate that cross-shell excitations dominate at high spin, where negative-parity structures exist. Positive-parity states are compared to the results of shell-model calculations using the USD, USDA, and USDB effective interactions. The negative-parity levels are compared to predictions of the WBP interaction and the recently-developed WBP-a Hamiltonian, by allowing 1p-1h excitations to fp-shell orbitals. The results suggest that single-neutron excitations to the 0f7/2 orbital play a significant role at high spin.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Structure of neutron-rich tungsten nuclei and evidence for a 10{sup -} isomer in {sup 190}W.
Isomers in the neutron-rich nucleus {sup 190}W have been characterized. A 10{sup -} state from the 9/2{sup -}[505] {circle_times} 11/2{sup +}[615] two-neutron configuration with a 240-{micro}s lifetime decays via a K-allowed, 97-keV, M2 transition to an 8{sup +} state with a 160-ns lifetime from the 9/2{sup -}[505] {circle_times} 7/2{sup -}[503] neutron configuration. New states have also been identified in {sup 188}W, including a K{sup {pi}} = 8{sup -}, 158-ns isomer from the 9/2{sup -}[614] {circle_times} 7/2{sup +}[404] two-proton configuration. The K hindrance is observed to decrease with increasing neutron number, consistent with a trend toward increasing triaxial softness.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
K-hindered decay of a six-quasiparticle isomer in {sup 176}Hf.
The structure and decay properties of high-K isomers in {sup 176}Hf have been studied using beam sweeping techniques and the Gammasphere multidetector array. A new {Delta}K = 8 decay branch, from a K{sup {pi}} = 22{sup -}, six-quasiparticle, isomeric (t{sub 1/2} = 43 {micro}s) state at 4864 keV to the 20{sup -} state of a K{sup {pi}} = 14{sup -} band, has been identified. The reduced hindrance factor per degree of K forbiddenness for this decay is measured to be unusually low (f{sub ?} = 3.2), which suggests K mixing in the states involved. The deduced interaction matrix elements are discussed within the context of relevant K-mixing scenarios. The 3266-keV state, previously interpreted as a K{sup {pi}} = 16{sup +} intrinsic state, is reassigned as the J{sup {pi}} = 16{sup +} member of the band based on the K{sup {pi}} = 15{sup +} state at 3080 keV. The systematics of f{sub ?} values as a function of the degree of forbiddenness is discussed in light of this change.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Coulomb excitation of a {sup 242}Am isomeric target : E2, E3 strengths, rotational alignment, and collective enhancement.
A 98% pure {sup 242m}Am (K=5{sup -}, t{sub 1/2} = 141 years) isomeric target was Coulomb excited with a 170.5-MeV {sup 40}Ar beam. The selectivity of Coulomb excitation, coupled with the sensitivity of Gammasphere plus CHICO, was sufficient to identify 46 new states up to spin 18 {h_bar} in at least four rotational bands; 11 of these new states lie in the isomer band, 13 in a previously unknown yrast K{sup {pi}} = 6{sup -} rotational band, and 13 in a band tentatively identified as the predicted yrast K{sup {pi}} = 5{sup +} band. The rotational bands based on the K{sup {pi}} = 5{sup -} isomer and the 6{sup -} bandhead were populated by Coulomb excitation with unexpectedly equal cross sections. The {gamma}-ray yields are reproduced by Coulomb excitation calculations using a two-particle plus rotor model (PRM), implying nearly complete {Delta}K = 1 mixing of the two almost-degenerate rotational bands, but recovering the Alaga rule for the unperturbed states. The degeneracy of the 5{sup -} and 6{sup -} bands allows for precise determination of the mixing interaction strength V, which approaches the strong-mixing limit; this agrees with the 50% attenuation of the Coriolis matrix element assumed in the model calculations. The fractional admixture of the I{sub K}{sup {pi}} = 6{sub 6{sup -}} state in the nominal 6{sub 5{sup -}} isomer band state is measured within the PRM as 45.6{sub -1.1}{sup +0.3}%. The E2 and M1 strengths coupling the 5{sup -} and 6{sup -} bands are enhanced significantly by the mixing, while E1 and E2 couplings to other low-K bands are not measurably enhanced. The yields of the 5{sup +} band are reproduced by an E3 strength of {approx}15 W.u., competitive with the interband E2 strength. Alignments of the identified two-particle Nilsson states in {sup 242}Am are compared with the single-particle alignments in {sup 241}Am.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Rotational bands in odd-A Cm and Cf isotopes: Exploring the highest neutron orbitals
Rotational bands have been identified up to high spins ({approx_equal}28({h_bar}/2{pi})) in the odd-A nuclei {sup 247,249}Cm and {sup 249}Cf through inelastic excitation and transfer reactions around the Z=100 region where stability results from shell effects. The [620]1/2 Nilsson configuration in {sup 249}Cm is the highest-lying neutron orbital, from above the N=164 spherical subshell gap, for which high-spin rotational behavior has been established. The data allow for an unambiguous experimental assignment of configurations to the observed bands, unusual for odd-A nuclei near Z=100. The high-spin properties are described in terms of Woods-Saxon cranking calculations.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Structure of three-quasiparticle isomers in {sup 169}Ho and {sup 171}Tm.
A three-quasiparticle isomer with {tau}=170(8) {micro}s and K{sup {pi}} = (19/2{sup +}) has been identified in the neutron-rich isotope {sup 169}Ho. The isomer decays with K-forbidden transitions to members of a band associated with the 7/2-[523] proton configuration, whose structure is characterized through analysis of the in-band {gamma}-ray branching ratios. In the isotone {sup 171}Tm, the rotational band based on the known 19/2{sup +}, three-quasiparticle isomer has also been observed. Alternative one-proton two-neutron configurations for the isomer in {sup 169}Ho are discussed in terms of multiquasiparticle calculations and through a comparison with the structures observed in {sup 171}Tm.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗