Showing papers by "R. Wadsworth published in 1995"

In-beam gamma -ray spectroscopy above 100Sn using the new technique of recoil decay tagging

Abstract: A novel method of selecting \ensuremath{\gamma}-ray transitions in heavy nuclei (Ag100) at the proton drip-line has been attempted. The characteristic charged-particle radioactivity of these nuclei (alpha decay, ground-state proton decay, and \ensuremath{\beta}-delayed proton emission) has been used to tag \ensuremath{\gamma}-ray transitions recorded by the highly efficient Eurogam spectrometer. The $^{58}\mathrm{Ni}$${+}^{54}$Fe and $^{58}\mathrm{Ni}$${+}^{58}$Ni fusion-evaporation reactions, at a beam energy of 240 MeV, have been used to populate specific states of these neutron-deficient nuclei and results are presented for $^{108,109}\mathrm{Te}$, $^{109}\mathrm{I}$, and $^{113}\mathrm{Xe}$, where \ensuremath{\gamma}-ray transitions have been identified. In the case of $^{109}\mathrm{I}$, this represents the first observation of \ensuremath{\gamma}-rays from a ground-state proton emitter.

Smooth Termination of Collective Rotational Bands.

Abstract: Rotational cascades observed in neutron-deficient nuclei with $Z\ensuremath{\approx}51$ are described as the first examples of smoothly terminating bands in heavy nuclei, since they show a continuous transition from high collectivity to a noncollective state. Excellent agreement between experiment and calculations in the Nilsson-Strutinsky cranking model is obtained for the ${}_{51}^{109}{\mathrm{Sb}}_{58}$ nucleus.

Intruder bands in (Z=53) I113:Band termination interpretation

Abstract: High spin states in {sup 113}I were studied with the {sup 58}Ni({sup 58}Ni, 3{ital p}{gamma}) reaction at 240 MeV, using the EUROGAM array in Daresbury, United Kingdom, with 45 Ge detectors. A total of five deformed intruder bands extending to spins {ital I} ranging from (73/2--89/2){h_bar} have been identified, all of which reach frequencies {h_bar}{omega}{gt}1.0 MeV and excitation energies {similar_to}25 MeV. A gradual dropoff of the dynamic moment of inertia {ital scrI}{sup (2)} with increasing rotational frequency has been observed and attributed to a mode of ``soft`` band termination involving valence particle alignment. In this process, the nucleus progresses gradually, over many transitions, towards a noncollective oblate ({gamma}=+60{degree}) band terminating state.

Observation of excited superdeformed bands in 132Ce and evidence for identical bands in the mass 130 region.

Abstract: Two excited superdeformed (SD) bands have been observed in ${}^{132}\mathrm{Ce}$ using the Eurogam gamma-ray spectrometer. Possible configurations are proposed in terms of particle-hole excitation from a theoretical analysis based on the cranking approximation with the Woods-Saxon deformed potential. From relationships between $\ensuremath{\gamma}$-ray energies of SD bands in ${}^{132}\mathrm{Ce}$ and ${}^{131}\mathrm{Ce}$, the existence of identical bands and approximately quantized alignments is established in the mass 130 region.

Excited superdeformed bands in 133pr : identification of the g9/2 proton orbital

Abstract: Four superdeformed bands, consisting of [gamma]-ray cascades of up to 15 transitions, have been found in the light rare earth nucleus [sup 133]Pr. This is the first observation of multiple superdeformed bands in the [ital A][approx]130 mass region. These bands are populated weakly at 0.2%--1% of the reaction channel intensity. The single particle configurations have been identified as [pi][ital g][sub 9/2]9/2[sup +] for bands 1 and 2 and [pi][ital h][sub 11/2][532]5/2[sup [minus]] for bands 3 and 4. Dipole linking transitions between two superdeformed bands have been observed, branching ratios have been measured, and [ital B]([ital M]1)/[ital B]([ital E]2) ratios deduced. A [ital B]([ital M]1) value for the [pi][ital g][sub 9/2]9/2[sup +] orbital of (1.7[plus minus]0.2)[mu][sup 2][sub [ital N]] (where [mu][sub [ital N]] is the nuclear magneton) has been extracted, based on the measured quadrupole moment for the [sup 132]Ce core.

Intruder bands in Te 114 : Smooth band termination

Abstract: The nucleus $^{114}\mathrm{Te}$ has been studied in heavy-ion \ensuremath{\gamma}-ray spectroscopy experiments performed with the early implementation of the GAMMASPHERE multidetector array. Three rotational intruder bands have been observed up to high spins. The yrast band, involving the 4p2h (${\mathit{h}}_{11/2}$${)}^{2}$(${\mathit{g}}_{7/2}$${)}^{2}$(${\mathit{g}}_{9/2}$${)}^{\mathrm{\ensuremath{-}}2}$ proton configuration, reaches I=48\ensuremath{\Elzxh} at an excitation energy of 30.3 MeV, the highest observed spin connected by \ensuremath{\gamma} rays in this mass region. The band properties are interpreted in the framework of smooth band termination.

First observation of a rotational band in odd 50Sn nuclei: 111Sn.

Abstract: Proton particle-hole excitations across the {ital Z}=50 closed shell are responsible for inducing low-lying prolate deformation in even-{sub 50}Sn nuclei. Although related rotational bands have been studied in odd-{sub 51}Sb isotopes involving the coupling of a valence proton, none had been found in odd-{sub 50}Sn nuclei. Using the {sup 96}Ru({sup 19}F,3{ital pn}) reaction, a decoupled band has been observed in {sup 111}Sn extending to a spin-parity of (67/2{sup {minus}}) and feeding out at 23/2{sup {minus}} into spherical states. This band is interpreted as the {nu}{ital h}{sub 11/2} valence orbital coupled to the deformed [({pi}{ital g}{sub 7/2}){sup 2}{direct_product}({pi}{ital g}{sub 9/2}){sup {minus}2}] {sup 110}Sn core. The extracted band properties are compared to those in {sup 111}Sb with the same deformed core.

Evidence for the pi h11/2(X) nu h11/2 configuration in doubly odd 116I

Abstract: Two (signature partner) rotational bands have been extended to high spin in doubly odd 116 using high-fold gamma -ray coincidence data collected with the Eurogam II spectrometer. The observation of the bands to a high rotational frequency (0.7 MeV/h(cross)), without evidence of low-frequency particle alignments, suggests a pi h11/2(X) nu h11/2 structure with double blocking of the alignments. This represents the first observation of such a structure in doubly odd iodine isotopes.

Lifetime measurements within the superdeformed minimum of Ce 133 and Ce 132

Abstract: The average transitional quadrupole moment of two superdeformed bands in $^{133}\mathrm{Ce}$ and the yrast superdeformed band in $^{132}\mathrm{Ce}$ have been measured as ${\mathit{Q}}_{\mathit{t}}$=75\ifmmode\pm\else\textpm\fi{}08 e b using the Doppler shift attenuation method and exploiting target-thickness induced Doppler broadening These results are consistent with total Routhian surface calculations and indicate that the occupation of the \ensuremath{ u}[530]1/2 orbital has little polarizing effect upon the shape of the superdeformed core

Highly excited Delta I=1 structures in 193Hg

Abstract: Highly excited states in the nucleus 193Hg have been investigated by in-beam gamma -ray spectroscopic techniques using the EUROGAM array. The reaction 150Nd(48Ca,5n) at a beam energy of 213 MeV was used to populate states of 193Hg. The level scheme has been considerably extended (up to 10.7 MeV) and enriched from earlier studies. Two new structures of competing dipole and quadrupole transitions were observed. Experimental B(M1)/B(E2) ratios were determined for the two structures and compared with theoretical estimates. They were also compared with similar structures in the neighbouring Hg and Pb nuclei.

Measurement of the mean lifetimes of low spin states in the superdeformed band in 133 Nd

Abstract: The mean lifetimes of the lower spin states of the superdeformed band in133Nd have been measured with the coincidence recoil distance method. The reaction used to populate the band was105Pd (32S, 2p2n)133Nd at a beam energy of 152 MeV, and the gamma-rays were detected with the POLYTESSA array. The differential decay curve method was used to analyse the data and transition quadrupole moments,Q 0, were extracted from the measured lifetimes. The results obtained from the three lowest transitions in the superdeformed band are:Q 0=6.3±0.9 eb (21+/2→17+/2),Q 0=6.7±1.1 eb (25+/2→21+/2) andQ 0>5.0 eb (29+/2→25+/2). These results are consistent with previous results for the high spin members of the band, and are compared to theoretical calculations of total routhian surfaces. The calculation of reduced transition probabilities for the transitions that feed out of the band, allows the effect of hindrance due to K-forbiddenness to be investigated.

M1 transitions between superdeformed states in 194, 195Tl: The fingerprint of the i13/2 proton intruder orbital

Abstract: Recent data from the EUROGAM array have revealed dipole transitions linking signature partner superdeformed bands in 194Tl and 195Tl nuclei Measurements of the decay branching ratios, taken together with the average quadrupole moment of the neighboring superdeformed nuclei, enable the absolute M1 strengths to be determined From these data and using the strong coupling model, we find that two SD bands in 195Tl are due to the 81st proton being in the [642]5/2+ orbital and four from the six SD bands in 194Tl correspond to a configuration where the intrinsic spins of the single proton and single neutron are aligned

Shell structure and shape coexistence in pb-195

Abstract: Pb-195 was investigated utilizing the reactions Dy-164(S-36, 5n)Pb-195 and Dy-164(S-34, 3n)Pb-195 at beam energies of 170 and 160MeV respectively. Two new dipole bands which feed into the yrast 25/2(+) state, were found in Pb-195. The connection between the bands and the spherical states was established and thus spins and energies of the involved collective states were determined. The deformation is understood as mainly due to excitations of protons across the Z = 82 shell gap. The observed backbends are interpreted as alignment of i(13/2) neutrons.

Excited superdeformed band in 142Sm identical to 146Gd.

Abstract: A study of superdeformation in the nucleus $^{142}\mathrm{Sm}$ has revealed a superdeformed band, which is isospectral for \ensuremath{\gamma}-ray energies above \ensuremath{\sim}1 MeV with a $^{146}\mathrm{Gd}$ band. The relative alignment of these bands is compared to Hartree-Fock calculations. This second case of A\ensuremath{\sim}150 2p2n isospectrality is discussed in terms of specific orbitals which are often associated with identical bands.

Yrast bands in I 117 and Xe 116 − 118 : Anomalous quasiparticle alignment frequencies and band termination

Abstract: The yrast bands of neutron-deficient $^{117}\mathrm{I}$ and $^{116}\mathrm{Xe}$ have been extended to I\ensuremath{\sim}34\ensuremath{\Elzxh}, and $^{117,118}\mathrm{Xe}$ to I\ensuremath{\sim}46\ensuremath{\Elzxh}, using highfold \ensuremath{\gamma}-ray coincidence data collected with the Eurogam II spectrometer. Systematic quasiparticle alignment frequencies are discussed and compared to theoretical cranked Woods-Saxon calculations. The first pair alignment is attributed to ${\mathit{h}}_{11/2}$ protons despite theoretical expectations for ${\mathit{h}}_{11/2}$ neutron alignment; the neutron alignment appears significantly delayed. At higher spins, the $^{117}\mathrm{I}$ and $^{117}\mathrm{Xe}$ isobars exhibit contrasting forms of band termination.

High-spin states in doubly odd 112I

Abstract: High-spin states have been identified in doubly odd 112I, up to a spin approaching 30h(cross), using the 58Ni(58Ni,3pn gamma ) reaction. Gamma-ray coincidence data were collected with the Eurogam spectrometer coupled to the Daresbury recoil separator. The results represent the lightest doubly odd iodine isotope for which gamma -ray transitions have been identified.

Confirmation of the existence of a superdeformed rotational band in192Pb

Abstract: A controversy has arisen over the existence, or not, of a superdeformed rotational band in192Pb. We have carried out an experiment using the168Er(3OSi, 6n)192Pb reaction at 159 MeV and have observed the coincidentγ rays in the EUROGAM 2 spectrometer. We confirm the original result of Henry et al. that a superdeformed band exists in192Pb.

Two proton high spin excitations and dipole bands in 192hg

Abstract: The 192Hg nucleus has been produced via the 36S(160Gd, 4n) reaction at a beam energy of 159 MeV. The γ-rays have been detected with the multi-detector array EUROGAM phase 1. The level scheme has been extended up to 10.4 MeV excitation energy and spin 34. Two new structures, composed of competing ΔI = 1 and ΔI = 2 transitions, have been observed and, for the first time in Mercury isotopes, some links with the previously known low-lying states have been established. These experimental results are discussed in terms of mean-field Hartree-Fock plus BCS calculations and the new bands are proposed to originate from deformation-aligned quasi-proton excitations. π(i13/2 * h9/2)kπ=11 and π(h9/2)2kπ=8+ coupled to rotation-aligned quasi-neutrons v(i13/2)n and quasi-proton π(h11/2)2 excitations.

Observation and lifetime of the first excited superdeformed band in $^{192}$Hg

Abstract: An excited superdeformed (SD) band has been observed in 192Hg with the UROGAM spectrometer. It is interpreted as a two quasi-neutron excitation in the 192Hg SD nucleus. Nuclear level lifetimeshave been measured using the Dopple-shift attenuation method. An average transition quadrupole moment of 19.5 ± 1.5 b has been obtained which is similar to the value measured in the yeast SD band. These results indicate that the deformation in the 192Hg SD nucleus is stable despite changes in the occupancy of high-N-orbitals.