Publications (18)27.8 Total impact
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Article: Close near-degeneracy in a pair of four-quasiparticle bands in 194Tl
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ABSTRACT: A pair of rotational bands associated with the configuration at lower spins and with the configuration at higher spins is found in 194Tl. The two 4-quasiparticle bands show exceptionally close near-degeneracy in the excitation energies. Furthermore close similarity is also found in their alignments and B(M1)/B(E2) reduced transition probability ratios. Such close near-degeneracy probably indicates chiral geometry in the angular momentum space.Physics Letters B 01/2013; · 3.95 Impact Factor -
Dataset: Candidate chiral bands in 198Tl
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Article: In-beam spectroscopy of 72Ge
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ABSTRACT: Abstract. High-spin states in the nucleus 72Ge were investigated via the 70Zn(�, 2n)72Ge reaction at a beam energy of 30MeV, using the AFRODITE spectrometer. One aim of the study was to search for tetrahedral states. There was no evidence for such states in our coincidence data. The existing decay scheme was substantially revised and extended. Several �-ray placements and level spin-parities were changed, and some 30 new transitions were added to the level scheme. One new negative-parity rotational band was identified. The new band is likely the unfavoured signature partner of the band built on the previously known I� = 3− state at 2515 keV. The two negative-parity bands are interpreted as involving an aligned octupole vibration which evolves to a four-quasiparticle structure at higher spins. The upbend in the yrast band is interpreted as the AB neutron alignment. The band structures are discussed with reference to Cranked Shell Model calculations, the aligned angular momenta, experimental routhians, and moments of inertia. 1 IntroductionEuropean Physical Journal A 07/2012; 48(48: 99):99. · 2.19 Impact Factor -
Article: Congruent band structures in 154Gd : Configuration-dependent pairing, a double vacuum and lack of -vibrations
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ABSTRACT: We have used the (a \alpha, 2n) and (a \alpha, 4n) reactions and the AFRODITE g \gamma -ray spectrometer to make a comprehensive study of the nucleus 154Gd below 20(h/2p)\ensuremath 20\hbar . While the first excited 02 + state at 681 keV is usually considered to be the head of a Kp=0+\ensuremath K^{\pi}=0^{+} b \beta -vibrational band, we propose that the data are best described as two separate vacuum states, the ground state and the 681 keV level, each with its own g \gamma and octupole vibrations, pairing and alignments. The implications of this finding, for understanding the structure of transitional rare-earth nuclei, are discussed.European Physical Journal A 04/2012; 47(1):1-6. · 2.19 Impact Factor -
Article: Candidate chiral bands in 198Tl
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ABSTRACT: High-spin states in 198Tl were studied using the 197Au(a \alpha, 3n) reaction. The level scheme was considerably extended including two new bands and several non-yrast levels. One of the new bands is possibly a chiral partner to the yrast p \pi h 9/2⊗ n \nu i 13/2 -1band. Two-quasiparticle-plus-triaxial-rotor model calculations suggest an aplanar orientation of the total angular momenta for these bands, thus supporting possible chirality.European Physical Journal A 04/2012; 45(1):39-50. · 2.19 Impact Factor -
Article: The first candidate for chiral nuclei in the $A\sim80$ mass region: $^{80}$Br
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ABSTRACT: Excited states of $^{80}$Br have been investigated via the $^{76}$Ge($^{11}$B, $\alpha$3n) and $^{76}$Ge($^{7}$Li, 3n) reactions and a new $\Delta I$ = 1 band has been identified which resides $\sim$ 400 keV above the yrast band. Based on the experimental results and their comparison with the triaxial particle rotor model calculated ones, a chiral character of the two bands within the $\pi g_{9/2}\otimes \nu g_{9/2}$ configuration is proposed, which provides the first evidence for chirality in the $A\sim80$ region.Physics Letters B 09/2011; 703(1):40-45. · 3.95 Impact Factor -
Article: Electric dipole moments in 230,232U and implications for tetrahedral shapes
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ABSTRACT: The nuclei 230U and 232U were populated in the compound nucleus reactions 232Th(α,6n) and 232Th(α,4n), respectively. Gamma rays from these nuclei were observed in coincidence with a recoil detector. A comprehensive set of in-band E2 transitions were observed in the lowest lying negative-parity band of 232U while one E2 transition was also observed for 230U. These allowed B(E1;I-→I+-1)/B(E2;I-→I--2) ratios to be extracted and compared with systematics. The values are similar to those of their Th and Ra isotones. The possibility of a tetrahedral shape for the negative-parity U bands appears difficult to reconcile with the measured Q2 values for the isotone 226Ra.Phys. Rev. C. 10/2010; 82(4-041305(R) (2010)). -
Article: Nonzero quadrupole moments of candidate tetrahedral bands.
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ABSTRACT: Negative-parity bands in the vicinity of 156Gd and 160Yb have been suggested as candidates for the rotation of tetrahedral nuclei. We report the observation of the odd and even-spin members of the lowest energy negative-parity bands in 160Yb and 154Gd. The properties of these bands are similar to the proposed tetrahedral band of 156Gd and its even-spin partner. Band-mixing calculations are performed and absolute and relative quadrupole moments deduced for 160Yb and 154Gd. The values are inconsistent with zero, as required for tetrahedral shape, and the bands are interpreted as octupole vibrational bands. The failure to observe the in-band E2 transitions of the bands at low spins can be understood using the measured B(E1) and B(E2) values.Physical Review Letters 01/2010; 104(2):022501. · 7.37 Impact Factor -
Article: Proton capture to continuum states of^{209} Bi
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ABSTRACT: High resolution spectra from fast proton radiative capture in 208Pb are presented for three energies of the incident protons: 14.8, 15.7, and 16.9 MeV. The collected spectra were analyzed with emphasis on the region of capture to unbound states in order to search for possible structure and to study the competition between two basically different phenomena: the statistical process versus the fluctuation effects in radiative capture to unbound final states. In the energy region of incoming protons considered in our experiment, the statistical processes modeled with a Hauser-Feshbach calculation dominate. The spectra of primary γ rays, coincident with the low energy one from the decay of the f7/2 single-particle state, show a prominent structure which is predominantly of g9/2 character. The calculations of continuum spectra based on the extended direct-semidirect model and the Hauser-Feshbach statistical approach successfully reproduce the experimental data.Phys. Rev. C. 04/2006; 73(4). -
Article: Capture cross sections for very heavy systems
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ABSTRACT: In intermediate-mass systems, collective excitations of the target and projectile can greatly enhance the subbarrier capture cross section σ cap by giving rise to a distribution of Coulomb barriers. For such systems, capture essentially leads directly to fusion [formation of a compound nucleus (CN)], which then decays through the emission of light particles (neutrons, protons, and alpha particles). Thus, the evaporation-residue (ER) cross section is essentially equal to σ cap. For heavier systems, the experimental situation is significantly more complicated owing to the presence of quasifission (QF) (rapid separation into two fragments before the CN is formed) and by fusion-fission (FF) of the CN itself. Thus, three cross sections need to be measured in order to evaluate σ cap. Although the ER essentially recoil along the beam direction, QF and FF fragments are scattered to all angles and require the measurement of angular distributions in order to obtain the excitation function and barrier distribution for capture. Two other approaches to this problem exist. If QF is not important, one can still measure just the ER cross section and try to reconstruct the corresponding σ cap through use of an evaporation-model code that takes account of the FF degree of freedom. Some earlier results on σ cap obtained in this way will be reanalyzed with detailed coupled-channels calculations, and the “extra-push” phenomenon discussed. One may also try to obtain σ cap by exploiting unitarity, that is, by measuring instead the flux of particles corresponding to quasielastic (QE) scattering from the Coulomb barrier. Some new QE results obtained for the 86Kr + 208Pb system at iThemba LABS in South Africa will be presented.Physics of Atomic Nuclei 01/2006; 69(7):1093-1100. · 0.57 Impact Factor -
Article: Aspects of nuclear physics research at iThemba LABS, South Africa
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ABSTRACT: A summary of recent results of research at iThemba LABS is given. This includes high-resolution measurements of the fine structure of isoscalar giant quadrupole resonances, a new programme to measure fusion barrier distributions for reactions leading to the formation of superheavy elements, measurements of high-energy γ-rays from (p,γ) reactions using the AFRODITE array of HpGe clover detectors, and a comparison of the vibrational band structures of 160Yb, measured with the AFRODITE array, with the X(5)-β2n model. A summary of future developments is given.Journal of Physics G Nuclear and Particle Physics 09/2005; 31(10):S1747. · 4.18 Impact Factor -
Article: Candidate chiral bands in
http://dx.doi.org/10.1140/epja/i2010-10994-x. -
Article: Barrier distribution for a ‘superheavy’ nucleus–nucleus collision
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ABSTRACT: Large-angle quasielastic scattering has been studied in a high-Z1Z2 nuclear reaction of the type leading to superheavy-element production by cold fusion. We show that despite the presence of strongly dissipative channels, and the complete absence of fusion, the notion of an external barrier distribution, determined by strong coupling to collective excitations of target and projectile, is still valid. Furthermore, our method allows us to deduce some properties of the deep-inelastic processes in this system.Physics Letters B. -
Article: A double vacuum, configuration dependent pairing and lack of β-vibrations in transitional nuclei: Band structure of N = 88 to N = 91 Nuclei
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ABSTRACT: We have used the AFRODITE γ-ray spectrometer to measure yrare structures in 152,154,155Gd using (α,xn) reactions. The measurements on 152,154Gd have shown that there is no convincing evidence that the lowest excited states are β-vibrations based on the ground state. Rather these levels may be regarded as a “second vacuum” each with its own γ and octupole vibrations. This is confirmed by the blocking of the coupling of this core state in 154Gd to the [505]11/2− single-particle quasi-neutron orbital in 155Gd. The coupling of this orbital to the 2+γ-vibration is observed since this coupling is not Pauli-blocked.Nuclear Physics A. 834:45. -
Article: Measuring high-energy γ-rays with Ge clover detectors
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ABSTRACT: Gamma-rays with energies up to 20.7 MeV produced by the capture reaction were measured with the AFRODITE germanium detector array. The measured full energy peak efficiency of the array is 1.5% at 1.3 MeV and 9(2)×10-4 at 15 MeV. The FWHM of capture γ-ray peaks was 50 keV up to the highest measured energies. The efficiency curve has been calculated using a Monte-Carlo simulation code GEANT. The simulation and measurements agree very well.Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 557(2):523-527. · 1.21 Impact Factor -
Article: Possible chirality in the doubly-odd^{198} Tl nucleus: Residual interaction at play
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ABSTRACT: A candidate for chiral bands was found in 198Tl for the first time in a mass region of oblate (or nonaxial with γ⩾30°) deformed nuclei. Two bands show very similar quasiparticle alignments, moments of inertia, and B(M1)/B(E2) ratios. They have a relative excitation energy of about 500 keV and different patterns of energy staggering. Calculations using the two-quasiparticle-plus-triaxial-rotor model with residual proton-neutron interaction included show that a triaxial deformation with γ~44° agrees very well with all the experimental observations. Furthermore, considerable energy staggering for both partner bands was calculated for this πh9/2⊗νi13/2-1 configuration at γ~30°, suggesting that chiral bands may have substantial energy staggering.Phys. Rev. C. 78(2). -
Article: Stopping power of Nd, Pm and Sm ions in Cd determined with γ-ray lineshape analysis
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ABSTRACT: A semi-thick target has been used to measure the stopping power for recoils, produced in heavy ion induced reactions, by γ-ray lineshape analysis. The target thickness has been chosen such that the recoils are partially slowed down in the target and subsequently escape into vacuum. For transitions deexciting long-lived states the Doppler lineshape then depends only on the stopping power. Stopping-power parameters have been determined for Pm, Sm and Nd nuclei in Cd. The results for the electronic stopping power agree within uncertainties with the predictions of the Lindhard, Scharff and Schiøtt theory. The nuclear stopping power can be well described in the framework of the Ziegler, Biersack and Littmark parameterization using the software package Stopping and Range of Ions in Matter (SRIM)2006.Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment. -
Article: Decay of^{6} Be populated in the^{6} Li (^{3} He,^{3} H) charge-exchange reaction
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ABSTRACT: A complete kinematic measurement of the 6Li(3He,3H)αpp reaction is performed at Elab=50 MeV, to investigate the cluster structure of 6Be. The energy and angular distributions of the breakup particles, emitted from the ground and first excited states of 6Be, are compared to three-body resonance and two-body sequential decay calculations. The results indicate a rather pure three-body configuration of the 2+ state of 6Be but they are not conclusive regarding the decay mode of the 0+ state. No branching ratio between sequential and three-body decay paths could be extracted. Excitations above the 2+ state were populated at Ex=1.67–23 MeV. Decay through 5Li-p and 2He-4He channels is identified in the continuum of 6Be, and the reaction mechanism is discussed.Phys. Rev. C. 81(5).
Top co-authors
Top Journals
Institutions
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2012
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University of Cape Town
Cape Town, Province of the Western Cape, South Africa
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2005–2012
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iThemba Labs - Laboratory for Accelerator Based Science
Cape Town, Province of the Western Cape, South Africa
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