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The empirical pairing gaps derived from four different odd-even mass staggering formulas are compared. By performing single-$j$ shell and multi-shell seniority model calculations as well as by using the standard HFB approach with Skyrme force we show that the simplest three-point formula $\Delta_C^ {(3)}(N)=\frac{1}{2}\left[B(N,Z)+B(N-2,Z)-2B(N-1,Z)\right]$ can provide a good measure of the neutron pairing gap in even-$N$ nuclei. It removes to a large extent the contribution from the nuclear mean field as well as contributions from shell structure details. It is also less contaminated by the Wigner effect for nuclei around $N=Z$. We also show that the strength of $\Delta^ {(3)}_C(N)$ can serve as a good indication of the two-particle spatial correlation in the nucleus of concern and that the weakening of $\Delta^ {(3)}_C(N)$ in some neutron-rich nuclei indicates that the di-neutron correlation itself is weak in these nuclei., Comment: 10 pages, 9 figures

High-spin states of the neutron deficient (100)pd nucleus have been investigated via the Cr-50(Ni-58, 4p alpha) and Zn-70(S-36,6n) heavy-ion induced reactions. For the detection of evaporated particles and gamma rays the NORDBALL array equipped with ancillary detectors and the EUROGAM II detector system were utilized. By the use of in-beam spectroscopic methods 89 transitions belonging to (100)pd have been observed, 49 of which were identified for the first time. The level scheme has been extended up to E-X approximate to 16 MeV excitation energy and I approximate to 25h. The experimental results were compared with the predictions of cranked shell model calculations. Maximal spin alignments were found in the (pig(9/2))(12+)(-4) (vd(5/2),g(7/2)(3)h(11/2))(13-) and (pig(9/2)(-3)p(1/2))(11-)(vd(5/2)g(7/2)(3)h(11/2))(13-) configurations. Possible existence of octupole correlations is discussed on the basis of 6 (E 1)lB(E2) values deduced for the decays of high-spin levels in the yrast negative-parity band. (C) 2001 Elsevier Science B.V. All rights reserved.

High-spin states in the neutron-deficient nuclide I-111 have been populated with the fusion reaction Ni-58 + Ni-58 at a beam energy of 210 MeV, in an experiment performed at the Tandem Accelerator of the Laboratori Nazionali di Legnaro. The gamma spectrometer GASP was used in time coincidence with the ISIS Si-ball and the CAMEL recoil mass spectrometer for the positive identification of the nuclide. Gamma transitions and structure details previously attributed to I-111 by Other authors are only partially confirmed. The obtained level scheme includes new rotational bands and a new low lying structure which suggests the presence of octupole correlations at predicted rotational frequency values.

Two excited J(pi)=0(+) states in Pb-186 populated in the a-decay of Po-190 have been identified through alpha -particle/conversion electron coincidences in an experiment at the velocity filter SHIP ...

A superdeformed band has been discovered in 143Gd consisting of 15 transitions. It does not show the band crossing observed in the neighbouring heavier Gd isotopes and it is degenerate with the superdeformed band in 143Eu. In contrast to other degenerate bands at superdeformed shape, the configurations of the bands are quite different here. They result from the exchange of a ν64 with a π[404]9/2 configuration. In addition, a collective band has been observed which adopts a well-deformed triaxial shape at high spins according to calculations. The transition from highly to superdeformed shapes proceeds via triaxial shapes.

High-spin states in Nb-85 were studied using the GAMMASPHERE Ge detector array and the MICROBALL charged-particle detector system. Three gamma-ray cascades with collective rotational characteristics were observed. One of the bands exhibits a forking at the top, most likely reflecting the termination of one branch into a favoured non-collective, near spherical state. The data are interpreted in terms of cranked Strutinsky-type calculations. (C) 1999 Elsevier Science B.V.

Excited states up to I ≈ 20 in 115 Sn and 116 Sn, populated via the ( 18 O, αxn) reactions, have been studied using the DORIS Ge detector array in conjunction with charged particle detectors. In both nuclei, spherical as well as regular, deformed level structures were found. The spherical states are interpreted to arise from pure neutron configurations, while the deformed, intruder bands obviously involve proton 2p-2h excitations across the Z = 50 shell gap.

Excited states of Co-56 were studied in the reaction Al-27(S-32,2p1n) Co-56. The NORDBALL array with a Neutron Wall and a Silicon Ball was employed. The excited states were interpreted in terms of ...

High-spin states of the isotopes In-103,In-105,In-107,In-109 have been investigated using in-beam gamma-ray spectroscopic methods. Results from three different experiments are presented. Targets of Fe-54, Cr-50, and Mo-92 were bombarded by a 270 and 261 MeV Ni-58 beam and by a 95 MeV F-19 beam, respectively. Reaction channel separation was achieved with a charged-particle detector array and in the first two experiments also with a 1 pi neutron detector system. As a result of these experiments the level schemes of In-103,In-105,In-107,In-109 were significantly extended. Excited states of these odd-A indium isotopes are discussed within the framework of the nuclear shell model and the hole-core coupling scheme. The systematics of excited states of light odd-A indium isotopes is also discussed. (C) 1997 Elsevier Science B.V.

Excited states of Cd-103 were studied in the reaction Ni-58(Cr-50,4pn)Cd-103. The NORDBALL array with neutron and charged particle detectors was employed for the detection of gamma rays and light evaporated particles. The level scheme of Cd-103 was significantly extended. A strong cascade of E2 transitions connecting negative parity states was found, Shell model calculations were performed and positive parity excited states were interpreted in terms of neutron-particle and proton-hole excitations with respect to the doubly magic N = Z = 50 core. The negative parity band was well reproduced by the total routhian surface calculations. (C) 1997 Elsevier Science B.V.

Five superdeformed (SD) bands have been identified in 144Gd with the GASP array using the reaction 100 Mo + 48 Ti . The J(2) moments of inertia of the four excited SD bands are flat as a function of rotational frequency, indicating the blocking of the N = 6 proton crossing observed in the yrast SD band. According to cranked shell model calculations, the excited bands have two-quasiproton configurations with one proton in the N = 6 orbital and the other in each signature partner of the [404]9 2 + or [411]3 2 + orbitals, lying above the Z = 64 gap. The interpretation of the excited SD bands in terms of proton excitations points to the high stability of the N = 80 shell closure at SD shapes. The population of the yrast SD band as a function of the beam energy has been also investigated. Our data shows that the intensity of the yrast SD band is not configuration dependent, being comparable to that of yrast SD bands in the neighboring nuclei. By comparing experimental and theoretical alignments, spins and configurations are assigned to all observed SD bands in the A = 140–150 region.

Excited states of Cd-101 were studied in the reaction Ni-58(Cr-50,2p alpha n)Cd-101. The NORDBALL array with a Neutron Wall and a Silicon Ball was employed for the detection of gamma-rays and evaporated particles, The level scheme of Cd-101 was significantly extended, Shell-model calculations were performed and the excited states were interpreted in terms of neutron particle and proton hole excitations with respect to the doubly magic N = Z = 50 core.

Lifetimes of high-spin states in 164 Yb have been measured by the Doppler-broadened line shape (DBLS) method using the reaction 124 Sn( 44 Ca, 4n) 164 Yb at a bombarding energy of 189 MeV. The Q t values from the data decrease sharply over the rotational frequency range 0.3 to 0.5 MeV ( I = 20 to 32 ħ). At the same rotational frequencies the kinematic moment of inertia is nearly constant, increasing slightly at the higher frequencies h ω = 0.40 to 0.55 MeV . These observations can be understood qualitatively as a rotationally-induced deoccupation of strongly shape polarizing anti-aligned high- j , low-Ω orbitals i.e., those in which the nucleons are moving in the opposite direction to the rotation. A summary of the Q t values extracted from the lifetime data for the yrast decay sequences of the even- A nuclei 160–168 Yb are compared with the deformations from self-consistent cranking TRS calculations. The TRS calculations show the expected neutron number dependence, i.e., Δβ 2 β 2 ( h ω = 0) is smaller with increasing N and the corresponding loss of collectivity, Q t vs ħ ω, has a smaller slope at the larger ħ ω with increasing N . For 164,166,168 Yb the loss of collectivity from the DBLS measurements occurs at higher ħ ω than predicted and the slope of Q t vs ħ ω is much larger than predicted. The enhanced E1/E2 branching ratios and the large transition dipole moments for decay of the states in the (−, 1) band to the ground band and the (+, 0) band in 164 Yb suggest an octupole instability for 164 Yb.

Rotational bands built on neutron h 11 2 states are investigated within the Strutinsky mean-field concept. Calculated deformation trajectories of one-, three- and five-quasiparticle configurations are presented. The experimentally observed signature splitting and crossing frequencies in XeBaCe nuclei are compared to theoretical predictions. The systematics of the observed band structures can only be accounted for by allowing a smooth transition from prolate to oblate structure via triaxial shapes as the number of neutrons increases.

A spectroscopic study of Sb-108 has been carried out as a part of a larger experiment where in total twenty-nine different residual nuclei were identified. This study gives the first information on excited levels in this nucleus. A low-lying two-quasi-particle multiplet together with some of the higher lying levels are discussed within the framework of the nuclear shell model. A rotational, strongly coupled, band is identified as, most likely, being built on the deformed pig9/2(-1) nuh11/2 configuration in accordance with total routhian surface calculations. Tentative spins and parities are presented as well as B(M1)/B(E2) ratios for some of the transitions in the strongly coupled band.

Pairing-deformation self-consistent total routhian surface type calculations are presented. The total routhian is calculated using the Strutinsky shell-correction method on a lattice in deformation space. At each grid point and each rotational frequency we calculate the pairing energy self-consistently by means of the Lipkin-Nogami theory. The accuracy of the Lipkin-Nogami approach is tested by comparison with particle-number projected calculations at fix deformation. Finally, examples of lattice calculations for the yrast superdeformed bands in 144Gd, 152Dy and 194Hg are shown.

High-spin states in 144 Gd have been investigated through the 108 Pd( 40 Ar, 4n) reaction. An extended level scheme is proposed. A ridge structure has been observed in a γγ coincidence matrix, consistent with energy spacings in the superdeformed bands of neighbouring nuclei. Several discrete high-spin sequences have been observed. They are interpreted in the context of cranked-Strutinsky-type calculations in terms of different coexisting shapes.

The neutron-deficient nucleus Te-110 has been investigated by in-beam spectroscopic methods using the NORDBALL multi-detector array. Except for the energy of one level observed in alpha-decay, exci ...

The nucleus 110Cd has been studied using the 13C and 18O induced reactions and the NORDBALL multi-detector array. The yrast band was observed to Iπ = 28+ and a large number of side bands were identified. The h 11 2 neutrons were found to play a dominant role in the observed rotational structures. The weakly populated yrast 3187 keV 8+ state is interpreted as arising from the proton g 9 2 −2 configuration. Collective bands, where the band members are connected by ΔI = 1 transitions and where the ΔI = 2 transitions are not observed, are also reported. These bands are interpreted to have prolate configurations involving both the h 11 2 neutrons and the g 9 2 protons. The experimental data are discussed in terms of the total routhian surface and cranked shell model calculations.

High-spin structures of the neutron-deficient 117,118,120 Xe isotopes have been studied by in-beam γ-ray spectroscopic techniques. The final nuclei have been identified by means of charged-particle detectors. Collective rotational bands based on the neutron d 5 2 , g 7 2 and h 11 2 configurations have been identified in 117 Xe. In the even 118 Xe and 120 Xe nuclei several new side bands were observed and the previously known bands were extended. The present level schemes include two positive-parity bands constructed to high spin in 118 Xe, whereas in 120 Xe three such bands were observed. In order to explain these bands, both proton and neutron ( h 11 2 ) 2 alignments, as well as the shape degree of freedom, have to be invoked. The possible occurrence of a γS-band is addressed and a pronounced structural change for more heavy Xe-isotopes is discussed. All negative-parity side bands are interpreted in terms of proton two-quasiparticle excitations. The experimental data are compared with total routhian surface calculations.

The routhian diagrams calculated by means of the cranked shell model are known to be a rather successful tool for describing the experimental alignment pattern observed for rotational band structures. However, this mean-field approach seems to fail when applied to band structures built on intruder configurations. It is investigated whether the observed anomalies are affected by the neutron-proton interaction which is only partly and indirectly included in the standard mean-field approach.

The nucleus 108Cd has been studied using a heavy-ion reaction and the NORDBALL multi-detector array. The yrast band has been extended to Ip = 24+, and nine quasi-rotational side bands have been identified. The n h 11 2 negative-parity orbital is found to play a dominant role both in the low-spin as well as in the high-spin structure of 108Cd. The first band crossing in the yrast band is caused by the h 11 2 neutrons. The 10+ state of the aligned n( h 11 2 ) 2 configuration decays strongly into a low-lying low-spin intruder-like positive-parity band. It is suggested that this intruder structure is dominated by a non-aligned n( h 11 2 ) 2 configuration. For the first time in this mass region, we report a collective, possibly prolate, M1 band built on the p( g 9 2 ) −2 configuration. The experimental results are discussed in terms of total-routhian-surface and cranked-shell-model calculations.

The nucleus 108Sn has been populated via the 54Fe(58Ni, 4p) reaction channel at a beam energy of 243 MeV. The high-spin structure is dominated by three ΔI = 2 rotational sequences. These bands can be interpreted in terms of particle-hole excitations involving the proton g 7 2 , g 9 2 and h 11 2 orbitals and also aligned neutrons from the bottom of the h 11 2 shell. Lifet measurements have also been performed using the Doppler-shift attenuation method. These data have enabled quadrupole moments to be deduced for the two strongest bands. The results yield Q0 = 2.6±0.4 e·b for the positive-parity band and 3.4 ± 0.6 fse·b for one of the proposed negative-parity bands. These values yield quadrupole deformations of β2 = 0.20 and 0.26, respectively, for the two bands. The results obtained are discussed in terms of Woods-Saxon and total routhian surface calculations.

Lifetime measurements on the 5/2[sup +][402] band in [sup 173]Re have been carried out by the recoil-distance technique. Lifetimes of the stretched E2 transitions in both signatures of this band were measured up to a spin of 35/2[sup +] as were the lifetimes of most of the M1 transitions between the two signatures. Large deformation increases were found in both signatures at low rotational frequencies ([Dirac h][omega] [approx] 160 keV) as predicted by TRS calculations and, thus, offer new insight into the behavior of the 5/2[sup +][402] band in [sup 173]Re and perhaps into other light nuclei in this region. The B(M1) values also show an increase at about the same rotational frequency. Particle-rotor calculations can account qualitatively for the M1 and E2 transition rates at low spins, but the enhanced MI rates remain a puzzle.

Several methods employed to parametrize the spins of the superdeformed bands in terms of their observed gamma-ray energies are closely examined. It is pointed out that, since the proposed fitting procedures are variants of the Harris expansion formula, it is questionable if they could be applied to determine the unknown spins of the excited bands due to the possible presence of non-zero initial alignments. A comprehensive study of the normally deformed excited bands indicates that no correlation exists between the fitted spins and the experimentally known values. By analogy, we may conclude that there is no firm evidence for the presence of anomalous relative alignments between the superdeformed identical bands in the Hg region. Additionally, a critical review of several of the models and scenarios that purport to explain the origin of the identical bands, or the patterns of the fitted alignments is presented. It is concluded that none of these models can satisfactorily explain the identical moments of inertia in neighboring nuclei and the systematics of the observed or fitted alignments. Thus, the important question of the microscopic origin of the identical bands remains to be investigated.

Lifetimes of states associated with the ridges in the Eγ-Eγ correlation spectrum of 164Yb and 170Hf were measured by the Doppler-shift attenuation method. The results indicate a reduction of the quadrupole collectivity associated with γ-rays with energy from 0.7 to 0.9 MeV and an enhancement of collectivity for those above 0.9 MeV. The reduced and the enhanced collectivity can be understood as rotationally-induced modifications of the occupation of high-j orbitals originating from below and above the Fermi surface, respectively. The effect of rotational damping on the lifetimes of continuum states is also discussed.

Rotational band structures have been observed in the odd-A Cs isotopes 119Cs, 121Cs and 123Cs. The previously known bands have been extended to higher spin values and several new bands have been es ...

Excited states in 167 W and 168 W, populated in the 142 Nd ( 30 Si , x n ) reaction, were investigated at high spins using the ESSA30 array with 29 Compton-suppressed Ge detectors. The level schemes are extended to higher spins and several new rotational bands are identified. The results are discussed in comparison with existing data on the neighbouring N = 93 and 94 isotones and on the Z = 74W isotopes. A theoretical analysis is presented within the framework of a pairing-self-consistent cranked shell model using shape parameters obtained from total-routhian-surface calculations.

High-spin states of 174Os have been investigated by means of the (Nd(S, 4n)Os)-Nd-146-S-32-Os-174 reaction using the ESSA30 multidetector system. The decay is dominated by the ground-state positive ...

High-spin states in 107Cd were populated by the reaction 94Zr(17O, 4n) at 80 MeV and their decay was studied by γ-spectroscopic methods using the NORDBALL multi-detector array. The experiment included γ-ray yields, γ-coincidences and γ-ray angular correlation measurements. Rotational bands were observed up to spin 39 2 and ( 51 2 + ) in the negative- and positive-parity bands, respectively. Calculations of total routhian surfaces and quasi-particle routhians have been performed and are used for assigning quasiparticle configurations to the bands. In the g 7 2 neutron band, a h 11 2 neutron alignment is observed. A large signature spliting is expected in the ν g 7 2 band before the band crossing. For the band built on a single ν h 11 2 quasiparticle, an alignment of either g 7 2 or h 11 2 neutrons is proposed. Shape change effects are discussed.

High-spin states have been observed in the odd-odd isotope 120Cs in 32S-induced reactions. The previously known band is extended to higher spin and several new bands are identified. Band-head configurations have been inferred by comparing the band properties with known bands in the odd nuclei 121Cs and 121Ba. A remarkable signature inversion is observed in the π h 11 2 ⊗ν h 11 2

Experiments using the 160 Gd ( 34 S , x n ) reaction at 159 and 162 MeV have revealed two γ-ray cascades which have properties similar to those of the previously reported superdeformed bands in the heavier Hg isotopes. These sequences have been identified as superdeformed structures in 189Hg and 190Hg. For the band in 190Hg, an average quadrupole moment Q0 = 18 ± 3 e · b was obtained from a DSAM measurement. The data are interpreted within the cranked Woods-Saxon formalism and the rise in the dynamical moment of inertia for both bands is linked to the presence of pair correlations and quasiparticle alignments. For the 189–194Hg superdeformed bands, the variations in the observed population intensity and in the rotational frequencies at which decay out of the bands occur are found to be consistent with the calculated change in the well depth of the superdeformed minimum with mass.

Energies of superdeformed states in nuclei around 192 Hg are calculated using the Strulinsky shell correction method with an average Woods-Saxon potential and a monopole pairing force. The influence of various terms in the model hamiltonian on the excitation energy of the superdeformed minimum is analysed. The systematics of calculated excitation energies of shape-isomeric minima and barrier heights in even-even HgRa nuclei are given together with predictions for one-quasiparticle band-head energies in odd-A Au, Hg, Tl, Pb and Bi nuclei. Possible occurrence of hyperdeformed states in very neutron-deficient PoRa isotopes is investigated. It is shown that the presence of these exotic configurations is strongly related to the magnitude of pairing correlations at strongly elongated shapes. Finally, the role played by reflection-asymmetric deformations at superdeformed shapes is discussed.

The high-spin structure of 121Ba has been investigated by in-beam gamma-ray spectroscopic techniques. Rotational bands based on the h11/2, g7/2 and tentatively d5/2 quasineutron configurations have ...

States of 126Ba up to spin 36+ were populated in the reaction 96Zr(34S, 4n)126Ba at 155 MeV and up to spin 20+ in the reaction 116Sn(13C, 3n)126Ba at 56 MeV. Gamma-ray spectroscopy was performed with the 8π spectrometer, an instrument comprising 20 Campton-suppressed HPGe detectors and 71 BGO ball elements. A level scheme organized into fifteen rotational bands is proposed on the basis on the γγ-coincidence and γ-ray angular correlation data. The competing π h 11 2 and v h 11 2 band crossings are investigated and interpreted in terms of cranked shell model and total routhian surface calculations. Amongst the topics discussed are (i) a comparison of DCO and spin orientation techniques for determining spins and multipolarities. (ii) the addivity property of quasiparticle energies or routhians, and (iii) analysis of B(M1)/B(E2, ratios between signature partner bands. The possibility of couplings between vibrational and aligned quasiparticle structures is explored.

High-spin states of 125 Xe have been populated via the 116 Cd( 13 C, 4n) reaction and were investigated by in-beam gamma-ray spectroscopy with the OSIRIS Compton-suppressed spectrometer. The yrast band was observed up to a tentative ( 47 2 − ) level. Four band structures of negative parity and six band structures of positive parity were found. Four band crossings were established. In addition three bands, which are decaying into all the other structures, have been observed. One-and three-quasiparticle routhians were constructed from the experimental data. Proton and neutron configurations for the observed bands have been assigned by comparison with theoretical routhians. Total routhian surfaces have been compiled to give an indication of core deformation. Low-spin states have been compared with triaxial rotor-plus-particle calculations.

Three superdeformed (SD) bands have been observed in 194Hg and four (or five?) SD bands in 193Hg using the 150Nd + 48Ca reaction. All bands except for two in 193Hg show a steady increase in dynamical moment of inertia J (2) with rotational frequency. The two exceptional bands form a classical pair of strongly interacting bands. It is suggested that the strong interaction between the bands is caused by a softness to octupole deformation. Evidence is found for the existence of dipole transitions connecting bands of opposite signature in 193Hg. The strengths of these transitions suggest that they are probably E1 supporting the importance of the role of octupole vibrations. These data suggest the wider importance of octupole softness in enhancing E1 transitions in the SD feeding and decay mechanisms. The spectroscopy of the observed SD bands in 193,4Hg are discussed in detail and attention is drawn to the “identical” energies of γ-rays in these isotopes with those in lighter isotopes. The similarities in bands relate to the neutron sub-shell closure for SD nuclei at N=112.

High-spin states in 124 Ba were populated via the 94 Zr( 34 S, 4n) 124 Ba and 110 Cd( 16 O, 2n) 124 Ba reactions at 145 MeV and 60 MeV, respectively. The yrast band has been extended to 34 ħ and seven side bands have been observed. Gamma-ray angular distributions with respect to the nuclear spin axis have been used along with directional correlation (DCO) ratios to establish spins and multipolarities. The competing π h 11 2 and ν h 11 2 crossings in this region are investigated and an interpretation of the observed crossings is proposed within the framework of the cranked shell model (CSM) and total routhian surface (TRS) calculations.

High-spin states in the transitional nucleus 184Pt were populated via the reactions 154Sm(34S, 4n)184Pt and 172Yb(16O, 4n)184Pt. The yrast band was extended up to I = 28 ħ and six new side bands built on both neutron and proton quasiparticle configurations were observed. Shell correction-type calculations indicate variations of the nuclear shape in different bands, especially as a result of band crossings due to the process of angular momentum alignment. Comparison of the band characteristics are made between 184Pt and eight adjacent nuclei. The pattern of band crossings in these nine nuclei is considered from the viewpoint of blocking comparisons and of theoretical calculations. The competition between low-frequency vi 13 2 and πh 9 2 band crossings is discussed.

Three superdeformed bands have been observed in 194Hg. The dynamical moment of inertia J (2) of all three bands is observed to increase by 30–40% over the frequency range ħω = 0.1–0.4 MeV. This phenomena can be understood in terms of the gradual alignment of pairs of high-j intruder orbitals within the framework of the cranked Woods-Saxon and Nilsson models including pairing. The calculations together with the observed J (2) behaviour of the three bands indicate that pairing correlations in the superdeformed minimum are rather weak.

Angular momentum alignment processes in nuclei around 180 Os are discussed within the deformed shell model using the Woods-Saxon potential and the monopole pairing interaction. The crossings between bands of different structures are analysed and predictions are made for the possible observation of highly deformed intruder bands. Predictions of Q o moments are presented for the highly deformed bands and tables of calculated deformations for 178–183 Os are given.