Intruder Orbitals Research Articles

Cluster structures of the ground and excited states of12Bestudied with antisymmetrized molecular dynamics

The structures of the ground and excited states of 12Be were studied with antisymmetrized molecular dynamics. The ground state was found to be a state with a developed 2-alpha core with two neutrons occupying the intruder orbits. The energy levels of the newly measured spin-assigned states were described well, except for the $1^-_1$ state. The calculations indicated that many exotic cluster structures appear in the low-energy region. The widths concerning alpha and 6He decays were discussed by using reduced width amplitudes.

Evidence for a1g9/2rotational band in51Mn

A terminating rotational band has been identified in Mn-51 following the Si-28(S-32,2alpha1p)Mn-51 fusion-evaporation reaction at 130 MeV beam energy. Spins and tentative positive parities of the band members are assigned based on angular distribution and correlation measurements of transitions, which connect the rotational structure with previously known yrast states. Configuration-dependent cranked Nilsson-Strutinsky calculations suggest a configuration of the band, which comprises one particle in the 1g(9/2) intruder orbit.

Search for the Jacobi shape transition in rapidly rotating nuclei

We have studied quasicontinuous gamma radiation from reactions of ${}^{48}\mathrm{Ca}$ beams on targets of ${}^{50}\mathrm{Ti},$ ${}^{64}\mathrm{Ni},$ ${}^{96}\mathrm{Zr},$ and ${}^{124}\mathrm{Sn}.$ The bombarding energies were in the range 195--215 MeV, and were chosen so as to bring in a high angular momentum without severe fragmentation of the cross sections for fusion residues. Experiments were performed both with the 8PI spectrometer at the LBNL 88-Inch Cyclotron, and with Gammasphere at the ANL ATLAS accelerator. The results have been analyzed in a variety of ways, with the consistent result that in all but the heaviest target, the nuclear moments of inertia tend to increase with increasing angular momentum up to the highest values observed. This could come from a transition towards a highly deformed Jacobi-like shape, or possibly be due to the appearance of high-$j$ intruder orbitals at the Fermi surface. We will argue that these effects can be equivalent.

Blocking effect and the moments of inertia of high-Kmultiquasiparticle bands in172,173,174,175Hf

The series of high-$K$ multiquasiparticle bands in ${}^{172,173,174,175}\mathrm{Hf}$ are investigated using the particle-number-conserving (PNC) method for treating the cranked shell model with monopole and quadrupole pairing interactions. The experimental results, including the moments of inertia and angular momentum alignments relative to the ground state band ${}^{172}\mathrm{Hf}(\mathrm{g}.\mathrm{s}.\mathrm{b}.)$ are reproduced quite well by the PNC calculation, in which no free parameter is involved. The difference in the $\ensuremath{\omega}$ variation of the moments of inertia between the high-$K$ multiquasiparticle bands and ${}^{172}\mathrm{Hf}(\mathrm{g}.\mathrm{s}.\mathrm{b}.)$ is mainly due to the blocking effects of high j intruder orbitals near the Fermi surface. The $\ensuremath{\omega}$ variation of the occupation probability of each cranked orbital and the contribution to moment of inertia from each major shell and from each cranked orbital are investigated.

Measurement of transition quadrupole moments of high-spin states in the [formula omitted] isotones 133Pr, 132Ce and 131La

The Doppler-Shift Attenuation Method has been used to extract transition quadrupole moments of high-spin bands in the N=74 isotones 133Pr, 132Ce and 131La, produced in the 37Cl + 100Mo reaction. The results appear to be configuration dependent and, for 133Pr and 132Ce, the involvement of Ω=1/2 νh 9/2 and νf 7/2 intruder orbitals appears to enhance the collectivity at high spin ( I>25 ℏ ).

Well-Deformed Oblate K Isomers Induced by Oblate IntruderStates

With configuration-constrained potential energy surfaces,it has been found that the neutron-deficient Z≈82 nucleican have well-deformed oblate K isomers which are formed byexciting quasiparticles to the proton i13/2 and h9/2oblate intruder orbitals. To achieve the configuration-constrainedcalculations, the maximum-overlap method of wavefunctions is proposed.In the present calculations of quasiparticle states,quadrupole pairings are included.The observed Kπ = 11- isomer in 188Pb is confirmed to beoblately deformed, with energy consistent with the experiment.Some low-lying oblate isomeric states are predicted for 187Tl.

Superdeformed and highly deformed bands in65Znand neutron-proton interactions in Zn isotopes

Superdeformed and highly deformed rotational bands were established in Zn-65 using the Ca-40(Si-29,4p)Zn-65 reaction, and averaged quadrupole moments were measured for two of these bands. The configurations of these bands were assigned based on Hartree-Fock calculations. One of the three bands exhibits at low (h) over bar omega a rise in the J((2)) dynamic moments of inertia that is similar to the alignment gain observed in Zn-60. A comparison of the rotational band configurations and their J((2)) moments of inertia for light Zn isotopes suggests that the rise in J((2)) is most likely caused by np interactions associated with the valence protons and neutrons occupying the g(9/2) intruder orbits.

Superdeformation in 91Tc

A high-spin rotational band with 11 γ-ray transitions has been observed in 91Tc. The dynamical moment of inertia as well as the transition quadrupole moment of 8.1 +1.9 −1.4 e b measured for this band show the characteristics of a superdeformed band. However, the shape is more elongated than in the neighbouring A=80–90 superdeformed nuclei. Theoretical interpretations of the band within the cranked Strutinsky approach based on two different Woods–Saxon potential parameterisations are presented. Even though an unambiguous configuration assignment proved difficult, both calculations indicate a larger deformation and at least three additional high- N intruder orbitals occupied compared to the lighter SD nuclei.

Particle-rotor model calculations of superdeformed bands inA=150and 190 regions

Quite a few superdeformed bands in the mass regions of 150 as well as 190, having different trends in the dynamic moments of inertia with increasing angular momentum, are studied in a simple version of the particle-rotor model. Transition energies, dynamic, and kinematic moments of inertia have been calculated. A reasonably good agreement with the experimental data justifies the application of this version of the model in the new regime of nuclear structure. Electromagnetic moments calculated within the same model also agree with the experimental results. The role of high-$j$ intruder orbitals in the structure of the superdeformed bands in both the regions has been reinvestigated.

Quadrupole moment of the yrast superdeformed band in144Gd

A mean lifetime measurement using the Doppler shift attenuation method has been performed at GASP in order to measure the quadrupole moment of the yrast superdeformed band of ${}^{144}\mathrm{Gd}$. The extracted intrinsic quadrupole moment above the backbending, ${Q}_{0}{=13.7}_{\ensuremath{-}0.9}^{+1.1} e$ b, is in nice agreement with cranked shell model predictions. To fit the data properly below the backbending a smaller ${Q}_{0}$ value is required ${(11.6}_{\ensuremath{-}1.0}^{+1.4})e$ b. This is consistent with the predicted lower deformation in the second well for the configuration without high-$N$ $(\ensuremath{\pi}N=6$ and $\ensuremath{\nu}N=7)$ intruder orbitals occupied.

Transition quadrupole moments at high spin in133Ndrotational bands

Lifetime measurements on the high-spin rotational bands in ${}^{133}\mathrm{Nd}$ have been carried out by the Doppler-shift attenuation method using the GASP array. The ${}^{133}\mathrm{Nd}$ nuclei were populated in the ${}^{104}\mathrm{Pd}{(}^{32}\mathrm{S},2pn)$ reaction at a bombarding energy of 135 MeV. The Doppler-broadened line shapes were analyzed for Au-backed and Gd-backed targets. Values of ${Q}_{t}=4.7(5)$, 5.6(5), and $7.4(4) e\mathrm{b}\mathrm{}$ have been determined for the mean transitional quadrupole moments of the ground-state band, the negative-parity band involving the neutron ${h}_{9/2}{/f}_{7/2}$ intruder orbitals, and the band built on the neutron $[660]{1/2}^{+}$ intruder orbital, respectively. The presently determined quadrupole moments are compared with those of the low-spin members of the bands derived elsewhere from recoil distance method measurements. The observed polarizing effects of the neutron intruder orbitals are compared with cranked shell model predictions.

High spin states in

The yrast states of the beta-stable nucleus have been studied using the reaction at a beam energy of 65 MeV. Transitions in this nucleus were identified unambiguously using high-efficiency recoil-gated gamma-ray spectra obtained from the AYEBALL gamma-ray array in conjunction with the Argonne fragment mass analyser coupled to a split anode ionization chamber. The decay scheme is presented and it is hoped that such a study will shed light on the competition between core breaking and population of the neutron intruder orbitals in the generation of high angular momentum states in this region.

Quadrupole deformation of barium isotopes

The B(E2:0 1 +→2 1 +) values of the Ba isotopes ( Z=56) exhibit a sharp increase in deformation as the neutron numbers approach the mid-shell value of N=66. This behavior is anomalous because the 2 + 1 level energies are very similar to those of the neighboring isotopes. By means of the axially-symmetric deformed Woods-Saxon (WS) hamiltonian plus the BCS method, we investigated the systematics of B(E2) of the Ba isotopes. We showed that 15% of the B(E2) values at N=66 was due to the level crossing, occurring at the deformation β WS∼0.3, between the proton orbits originating from the orbits Ω π=1/2 −(h11/2) and 9/2 +( g9/2) at zero deformation. The latter of these two was an intruder orbit originating from below the energy gap at Z=50, rising higher in energy with the deformation and intruding the Z=50–82 shell. These two orbits have the largest magnitude of the quadrupole moment with a different sign among the orbits near and below the Fermi surface. Occupancy and non-occupancy of these orbits by protons thus affect B(E2:0 1 +→2 1 +) significantly.

Influence of valence neutron configuration on quadrupole deformation in doubly-odd134Pr

Lifetimes of levels in two doubly-decoupled bands of ${}^{134}\mathrm{Pr}$ involving neutron intruder orbits have been measured using Doppler-shift techniques. The deduced transition quadrupole moments are consistent with total Routhian surface predictions for the suggested configurations of both bands. The negative-parity band with one neutron in the ${i}_{13/2}$ orbit has a much larger quadrupole deformation than the positive-parity band with a neutron in the ${(f}_{7/2}{/h}_{9/2})$ orbit, reflecting the strong polarizing effect of the $N=6$ intruder orbit on the nuclear shape.

Step-wise occupation of theνi13/2intruder orbital in the doubly-odd nucleus132Pr

Five rotational bands involving intruder and/or extruder orbitals have been observed in the doubly-odd nucleus ${}^{132}\mathrm{Pr}.$ One of them is observed for the first time. The properties of the new band are similar to those of the excited highly deformed band of ${}^{134}\mathrm{N}\mathrm{d},$ which has a configuration involving one ${i}_{13/2}$ neutron at low frequency that is crossed at high frequency by a configuration with two ${i}_{13/2}$ neutrons. This behavior indicates that the stepwise occupation of $\ensuremath{\nu}{i}_{13/2}$ intruder orbitals is an efficient mechanism for building high angular momentum in the light $A\ensuremath{\sim}130$ nuclei.

Comparative quadrupole moments of superdeformed bands in 193−196Pb

Lifetimes have been measured for states in five superdeformed yrast bands in 193−196Pb using the Gammasphere spectrometer array. Centroid shifts and lineshapes of transitions showing attenuated Doppler shifts have been analyzed. Transition quadrupole moments are deduced for the five bands including, for the first time, bands of two odd-A Pb isotopes. The quadrupole moments are found to be remarkably similar for the yrast bands in 194Pb (band 1), 195Pb (bands 1 and 2) and 196Pb (band 1) (Q t ≈20 b). However, for 193Pb a smaller value, Q t =17.3(7) b, is obtained. This can be explained in terms of the occupation of the neutron N = 7 intruder orbitals.

Deformation of highly deformed bands in Nd nuclei

The quadrupole moments of highly deformed bands in134,135Nd and131Ce have been determined using the Doppler-shift attenuation method. The obtainedQ0 values show no clear dependence on the number of occupiedvi13/2 intruder orbitals. Cranked shell model calculations indicate that the charge-quadrupole moments of the highly deformed bands decrease with increasing spin. The expected increase in deformation induced by thevi13/2 orbital in the two bands of134Nd is attenuated by a rotation-induced shrinking effect driving the nucleus towards smaller deformation.

Lifetime measurements of highly deformed bands in134,135Ndand131Ce

The quadrupole moments of highly deformed bands in ${}^{134,135}\mathrm{Nd}$ and ${}^{131}\mathrm{Ce}$ have been determined using the Doppler-shift attenuation method. The obtained ${Q}_{0}$ values show no clear dependence on the number of occupied $\ensuremath{\nu}{i}_{13/2}$ intruder orbitals. Cranked shell model calculations indicate that the charge-quadrupole moments of the highly deformed bands decrease with increasing spin. The expected increase in deformation induced by the $\ensuremath{\nu}{i}_{13/2}$ orbital in two bands of ${}^{134}\mathrm{Nd}$ is attenuated by a rotation-induced shrinking effect driving the nucleus towards smaller deformation.

Hartree-Fock-Bogoliubov description of quasiparticle excitations in the superdeformed wells of191,192Hg and192,193Tl

The properties of superdeformed bands in {sup 191,192}Hg and {sup 192,193}Tl have been studied using the cranked Hartree-Fock-Bogoliubov method with the Lipkin-Nogami prescription, the Skm{sup {asterisk}} interaction, and a surface-delta, density-dependent pairing force. In particular, quasiparticle excitations involving intruder orbitals are analyzed in detail. Comparisons between data and calculations are performed for J{sup (2)} moments, quadrupole moments, spins, transition energies, and alignments. {copyright} {ital 1998} {ital The American Physical Society}

Identical superdeformed bands

The phenomenon of identical bands is studied by analyzing the distributions of fractional changes in the dynamical moments of inertia of pairs of bands in superdeformed (SD) nuclei. These distributions are found to exhibit a peak with a centroid at nearly zero. Their widths increase in going from the SD bands in the massA∼150, to the SD bands in the mass ∼190 and to the normally-deformed bands in the rare-earth region. These differences may be attributed to the weaker pairing correlations and the stabilizing role of intruder orbitals on the structures of SD bands. Precise level lifetimes have been measured for various pairs of identical SD bands in Gd and Dy isotopes. By comparing the derived quadrupole moments with calculations performed in the framework of the cranking Skyrme-Hartree-Fock model, it is shown that, independently of the intrinsic configuration and of the proton and neutron numbers, the charge moments calculated with respect to the doubly-magic SD core of 152 Dy can be expressed in terms of independent contributions from the individual hole and particle orbitals.