Oblate Band Research Articles

Reevaluation of structures in Se70 from combined conversion-electron and γ -ray spectroscopy

In the selenium isotopes various shape phenomena are present, in particular, the emergence of a dominant oblate deformation in the most neutron-deficient isotopes has been observed. The scenario of shape coexisting oblate and prolate bands has been proposed across the isotopic chain, with the crossing point of such bands being located near Se70, where no coexistence has yet been identified. To determine the presence or absence of any low-lying 0+ state in Se70, confirm the level structure, and interpret the nuclear deformation with theoretical models. A combined internal-conversion-electron and γ-ray spectroscopy study was undertaken with the SPICE and TIGRESS spectrometers at the TRIUMF-ISAC-II facility. Nuclear models were provided by the generalized triaxial rotor model (GTRM) and the collective generalised Bohr Hamiltonian (GBH). Despite a comprehensive search, no evidence was found for the existence of a 0+ state below 2 MeV in Se70. Significant discrepancies to the previously established positive-parity-level scheme were found. GBH calculations using UNEDF1 mass parameters were found to reproduce the revised low-lying level structure well. Se70 does not have a well-defined axial shape. The 22+ state at 1601 keV resembles a quasi-γ excitation rather than a member of a shape coexisting band; the presence of such a band is all but ruled out.Published by the American Physical Society2024

Evidence for spherical-oblate shape coexistence in Tc87

Excited states in the neutron-deficient nucleus Tc87 have been studied via the fusion-evaporation reaction Fe54(Ar36,2n1p)Tc87 at 115 MeV beam energy. The AGATA γ-ray spectrometer coupled to the DIAMANT, NEDA, and Neutron Wall detector arrays for light-particle detection was used to measure the prompt coincidence of γ rays and light particles. Six transitions from the deexcitation of excited states belonging to a new band in Tc87 were identified by comparing γ-ray intensities in the spectra gated under different reaction channel selection conditions. The constructed level structure was compared with the shell model and total Routhian surface calculations. The results indicate that the new band structure in Tc87 is built on a spherical configuration, which is different from that assigned to the previously identified oblate yrast rotational band.1 MoreReceived 10 September 2021Revised 24 January 2022Accepted 23 March 2022DOI:https://doi.org/10.1103/PhysRevC.106.034304Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Funded by Bibsam.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasElectromagnetic transitionsLow & intermediate energy heavy-ion reactionsNuclear structure & decaysNucleon distributionTransfer reactionsProperties59 ≤ A ≤ 89Nuclear Physics

Decoherence of collective motion in warm nuclei

Collective states in cold nuclei are represented by a wave function that assigns coherent phases to the participating nucleons. The degree of coherence decreases with excitation energy above the yrast line because of coupling to the increasingly dense background of quasiparticle excitations. The consequences of decoherence are discussed, starting with the well studied case of rotational damping. In addition to superdeformed bands, a highly excited oblate band is presented as a new example of screening from rotational damping. Suppression of pair correlation leads to incoherent thermal M1 radiation, which appears as an exponential spike (LEMAR) at zero energy in the γ strength function of spherical nuclei. In deformed nuclei a Scissors Resonance appears and LEMAR changes to damped magnetic rotation, which is interpreted as partial restoration of coherence.

Deformed band structures at high spin in Tl200

High-spin band structures of $^{200}\mathrm{Tl}$ have been studied by $\ensuremath{\gamma}$-ray spectroscopic methods using the $^{198}\mathrm{Pt}(^{7}\mathrm{Li},5n)^{200}\mathrm{Tl}$ reaction at 45 MeV of beam energy. The level scheme of $^{200}\mathrm{Tl}$ has been extended significantly and several new band structures have been established with the observation of 60 new transitions. The $\ensuremath{\pi}{h}_{9/2}\ensuremath{\bigotimes}\ensuremath{\nu}{i}_{13/2}$ oblate band has been extended beyond the particle alignment frequencies. The band structures and the other excited states have been compared with the neighboring odd-odd Tl isotopes. Total Routhian surface calculations have been performed to study the deformation and shape changes as a function of spin in this nucleus. These calculations could reproduce the particle alignment frequency and suggest that the neutron pair alignment in $\ensuremath{\nu}{i}_{13/2}$ orbital induces $\ensuremath{\gamma}$ softness in $^{200}\mathrm{Tl}$.

High-spin structures in theXe129nucleus

High-spin states in the Xe-129 nucleus are studied with the reaction Sn-124(Be-9,4n) at a beam energy of 36 MeV. The level scheme is extended significantly. For the positive-parity band, the alpha = +1/2 and the alpha = -1/2 signature components are combined to form a complete band structure based on the 3/2(+) state with spin and parity up to 21/2(+). For the negative-parity band based on the 11/2(-) state, the alpha = +1/2 signature component is newly established and both the alpha = +1/2 and the alpha = -1/2 signature components also form a complete band structure up to the 35/2(-) state. The positive-and negative-parity bands are proposed to originate from nu d(3/2) 3/2(+)[402] and nu h(11/2)11/2(-)[505] Nilsson configurations, respectively. A backbending is observed in the negative-parity band, which originates from the alignments of two h(11/2) protons according to crank shell model calculations. Based on the total Routhian surface and quasiparticle triaxial rotor model calculations, the negative-parity band is interpreted as a triaxially deformed shape with gamma approximate to -30 degrees, while the positive-parity band is associated with. softness, in accordance with previous studies. In the high-spin states, three decoupled bands and one oblate band with gamma approximate to -60 degrees are newly identified. The systematics and other characteristics of these bands are discussed.

Algebraic benchmark for prolate-oblate coexistence in nuclei

We present a symmetry-based approach for prolate-oblate and spherical-prolate-oblate shape coexistence, in the framework of the interacting boson model of nuclei. The proposed Hamiltonian conserves the SU(3) and $\overline{\rm SU(3)}$ symmetry for the prolate and oblate ground bands and the U(5) symmetry for selected spherical states. Analytic expressions for quadrupole moments and $E2$ rates involving these states are derived and isomeric states are identified by means of selection rules.

High-spin states and possible “stapler” band inIn115

High-spin states of $^{115}\mathrm{In}$ have been studied using the $^{114}\mathrm{Cd}$ ($^{7}\mathrm{Li},\ensuremath{\alpha}2\mathrm{n}$) reaction at a beam energy of 48 MeV. A total of 13 new transitions have been observed and added to the level scheme of $^{115}\mathrm{In}$. Most of the states in $^{115}\mathrm{In}$ can be interpreted in terms of the weak coupling of a ${g}_{9/2}$ proton hole to the core states of $^{116}\mathrm{Sn}$ or a ${g}_{7/2}$ proton to the core states of $^{114}\mathrm{Cd}$. A $\ensuremath{\Delta}I=1$ band with the $\ensuremath{\pi}{({g}_{9/2})}^{\ensuremath{-}1}\ensuremath{\bigotimes}\ensuremath{\nu}{({h}_{11/2})}^{2}$ configuration was suggested as an oblate band built on the ``stapler'' mechanism with the aid of the tilted axis cranking model based on covariant density functional theory.

Shape-coexisting rotation in neutron-deficient Hg and Pb nuclei

For a shape-soft nucleus, the deformation change with increasing angular momentum of rotation can be significant. Total-Routhian-surface (TRS) calculations include the shape changes, but angular momentum is not conserved (neither is it a good quantum number, nor is it kept unchanged in the whole TRS mesh). In the projected shell model (PSM), the angular momentum appears as a good quantum number, but calculations have usually been performed with fixed deformation. In the present work, by performing angular-momentum projection on the mean-field potential-energy surface (PES), we can obtain an angular-momentum-conserved PES which gives deformation for a rotational state at a given spin. In order to investigate the shape-changing effect, we have chosen neutron-deficient Hg and Pb isotopes in which shape coexistence occurs. We interpret the irregular rotational behavior of the oblate bands at low spin as arising from deformation changes which are induced by collective rotation. At higher spin, the oblate rotational spectrum can also be influenced by the crossing between the $K=0$ ground-state band and a low-$K$ two-quasineutron band. Calculated $g$ factors for the states of oblate bands are given for future experimental testing, and the intrinsic structures of high-$K$ oblate states are investigated.

High spin spectroscopy of201Tl

The high spin structure of ${}^{201}$Tl has been studied by $\ensuremath{\gamma}$-ray spectroscopic method using the ${}^{198}$Pt(${}^{7}$Li,4$n$)${}^{201}$Tl reaction at 45 MeV beam energy. The level scheme of ${}^{201}$Tl has been considerably extended through the observation of 31 new transitions. Several new band structures have been established. The 9/2${}^{\ensuremath{-}}$ oblate band has been significantly extended beyond the particle alignment frequencies. The band structures and the other excited states have been compared with the neighboring odd-$A$ Tl isotopes and with the even-even core nucleus ${}^{200}$Hg. The total Routhian surface calculations have been performed to study the deformation and shape changes as a function of spin in this nucleus.

High-spin structures in the139Pr nucleus

Background: ${}^{139}$Pr is located in a transitional region of neutron number close to the $N=82$ shell. The study of its high-spin states and collective bands is important for systematically understanding the nuclear structural characteristics in this region.Purpose: To investigate the high-spin levels and to search for oblate bands in ${}^{139}$Pr.Methods: The high-spin states of ${}^{139}$Pr have been studied via the reaction ${}^{124}$Sn(${}^{19}$F,4$n$) at a beam energy of 80 MeV. The experiment was carried out at the HI-13 Tandem Accelerator at the China Institute of Atomic Energy (CIAE). The data analysis was done by using the $\ensuremath{\gamma}$-$\ensuremath{\gamma}$ coincidence method.Results: The level scheme of ${}^{139}$Pr has been expanded with spin up to 45/2$\ensuremath{\hbar}$. A total of 39 new levels and 45 new transitions are identified. Four collective band structures at high-spin states have been newly established. From systematic analysis, one of the bands is proposed as a double decoupled band; two bands are proposed as oblate bands with $\ensuremath{\gamma}\ensuremath{\sim}\ensuremath{-}{60}^{\ensuremath{\circ}}$; another band is suggested as an oblate-triaxial band with $\ensuremath{\gamma}\ensuremath{\sim}\ensuremath{-}{90}^{\ensuremath{\circ}}$. The other characteristics for these bands are discussed.Conclusions: A new level scheme in ${}^{139}$Pr has been established and the collective bands at high spin have been identified. The result shows that the strong oblate shape-driving effect is caused by neutrons at the high-spin states in ${}^{139}$Pr.

High-Spin States in 141Pm

The high-spin states of 141Pm nucleus have been studied by using in-beam γ-ray spectroscopy technology through the126Te(19F, 4n) reaction at a beam energy of 90 MeV. The previous level scheme has been extended with spin up to 49/2 ℏ. Many new levels and transitions are identified. Five collective band structures are observed. Based on systematic comparison with the neighboring nuclei, two bands with strong ΔI = 1 M1 transitions inside the bands are proposed as the oblate bands with γ∼−60°, and three bands with large signature splitting have been suggested as the oblate-triaxial deformation with γ∼−90°. The characteristics for these bands have been discussed.

Identification of a New Four-Quasiparticle State in Odd-Odd 186Au

The high-spin level structure of 186Au is re-investigated by means of in-beam γ-ray spectroscopy via the 172Yb(19F, 5nγ)186Au reaction. The oblate bands previously reported are revised and extended to higher-spin states. A new Iπ = (20+) excited state has been identified and assigned to the πh−111/2 ⊗ νi−213/2h−19/2 configuration. The total Routhian surface and cranked shell model calculations suggest that the πh−111/2 ⊗ νi−213/2h−19/2 band in 186Au has a non-axial deformation.

Infrared spectra of ethylene clusters: (C2D4)2 and (C2D4)3

Spectra of ethylene dimers and trimers are studied in the ν(11) fundamental band region of C(2)D(4) (≈2200 cm(-1)) using a tuneable quantum cascade laser to probe a pulsed supersonic slit jet expansion. The dimer spectrum is that of a prolate symmetric top perpendicular band, with a distinctive appearance because the A rotational constant is almost exactly equal to six times the B constant. The analysis supports the previously determined cross-shaped dimer structure with D(2d) symmetry. An ethylene trimer has not previously been observed with rotational resolution. The spectrum is that of an oblate symmetric top parallel band. It leads to a proposed trimer structure which is barrel shaped and has C(3h) or C(3) symmetry, with the ethylene monomer C-C axes approximately aligned along the trimer symmetry axis.

Observation of high-spin oblate band structures inPm141

The high-spin states of $^{141}\mathrm{Pm}$ have been investigated through the reaction $^{126}\mathrm{Te}$($^{19}\mathrm{F}$,4$n$) at a beam energy of 90 MeV. A previous level scheme has been updated with spins up to 49/2$\ensuremath{\hbar}$. Six collective bands at high spins are newly observed. Based on the systematic comparison, one band is proposed as a decoupled band; two bands with strong $\ensuremath{\Delta}I=1$ $M1$ transitions inside the bands are suggested as the oblate bands with $\ensuremath{\gamma}$ $~$$\ensuremath{-}{60}^{\ifmmode^\circ\else\textdegree\fi{}}$; three other bands with large signature splitting have been proposed with the oblate-triaxial deformation with $\ensuremath{\gamma}$$~$ $\ensuremath{-}{90}^{\ifmmode^\circ\else\textdegree\fi{}}$. The triaxial $n$-particle-$n$-hole particle rotor model calculations for one of the oblate bands in $^{141}\mathrm{Pm}$ are in good agreement with the experimental data. The other characteristics for these bands have been discussed.

High-spin states and collective band structures in the odd–odd 140Pm nucleus

The high-spin states of 140Pm have been investigated through the reaction 126Te(19F, 5n) at a beam energy of 90 MeV. A previous level scheme based on the 8− isomer has been updated with spin up to 23 ℏ. A total of 22 new levels and 41 new transitions were identified. Six collective bands were observed. Five of them were expanded or re-constructed, and one of them was newly identified. The systematic signature splitting and inversion of the yrast πh11/2⊗νh11/2 band in Pr and Pm odd–odd isotopes has been discussed. Based on the systematic comparison, two ΔI = 2 bands were proposed as double-decoupled bands; other two bands with strong ΔI = 1 M1 transitions inside the bands were suggested as oblate bands with γ ∼ −60°; another band with large signature splitting has been proposed with oblate-triaxial deformation with γ ∼ −90°. The characteristics for these bands have been discussed.

α clustering and superdeformation of28Si

We have studied positive-parity states of 28Si using antisymmetrized molecular dynamics and multiconfiguration mixing with constrained variation. Applying constraints to the cluster distance and the quadrupole deformation of the variational calculation, we have obtained basis wave functions that have various structures such as α-24Mg and 12C-16O cluster structures as well as deformed structures. Superposing those basis wave functions, we have obtained a oblate ground state band, a β vibration band, a normal-deformed prolate band, and a superdeformed band. It is found that the superdeformed bands contain large amounts of the α-24Mg cluster components. The results also suggest the presence of two excited bands with the developed α-24Mg cluster structure, where the inter-cluster motion and the 24Mg-cluster deformation play important roles.

Cluster structures and superdeformation inSi28

We have studied positive-parity states of 28 Si using antisymmetrized molecular dynamics and multiconfiguration mixing with constrained variation. Applying constraints to the cluster distance and the quadrupole deformation of the variational calculation, we have obtained basis wave functions that have various structures such as α- 24 Mg and 12 C- 16 O cluster structures as well as deformed structures. Superposing those basis wave functions, we have obtained a oblate ground-state band, a β vibration band, a normal-deformed prolate band, and a superdeformed band. It is found that the normal-deformed and superdeformed bands contain large amounts of the 12 C- 16 O and α- 24 Mg cluster components, respectively. The results also suggest the presence of two excited bands with the developed α- 24 Mg cluster structure, where the intercluster motion and the 24 Mg-cluster deformation play important roles.

CLUSTER STRUCTURES IN 28Si

Positive-parity states of 28 Si are studied with antisymmetrized molecular dynamics and multi-configuration mixing focusing on clustering and deformations. By superposition of wave functions obtained by energy variation imposing constraints on quadrupole deformation parameter β and distance between α and 24 Mg , and 12 C and 16 O clusters, two oblate bands, two prolate bands and developed α-24 Mg band are obtained. It is found that cluster structure components are contained in those bands.

High spin band structure in 139Nd

High-spin states in 139Nd nucleus have been reinvestigated with the reaction 128Te (16O, 5n) at a beam energy of 90 MeV. The level scheme has been expanded with spin up to 47/2 ħ. At the low spin states, the yrast collective structure built on the vh11/2−1 multiplet shows a transitional shape with γ ≈ 32° according to calculations of the triaxial rotor-plus-particle model. Three collective oblate bands with γ ∼ −60° at the high spin states were identified for the first time. A band crossing is observed around ħω ∼0.4 MeV in one oblate band based on the 25/2− level.

High-spin states and collective oblate bands inNd139

High spin states in {sup 139}Nd nucleus have been reinvestigated with the reaction {sup 128}Te ({sup 16}O, 5n) at a beam energy of 90 MeV. The level scheme has been expanded with spin up to 47/2({Dirac_h}/2{pi}). At low spin, the yrast collective structure built on the {nu}h{sub 11/2}{sup -1} multiplet shows a triaxial shape with {gamma}{approx_equal}32 deg. according to calculations of the triaxial rotor-plus-particle model. The configurations for some high spin states have been discussed from a systematic comparison with neighboring nuclei. Three collective oblate bands with {gamma}{approx}-60 deg. at high spin were identified for the first time. A band crossing is observed around ({Dirac_h}/2{pi}){omega}{approx}0.4 MeV in one oblate band based on the 25/2{sup -} level. The characteristics for these oblate bands have been discussed.