Negative-parity Side Bands Research Articles

High spin states of 195Hg

High spin states in 195Hg, populated using the 150Nd(48 Ca,3n) reaction, were investigated by γ-ray spectroscopic techniques. Two backbendings are observed in the yrast band considered to be caused by the alignment of a pair of i13/2 and p3/2 neutrons respectively. The strong backbending behaviour observed in the negative parity side band is consistent with the alignment of a pair of i13/2 neutrons. The similarity with the corresponding backbendings in the neighbouring isotopes is impressive.

Reduction of collectivity at very high spins inNd134: Expanding the projected-shell-model basis up to 10-quasiparticle states

Background: The recently started physics campaign with the new generation of $\ensuremath{\gamma}$-ray spectrometers, ``GRETINA'' and ``AGATA,'' will possibly produce many high-quality $\ensuremath{\gamma}$ rays from very fast-rotating nuclei. Microscopic models are needed to understand these states.Purpose: It is a theoretical challenge to describe high-spin states in a shell-model framework by the concept of configuration mixing. To meet the current needs, one should overcome the present limitations and vigorously extend the quasiparticle (qp) basis of the projected shell model (PSM).Method: With the help of the recently proposed Pfaffian formulas, we apply the new algorithm and develop a new PSM code that extends the configuration space to include up to 10-qp states. The much-enlarged multi-qp space enables us to investigate the evolutional properties at very high spins in fast-rotating nuclei.Results: We take $^{134}\mathrm{Nd}$ as an example to demonstrate that the known experimental yrast and the several negative-parity side bands in this nucleus could be well described by the calculation. The variations in moment of inertia with spin are reproduced and explained in terms of successive band crossings among the 2-qp, 4-qp, 6-qp, 8-qp, and 10-qp states. Moreover, the electric quadrupole transitions in these bands are studied.Conclusions: A pronounced decrease in the high-spin $B(E2)$ of $^{134}\mathrm{Nd}$ is predicted, which suggests reduction of collectivity at very high spins because of increased level density and complex band mixing. The possibility for a potential application of the present development in the study of highly excited states in warm nuclei is mentioned.

Reinvestigation of high spin states and proposed octupole correlations inCe147

High spin states in neutron-rich $^{147}\mathrm{Ce}$ have been reinvestigated by measuring the triple- and fourfold $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ coincidence data observed in the spontaneous fission of $^{252}\mathrm{Cf}$. The positive-parity band is expanded to spin (45/2) $\ensuremath{\hbar}$ and the negative-parity side band is expanded to spin (39/2) with a new third band connected to it. An octupole band structure with $s=+i$ was proposed. The systematic characteristics of the octupole correlations have been discussed. Reflection asymmetric shell model calculations for the octupole band structure of $^{147}\mathrm{Ce}$ are in good agreement with the experimental data.

In-beam gamma spectroscopy of 73As nucleus

The high spin level structure of 73As nucleus is studied by populating the nucleus in 64Ni(12C,p2n)73As reaction. Level scheme is revised significantly. Positive parity sequence is extended up to 33/2+ and a negative parity side band is identified and extended up to high spins 37/2−. In addition about 15 new energy levels and a total of about 25 new gamma transitions were placed in the level scheme.

Possible magnetic and antimagnetic rotations inDy144

High spin states of $^{144}\mathrm{Dy}$ have been studied through in-beam $\ensuremath{\gamma}$-ray spectroscopy by using the reaction $^{92}\mathrm{Mo}(^{56}\mathrm{Fe},2p2n)$. It has been found that the continuation of the ground-state band forks into three $\ensuremath{\Delta}I=2$ bands above the ${8}^{+}$ state. This forking has been attributed to the alignments of $\ensuremath{\pi}{h}_{11/2}^{2}$ or $\ensuremath{\nu}{h}_{11/2}^{\ensuremath{-}2}$ configurations with the help of the systematics in neighboring nuclei. Additionally a negative-parity sideband of $\ensuremath{\Delta}I=2$ cascades has been observed to start from the ${5}^{(\ensuremath{-})}$ state and continue to a dipole band above the (${13}^{\ensuremath{-}}$) state through another negative-parity sideband of $\ensuremath{\Delta}I=2$ cascades in between. These structures have been discussed from the viewpoint of a competition between ``Magnetic Rotation'' and ``Anti-magnetic Rotation'' based on a classical particles-plus-rotor model.

Excited states and deformation of 112Xe

Abstract Excited states have been identified in the very neutron-deficient 112 Xe nucleus. The ground-state band has been observed up to spin 12 ℏ, and a negative-parity side band has been observed up to 13 ℏ. The bands have been assigned to 112 Xe by detecting gamma rays in coincidence with evaporated charged particles and neutrons. The energy of the 2 + 1 state in 112 Xe is lower than would be expected from both calculations and from systematics of the neighboring xenon isotopes, which indicates a larger than expected ground-state deformation. The observations of a low-lying 3 − state and large deduced B (E1) values are suggestive of increased octupole correlations, which may explain the apparently large ground-state deformation. Constrained Hartree–Fock+BCS calculations have been performed which show qualitative agreement with the trend in deformation of the very neutron-deficient xenon isotopes.

Pronounced shape change induced by quasiparticle alignment

Mean lifetimes of high-spin states of Kr-74 have been determined using the Doppler-shift attenuation method. The high-spin states were studied using the Ca-40(Ca-40, alpha 2p) reaction at a beam energy of 160 MeV with the GASP gamma-ray spectrometer. The ground-state band and negative parity side band show the presence of three different configurations in terms of transitional quadrupole deformations. A dramatic shape change was found along the ground-state band after the S-band crossing. The deduced quadrupole deformation changes are well reproduced by cranked Woods-Saxon Strutinsky calculations.

Systematics of even-even Tz= 1 nuclei in the A= 80 region: High-spin rotational bands in 74Kr, 78Sr, and 82Zr

High-spin states of T-z = 1 nuclei were studied with the reactions Ni-58(Si-28,3 alpha)Kr-74,Ni-58(Si-28,2 alpha)Sr-78, and Ni-58(Si-28,2p2n)Zr-82 at 130 MeV beam energy. The Gammasphere array in conjunction with the 4 pi charged-particle detector array Microball was used to detect gamma rays in coincidence with evaporated light charged particles. The known pi = +, alpha = 0 yrast bands were extended to I = 28 (h) over bar at 20 MeV excitation energy. For all three nuclei, a number of positive- and negative-parity sidebands were established; altogether 15 new rotational bands were found. The data are discussed using the pairing-and-deformation self-consistent total Routhian surface (TRS) model: High-spin structures of Kr-74 and Sr-78 are governed by the shell gaps at large prolate deformation while Zr-82 seems to exhibit shape coexistence. Nearly identical bands were established which may be explained as arising from the fp orbits acting as spectators at very elongated shapes. The experimental data in these T-z = 1 nuclei are in good agreement with predictions of the TRS model using conventional T = 1 like-nucleon pairing correlations. (Less)

Competing proton and neutron alignments in neutron-deficient Xe-nuclei

High-spin structures of the neutron-deficient 117,118,120Xe 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 117Xe. In the even 118Xe and 120Xe 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 118Xe, whereas in 120Xe 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.

Prolate and oblate rotational bands in 136Sm

Six rotational bands have been established in 136Sm following heavy-ion induced reactions carried out at the Nuclear Structure Facility, Daresbury. The yrast band has been observed up to Ipi =(24+) and undergoes three band-crossings. A band was observed built on the low-lying 2+ gamma-vibrational state, in addition to two negative parity sidebands built on two-quasiproton excitations. A dipole band showing the characteristics of collective oblate rotation was also established. The results are discussed in terms of total Routhian surface and cranked Woods-Saxon model calculations.

High-spin states and intrinsic structure in 174Os and 175Os: Alignments and strong interaction

Abstract High-spin states have been identified in 174Os and 175Os using (HI, x>n) reactions and gamma-ray techniques. Rotational bands built on low-lying intrinsic states in 175Os have been observed for the first time. The intrinsic states are identified as a mixed i 13 2 neutron state (nominally 7 2 + [633] ) and the 1 2 − [521] and 5 2 − [512] Nilsson configurations. Differences in the alignment of the two signatures of the decoupled band built on the 1 2 − [521] state and the comparison between the signature splitting of routhians and if B(M1)/B(E2) values in the i 13 2 band suggest softness towards gamma deformation. Crossing transitions between the negative-parity bands at intermediate spin have been observed and the interaction strength derived from the branching ratios. A comparison of the properties of bands in 174Os and 175Os shows some expected similarities, but also surprising differences which point towards the need for a more comprehensive treatment in strong interaction cases. Depending on the choice of reference the alignment in the i l3 2 band might show degenerate BC and AD crossings at a relatively low frequency. This may be consistent with the alignment observed in the negative parity side bands in 174Os.

High spin spectroscopy of124Ba andh 11/2 neutron alignment

The level scheme of the yrast band of124Ba has been extended up to spin 32ħ. Transitions in the two negative parity side bands are observed up to spin 27ħ and tentatively 20ħ. The second backbend observed in the S-band and the backbends in the side bands are explained as due to neutrons. The structure of the sidebands is discussed and compared with deformation self-consistent calculations, Total Routhian Surfaces (TRS).

The structure of states at very high spin in 157Er

High spin states of 157Er have been populated using the 114Cd+48Ca reaction at a 48Ca bombarding energy of 200 MeV. Gamma rays were detected in an array (TESSA2) of six escape-suppressed Ge spectrometers and a 50-element BGO multiplicity and summed energy detector. Decay scheme and angular correlation data were obtained from the gamma - gamma BGO coincidence data The yrast band has been identified up to 65/2+ and two negative parity sidebands have been identified (one previously unobserved) up to 69/2- and 63/2-. An additional sideband of six transitions was also observed. The band structures are interpreted in terms of quasiparticles in the framework of the cranked shell model.

Band crossings in even-even 138Sm and 134Nd.

The rotational alignment of ${h}_{11/2}$ quasiparticles has been observed in the even-even $^{138}\mathrm{Sm}$ and $^{134}\mathrm{Nd}$ nuclei by means of heavy-ion fusion-evaporation reactions. The structures observed are compared to those found recently in $^{136}\mathrm{Nd}$. The ground-state bands of the $^{136}\mathrm{Nd}$ and $^{138}\mathrm{Sm}$ isotones (N=76) possess significant triaxial shapes (\ensuremath{\gamma}\ensuremath{\sim}-25\ifmmode^\circ\else\textdegree\fi{}). Rotational alignment of ${h}_{11/2}$ protons induces a shape change towards the prolate (\ensuremath{\gamma}=0\ifmmode^\circ\else\textdegree\fi{}) shape, while ${h}_{11/2}$ neutron alignment induces a change towards the oblate (\ensuremath{\gamma}=-60\ifmmode^\circ\else\textdegree\fi{}) collective shape. In the more deformed $^{134}\mathrm{Nd}$ nucleus, band crossings are observed in both the ground-state band and the \ensuremath{\gamma}-vibrational band. The crossing in the ground-state band is attributed to ${h}_{11/2}$ protons, while a second band that feeds into both the ground-state band and the \ensuremath{\gamma}-vibrational band is built on the [${h}_{11/2}$${]}^{2}$ neutron-aligned configuration. In addition, negative-parity sidebands built on two-quasiproton configurations were seen in these nuclei. The systematics of the proton and neutron alignments are discussed.

Band crossings in the gamma -soft nucleus 136Nd.

High-spin states have been populated in /sup 136/Nd using the /sup 116/Cd(/sup 24/Mg,4n..gamma..) /sup 136/Nd reaction. Several rotational bands have been observed based on two- and four-quasiparticle configurations. The ground-state band backbends sharply at a frequency h-dash-bar..omega.. = 0.34 MeV due to the alignment of a pair of h/sub 11/2/ protons. Both signatures of a negative parity side band based on a ..pi..h/sub 11/2/x..pi..g/sub 7/2/ configuration were observed to backbend at a frequency h-dash-bar..omega.. = 0.44 MeV, again due to the alignment of a pair of h/sub 11/2/ protons. All these bands are prolate because of the stabilizing influence of the h/sub 11/2/ quasiprotons on the ..gamma..-soft core. In addition to the h/sub 11/2/ proton alignment in the ground-state band, a rotational band is observed built on a second 10/sup +/ state, which involves the alignment of a pair of h/sub 11/2/ neutrons. An alignment of neutrons from the upper h/sub 11/2/ midshell drives the nucleus towards a collectively rotating oblate (..gamma.. = -60/sup 0/) shape. Both signatures of a band based on a ..nu..h/sub 11/2/x..nu..g/sub 7/2/ configuration were observed, also possibly oblate. The observation of both neutron and proton low-frequency alignments is consistent with cranked-shell model calculations and theoreticalmore » estimates of the prolate-oblate energy difference for this nucleus.« less

Near-yrast discrete line -ray spectroscopy of159Er and160Er to I or 40

High-spin states in 159Er and 160Er have been populated using the 116Cd+48Ca reaction at a 48Ca bombarding energy of 210 MeV. Gamma rays were detected in an array (TESSA2) of six escape suppressed germanium spectrometers and a 50 element BGO multiplicity and summed energy detector. Decay scheme and angular correlation data for these nuclei were deduced from the gamma - gamma -BGO coincidence data. In 159Er the yrast band has been identified up to Ipi =89/2+ and three side bands have been observed to 69/2-(77/2-), 43/2+ and 51/2-(59/2-). In 160Er the lowest energy positive-parity band has been observed up to 38+(40+) and the two lowest energy negative-parity side bands up to 40-(44-) and 39-(41-). The ground state level sequence has been observed up to (28+). The spin and parity of the low-lying states (I<20h(cross)) in the bands observed in 160Er were established by measuring gamma -ray angular distributions and internal conversion coefficients following the 148Nd(16O, 4n) reaction at a beam energy of 80 MeV. The band structures can be understood in terms of aligning quasiparticles using the cranked shell model. The alignment of the first pair of h11/2 protons is observed in the three lowest energy bands in each nucleus. The increase in crossing frequency of this alignment with neutron number is further evidence for the increase in deformation for the light Er isotopes. Evidence is presented for the competition of favoured single-particle configurations with the collective structures at the highest spins.

High-spin states in172Hf

The reaction 128Te(48Ca,4n) has been used to extend the yrast sequence of 172Hf up to Ipi =30+. In addition, two negative-parity sidebands have been observed to Ipi =24- and 25-. The aligned angular momentum of these bands is seen to increase approximately linearly with rotational frequency at high frequencies.

Rotational alignment in158Er and159Er

High-spin states in 158Er have been populated with the reactions 145Nd(16O,3n) and 146Nd(16O,4n) and high-spin states in 159Er by the reaction 145Nd(18O,4n). Gamma-gamma coincidence and angular correlation data were obtained using an array of five Ge(Li)-NaI(Tl) escape-suppressed spectrometers. Linear polarisation data were obtained using a sectored Ge(Li) detector and internal conversion data were obtained using a mini-orange spectrometer. In 158Er the yrast band has been observed up to Jpi =26+ and three negative-parity side bands have been observed up to spins Jpi =23-, 22- and 15- respectively. The ground-state band has been observed to continue after the first backbend in the yrast sequence to Jpi =20+ and the odd- and even-spin members of the gamma-vibrational band were observed up to 7+ and 10+, respectively. In 159Er the yrast band has been observed up to Jpi =57/2+ and four side bands have been identified to spins Jpi =15/2-, 39/2+, 49/2-, and 51/2- respectively. The bands observed are interpreted as configurations of quasiparticles using the cranked shell model. Residual quasiparticle interactions are calculated as a function of rotational frequency.

Rotational Behavior at High Spin inHf168

The yrast sequence in $^{168}\mathrm{Hf}$ has been extended to ${I}^{\ensuremath{\pi}}={34}^{+}$ and two negative-parity side-bands have been observed to ${I}^{\ensuremath{\pi}}={33}^{\ensuremath{-}} \mathrm{and} {30}^{\ensuremath{-}}$. At high rotational frequencies all three bands have little spin alignment and have moments of inertia which remain constant as a function of frequency. Possible evidence for the vanishing of pairing correlations is discussed.

Strongly interacting bands in 170Hf

High-spin states in 170Hf have been excited by the reaction 158Gd( 16O, 4n). The yrast band has been observed up to the I π = 26 + member and a new odd-spin negative-parity side band is established up to I π = 25 −. The consequences of strong-band interactions in this nucleus are discussed. The band crossings observed are compared with those in neighbouring odd- A nuclei and the cranked shell model. An apparent failure of this model with constant pairing to predict band-crossing frequencies and alignments simultaneously in the yrast and side bands is discussed. Average values of 2 L (2) band/ħ 2 are presented for the “rotational-like” continuum cascades at the angular frequencies of the pairing phase transition and are compared with similar values for the yrast cascade.