Positive-parity Yrast Band Research Articles

A new type of band crossing at large deformation

A detailed study of the positive parity yrast and yrare rotational bands in 77Rb is presented. Using the reaction 40Ca( 40Ca, 3 p) and the EUROGAM I spectrometer, γγγ coincidences enabled us to follow both bands over a large spin range and to measure many E2 strengths. The moments of inertia and transition quadrupole moments indicate that a more deformed band ( ϵ 2 ≈ 0.38) is crossed by a less deformed one ( ϵ 2 ≈ 0.29). Since the yrare band starts at the extremely low spin value of I = 9 2 , the conventional band crossing mechanism of two aligning high- j quasiparticles is excluded. The frequency-dependent equilibrium shapes were calculated with Nilsson-Strutinsky type calculations using a diabatic tracing of configurations near the neutron Fermi level. This is the first observation of such a band crossing.

Alpha -cluster structure of the yrast bands of 44Ti.

The Bloch-Brink microscopic {alpha}-cluster model is used to investigate the structure of the positive and negative parity yrast bands of {sup 44}Ti. Unlike the resonating group method or the local potential model, we approximate the wave function of {sup 44}Ti by an intrinsic configuration which is obtained from a variational principle. States with good angular momenta are constructed using angular momentum projection techniques. The wave function is completely antisymmetrized and the center of mass motion is treated properly. The calculated {ital E}2 transition strengths reproduce experiment very well, but the energy spectra are in a poor agreement with the data. {copyright} {ital 1996 The American Physical Society.}

128,130,132,134La in the axially symmetric rotor model.

Recent experimental data on positive-parity yrast states, based on the {pi}{ital h}{sub 11/2}{direct_product}{nu}{ital h}{sub 11/2} configuration, of odd-odd {sup 128,130,132,134}La are compared in a systematic way with the energies and transition probabilities calculated within the framework of the two quasiparticle, axially symmetric core model. It is found that very good agreement is obtained in {sup 128,130}La assuming prolate deformation. Influence of the neutron Fermi level on the level properties in these two nuclei has also been studied. In the case of {sup 132,134}La, this model seems to indicate that this positive parity yrast band may have an oblate deformation.

High-spin bands in 80Kr.

High-spin states in $^{80}\mathrm{Kr}$ were studied using the $^{65}\mathrm{Cu}$${(}^{18}$O,p2n) reaction at 65 MeV. Prompt \ensuremath{\gamma}-\ensuremath{\gamma} coincidences were measured using the Pitt-FSU detector array with nine high-purity Compton-suppressed Ge detectors and a 28-element Bismuth Germanate multiplicity filter. The previous level scheme has been extended by 16 new states up to spins and parities of ${20}^{+}$ and (${19}^{\mathrm{\ensuremath{-}}}$). The known band crossing in the positive-parity yrast band of $^{80}\mathrm{Kr}$ near \ensuremath{\Elzxh}\ensuremath{\omega}\ensuremath{\approxeq}0.5 MeV may be due to a ${\mathit{g}}_{9/2}$ neutron alignment, rather than a ${\mathit{g}}_{9/2}$ proton alignment as had been previously thought, based on the predicted oblate shape by Hartree-Fock-Bogolyubov calculations. No evidence was found for a second crossing in the yrast sequence up to \ensuremath{\Elzxh}\ensuremath{\omega}=0.9 MeV. The new extension of the ground-state band through nonyrast states points to another band crossing at \ensuremath{\Elzxh}\ensuremath{\omega}\ensuremath{\approxeq}0.62 MeV which may be caused by a ${\mathit{g}}_{9/2}$ proton alignment. A new negative-parity high-spin band shows that rotational bands do form at excitations above the strongly mixed low-spin levels as in the isotone $^{82}\mathrm{Sr}$.

Negative-parity structures and lifetime measurements in 71As.

High-spin states up to (29/${2}^{+}$) and (29/${2}^{\mathrm{\ensuremath{-}}}$) in $^{71}\mathrm{As}$ have been studied via the $^{58}\mathrm{Ni}$${(}^{19}$F,\ensuremath{\alpha}2p) reaction at 62 MeV using in-beam \ensuremath{\gamma}-ray spectroscopic techniques. Based on a particle-\ensuremath{\gamma} coincidence measurement a new decay sequence of negative parity has been assigned to $^{71}\mathrm{As}$. Lifetimes for some E2 transitions were extracted from Doppler-shifted line shapes observed in coincidence mode, and experimental transition quadrupole moments were determined. They indicate a moderate to high collectivity in both the positive-parity yrast band and the new negative-parity decay sequence at high spins. Total Routhian surfaces calculated for states of both parities reveal strong triaxial shapes at high spins. A noncollective structure of 19/${2}^{(\mathrm{\ensuremath{-}})}$ and 21/${2}^{(\mathrm{\ensuremath{-}})}$ states is interpreted as a three-quasiparticle excitation.

High spin structure of 84Y.

The high spin levels of $^{84}\mathrm{Y}$ have been established up to 12 MeV excitation energy and probable spin of ${25}^{+}$ through the reaction $^{59}\mathrm{Co}$${(}^{28}$Si,2pn${)}^{84}$Y using in-beam \ensuremath{\gamma}-ray spectroscopic techniques. The \ensuremath{\gamma} transitions in $^{84}\mathrm{Y}$ have been identified through the particle-gamma coincidences. The level scheme was constructed through the \ensuremath{\gamma}-\ensuremath{\gamma} coincidences, angular distribution, and directional correlation orientation ratios. The positive parity yrast band built on the (\ensuremath{\pi}${\mathit{g}}_{9/2}$\ensuremath{\bigotimes}\ensuremath{\nu}${\mathit{g}}_{9/2}$) configuration shows a large signature splitting and a signature inversion at I\ensuremath{\sim}11\ensuremath{\Elzxh}. The measured B(E2; I\ensuremath{\rightarrow}I-2) values give a deformation ${\mathrm{\ensuremath{\beta}}}_{2}$\ensuremath{\sim}0.17. The negative parity band shows a signature inversion at I\ensuremath{\sim}11\ensuremath{\Elzxh} and B(M1; I\ensuremath{\rightarrow}I-1) enhancement for Ig11\ensuremath{\Elzxh}. The projected Hartree-Fock and the cranked shell model calculations provide an understanding for the observed properties of the energy levels in $^{84}\mathrm{Y}$.

Nuclear structure of odd-oddY82

Excited states in $^{82}\mathrm{Y}$ have been investigated via the $^{58}\mathrm{Ni}$${(}^{28}$Si,3pn${)}^{82}$Y and $^{56}\mathrm{Fe}$${(}^{29}$Si,p2n${)}^{82}$Y reactions using beams provided by the Florida State University Tandem-LINAC with energies of 95 to 135 MeV. Particle-\ensuremath{\gamma} coincidences with a full-sphere charged-particle detector and an excitation curve were used to assign \ensuremath{\gamma} rays to $^{82}\mathrm{Y}$. The $^{56}\mathrm{Fe}$${(}^{29}$Si,p2n${)}^{82}$Y reaction at 95 MeV was used to measure \ensuremath{\gamma}-\ensuremath{\gamma} coincidences with five Compton-suppressed Ge detectors. Lifetimes in the ns and ps ranges were measured using direct timing and the Doppler-shift attenuation method. A level scheme with 39 states was constructed up to tentative spins and parities of (${15}^{\mathrm{\ensuremath{-}}}$) and (${21}^{+}$). In the positive-parity yrast band a phase reversal was observed in the signature splitting at I=10\ensuremath{\Elzxh}. The B(M1) strengths in this band alternate with signature between about 0.1 and 1.0 ${\mathrm{\ensuremath{\mu}}}_{\mathit{N}}^{2}$. The B(E2) strengths of about 90 Weisskopf units are in good agreement with those predicted by Hartree-Fock-Bogolyubov calculations. Particle-rotor calculations indicate a prolate or triaxial core shape for the positive-parity band.

Tests of microscopic calculations of multiple band structures and large deformations in 68Ge and 72Se.

High-spin states in $^{68}\mathrm{Ge}$ and $^{72}\mathrm{Se}$ have been studied by in-beam, gamma-ray spectroscopy. In $^{68}\mathrm{Ge}$ the three known bands beginning at ${8}^{+}$ are found to exhibit crossing transitions and other new positive-parity bands are discovered to feed into them. In $^{72}\mathrm{Se}$ only a single positive-parity yrast band is seen to high spin. Detailed, more realistic microscopic model calculations have been carried out for $^{68}\mathrm{Ge}$ and $^{72}\mathrm{Se}$. There is striking agreement between the calculations and the new data for $^{68}\mathrm{Ge}$, including a predicted new band with superdeformation, \ensuremath{\beta}\ensuremath{\sim}0.42. The calculations for $^{72}\mathrm{Se}$ predict similar multiple band structures that are not seen experimentally.

In-beam spectroscopy and microscopic calculations in the A = 60−80 region

High spin states in 68Ge and 72Se have been studied via 46Ti(68 MeV 25Mg, 2p) 68Ge and 47Ti(95 MeV 28Si, 2pn) 72Se reactions with the Holifield heavy ion tandem. Gamma-gamma coincidence experiments were carried out in the spin spectrometer and the close geometry Compton-suppressed Ge ball. In 68Ge the three known bands built on 8 + states were found to exhibit crossing transitions, and other new bands were discovered to feed into them. In 72Se only a single positive-parity yrast band is seen to have high spins. Detailed microscopic model calculations have been carried out for 68Ge and 72Se.

The unpaired spectroscopy of 161,162Er at spins up to 50ħ

High spin states in 161Er and 162Er have been populated using the 130Te + 36S reaction at a bombarding energy of 170 MeV. In 161Er, three rotational bands were extended from I≈35ħ to I≈50ħ. In 162Er the positive parity yrast band was observed to I=44ħ. These rotational sequences are compared to a simple unpaired model which was successful in predicting the behaviour of similar high spin data on 159,160Er. These studies of the light Er isotopes constitute the highest spins observed in normal deformed nuclei and yield direct and specific information on the single-neutron spectrum of states.

On the coexistence of oblate and prolate deformed bands in83Zr

Deformation parameters of the positive parity yrast band and negative parity bands in83Zr are deduced from lifetimes andE2/M1 mixing ratios. Lifetimes of high spin states have been determined from recoil distance Doppler shift and Doppler shift attenuation measurements using the54Fe(32S,2pnγ) 83Zr reaction. Ten lifetimes and five lifetimes limits were determined. The positive parity band, built on theg 9/2 K=5/2 orbital has an average deformation ¦β 2¦=0.28(2), and shows a reduction ofE2 transition strengths in the observed backbend region at Iπ≈21/2+. In contrast, theE2 strengths in the negative parity states show a steady increase up to Iπ≈=15/2−. These states are more strongly deformed than the positive parity states (¦β 2¦=0.33(3)). TheE2/M1 mixing ratios show that the negative parity band hasK=3/2 and is prolate, and favour oblate deformation for the positive parity yrast band. In theK=1/2− band theE2 strength of the 7/2−→3/2− transition yields a deformation ¦β 2¦=0.26(5). The band structure is compared with calculations within the Hartree-Fock Bogoliubov cranking model.

Side-feeding times and level lifetimes measured in the reaction 54Fe( 32S, 3p) 83Y

We have performed recoil-distance and Doppler-shift-attenuation lifetime measurements in the reaction 54Fe( 32S, 3p) 83Y at 103–107 MeV beam energy. By analyzing the Doppler-broadened line shapes of the 964 keV 21 2 + → 17 2 + yrast transition in coincidence with feeding and depopulating transitions, the average side-feeding time and lifetime of this state have been deduced as τ F = 0.6(2) ps and τ = 0.62(10) ps, respectively. A similar analysis of the 811 keV 17 2 +→ 13 2 + transition gave values of τ F = 0.65(25) ps and τ = 1.5(2) ps, respectively for the 17 2 + yrast state. Feeding times of the higher transitions in the positive-parity yrast band have been estimated from their Dopplerbroadened lineshapes measured in coincidence with depopulating transitions only. The feeding times are compared with the results of Monte Carlo calculations of the time development of the continuum γ-ray flux, and a damping width of 2–3 MeV has been deduced. The measured level lifetimes support the regular band structures with the deformation β 2 = 0.33 previously inferred from the transition energies.

Band crossings and near-rigid rotation in 76Kr and 78Kr

High-spin states in 78Kr and 76Kr were studied via the reactions 58Ni( 24Mg, 4p) 78Kr and 58Ni( 24Mg, α2p) 76Kr at 110 MeV beam energy. An array of fifteen Compton suppressed Ge detectors was used in the POLYTESSA framework to collect γγ-coincidence spectra. Rotational bands were observed up to probable spins of 24 + and 22 − in 76Kr, and 24 + and 21 in 78Kr. While the positive-parity yrast bands show strong variations in the moments of inertia caused by the alignments of pairs of ( g 9 2 ) proton and neutron quasiparticles, the negative-parity bands display a constant moment of inertia throughout most of their frequency range. The band crossings and alignments in both nuclei are discussed in the context of the cranked shell model. From an examination of the systematics of crossing frequencies along the Kr isotopes, the previous suggestion that protons align first is confirmed. The intrinsic structures of the bands are also analyzed with the Woods-Saxon-Strutinsky cranking model. The interplay between neutron and proton excitations, the shape changes induced by quasiparticle alignment, and the possible reduction of the static proton pairing in the negative-parity bands are discussed in detail. Three lifetimes in the 76Kr ground band were remeasured by using the Doppler shift attenuation method, indicating a constant deformation of β 2 = 0.33 up to spin 10 +.

Transition probabilities in the band crossing region of79Kr

A change of the M1 transition probabilities is observed in the band crossing region in the positive-parity yrast band of 79Kr. This change is compared with that observed in the neighbouring nucleus 81Kr and is interpreted on the basis of a semiclassical coupling scheme of particle angular momenta.

High-spin yrast and non-yrast bands in 176Os, 178Os and 180Os

High-spin yrast and non-yrast states have been identified in 176Os, 178Os and 180Os using ( 16 O, xn) reactions, and γ-ray techniques. Band crossing anomalies are observed in each of the positive-parity yrast bands. The magnitude of these anomalies decreases with decreasing neutron number, an effect attributed to the change in the moment of inertia of the ground state rotational bands. A 23 ns isomer, predominantly K π = 7 −, is identified at 1930 keV in 180Os. The configuration of this isomer is discussed on the basis of the properties of its rotational band. Negative parity, odd and even spin, sideband sequences are observed in each isotope. Their relationship to rotation-aligned octupole and 2-quasiparticle bands is discussed from their excitation energies, band spacings, and decay properties. Detailed calculations for Coriolis mixed bands are carried out for the likely 2-quasiproton and 2-quasineutron configurations. An anomaly observed at spin 17 in the odd-spin negative-parity sequence in 180Os is attributed to a band crossing with a fourquasiparticle configuration.

Collective band structure to high spin and shape coexistence inKr76

Excited states in /sup 76/Kr have been investigated via the /sup 66/Zn(/sup 12/C, 2n) reaction at a beam energy of 39 MeV. Gamma-ray energies, intensities, spins, parities, and mean lives of excited states were extracted from measurements of the ..gamma.. rays which were emitted in-beam. The energies of 25 excited levels were deduced from ..gamma..-..gamma.. coincidence data, and all but three of these levels are members of bandlike structures which persist to high spin. The Doppler shift attenuation method was used to extract the lifetimes of thirteen excited levels. The E2 strengths are highly collective. The positive parity yrast band to spin (12/sup +/) has been observed with a forward bend at low spin, but with no backbending in the moment of inertia below 12/sup +/. A positive parity band with even- and odd-spin members to (6/sup +/) and (9/sup +/), respectively, has been observed and is interpreted as a ..delta..J = 1 quasigamma band. Negative-parity bands built on 5/sup -/ and (6/sup -/) levels have been observed to spins of (13/sup -/) and (12/sup -/), respectively. Finally, the first excited 0/sup +/ state and a 2/sup +/ level built on it have been identified and are assigned to amore » near spherical configuration which coexists with the ground state that has an unusually large deformation.« less

Rotational bands in 170Yb observed following (α,xn) reactions

Abstract At least seven side bands, with Kπ = (0+), (1−), (2+), (3−), 4−, 6−, and 7−, have been identified in 170Yb up to high spin, using γ-ray techniques following (α, 2n) and (α, 4n) reactions. Backbending in the positive-parity yrast band is confirmed, and candidates for low-spin members of the S-band are found. The negative-parity yrast band has behaviour characteristic of both strong coupling and decoupling. These properties, together with those in the other negative-parity side bands, are interpreted as arising from Coriolis effects on the quasiparticle configurations, and the experimental results are compared with a two-quasipartiele-plus-rotor calculation. Evidence of ΔK = 2 mixing is found in the K = (3) side band. The relationship between the moments of inertia of the positiveparity and negative-parity yrast bands in the N = 100 isotones is discussed in the framework of a simple empirical model, giving insight into the presence or absence of backbending. The negativeparity yrast bands in the N = 100 isotones are compared. Their significant differences are qualitatively reprodued in two-quasiparticle-plus-rotor calculations.

Evidence for an Octupole Rotational Band inSe74

From in-beam $\ensuremath{\gamma}$-ray spectroscopy in $^{74}\mathrm{Se}$, a negative-parity band to (${11}^{\ensuremath{-}}$) with rotational properties built on a ${3}^{\ensuremath{-}}$ collective octupole state is established along with the positive-parity (Barrette et al.) yrast band to (${14}^{+}$). The negative-parity band has essentially a constant moment of inertia in sharp contrast to the positive-parity band. This is the first negative-parity band outside those in deformed nuclei with such pure rotational behavior.