Yrast Cascade Research Articles

Decay ofTI188and observed shape coexistence in the bands ofHg188

The decay of the isotope $^{188}\mathrm{Tl}$ (${T}_{\frac{1}{2}}=71\ifmmode\pm\else\textpm\fi{}1$ s) was studied via conversion-electron, gamma, gamma-gamma, and gamma-conversion-electron measurements. Levels of $^{188}\mathrm{Hg}$ were deduced. Two bands with quite different deformation were established. One band is built on a near spherical ground state and the other band is built on a well-deformed shape with an excited ${0}_{2}^{2}$ band head at 824.5 keV. The bands cross and coexist above and below the crossing point. Their crossing explains the unusual behavior of the moment of inertia of the yrast cascade.

Two discontinuities of the moment of inertia in the74Kr yrast levels at high spin

The levels of 74Kr were identified to a tentative 20+ level in the reaction 58Ni(19F, p2n gamma ) by using a neutron multiplicity filter. The moment of inertia of the yrast cascade exhibits the first high-spin double discontinuity in such light nuclei, above 10+ and 14+. Both discontinuities are interpreted as arising from the crossing of rotation aligned bands.

High-spin properties of 163Yb: Band crossings and signature-splitting in Yb nuclei

High-spin states in 163Yb have been studied by the 149Sm( 18O, 4n) 163Yb reaction with 84 MeV 18O ions from the NBI tandem accelerator. Gamma-gamma coincidence data were accumulated with an array of five Compton-suppressed Ge(Li) detectors resulting in an extension of our knowledge of high spin members of familiar rotational bands as well as of weakly populated side bands. A γγ time coincidence experiment, conversion electrons and angular distribution measurements were performed. Four side bands are observed besides the yrast cascade, which is followed up to its 49 2 + member. The yrast band of the 163Yb nucleus has been interpreted, following the systematics in this region of neutron deficient rare-earth nuclei, as the favoured member of a decoupled i 3 2 neutron band. The negative-parity band ( −, +) 1 based on the 3 2 −[521] state was reestablished. The other signature of this band is the newly introduced band ( −, − ) 1. The signature splitting in the negative-parity band is observed to be small; however, for large &z.xh;ω, band ( −, − ) 1 lies lower in energy than band ( −, +) 1; which is a unique observation in this mass region. The quasiparticle routhians and alignments obtained from these data are compared with results of cranked shell-model calculations and similar data for the heavier and lighter Yb nuclei. The rotational effects on M1 transitions connecting unfavoured members of the band, based on the i 13 2 orbital, with the favoured members are demonstrated.

Electromagnetic transition strengths and nuclear moments in123Xe

The level structure of123Xe has been studied with the114Cd (12C, 3n) and−Cd (13C, 4n) reactions using in-beamγ-ray spectroscopy. The yrast cascade on the low lying odd parity states originating from theh 11/2 neutron orbital has been investigated yielding lifetimesτ of 18(3) ps, 2.2 (4) ns and 17(2) ns for the 15/2−, 11/2− and 9/2− states, respectively. Theg-factor of thet 1/2=5.2 μs isomer has been measured to beg=−0.256(2). From the time differential observation of the electric quadrupole interaction in a114Cd single crystal the spin of this isomer has been determined to be 7/2 and from the coupling constante 2 Qq/h=116(2) MHz the quadrupole moment was deduced. The results are discussed in the framework of the triaxial rotor plus particle model.

Measurement of the linear polarization of continuum gamma rays from ( 32S, xn) reactions to prolate and oblate rare-earth nuclei

We describe the techniques and report the results of a measurement of the linear polarization, P γ of continuum γ-rays from Er and Dy nuclei produced in ( 32S, xn) reactions on 130Te and 124Sn targets. For the prolate 156–158Er isotopes we find P γ ≈ 0.35 in the γ-ray energy range if0.3 MeV ≦ E γ ≦ 1.5 MeV, thus confirming the E2 nature of the unresolved yrast cascades in rotational nuclei. In contrast, the rather small polarization P γ ≈ 0.05 for E γ ≦ 1.0 MeV measured for the oblate 151,152Dy isotopes, and the shape of the spectrum of continuum γ-rays suggest a rather direct feeding of the yrast states by statistical cascades. For 1.0 MeV ≦ E γ ≦ 1.5 MeV we find P γ ≈ 0.3 indicating collective rotations of the Dy nuclei at spin I ≧ 40 h z.shtsls; . Measurements of γ-ray angular distributions with a collimated NaI detector support these results. From the reduction of the P γ , and A 2 values of the γ-ray continuum in 156–158Er, compared to those measured for discrete E2 yrast transitions, we obtain estimates for the intensities of the M1 and statistical components in the continuum of these nuclei.

The low-seniority shell-model structure of 212At

Using the reaction 209Bi(α, n), states in the nucleus 212At were populated. The decay of excited states was studied using conventional in-beam γ-ray and conversion-electron spectroscopy. A great number of new levels were established and assigned spins and parities and the previously reported yrast cascade was confirmed. A new T 1 2 = 32 ± 1 ns isomer at 275.4 keV is reported and interpreted to be the 5 − state of the πj 0 + 2 h 9 2 ν g 9 2 configuration. The configurations were calculated as belonging to the neutron-proton seniority-2 multiplets where the available orbitals are h 9 2 , f 7 2 and i 3 2 for the protons and g 9 2 , i 11 2 and j 15 2 for the neutrons. Configuration mixing is shown to be essential in order to account for the experimental findings. On the basis of isomeric lifetimes the effective charges are discussed.

Evidence for Deformed Ground States in Light Kr Isotopes

The levels in $^{74,76}\mathrm{Kr}$ were studied with in-beam $\ensuremath{\gamma}$-spectroscopy techniques and the ${\ensuremath{\beta}}^{+}$ decay of $^{76}\mathrm{Rb}$. The energies of the ${2}_{1}^{+}$ states in $^{74,76}\mathrm{Kr}$ deviate from smooth behavior compared with the higher spin levels. The yrast cascade $B(E2)$'s are highly collective. The $^{74,76}\mathrm{Kr}$ ground states have unusually large deformation. The origin of this deformation and of shape coexistence in this region is described in terms of the protons driving the deformation.

High-spin yrast cascade inNi60

The role played by the /sup 1/g/sub 9/2/ orbital vis-a-vis the structure of the recently observed high-spin yrast cascade in /sup 60/Ni is examined. The anomalous high-spin sequence is correctly reproduced. A number of electromagnetic transitions are predicted.

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.

High-Spin Band Structure in 166Yb

High-spin states in 166Yb have been studied via the 154Sm(16O, 4n)166Yb reaction with 80 MeV 16O ions from the NBI tandem accelerator. γ-γ coincidence data were accumulated with an array of four Ge(Li) and four NaI(Tl) detectors to favour high multiplicity events. Totally about 3 × 108 events were accumulated in the coincidence experiment. Measurements of the angular distribution of the γ-radiation and the conversion coefficients were furthermore performed. Four weakly populated side bands are observed besides the yrast cascade which is followed up to its 24+ member. The ground state band has thus been identified to its 18+ state, while the crossing Stockholm band becomes the yrast band from its 16+ member. The S-band is established from its 12+ state. The observed level energies and transition rates are well reproduced in calculations within the particle rotor model.

Non-Collective Properties of the Continuum Gamma Rays Feeding Two High-Spin Isomers in 152Er

The decay and feeding of two yrast isomer (I ∼ 26ℏ and I ∼ 36ℏ respectively) in 152Er is measured. The isomeric delays are used to separate the spectrum of γ-rays from the highest angular momentum states from the spectrum due to the decay of low spin states. Strong dipole as well as quadrupole radiation dominates the yrast cascades which feed into the I ∼ 36ℏ isomer, and the correlation spectrum for the feeding transitions does not indicate any collective structure for spins below 50ℏ. It is tentatively concluded that the near magic nucleus 152Er remains spherical or oblate up to this high spin value.

High-Spin States in 204Bi and the Question of Many-Nucleon Configuration Mixing

The properties of high-spin levels in 204Bi have been studied using conventional in-beam γ-ray and conversion-electron spectroscopy on the products of the reactions 205Tl(α, 5n) 204Bi and 203Tl(α, 3n) 204Bi. The yrast cascade is followed to a 19+ level at 3808 keV and several non-yrast levels in the spin interval J = 9-16 are reported. The energies of the levels in 204Bi were calculated within the shell-model frame of multi-nucleon states, and the agreement is found to be good. A previously reported T1/2 = 1 ms isomeric state is shown to have a main configuration πh9/2ν(j-2)0+(i13/2-2)12+f5/2-1 and a spin-parity 17+. The configuration mixing for the neutron-hole components of the wave functions is discussed.

The dipole component in the yrast cascade and the multiplicity of statistical γ-rays

We have studied the quasi-continuum γ-rays following ( 16O, xn) reactions leading to the residual nuclei 162, 166Yb, 134Ce and and 122Xe by the γ-γ coincidence technique. In all cases studied, a dipole component was seen in the continuum spectra below ∼ 400 keV; it was particularly large in the 134Ce case. This component can be understood in terms of a simple model proposed previously. The average number of statistical γ-rays increases with increasing input angular momentum. This can be understood in terms of a corresponding increase of average excitation energy for γ-emission above the yrast line.

Low-Lying Yrast States inRn210andRn211and the Competition between Neutron-Hole and Proton Excitations

The $\ensuremath{\gamma}$-decay and nuclear structure of low-lying yrast and near yrast levels in $^{210}\mathrm{Rn}$ and $^{211}\mathrm{Rn}$ have been investigated. The cascade from the yrast ${6}^{+}$ state in $^{210}\mathrm{Rn}$ branches to two close-lying ${4}^{+}$ states, deduced to be complete mixtures of the ${4}^{+}$ states arising from the proton configuration, and from the neutron-hole intruder configuration. The influence of this proton-neutron-hole interaction on the yrast cascades in $^{211}\mathrm{Rn}$ and $^{206}\mathrm{Rn}$, $^{208}\mathrm{Rn}$ is discussed.

Protonh 2 11/2 and octupole excitations in148 66Dy82 and149 66Dy83

The yrast states of148Dy and149Dy have been studied by γ-ray and conversion electron measurements in (α, xn) and (16O,xn) reactions on enriched152Gd and135Ce targets. Level schemes to above 4 MeV for the two nuclei are reported. The πh211/2 spectrum identified in148Dy and the πh11/2 effective chargeeeff=1.52±0.05e, derived from the measuredE2 transition rate between the (πh211/2) 10+ and 8+ states, are discussed and compared with results for other two-particle nuclei. The yrast cascades in148Dy and149Dy continue above the (πh211/2) 10+ and πh211/2vf7/2) 27/2− states by ∼ 1 MeVE1 transitions de-exciting the lowest members of octupole multiplets built on these states. The energy shifts for the observed members of the πh211/2 × 3− multiplet are analyzed in terms of twoparticle-phonon exchange coupling using an empirical coupling strength extracted from the one valence particle nucleus147Tb. The dominantvf7/2×3− character of low-lying 13/2+ isomers in149Dy and otherN=83 nuclei is emphasized.

A dipole component in the yrast cascade and its possible interpretation

Quasi-continuum γ-rays following (heavy-ion, xn) reactions have been studied for the prolate final nuclei 161Yb, 174,175W. The effect of the discrete lines arising from the ground-state bands and other bands have been subtracted out so that the lower energy part of the continuum can be properly observed. This has revealed the presence of a low-energy component with strong negative anisotropy, suggesting a dipole component in the yrast cascade. A simple model resulting in such a component is considered and discussed. Some comments are made regarding the interpretation of moments of inertia derived from continuum spectra.

M1 Radiation in the Yrast Cascade

The influence of M1 modes of de-excitation on yrast and statistical cascades is studied. It is found that large g values are needed to fit recent experimental data and the presence of some structure in the "continuous" spectrum is predicted.

Possible Manifestations of Oblate Shapes at High Spin in the Yrast Region ofA=160−180Nuclei

The possible manifestations of an oblate-shape region at high spin in $A=160\ensuremath{-}180$ nuclei are considered. We focus attention on the fact that rotational band phenomena indeed can exist in nuclei with oblate shapes. From estimates of transition energies, the yrast cascade there could consist largely of low-energy, alternating $M1$ transitions within bands and $E2$ transitions between bands. If the recently found stretched-dipole transitions are confirmed as $M1$, these results may be an indication for the existence of oblate shapes at high spin.

On the yrast two proton-two neutron hole states in208Po

High-spin levels in208Po, populated in the208Pb(α,4n)-reaction, were studied usingα-particles in the energy region 41–51 MeV. The energies of levels above the 6+ level have an uncertainty of about 10 keV due to the fact that the 8+→6+ transition has not been observed so far, but this transition has previously been established to be converted neither in theK-shell nor in theL-shells. It was found that the yrast cascade ofγ-rays from a 19+ level at 5896+e keV feeds levels of lower spin which all can be explained as originating from two proton-two neutron hole configurations. In the higher part of the cascade it is mainly the neutron holes which change their configuration, while the lower part of the cascade is dominated by changes in the proton configuration. The yrast levels in the angular momentum regionJ=8–19 vary practically linearly with energy in the region 1.5–6 MeV. No isomeric traps were found above the 11− level at 2699+e keV.

Evidence for a Dipole Component in the Yrast Cascade

The shapes and angular distributions of continuum ..gamma..-rayspectra for /sup 174,175/W and /sup 161/Yb, produced through (/sup 16/O, xn) reactions, have been measured down to E/sub ..gamma../approx. = 300 keV. Large negative anisotropics are observed at the lower energies, suggesting the existence of a dipole component in the yrast cascade. A simple model give good agreement with the results.