Rotational sidebands in 166Er

Rotational sidebands in ¹⁶⁶Er were observed using the 24 MeV ¹⁶⁴Dy(α, 2nγ) reactions. The ground-state band was observed up to spin 16⁺ and does not backbend. A strong backbend is, however, observed in a K^π = (O⁺) sideband, indicating that the 12⁺ state of the previously unknown S-band is at 2656 keV. The γ-band shows significant rotational alignment above I = 10⁺. Levels of at least two negative-parity bands, one of which is primarily the K^π = 2^? octupole vibration, are also observed.     

(α, 2nγ) Studies of γ-vibrational and other side-bands in 162, 164, 166Er

Levels in ^{162, 164, 166}Er have been studied using the (α, 2nγ) reaction at an energy of 24 MeV. Singles spectra, γ-γ coincidence spectra, and angular distributions were obtained using Ge(Li) detectors. Transitions from levels in the γ-vibrational bands up to the 8^+' in ^{162, 164}Er and 10^+' in ¹⁶⁶Er were observed and M1/E2 mixtures were determined for many of these transitions. There is a relative shifting upward of the even-spin levels in the γ-band of ¹⁶⁶Er while the analogous levels of ^{162, 164}Er are shifted downward with the effect being most pronounced for¹⁶²Er. The standard phenomenological band-mixing parameters z₂ and z₀₂ were obtained from γ-ray branching ratio data and the values are probably correlated with the staggering of levels in the γ-bands. The ratios of the intraband and interband E2 transition strengths which are related to the intrinsic quadrupole moments of the ground-state and γ-bands are discussed. A number of other levels are observed in ¹⁶², 164Er and some of these correspond to negative parity states reported in decay studies.     

Lifetimes and structures of some levels above 1.5 MeV in 168Yb, 164Er and 158, 160, 162Dy

The following lifetime measurements have been performed in ¹⁶⁸Yb: 62 ns (7^?, 2222.5 keV), 0.34ns (6^?, 2111.1 keV), 81.7 ns (5^?, 1998.7 keV), ≦ 0.14 ns (4⁺, 2203.8 keV); in ¹⁶⁴Er: 23.3 ns (7^?, 1985.0 keV), 0.22 ns (6^?, 1744.4 keV), ≦ 0.08 ns (5^?, 1664.2 keV); in ¹⁵⁸Dy: ≦ 0.11 ns (4⁺, 1895.3 keV); in ¹⁶⁰Dy: 0.18 ns (4⁺, 1694.0 keV); in ¹⁶²Dy: 1.93 ns (5^?, 1485.9 keV). The experimental reduced transition probabilities are discussed in the framework of current nuclear models.     

Anomalous moments of inertia for high-spin levels in the beta vibrational band of 154Gd

The levels of ¹⁵⁴Gd populated by the ¹⁵⁴Sm(α,4n) reaction have been studied with γ-γ coincidence techniques. The energy spacings of the β-band levels of spin > 8 are found to differ from the rotational picture in a manner similar to that recently observed in the ground-state bands of some other rare-earth nuclei. The ground-state band of ¹⁵⁴Gd does not show the anomaly;     

High-spin,discrete line spectroscopy of 162Yb and 164Hf and the systematics of N = 92 isotones

The high spin states of ¹⁶²Yb and ¹⁶⁴Hf have been studied using ¹²²Sn(⁴⁴Ca, 4n) and ¹²⁰Sn(⁴⁸Sn(⁴⁸Ti, 4n) reactions respectively. The resulting new level schemes for ¹⁶²Yb and ¹⁶⁴Hf are presented and compared with existing data for ¹⁶⁰Er. The systematic behaviour of these three N = 92 isotones at large angular momentum yields information both on modifications of the single-neutron spectrum of states resulting from the polarisation of the nuclear field and on the single-proton spectrum of states.     

Band crossings at low rotational frequency in 166Yb

The ¹⁶⁶Er(³He, 3nγ) and ¹⁶⁴Er(α, 2nγ) reactions were used to populate rotational side-bands in the N = 96 nucleus ¹⁶⁶Yb. The aligned $\text{v(}\text{i}{}_{\mathrm{<mtext>13</mtext><mtext>2</mtext>}}\text{)}{}^2$ S-band was observed from I^π = 8⁺to 12⁺, while the β- and γ-vibrational bands were observed from the bandheads to I^π = 10⁺and 12⁺, respectively. Negative-parity bands with K^π = 0^?, (2^?) and 5^? were also observed. Band crossings and interaction effects are seen in both the positive- and negative-parity excitations below about $\text{h}\text{?}\text{ω = 0.25}\text{MeV}$, the frequency at which the first yrast backbend occurs.     

Study of Coriolis-decoupled bands in 155, 157, 159Dy and 155, 157, 159Er using (α, xnγ) reactions

Levels in ^{155, 157, 159}Dy and in ^{155, 157, 159}Er have been populated using (α, xnγ) reactions where x = 5, 7 or 9. The resulting γ-rays have been investigated using in-beam y-spectroscopic techniques. Mixed positive-parity bands were predominantly populated and are identified with the Coriolis-deeoupled bands described in the framework of the rotation-alignment model of Stephens and coworkers. In the case of ¹⁵⁷Dy the band was observed up to the $\text{49}\text{2}$⁺ state. The absence of the backbending effect in these nuclei can be explained by the blocking of certain $\text{13}\text{2}$ neutron orbitals near the Fermi surface which are essential for the development of the baekbending mechanism.     

Energies and electromagnetic moments of deformed nuclei in the rare-earth region

Strutinsky-type cranking calculations with inclusion of pairing correlations have been performed for the rare-earth nuclei ^{156, 158, 164}Dy and ¹⁶⁴Er. The pairing effects contribute significantly and with their inclusion the calculated yrast spectra agree very well with experiments. Using Hartree-Fock-Bogouliubov cranking wave functions we have calculated the magnetic moments and quadrupole moments for states up to spin $\text{I = 20}\text{h}\text{?}$. The quadrupole moments are found to be constant over the whole spin range. The gyromagnetic factors g(I) show a strong I-dependence for ^{156, l58}Dy, a weaker one for ¹⁶⁴Er and none for ¹⁶⁴Dy. The sensitivity of this spin dependence on the single-particle occupation and the pairing degrees of freedom is studied. It is found that the spin variation of the gyrofactors is a rotational alignment effect.     

Towards a shell model description of the low-energy structure of deformed nuclei I. Even-even systems

A microscopic shell model theory for the low-lying collective states of heavy deformed nuclei is described. Basis selection is guided by the Bohr-Mottelson-Nilsson picture of collective motion in nuclei. The necessary further truncation is achieved by exploiting an SU(3) symmetry inherent to the structure of the normal parity states and by restricting abnormal parity configurations to states with low seniority. An effective interaction comprised of operators which form an integrity basis for the SU(3) → R(3) algebra is shown to be sufficient to reproduce almost exactly, within a single leading irreducible representation of SU(3), the ground and gamma band rotational structure of eight rare earth (¹⁶⁰Dy, ¹⁶²Dy, ¹⁶⁴Dy, ¹⁶⁴Er, ¹⁶⁶Er, ¹⁶⁸Er, ¹⁶⁶Yb, ¹⁶⁸Yb) and four actinide (²³²Th, ²³⁴U, ²³⁶U, ²³⁸U) nuclei. The concomitant interband and intraband E2 strengths are also shown to be accurately reproduced. Extensions of the theory and necessary further theoretical investigations are reviewed.     

Energy levels in the nucleus 156Dy through the 159Tb(p, 4nγ)156Dy reaction

Using the ¹⁵⁹Tb(p, 4nγ)¹⁵⁶Dy reaction at E_p = 27 to 51 MeV and standard on-line γ-ray spectroscopy methods, the energies and decay properties of members of various rotational bands in the nucleus ¹⁵⁶Dy have been investigated, i.e. the ground-state band up to 14_g⁺, the β-vibrational band up to 14_β⁺,the γ-vibrational band up to 11_γ⁺,and two other bands, one with odd-spin levels up to 11, the other with even-spin levels up to 10. The results are compared with various calculations in the framework of the collective model, and no satisfactory fit is obtained; possible improvements of the model to remove these discrepancies are suggested.     

Feeding times of high-spin states in 152, 154Dy: Probes of nuclear structure above the yrast line

Measurements of feeding times of high-spin yrast states up to spin 30 ? in ¹⁵⁴Dy and 36 ? in ¹⁵² Dy were utilized to obtain information about possible spin-dependent shape changes. The reactions ²⁵Mg(¹³⁴Xe, 5n), ¹²⁴Sn(³⁴S, 4n) and ²⁵Mg(¹³²Xe, 5n), ¹²²Sn(³⁴S, 4n) were used to populate the high-spin states in ¹⁵⁴Dy and ¹⁵²Dy, respectively. Feeding times as well as lifetimes were determined with the recoil-distance technique. In ¹⁵²Dy only long feeding times (? 10 ps) could be identified, indicating that the aligned-particle yrast states are fed through configurations of similar character, with little direct population from collective cascades in the continuum region. In ¹⁵⁴Dy discrete states with I ? 30 ? have lifetimes which are characteristically collective, whereas the preyrast cascades exhibit both fast (?1 ps) and slow (～10 ps) feeding components. The latter imply a change with increasing spin from collective to aligned-particle character, probably associated with a prolate to oblate shape transition.     

Remeasurement of lifetimes for high spin states in 154Er

Lifetimes of high spin states populated by the ¹²⁶Te(³²S, 4n)¹⁵⁴Er reaction were measured with the recoil distance method. The data require that the decay scheme of ¹⁵⁴Er proposed by previous workers be modified. The measured lifetimes show that there is little evidence for any collective behaviour in segments of the yrast line and that, contrary to previous speculation, the yrast states are probably made up of states of differing single particle configurations as in the case of other neighbouring nuclei.     

Measurement of g-factors in 170Yb, 172Yb and 174Yb by transient field interaction at high recoil velocities

A beam of ⁴⁰Ca ions at 168 MeV has been used to Coulomb excite high-spin states in ¹⁶⁹Tm and ^{170, 172, 174}Yb which recoiled through thin polarized iron at ν/c ≈ 3.5%. An enhanced transient field ≈ 6 times larger than the Lindhard-Winther prediction was observed and calibrated against the known g-factors in ¹⁶⁹Tm. Individual g-factors could not be extracted because of strong feeding; nevertheless, deviations from rotational behaviour (constant g-factors) would be detectable. Empirical fits to microscopic calculations of the rotational g-factors below spin 12⁺ suggest that if deviations from the simple rotational formula occur, they should be of the form g(J) = g₀(1 + αJ²). We find the following values in ^{170, 172, 174}Yb: α × 10³ = ?0.5±1.5, +1.0±1.5. The uncertainties are the level at which deviations from rotational behaviour are expected to occur. The results are discussed in terms of Coriolis anti-pairing effects.     

Investigation of α-particle emission from the interactions of fast neutrons with 161Dy and 163Dy nuclei

The energy spectra of α-particles emitted in the ¹⁶¹Dy(n, α)¹⁶¹Gd and ¹⁶³Dy(n,α)¹⁶⁰Gd reactions have been measured at neutron energies equal to 14.1 and 18.2 MeV. The results have been analysed in terms of Hauser-Feshbach, pre-equilibrium and knock-on models. The experimental data can be described assuming the existence of preformed α-clusters in target nuclei and the processes involving only few degrees of freedom.     

High-spin states in the single-closed-shell nucleus 142Nd

The excited states of ¹⁴²₆₀Nd₈₂ have been studied using the ¹⁴⁰Ce(α, 2nγ)¹⁴²Nd and ¹⁴²Ce(α, 4γ)¹⁴²Nd reactions. Singles γ-ray, γ-γ coincidence spectra and angular distributions of γ-rays with respect to the beam direction have been measured. Excited states up to 6.7 MeV with spin values up to 14 are populated. The energy spacings between the lower excited states with spin values up to 8 are similar to those found in the lighter N = 82, even-Z isotones. The majority of the observed states with spin values up to 10 can be explained as two-quasiparticle states. Several of the highest-spin states can be explained qualitatively as fourquasiparticle states. Strong population of the highest excited states (at about 5.7 MeV) is noted, like in other N = 82 isotones. The observed levels in ¹⁴²Nd are compared with the shell model predictions using a simple δ-force interaction between two nucleons.     

Etude du noyau 63Cu par la reaction 60Ni(α, pγ)63Cu

The level structure and the decay properties of low-lying levels in ⁶³Cu have been investigated via the ⁶⁰Ni(α, pγ)⁶³Cu reaction at E_α = 11.7MeV. Using a Ge(Li) detector, the correlations of twenty-five primary γ-rays in coincidence with protons, stopped in an annular detector at approximatively 180° with respect to the beam direction, were measured. From these measurements, branching ratios, γ-ray mixing ratios and spin assignments have been obtained for most of the levels up to 2.4 MeV excitation.     

Discrete levels in 154Dy above spin 30ħ: Evidence for terminating band structures

High-spin states in ¹⁵⁴Dy have been studied using the TESSA2 γ-rays spectrometer following the ¹¹⁰Pd(⁴⁸Ca,4n)¹⁵⁴Dy reaction at a beam energy of 210 MeV. States up to 44⁺ and 37⁻ have been observed. Below spin 30 the data display regular rotational behaviour which can be interpreted in terms of the cranked shell model. Above spin 30, sequences of levels connected by stretched E2 transitions, which show large gains in energy when compared to a rotating liquid drop reference, are lowest in energy for both parities. Particularly low energy levels are observed for spin I^π = 36⁺ and 42⁺ and in addition dipole transitions are found connecting negative-parity states around spin I = 35. The experimental data for I ≳ 30 are compared with calculations, based on the Nilsson-Strutinsky cranking formalism, in which it is possible to trace fixed configurations through a sequence of spins. For the high-spin positive-parity sequence, the similarity with the ¹⁵⁶Er spectrum is discussed.     

Experimental study of high spin states in the ground state bands of158,160Er,164,166Yb,and168Hf

Ground state rotational bands in the deformed doubly-even rare earth nuclei^158,160Er,^164,166Yb, and¹⁶⁸Hf have been observed in (α, 8n γ) reactions. The γ-spectra associated with these reactions were studied in-beam using conventional spectroscopic methods. In all five nuclei the nuclear moment of inertia of the ground state rotational states was found to increase abruptly as higher spin states were attained. In a plot of the moment of inertia as a function of the angular velocity all these five nuclei display “backbending” curves. Moreover, in¹⁵⁸Er and¹⁶⁶Yb, the curves after passing through a maximum bend subsequently downwards.     

On the decay of compound nuclei following alpha-particle and12C induced reactions

Multiple coincidence rates have been measured using a detector system consisting of a Ge(Li) spectrometer and eight NaI(Tl) or eight liquid scintillators. Reactions induced byα-particles with energies of 51–55 MeV and 118 MeV¹²C ions are studied. The data are analysed to give the first and second central moments of the distribution of the number ofγ-rays feeding individual levels in the final nuclei. When these numbers are compared to spin distributions calculated with the statistical model code GROGI the relative importance of dipole and quadrupole deexcitation modes can be ascertained. In particular, in the¹²²Te(α, 4n)¹²²Xe reaction theγ-decay prior to the entry into the ground band is well described as a statistical process proceeding to 50% by dipole and 50% by quadrupole radiation. In the¹⁶⁶Er(α,4n)¹⁶⁶Yb and¹⁹²Os(α,4n)¹⁹²Pt reactions the relative amount of quadrupole radiation is larger and it seems that the dipole and quadrupole decay takes place via separate cascades. In the¹⁶⁴Dy(¹²C, 7-8n) reactions the average multiplicity is independent of spin, suggesting that the nucleus forgets the spin of the entry state before the process enters into the ground band. In the¹⁷⁶Yb(¹²C, 8n)¹⁸⁰Os reaction, finally, the nucleus definitely retains memory of the entry state during the decay. In this last case the multiplicity measurement is combined with aγ-ray singles measurement to give an average excitation energy prior to theα-decay and the average moment of inertia characterising the decay of the high-spin states.     

Multipolarity of the continuum γ-rays in 152Dy and 160Er via linear polarisation measurements

The multipolarity of the continuum transitions in ¹⁵²Dy and ¹⁶⁰Er has been deduced from γ-ray linear polarisation and angular distribution measurements. The yrast part for ¹⁶⁰Er agrees with the predicted stretched E2 cascade. The statistical parts for both ¹⁵²Dy and ¹⁶⁰Er consist of a mixture of dipolar stretched and non-stretched transitions, predominantly of electrical character. Below 1.5 MeV, the data of ¹⁵²Dy exhibit a different structure, and reveal a stretched M1 component in the low-energy region.