A study of the rotational side-bands in 162Dy

Rotational side-bands in ¹⁶²Dy have been studied using the ¹⁶⁰Gd(α, 2nγ)¹⁶²Dy reaction. Seven side-bands are observed, with K^π = 2⁺, 2^?, (0)^?, 0⁺, 5^?, 4⁺ and (6^?). Four of these bands have collective structure at low spin: the K^π = 2⁺γ-vibrational band, the K^π = 0⁺β-vibrational band, and the K^π = 2^? and (0)^? octupole vibrational bands. Of the remaining bands, the 4⁺ band is deformation coupled while the 5^? and (6^?) bands are rotation-aligned. Several bandcrossings are observed in this nucleus. The β and γ-bands are crossed at $\text{I = 6}\text{h}\text{?}\text{and}\text{12}\text{h}\text{?}$, respectively, by a highly aligned ($\text{i}{}_{\mathrm{<mtext>13</mtext><mtext>2</mtext>}}$)² S-band; extrapolation of this S-band to higher spin suggests that it crosses the g.s.b. between $\text{I = 18}\text{h}\text{?}\text{and}\text{20}\text{h}\text{?}$. The 2^? octupole band is crossed by the 5^? band at $\text{I = 9}\text{h}\text{?}$ and again by the (6^?) band at $\text{I = 12}\text{h}\text{?}$. The latter bandcrossings are discussed in terms of two-quasiparticle plus rotor calculations.     

High spin states in 74Se

Gamma-ray transitions from the bombardment of ⁶⁴Ni by ¹⁶O up to 81 Me V were studied in a variety of experiments. Four transitions in coincidence with four previously known transitions in ⁷⁴Se were discovered. Information on energies, angular distributions, intensities multiplicities and lifetimes is given for these transitions. Excitation functions for this and several other channels are presented. Statistical calculations are in satisfactory agreement with experiment.     

High spin states in 57Co

High spin states of ⁵⁷Co have been studied via prompt γ-ray spectroscopy in the reactions ⁴⁸Ti(¹²C, p2n) and ⁵⁴Fe(α, p) at 26–48 MeV and 12–24 MeV, respectively. The energies and decay modes of these levels were determined from the analysis of γ-ray singles and γ-γ coincidence spectra, excitation functions, angular distributions and correlations. The relevant lifetimes were measured by the Doppler-shift attenuation method. The new levels established in this work are at 4037, 4814 and 5918 keV with the most probable J^π assignment of $\text{15}\text{2}{}^{\mathrm{?}}$, if $\text{17}\text{2}{}^{\mathrm{?}}$ and $\text{19}\text{2}{}^{\mathrm{?}}$, respectively. The previously known level at 2524 keV was assigned to have $\text{J}{}^{\mathrm{\pi }}\text{=}\text{13}\text{2}{}^{\mathrm{?}}$. These together with the known $\text{9}\text{2}{}^{\mathrm{?}}$(1224 keV) and $\text{11}\text{2}{}^{\mathrm{?}}$(1690 keV) levels constitute the yrast states of ⁵⁷Co. The measured lifetimes of the above six levels are (in order of increasing energies) 0.085±0.030, 0.32±0.10, 0.16±0.06, 0.10_?0.07^+0.06, 1.5_?0.5⁴ and 0.17_?0.07^+0.08 ps, respectively. Comparisons with some theoretical calculations are presented.     

Yrast isomers and very high spin states in 148, 149, 151, 152Dy and 147Gd

We have studied level schemes for the feeding and decay of high-spin isomers in the nuclei ^{148, 149, 151, 152}Dy and ¹⁴⁷ Gd. A variety of techniques involving γ-ray spectroscopy with pulsed heavy-ion beams have been applied including linear polarization, γ-γ correlations and recoil distance measurements.The general aspect of these level schemes is in accordance with the idea that the states near and along the yrast line can be described by a spheroidal shell model in which the nucleus is axially symmetric with respect to the total spin. In these cases the spin is generated not by collective rotation, but by alignment of many single-particle orbits around a common axis.     

High spin states in 105Ag

High spin states in ¹⁰⁵Ag have been studied using the ¹⁰³Rh(α, 2nγ)¹⁰⁵Ag reaction. A collective band on the $\text{9}\text{2}{}^{\mathrm{+}}$ state at 53.2 keV and a negative parity band with spins ranging from $\text{15}\text{2}$ to $\text{27}\text{2}$ were observed. Furthermore a $\text{15}\text{2}{}^{\mathrm{+}}$ isomeric state with $\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext><mtext>\; =\; 6.0\pm 0.2\; </mtext><mtext>ns</mtext>}}$ at 1733.8 keV was identified. The g-factor deduced for this state is g = 0.58±0.06. A comparison of the experimental results with theoretical calculations indicates that the properties of most of the positive parity states are reasonably well described by the cluster-vibrational model as well as by the triaxial rotator model.     

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.     

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.     

High spin states of 83Sr

The reactions ⁸⁰Kr(α, n)⁸³Sr and ⁸²Kr(α, 3n)⁸³Sr were used to populate excited states of high spin in ⁸³Sr. The de-excitation of these states was studied by in-beam γ-ray spectroscopy. A number of new high-spin states have been observed. The results are discussed within the framework of the Coriolis-coupling model and the cluster-core coupling model.     

(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.     

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.     

The g-boson and the collective states of 168Er

Comparison between the IBM-I, IBM-II and the U(15) theories of the ¹⁶⁸Er spectrum, together with an analysis of the boson weights, supports the incorporation of the g-boson.     

One- and three-quasiparticle states in 171Hf and high spin rotational bands

The $\text{1}\text{2}{}^{\mathrm{?}}\text{[521]}\text{and}\text{7}\text{2}{}^{\mathrm{+}}\text{[633]}$ one-quasiparticle bands in the N = 99 nucleus ¹⁷¹Hf have been identified to spins of about $\text{45}\text{2}$ using (heavy ion, xn) reactions. The moments of inertia of these bands are consistent with the absence of backbending in the N = 98 core nucleus. The half-life of the $\text{5}\text{2}{}^{\mathrm{?}}\text{[512]}$ intrinsic state was measured as 63.6 ns. The strength of the $\text{5}\text{2}{}^{\mathrm{?}}\text{[512] →}\text{7}\text{2}{}^{\mathrm{+}}\text{[633]}\text{E}\text{1}$ transition is discussed. Two three-quasiparticle isomers with spins and parities $\text{19}\text{2}{}^{\mathrm{+}}\text{and}\text{23}\text{2}{}^{\mathrm{?}}$ have been identified and their suggested configurations are a $\text{7}\text{2}{}^{\mathrm{+}}\text{[633]}$ neutron added to the 6⁺ and 8^? two-quasiproton states of the core. The moment of inertia of a rotational band based on the $\text{23}\text{2}{}^{\mathrm{?}}$ isomer supports this suggestion, and shows the effect of partial rotation alignment of the $\text{i}{}_{\mathrm{<mtext>13</mtext><mtext>2</mtext>}}$ neutron.     

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.     

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.     

Intrinsic and rotational states in 173Ta

High spin rotational levels in ¹⁷³Ta are populated in the ¹⁶⁵Ho(¹²C, 4n)¹⁷³Ta and ¹⁷⁵Lu(α, 6n)¹⁷³Ta reactions. The de-excitation γ-ray cascades are studied with Ge(Li) detectors. The rotational bands, which are built on the $\text{7}\text{2}$⁺(404), $\text{5}\text{2}$⁺(402), $\text{9}\text{2}$^?(514) and $\text{1}\text{2}$^?(541) intrinsic states, are identified up to high spin values. A state, interpreted as a three quasi-particle state with a probable spin of $\text{21}\text{2}$ is located at 1713 keV. Its half-life is about 100 ns. The behaviour of the moment of inertia of each rotational band versus the rotational frequency is compared with that of the doubly even core.     

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.     

High spin states in 93mo populated in the (p,n) reaction

High spin states in the $\text{ν}\text{d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}\text{(π}\text{g}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}{}^2$) multiplet in ⁹³Mo were investigated with the ⁹³Nb(p, n) ⁹³Mo reaction. A new isomeric state at 2430 keV with a half-life of 3.53±0.18 ns is interpreted as the lower $\text{17}\text{2}{}^{\mathrm{+}}$ member of the multiplet. A number of further states have been identified. The results are compared to previous experiments and shell model calculations.     

Intrinsic states,high-spin rotational bands and rotation alignment in 177Os and 179Os

Low-lying intrinsic states and their associated rotational bands have been identified in ¹⁷⁷Os and ¹⁷⁹Os. They are the mixed i_{$\text{13}\text{2}$} neutron states and the $\text{1}\text{2}$^?[521] states in ¹⁷⁷Os and ¹⁷⁹Os, as well as the $\text{5}\text{2}$^?[512] state in ¹⁷⁷Os and the $\text{7}\text{2}$^?[514] state in ¹⁷⁹Os. The $\text{1}\text{2}$^? sta is assumed to be the ground state, the other intrinsic states giving rise to isomers. The in-band decay properties of the $\text{7}\text{2}$^?[514] band, and the i_{$\text{13}\text{2}$} bands show the effect of mixing. In the rotational bands in ¹⁷⁷Os a low frequency backbending anomaly is observed but no anomaly is observed in the i_{$\text{13}\text{2}$}. band. In ¹⁷⁹Os the i_{$\text{13}\text{2}$} band does backbend but at a higher frequency than in the yrast bands of the even neighbours. The systematics of the backbending frequencies, and the effects of blocking, are discussed. The rotation aligned angular momentum is deduced, and a comparison made between the i_{$\text{13}\text{2}$} bands and the s-bands in the even neighbours. The results broadly support the identification of the s-bands with the aligned (i_{$\text{13}\text{2}$})² configuration.     

Breakup process for 100 MeV 3He interacting with 165Ho and 166, 167Er nuclei

The projectile breakup process for 100 MeV ³He interacting with ¹⁶⁵Ho and ^{166, 167}Er nuclei was studied by measuring singles and coincidence spectra of the breakup fragments and following γ-rays. By comparing the data of the ³He-induced reaction with those of the α-induced reaction, the breakup process was found to make a large contribution to the charged-particle spectra of the ³He-induced reaction. Contributions of the elastic breakup, inelastic breakup, and breakup fusion processes were deduced individually from the particle-γ coincidence measurements.     

Continuous gamma radiation feeding high-spin isomers in 148, 149, 151, 152Dy and 147Gd

The energy, multipolarity and multiplicity spectra of the continuum gamma radiation feeding high-spin isomers in ^{148, 149, 151, 152}Dy and ¹⁴⁷Gd have been measured at several bombarding energies. The final nuclei were selected via a delayed coincidence technique while the use of a NaI(Tl) Compton shielded crystal allowed the primary gamma-ray spectra to be generated reliably from the observed spectra.It was found that: (i) the energy of both the gamma-ray statistical cascade and the neutron cascade increases with increasing excitation energy, the latter much more rapidly than expected; (ii) the multiplicity of gamma-ray cascades, in which a statistical transition was detected, is generally lower than that of the average cascade; (iii) stretched E2 spin-correlated transitions occur above $\text{J? 39}\text{h}\text{?}\text{in}{}^{152}\text{Dy}$ and above $\text{～ 50.5}\text{h}\text{?}\text{in}{}^{147}\text{Gd}$, indicating the onset of collectivity at these spins — in addition, a region of predominantly dipole emitting states is located below $\text{T? 44}\text{h}\text{?}\text{in}{}^{147}\text{Gd}$; (iv) effective moments of inertia derived from the bump edge are 50–100% larger than those deduced from the density of stretched E2 transitions in the bump; (v) in ^149,151Dy the bump edge is very sharp but no multiplicity features are observed; (vi) although the four Dy isotopes were populated at approximately the same excitation energy, they display considerable differences in their continuum properties.Probable interpretations of these observations are discussed, in particular we have suggested that several of the observed effects are consistent with the possible presence of high-K collective bands above the yrast line.