Contrasting high-spin yrast bands in 172Os and 174Os and an unexpected low-frequency anomaly in 172Os

The yrast bands of the neutron deficient isotopes ¹⁷²Os and ¹⁷⁴Os have been identified to spins of about 24. The yrast band in ¹⁷⁴Os shows no bandcrossing anomalies, confirming the shell effect observed in other N = 98 nuclei. In contrast, a strong backbend observed at a frequency of about 0.26 MeV in ¹⁷²Os is attributed to the s-band crossing. A weaker band-crossing is also observed at a lower frequency, about 0.24 MeV, in ¹⁷²Os. This unexpected anomaly may be due to either a deformation effect, or to a change in the s-band structure.     

High-spin structure in 169W and 170W

High-spin states in ^{169, 170}W have been populated in ${}^{154}\text{Gd}\text{(}{}^{20}\text{Ne}\text{, x}\text{n}\text{)}$ reactions. In-beam γ-ray spectroscopic techniques with multi-detector set-ups, multiplicity filters and an anti-Compton shield have been used. Levels up to about spin 30 (tentatively up to 36) in ¹⁷⁰W and up to $\text{57}\text{2}$ (tentatively up to $\text{61}\text{2}$) in ¹⁶⁹W have been identified. The data are interpreted within the framework of a pairing-selfconsistent cranking model. The nuclear shape evolution with increasing spin is studied theoretically within a configuration-controlled shell correction approach and also pairing effects are studied. The behaviour of the yrast states around 28⁺ in ¹⁷⁰W can be related in a model-dependent way to a reduction of the neutron-pairing correlations.     

High-spin positive-parity states in 179Hf studied by the 180Hf(τ, α)179Hf reaction AT 32 MeV

Full angular distributions are presented for states populated in the reaction ¹⁸⁰Hf(τ, α)¹⁷⁹Hf at 32 MeV beam energy. Positive-parity states associated with the i_{$\text{13}\text{2}$} unique parity intruder orbital are given special attention. Thus, angular distributions for the five first members of the [624$\text{9}\text{2}$] groundstate sequence are given, as well as for a number of more highly excited states, some being new assignments. The distribution of l = 6 transfer strength is quite characteristic, two $\text{13}\text{2}$⁺ states being substantially more populated than the rest. The characteristic features of the data are explained by a quasiparticle-rotor calculation employing deformed Woods-Saxon orbitals, but only if the hexadecapole shape parameter of the nuclear potential is β₄ ～ ?0.08. The often anomalous differential cross sections for $\text{I}{}^{\mathrm{\pi }}\text{≠}\text{13}\text{2}{}^{\mathrm{+}}$ band members are well accounted for by a rotor model CCBA calculation employing transfer form factors extracted from the orbitals of the deformed Woods-Saxon field, and including non-adiabatic Coriolis mixing effects.     

High-spin states in 92Tc

High-spin states in ⁹²Tc have been studied using the 33 MeV ⁹²Mo(³He, p2nγ)⁹²Tc reaction. Levels up to J = (13) are identified. The results are compared with various shell-model calculations.     

Absolute E1, ΔK = 1 transition probabilities between multi-quasiparticle high-spin states in 176Hf and 178Hf

Applying the generalized centroid shift method in (α, 2n) reactions, the half-lives of the 3080 keV 15⁺ state in ¹⁷⁶Hf and of the 1637 keV 5^? state in ¹⁷⁸Hf have been measured as $\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 0.20}{}^{\mathrm{+0.12}}{}_{\mathrm{?0.08}}\text{ns}$ and $\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 0.40 ± 0.10}\text{ns}$, respectively. B(El) values of K-allowed E1 transitions $\text{n}\text{9}\text{2}{}^{\mathrm{+}}\text{[624]→}\text{7}\text{2}{}^{\mathrm{?}}\text{[514]}$ are derived, and together with other data on similar transitions in odd-A nuclei, compared with predictions of the Nilsson plus pairing model. In ¹⁷⁶Hf, the 15⁺ and 14^? states at 3080 and 2866 keV, respectively, appear as quite pure deformed 4QP configurations. In the 2QP state at 1637 keV in ¹⁷⁸Hf, possible strong mixing of vibrational components is discussed coupled via 2QP K-admixtures arising from the partial alignment of the $\text{i}{}_{\mathrm{<mtext>13</mtext><mtext>2</mtext>}}$ neutron.     

High-spin states in the isotones 89Zr, 91Mo and 93Ru populated in α-particle-induced reactions

The reactions (α, 3nγ) and (α, α′nγ) on ⁸⁸Sr, ⁹⁰Zr and ⁹²Mo were used to populate excited states in ⁸⁹Zr, ⁹¹Mo and ⁹³Ru. The de-excitation of these states was studied by in-beam γ-ray spectroscopy. The short-lived radioactivity of the ⁹²Mo target was measured in order to study the decay of ⁹³Ru. Many new levels were observed, particularly in ⁹¹Mo. They are interpreted as due to the coupling of a $\text{g}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}$ neutron hole with known excited states in ⁹⁰Zr, ⁹²Mo and ⁹⁴Ru.     

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.     

High-spin yrast levels of 38Ar

High-spin states of ³⁸Ar have been studied with the ³⁵Cl(α, pγ)³⁸Ar reaction at E_α = 18 MeV and with the ²⁴Mg(¹⁶O, 2pγ)³⁸Ar reaction at $\text{E(}{}^{16}\text{O}\text{) = 38}\text{and}\text{45}\text{MeV}$. The ³⁸Ar level scheme is obtained with the former reaction from a proton-γ coincidence measurement. Gamma-gamma coincidence, γ-ray angular distribution and linear polarization experiments have been performed with a Ge(Li)-Na(Tl) Compton suppression spectrometer and a three-crystal Ge(Li) Compton polarimeter. Unambiguous spin-parity assignments of $\text{J}{}^{\mathrm{\pi }}\text{= 7}{}^{\mathrm{?}}\text{, 7}{}^{\mathrm{+}}\text{, 8}{}^{\mathrm{+}}\text{, 7}{}^{\mathrm{?}}\text{, 9}{}^{\mathrm{?}}\text{and}\text{11}{}^{\mathrm{?}}$to the ³⁸Ar levels at E_x = 7.51, 8.08, 8.57, 8.97, 10.17 and 11.61 MeV, respectively, are obtained. The 8.57 MeV, 8⁺ level has a mean life below 0.8 ps. Excitation energies, branching ratios, multipole mixing ratios and transition strengths are reported. The experimental results are compared with shell-model calculations.     

High-spin states and yrast isomerism in 153Er

Energy levels in ¹⁵³Er have been populated in the reaction ¹⁴⁴Sm(¹²C. 3n)¹⁵³Er. Isotopically enriched targets were bombarded with 53–65 MeV ¹²C ions and the emitted γ-ray and conversion electron spectra were investigated. From studies of excitation functions. γ- coincidences, γ-ray multiplicities, delayed γ-radiation and angular distributions, the level scheme of ¹⁵³Er has been constructed. The properties of the energy levels are discussed and compared with the results of calculations with a deformed shell model. The remarkable similarities and some important discrepancies of the level structure, when compared with adjacent, N = 85 nuclides, are emphasized.     

The level structure of 184Os from 184Ir decay and 185Re(p, 2nγ) in-beam spectroscopy

Levels of ¹⁸⁴Os populated in the decay of 3.1 h ¹⁸⁴Ir and in the ¹⁸⁵Re(p, 2nγ) reaction have been investigated. The measurements included γ-ray singles, β⁺ ray endpoint, conversion coefficient, β⁺-γ coincidence and detailed γ-γ coincidence determinations. The results have established an extensive ¹⁸⁴Os level scheme, which includes well developed ground state, γ-vibrational and K = 3 octupole bands and which accommodates all the intense transitions observed in both the radioactivity and in-beam γ-ray measurements. Deviations of the level energies in the K^π = 0⁺and K^π = 2⁺ bands and of the interband reduced transition probabilities from the predictions of the strong-coupling model are discussed in terms of the rotationvibration interaction, and the systematics of the octupole vibrational excitations in even-mass W and Os nuclei are reviewed. It is concluded that the ¹⁸⁴Ir ground state configuration has a spin of 5, and that it contains large admixtures of K = 0 or K = 1 character.     

Proton capture by 176Yb in the giant dipole resonance region

The proton capture cross section for the reaction ¹⁷⁶Yb(p, γ)¹⁷⁷Lu has been measured for incident proton energies between 6 and 24 MeV. The excitation function for this deformed nucleus agrees remarkably well with the results of previous studies on spherical nuclei, e.g. ¹⁴²Ce(p, γ)¹⁴³Pr. The results indicate that the giant dipole resonance (GDR) is strongly excited as predicted by the direct-semidirect (DSD) model. It is found that the model describes reasonably well the excitation function. In the low-energy proton range, where the excitation function increases rapidly with proton energy, the observed cross section is significantly higher than the DSD predictions. The difference can only partly be explained by compound nucleus contributions. In the high-energy end, the predicted cross section tends to be too high primarily due to an increasing contribution of direct capture to orbitals with large angular momenta.     

Cross-section measurements and thermonuclear reaction rates for 46Ti(p, γ)47V, 47Ti(p, γ)48V, 47Ti(p,n)47V and 48Ti(p, γ)49V

The yield of γ-rays from the reaction ⁴⁶Ti(p, γ)⁴⁷V has been measured as a function of bombarding energy over the range 0.72–3.00 MeV, from ⁴⁷Ti(p, γ)⁴⁸V over the range 0.74–3.50 MeV, and from ⁴⁸Ti(p, γ)⁴⁹V over the range 0.72–4.40 MeV. The yields of γ-rays following (p, p') reactions on all three targets were also measured and (p, p') cross sections were deduced for the first excited state proton groups for ⁴⁶Ti and ⁴⁸Ti and for the first ten proton groups for ⁴⁷Ti. The yield of neutrons from the reaction ⁴⁷Ti(p, n)⁴⁷V has been measured over the range from threshold to 4.40 MeV. All these data are compared with statistical-model calculations, and good agreement is achieved. Thermonuclear reaction rates for the (p, γ) and (p, n) reactions are calculated for the temperature range 5 × 10⁸–10¹⁰ K which includes the range of temperatures of interest in nucleosysnthesis calculations.     

Quasi-free (p,pα) scattering on 24Mg, 28Si, 40Ca and 58Ni at 157 MeV

Sixfold energy spectra have been measured for the (p, pα) reaction at 157 MeV on ²⁴Mg, ²⁸Si, ⁴⁰Ca and ⁵⁸Ni around quasi-free kinematic conditions. For the three s-d shell nuclei the experiment covered a map ranging from 0 to 220 MeV/c in recoil momentum and from 0 to 20 MeV in excitation energy of the final nucleus. Recoil momentum distributions have been obtained for the 0⁺ ground state and the 2⁺ first excited state of ²⁰Ne, ²⁴Mg and ³⁶Ar, and also for the states around 4.4 MeV (mainly 4⁺) of ³⁶Ar. The a spectroscopic factors extracted by a DWIA analysis are about three times larger than those predicted by the SU(3) model; however, they agree quite well in relative magnitude for a number of cases. The disagreement in shape between experiment and theory observed at low recoil momentum for the 2⁺ states might result from another reaction mechanism. The cross sections for ⁵⁸Ni are about a factor often smaller than those for ⁴⁰Ca. The ⁵⁸Ni(p, pα)⁵⁴Fe reaction seems to lead mainly to excited states of the final nucleus.     

Energy dependence of the 24Mg(16O, 12C)28Si reaction

Excitation functions for the reaction ²⁴Mg(¹⁶O, ¹²C)²⁸Si(g.s., 2⁺₁) were measured at 5°(lab) in the energy range 32 < E_c.m. < 49 MeV. Although the resonant structure, previously observed at lower energies, becomes progressively weaker, three new correlated maxima have been observed near E_c.m. = 37.5, 40.2 and 43.5 MeV. Angular distribution measurements at these energies yield spin assignments, from P²_j(cos θ) comparisons, of 27, 29 and 31, respectively. Attempts to find a consistent optical-model fit to the elastic scattering in the entrance channel and an exact finite-range DWBA fit to the four-nucleon transfer reaction in this energy range were unsuccessful. Such a failure is to be expected if strong couplings between the elastic channel and inelastic channels of either the initial or final system are important. The features of the resonance phenomena in the transfer reaction are discussed within a band crossing model framework.     

High-spin rotational bands and pairing reduction in 166Hf

Rotational properties of the ¹⁶⁶Hf nucleus have been studied by in-beam spectroscopic methods using the ¹⁵⁰Sm(²⁰Ne, 4n)¹⁶⁶Hf reaction. The ground band was observed up to the I^π = 18⁺ level, the s-band from the I^π = 12⁺ up to I^π = 22⁺ level; in addition two side bands were found. The results are interpreted in terms of the pairing-self-consistent Hartree-Fock-Bogolyubov cranking model. Quantitative agreement between theory and experiment is obtained for the crossing of the ground band and s-band. The calculations predict a reduction of about 50 % in the neutron pairing energy gap of yrast states in the crossing region. The kinematical ($\text{I}$⁽¹⁾) and dynamical ($\text{I}$⁽²⁾) moments of inertia are analysed.     

The (3He,n) reaction on 16O and 18O

The (³He, n) reaction on ¹⁶O and ¹⁸O has been used to study low-spin states in ¹⁸Ne and ²⁰Ne up to E_x ≈ 8 and 20 MeV, respectively. The measured neutron angular distributions have been analysed using DWBA. By a comparison with shell-model calculations in the (s, d) shell it is found that most of the two-proton transfer strength can be explained within that shell. Important contributions, however, from the (f, p) shell in low-lying negative parity states are also present.     

194Pt(12C, 12C′) reaction and the triaxial-rotor model

Elastic and inelastic scattering differential cross sections for the ¹⁹⁴Pt(¹²C, ¹²C′) reaction at a bombarding energy of 78 MeV have been measured. Data have been obtained for 0⁺, 2⁺, 4⁺, and 2^+′, states in ¹⁹⁴Pt. The data have been analyzed using a rigid asymmetric-rotor model in coupled-channels reaction calculations. It is found that satisfactory fits to all data can be obtained but only if the hexadecapole deformation is included in the asymmetric-rotor model.