Osmium Nuclei Research Articles

Lifetime measurements of excited states in 169,171,173Os: Persistence of anomalous B(E2) ratios in transitional rare earth nuclei in the presence of a decoupled i13/2 valence neutron

Lifetimes of low-lying excited states in the νi13/2+ bands of the neutron-deficient osmium isotopes 169,171,173Os have been measured for the first time using the recoil-distance Doppler shift and recoil-isomer tagging techniques. An unusually low value is observed for the ratio B(E2;21/2+→17/2+)/B(E2;17/2+→13/2+) in 169Os, similar to the “anomalously” low values of the ratio B(E2;41+→21+)/B(E2;21+→0gs+) previously observed in several transitional rare-earth nuclides with even numbers of neutrons and protons, including the neighbouring 168,170Os. Furthermore, the evolution of B(E2;21/2+→17/2+)/B(E2;17/2+→13/2+) with increasing neutron number in the odd-mass isotopic chain 169,171,173Os is observed to follow the same trend as observed previously in the even-even Os isotopes. These findings indicate that the possible quantum phase transition from a seniority conserving structure to a collective regime as a function of neutron number suggested for the even-even systems is maintained in these odd-mass osmium nuclei, with the odd valence neutron merely acting as a “spectator”. As for the even-even nuclei, the phenomenon is highly unexpected for nuclei that are not situated near closed shells.

Sdg-IBM1 Calculations for Osmium Nuclei

sdg-IBM1 Calculations for Osmium Nuclei

Nuclear-tidal waves in the osmium nuclei

Structures have been identified built upon the Kπ = 43/2(−) and Kπ = 43/2+ isomeric states in 183Os. Although these sequences appear quite irregular, their properties can be understood if the angular momentum is built up, to a large degree, from multiple phonons. These vibrations are found to compete with rotations in generating angular momentum.

High-K band crossings in osmium and rhenium isotopes

In a quantum many-body calculation using deformed Hartree–Fock and angular momentum projection formalism, the nature of K-mixing around the first band crossings in the high-spin structure of 182,184,186Os and 181,183Re is studied. For Z = 75 181,183Re (N = 106, 108) our calculation reproduces the observed lowering of the high-K (t-band) structure compared to the low-K (s-band) structure, arising due to the coupling of a pair of i13/2 neutrons. We find a transition in even–even osmium nuclei, at N = 108, from the conventional s-band crossing to the t-band crossing. The degree of high-K mixing obtained in the calculation provides a microscopic understanding of the K-forbidden transitions from the high-K bandheads to the yrast states with low-K intrinsic structure.

Reply to “Comment on ‘Two-phononγ-vibrational strength in osmium nuclei’ ”

The claim that the two-phonon gamma-vibrational configuration constitutes a major component for the I=4+ states in osmium nuclei is based on solid experimental evidence. A non-negligible two-quasiparticle or hexadecapole component must also exist in order to explain the data.

Comment on “Two-phononγ-vibrational strength in osmium nuclei”

Claims in a recent paper by Wu et al. [Phys. Rev. C 64, 014307 (2001)] for the $\ensuremath{\gamma}\ensuremath{\gamma}$ character of ${K}^{\ensuremath{\pi}}{=4}^{+}$ bands are based on a flawed argument. A paper they do not cite has explained this weakness in detail, and concluded a dominant g boson or hexadecapole structure is required to explain other data inconsistent with the $\ensuremath{\gamma}\ensuremath{\gamma}$ description.

Two-phonon γ-vibrational strength in osmium nuclei

Collective $E2$ strengths were measured to probe the two-phonon \ensuremath{\gamma}-vibrational character of ${I}_{K}^{\ensuremath{\pi}}{=4}_{4}^{+}$ states for ${}^{188,190}\mathrm{Os}.$ Lifetimes for states with spin up to ${8}^{+}$ of the ground-state band, ${6}^{+}$ of the \ensuremath{\gamma} band, and the ${4}^{+}$ state of the ${K}^{\ensuremath{\pi}}{=4}^{+}$ band were measured by the recoil-distance method following Coulomb excitation by a ${}^{58}\mathrm{Ni}$ projectile at ${E}_{\mathrm{lab}}=275 \mathrm{MeV}.$ These lifetime data are compared with the electromagnetic matrix elements measured by previous Coulomb excitation experiments. The good agreement between these independent measurements supports the conclusion, made by the Coulomb excitation work, that the electromagnetic properties of these osmium nuclei are consistent with the description of \ensuremath{\gamma}-soft collective models. In addition, it supports the evidence that the two-phonon \ensuremath{\gamma}-vibration configuration is the dominant component for the lowest-lying ${I}_{K}^{\ensuremath{\pi}}{=4}_{4}^{+}$ states in osmium nuclei.

Yrast states in194Os:The prolate-oblate transition region

Previously unidentified states in the neutron-rich nucleus ${}^{194}\mathrm{Os}$ have been populated following a deep inelastic reaction using a 780 MeV ${}^{136}\mathrm{Xe}$ beam on a thick ${}^{198}\mathrm{Pt}$ target. $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ coincidence events were collected using the $8\ensuremath{\pi}$ detector array. The yrast band in ${}^{194}\mathrm{Os}$ has been observed up to ${I}^{\ensuremath{\pi}}{=(10}^{+}),$ for the first time. This represents the heaviest osmium nucleus where in-beam \ensuremath{\gamma}-ray spectroscopy has been performed to date. The excitation energies of the new levels are compared to the systematics of the lighter even-even osmium nuclei. The evidence for a transition from prolate to oblate-deformed ground states in the heavy osmium nuclei is discussed and total Routhian surface calculations are presented. An alignment analysis together with cranked shell model calculations, suggest that the yrast states have a prolate shape, in contrast to earlier interpretations. Predictions for the neighboring even-even tungsten isotopes are also described.

Beta-decays of172?175Ir neutron-deficient isotopes

Theβ-decays of172–175Ir have been studied using a Hejet system through32S induced reactions,β half-lives have been measured and compared with those given fromα decays. The level schemes of osmium nuclei are deduced and for172Os and174Os, the 22+ level is possibly located. The observedβ-decay half-lives clearly indicate an isomerism phenomenon occuring in light Ir isotopes.

The quadrupole plus pairing interaction model in large configuration space for W, Os and Pt nuclei

The equilibrium deformations of tungsten, osmium and platinum nuclei are studied with the self-consistent quadrupole plus pairing interaction model by considering all the nucleons in nucleus explicitly. It is shown that similar results can be obtained by performing calculations with or without the assumption of an inert core. The only difference is in the strength of the quadrupole and pairing interactions to be employed in the respective calculations. The experimental static quadrupole moments and theB (E2) values are correctly reproduced by performing calculations with bare nucleon charge for all the nucleons.

Os188,190,192(t,p)reaction at 15 MeV

A study of the heavier osmium isotopes $^{190}\mathrm{Os}$, $^{192}\mathrm{Os}$, and $^{194}\mathrm{Os}$ has been made with the ($t,p$) reaction using a 15 MeV triton beam. The osmium nuclei may be considered as transitional nuclei going from a deformed to a spherical shape with increasing neutron number and the ($t,p$) reaction is used to probe this change. The study principally investigates the position and strength of the ${0}^{+}$ states, a number of new such states being observed. The nucleus $^{194}\mathrm{Os}$ is also reported in some detail for the first time with 15 excited levels observed and 7 spin assignments made.[NUCLEAR REACTIONS $^{188,190,192}\mathrm{Os}(t,p)$, ${E}_{t}=15$ MeV. Measured $\ensuremath{\sigma}(\ensuremath{\theta})$; enriched targets; DWBA calculations deduced $J$, $\ensuremath{\pi}$.]

Stabilization of the emission frequency of a CO2laser to within 10–12with the aid of narrow SF6and OsO4resonances

A long-term stability better than 10–12 (averaging time 100 sec) was achieved for the emission frequency of a cw CO2 laser, which was stabilized using narrow saturated-absorption resonances in an external low-pressure SF6 cell. Narrow molecular resonances of OsO4 were used as a second natural frequency standard. A nonlinear spectrometer with a stabilized CO2 laser and an external OsO4 absorption cell was found to be suitable for measurement of the energy of metastable osmium nuclei.

Possible Nuclear Structure Effects in Even-Even Osmium Nuclei

A compilation of the multipole mixing of 2′ + → 2 + transitions in 186Os, 188Os, 190Os and 192Os is given. In view of recent results obtained at our laboratory, indicating large anomalies in the conversion process of inhibited M1 transitions, the study has been extended for even-even Osmium nuclei. A series of experiments has been performed in which the conversion coefficients and transition probabilities were measured. The data of 2′ + → 2 + transitions were analysed by taking into account nuclear structure effects on the Ml internal conversion process. The M1 admixtures obtained are compared with Greiner′s calculations. Values of reduced transition probabilities B (E2, 0 → 2 +) , B (E2, 2′+ → 2+) , the mixing ratio δ = 〈2 || E2 || 2′〉 / 〈2 || M1 || 2′〉 and the transition branching ratio T (2′ → 2)/T(2′ → 0) are reported for second and higher 2+ states. The results are compared to the pairing-plus-quadrupole model calculations of Kumar and Baranger.

Magnetic Dipole Moments of First2+States in Even Osmium Nuclei Recoil-Implanted in Nickel

The ion-implantation-perturbed-angular-correlation technique has been used to measure the $g$ factors of the first ${2}^{+}$ excited states of four even osmium nuclei in a ferromagnetic nickel host. The observed angular precessions were: $\ensuremath{\omega}\ensuremath{\tau}(^{192}\mathrm{Os})=0.22\ifmmode\pm\else\textpm\fi{}0.01$, $\ensuremath{\omega}\ensuremath{\tau}(^{190}\mathrm{Os})=0.277\ifmmode\pm\else\textpm\fi{}0.009$, $\ensuremath{\omega}\ensuremath{\tau}(^{188}\mathrm{Os})=0.39\ifmmode\pm\else\textpm\fi{}0.03$, and $\ensuremath{\omega}\ensuremath{\tau}(^{186}\mathrm{Os})=0.38\ifmmode\pm\else\textpm\fi{}0.08$. The angular precession result for $^{192}\mathrm{Os}$ is in excellent agreement with the results of perturbed-angular-correlation measurements in nickel using radioactive sources. It was found that the internal field seen by the osmium nuclei implanted in nickel is in good agreement with the value obtained by room temperature NMR techniques. The available direct lifetime measurements for $^{186,188}\mathrm{Os}$ and lifetimes extracted from $B(E2)$ values for $^{190,192}\mathrm{Os}$ are used to obtain the gyromagnetic ratios: $g(^{192}\mathrm{Os})=0.39\ifmmode\pm\else\textpm\fi{}0.03$, $g(^{190}\mathrm{Os})=0.39\ifmmode\pm\else\textpm\fi{}0.03$, $g(^{188}\mathrm{Os})=0.29\ifmmode\pm\else\textpm\fi{}0.03$, and $g(^{186}\mathrm{Os})=0.24\ifmmode\pm\else\textpm\fi{}0.05$.

Experimental Tests of Recent Pairing-Plus-Quadrupole Model Calculations in the Osmium Nuclei

Experimental Tests of Recent Pairing-Plus-Quadrupole Model Calculations in the Osmium Nuclei

Ground state (quasi-) rotational levels in light Os, Pt and Hg nuclei

Energy levels in some neutron-deficient doubly even nuclei in the platinum region have been studied following heavy-ion reactions. Information of the ground state rotational (or quasi-rotational) bands in 178, 180, 182Os, 182, 184, 186, 188Pt and 188, 190Hg is presented. The levels suggest that the light platinum nuclei are very soft axially-symmetric rotors, rather different from the well-known γ-unstable or tri-axial heavy osmium nuclei. The energy-level systematics of this region are compared with those of the samarium region, and it is concluded that the two regions are basically similar, with the well-known 88–90 neutron discontinuity reasonably well duplicated between neutron-deficient osmium and platinum nuclei (proton number 76–78).