Nuclear levels in193Os

The previously completely unknown energy levels of¹⁹³Os have been investigated using the reactions¹⁹²Os(d, p)¹⁹³Os and¹⁹²Os(n, γ)¹⁹³Os. The neutron separation energy was measured to be 5583.5±2.0 keV. Most of the states below 500 keV can be qualitatively interpreted in terms of the Nilsson model; however the occurrence of four levels with significantl=1 transfer strength in this energy range is incompatible with that model. The Nilsson orbital systematics of the odd-A Os and W isotopes are compared.     

Mössbauer studies and nuclear quadrupole moments of186, 188, 189, 190 Os

Mössbauer transmission experiments with the 137, 155, 69 and 187 keV gamma rays of^{186, 188, 189, 190}Os, respectively, yielded the electric quadrupole splitting of these gamma resonance lines in OsO₂ and OsP₂. From the results the following ratios of quadrupole moments were derived:Q ₂ ⁺(¹⁸⁶Os, 137 keV):Q ₂ ⁺(¹⁸⁸Os, 155keV):Q ₂ ⁺(¹⁹⁰Os, 187 keV):Q _3/2 ^?(¹⁸⁹Os, g.s.)=(+1.100±0.020): 1.0:(+0.863±0.051): (?0.586±0.011) andQ _5/2 ^?(¹⁸⁹Os, 69 keV)/Q _3/2 ^?(¹⁸⁹Os, g.s.)=?0.735 ±0.012. The ratios for^{180, 188, 190}Os are, within their limits of error, in agreement with the expectation of the rotational model, indicating that the pairing-plusquadrupole model calculations of Kumar and Baranger predict too rapid a transition form rotational to vibrational nuclei. Applying this argument to¹⁸⁶Os in particular and using the measured ratios, one obtains a set of values for the quadrupole moments themselves, namelyQ ₂ ⁺ (¹⁸⁶Os)=?(1.50 ± 0.10)b,Q ₂ ⁺(¹⁸⁸Os)=?(1.36± 0.09) b,Q ₂ ⁺ (¹⁹⁰Os)=?(1.18 ± 0.08) b,Q _3/2 ^?(¹⁸⁹Os)=+ (0.80 ± 0.06) b, andQ _5/2 ^? (¹⁸⁹Os)=? (0.59 ± 0.05) b. For the electric field gradient at Os nuclei in Re metal a value ofV _zz=?(3.3 ± 0.6) · 10¹⁷ V/cm² was found. A measurement with a magnetically split source yielded δ=+ 0.685 ± 0.025 for theE2/M1 mixing parameter of the 69 keV transition of¹⁸⁹Os,g _5/2 ^?/g _3/2 ^?=0.895 ± 0.006 for the ratio of theg-factors of the 69 keV state and the groundstate, andH _i=?(1135 ± 20) kOe for the hyperfine field at Os nuclei in an iron matrix.     

Mössbauer isomer shifts and quadrupole splittings in osmium compounds

From Mössbauer spectra of the 36.2keV and 69.5keV gamma rays of¹⁸⁹Os, isomer shifts and electric quadrupole interactions for a number of chemical compounds of osmium in the oxidation states +8, +6, +4, +3, and +2 were obtained. The results show that the dependence of the isomer shifts on the oxidation state of Os follows the general pattern established for other transition elements like Ru and Ir. A comparison of the isomer shift data with electron densities at the Os nuclei resulting from relativistic self-consistentfield calculations for free Os ions yieldsΔ 〈r²〉 [36 keV]=?2.0 · 10^?3 fm² andΔ 〈r²〉 [69 keV]=?0.13 · 10^?3 fm³ for the changes of the mean-square nuclear charge radius of¹⁸⁹Os. These values are in qualitative agreement with an interpretation of the low-energy level scheme of¹⁸⁹Os in terms of two rotational bands.     

Alpha decay studies of very neutron deficient isotopes of Hf,Ta, W,and Re

Very neutron deficient isotopes in the Hf-Re region were produced by bombarding targets of^{107, 109}Ag,^{nat, 108, 110}Pd, and¹⁰³Rh with⁵⁸Ni accelerated by the linear accelerator UNILAC at GSI. After separation from the projectile beam by the velocity filter SHIP, the fusion products were implanted with their full recoil energy into a silicon surface barrier detector. The subsequent alpha decay of the implanted ions was measured with the same detector. In some of the experiments the evaporation residues were implanted into a position sensitive silicon surface barrier detector. With a newly developed position and time correlation technique parent daughter relationships, half lives, andα branching ratios of a large number of isotopes could be determined. In the investigated reactions the eleven new isotopes^161–164Re,¹⁶⁰W,^157–161Ta, and¹⁵⁶Hf could be identified. The measuredQ _α values, half lives, and a branching ratios are discussed. Two new high energyα. transitions of (7,408±10) keV and (7,804±15) keV were also found with half lives of 2.7 ms and 0.52 ms, respectively.A tentative mass and atomic number assignment givesA=153–156 andZ=70–72. The energies of the excited isomeric states are between 2 MeV and 3 MeV. From the systematics of alpha ground state transitions in even nuclei a hindrance of 10⁵ can be deduced for both transitions, possibly indicating orbital angular momenta on the order of 10? for the alpha emitting states. The isomers are proposed to belong to shell model isomers beyond¹⁴⁶Gd.     

Beta-decays of172–175Ir neutron-deficient isotopes

Theβ-decays of^172–175Ir 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.     

Disparity of measured gyromagnetic ratios of ground- and excited-band states in184W

Gyromagnetic ratios of levels in the ground-band and of the 2 ₂ ⁺ state in¹⁸⁴W were measured by means of the transient magnetic field perturbedγ-ray angular distribution technique. The levels of interest were Coulomb excited by beams of 220-MeV⁵⁸Ni and⁶³Cu ions. The results show theg-factor of the 2 ₂ ⁺ state to be approximately half the effectively constant value found for the ground-band levels. This disparity is compared with those previously obtained for the corresponding states in^188–192Os and¹⁸⁶W; these are discussed in the context of the interacting boson model.     

The charge radii of198Pt-183Pt

The reactions⁵⁸Ni+¹⁰²Pd→¹⁶⁰W and⁵⁸Ni+¹⁰⁶Cd→¹⁶⁴Os were investigated to search for new decay data of neutron deficient nuclei. Excitation energies of the compound nuclei covered a range from 47 to 89 MeV. Velocity separation of the evaporation residues and position time correlations with the a decays of the implanted nuclei were used. The following new decay data were measured:¹⁶²Os (E_α=(6611 ±30) keV, T_1/2=(1.9±0.7) ms);¹⁵⁸W (T_1/2=(0.9±0.3) ms);¹⁵⁸m_W (E=1.88 MeV, E_α=(8280±30) keV, T_1/2=(0.01-1) ms);^155mLu (E_α=(5575±10) keV); β decay of¹⁵⁶Ta (T_1/2 > 10 ms) to the 8⁺ yrast isomer in¹⁵⁶Hf. A cross section of 5μb was measured for the new isotope¹⁵⁶Ta produced in a p3n evaporation channel from¹⁶⁰W at 64 MeV excitation energy.     

Electric quadrupole and magnetic dipole moments of the first excited 2+ states of186Os and188Os

The spectroscopic quadrupole moments and the magnetic dipole moments of the lowest 2⁺ states in¹⁸⁶Os (137 keV) and¹⁸⁸Os (155 keV) have been determined by Mößbauer transmission experiments. The electric quadrupole momentsQ ₂₊(Os 186)=? (1.80±0.22) b andQ ₂₊(Os 188)=?(1.81±0.24) b as well as their ratioQ ₂₊(Os 188)/Q ₂₊(Os 186)=1.00±0.07 within the limits of error agree withB(E2) data, if a comparison on the basis of the rotational model is made. For the g-factors and their ratio g₂₊(Os 186)=0.281±0.008, g₂₊(Os 188)=0.305±0.015 andg ₂₊(Os 188)/g ₂₊(Os 186)=1.08±0.05 was obtained. All results are compared with recent model calculations.     

E2/M1 Mixing ratios of 2+-2+-transitions in58Fe,74Ge and186Os

E2/M1 mixing ratios of 2⁺-2⁺-transitions in⁵⁸Fe,⁷⁴Ge and¹⁸⁶Os were determined by measuring the angular correlation of the 2-2-0γγ-cascades in these nuclei. The following values were obtained:⁵⁸Fe(863 keV):δ=+1.10±0.20;⁷⁴Ge(609keV):δ=?2.9±0.8 and¹⁸⁶Os(630keV):δ=?13 _?3.5 ^+5.5 . These results are compared with theoretical predictions.     

Observation of highly enhancedE3-transitions in198,200Po. Evidence for octupole instability atZ=84 andN≦ 114?

^198,200Po have been studied by in beam γ-spectroscopy with the^182,184W(²⁰Ne,4n) reaction at 105 to 112 MeV. Both nuclei exhibit 8⁺, 11^?, and 12⁺ isomers for which lifetimes andg-factors have been measured, that determine their single particle structure. The a priori highly hindered (πh _9/2 i _13/211^?→πh ₉ ^2/2 8⁺)E3-transition becomes with 25 Weisskopf units very strong in¹⁹⁸Po suggesting that octupole vibrations or deformations are important in this region of nuclei.     

Evidence of new isotopes:169, 170Ir,166, 167, 168Re

At the Darmstadt UNILAC newα-emitting nuclei were produced in the bombardment of⁸⁹Y and⁹³Nb targets using⁸⁴Kr ions with energies in the range of 5.1 to 5.5 MeV/u, and 5.8 to 6.4 MeV/u, and using⁸⁶Kr ions with energies in the range of 5.6 to 6.0 MeV/u. Reaction recoils emitted from the targets were stopped and transported with argon to a collection site. Using three alpha detectors and spectrum multiscaling, energies and half-lives were measured. Ir and Re isotopes were identified by cross bombardments, excitation function data andα-systematics. The decay characteristics of the new species are as follows: Estimates onα-branching ratios have been obtained for^{169, 170}Os and^{166, 167, 168}Re.     

Studies of the low-lying levels in192Pt and192Os

The energy level schemes of¹⁹²Os and¹⁹²Pt have been established on the basis ofγ- γ coincidence studies using a dual parameter data collection system. Ge(Li) detectors were employed to study the gamma spectra produced in the E.C. andβ ^? decays of¹⁹²Ir to¹⁹²Os and¹⁹²Pt, respectively. Thirteen new transitions and three new levels at 1,146.95, 1,237.35 and 1,913.76 keV are suggested. Relative intensities from singles measurements, branching ratios,α(K) and log ft values were calculated and multipolarities, spins and parities deduced. Comparisons are made with predictions of the Interacting Boson Model calculated on the basis of an O(6) to SU(3) transition.     

Intensitätsmessung von Röntgen- und Kerngammastrahlung an einigen schweren Myonenatomen

Absolute intensities of the 5–4 muonic X-ray transition in muonic W, Os, and Bi, and relative intensities of other X-ray transitions have been measured and compared with cascade calculations. The excitation of several levels in W, Os, Ir, Tl, and Bi nuclei resulting from the muonic cascade process has been observed and absolute excitation probabilities have been determined. For the first 2⁺ level of¹⁸⁸Os and the first 3/2⁺-levels of^203,205Tl the excitation probabilities are larger than calculated. The results are discussed.     

Hot nuclei — Effective shapes and entrance channel effect

The γ-decay of the Giant Dipole Resonance in hot rotating compound nuclei has provided valuable information on the nuclear structure at finite temperature. A number of experimental results showing that the nuclear shapes change with temperature and angular momentum are here reviewed. In particular we concentrate on the temperature interval from 1 to 2 MeV, rotational frequencies from 0.2 to 1.5 MeV and on nuclei in the mass regionsA≈160–170 andA≈110, characterized by the prolate-oblate and spherical-oblate phase transitions, respectively. The possibility to study the shapes involved in the compound nucleus formation is also discussed. For this purpose long formation times are required and the nucleus¹⁷⁰W formed with the reaction⁶⁰Ni+¹¹⁰Pd, here studied, seems to be a good candidate. The gamma and particle decays were compared to those of the reaction⁴⁸Ti+¹²²Te. The comparison shows that in average the energy of the α-particles is larger for the more symmetric reaction, consistent with longer formation times and larger deformations in the pre-equilibrium stage.     

Nuclear levels and spectroscopy of191Os

Thirty-nine states, all except two of which are new, have been observed up to 1535 keV in¹⁹¹Os using the reactions¹⁹⁰Os(d,p)¹⁹¹Os,¹⁹²Os(d, t)¹⁹¹Os with 12 MeV deuterons and magnetic analysis with photographic emulsions of the outgoing particles, and¹⁹⁰Os(n, γ)¹⁹¹Os with thermal neutrons and three modes of gamma detection in the energy range from 40 keV to ～6 MeV. The neutron separation energy was determined as 5758.5±2.0 keV in agreement with the value of the (d, p) reaction. TheQ value for the (d, t) reaction was determined as ?1265±15 keV. Spins and parities are assigned for most of the states below 800 keV. The states below 463 keV are shown to be qualitatively but not quantitatively consistent with the expectations of the Nilsson model. Nilsson systematics of the 1/2^?: ¦510¦ and 3/2^? ¦512¦ orbitals in odd-A Os isotopes with increasing neutron number can be understood in terms of decreasing deformation. Anomalously large (d, p) and (d, t) cross sections populating 5/2^? and 3/2^? states at 134 and 142 keV, respectively, are explained as a tendency to restore the cross sections expected for a spherical nucleus. Evidence for triaxiality in¹⁹¹Os is observed in the form of a 5/2^? state at 273 keV which appears to be a rotational state built on the 9/2^? ground state.     

Ground and γ-vibrational bands in even Os isotopes (A = 182–188) studied by (α, χnγ) and (p, χnγ) reactions

Gamma rays following ¹⁸²W(α, 2n)¹⁸⁴Os, ¹⁸⁴W(α, 2n)¹⁸⁶Os, ¹⁸⁶W(α, 2n)¹⁸⁸Os, ¹⁸⁵Re (p, 2n)¹⁸⁴Os, ¹⁸⁵Re(p, 4n)¹⁸²Os, ¹⁸⁷Re(p, 2n)¹⁸⁶Os and ¹⁸⁷Re(p, 4n)¹⁸⁴Os reactions have been measured to study the ground and γ-vibrational bands in even Os isotopes (A = 182–188). In addition to the ground-state levels the γ-vibrational bands have been identified up to the 5⁺ members. The K = 4 band head is proposed in ¹⁸⁶Os and ¹⁸⁸Os.     

Isotope shift in osmium I: Experiment and theory

The isotope shift in the arc spectrum of osmium was studied in 10 lines for the highly enriched isotopes¹⁸⁸Os and¹⁹²Os and in 2 lines for natural¹⁹⁰Os by means of a photoelectric recording Fabry-Perot spectrometer with digital data processing. The observed shifts were analyzed by means of the parametric method. The field shift of 5d ⁷6s ⁵ F is 53.4 (9.6) mK larger than that of³ F, thus showing the influence of crossed-second-order effects produced by the interaction of the electrostatic operator and the field shift operator. Spin dependent effects were also able to be evaluated. The corresponding parameterz _s for the configuration 5d ⁷6s was found to be 2.5 (2) mK. The experimental data are compared with results from non-relativistic Hartree-Fock calculations. Except for a scaling factorS ^*(Z)=1.489 the calculated electron densities describe the measured isotope shifts of this work and of other authors.     

High spin states in94Ru and95Rh

The results of shell model calculations are presented and these support the experimental angular momentum assignments made for the high spin states in⁹⁴Ru and⁹⁵Rh. Only one disagreement between theory and experiment is encountered and this concerns the depopulation of the yrast 9^? state in⁹⁴Ru. A highly inhibitedE1 transition to the 8⁺ state is expected but not seen. The reason that seniority isomers are not observed in these nuclei is discussed.     

High-spin negative parity states in doubly even Hg,Pt and Os nuclei

A systematic study of the level structure of ²⁰⁰Hg, ¹⁹⁸Hg, ¹⁹⁶Hg, ¹⁹⁴Pt, ¹⁹²Pt, ¹⁹⁰Pt, ¹⁸⁸Os, ¹⁸⁶Os and ¹⁸⁴Os has been performed by means of (α, 2nγ) and (p, 2nγ) in-beam γ-ray spectroscopy. Much new information has been obtained about the individual level spectra, and arguments based on level energy and population intensity systematics have been used to trace the behaviour of interesting spectral features through this series of transitional nuclei. Of particular interest is a 5^?, 7^?, 9^? … sequence of levels which appears to be a recurring feature in the spectra of the doubly even Pt and Hg nuclei in the mass range A = 190–200. These negative parity levels are connected by enhanced E2 transitions and they are very strongly populated in the de-excitation of the residual nuclei formed in (α, 2n) reactions. It is proposed that the 5^?, 7^?, 9^? … sequences constitute rotation-aligned bands analogous to the decoupled bands identified by Stephens and co-workers in odd-A transitional nuclei. In the doubly even nuclei, the bands may arise from the coupling of an $\text{i}\text{13}\text{2}$ neutron with a low-j neutron partner (from the $\text{p}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$, $\text{p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ and $\text{f}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$ orbitals). Negative parity levels in the Os nuclei are populated much less strongly in the (α, 2nγ) reactions. While 5^? levels corresponding to those in the Pt and Hg nuclei appear to be fairly well established in ¹⁸⁶Os, ¹⁸⁸Os (and ¹⁹⁰Os), it is not yet clear whether the 5^?, 7^?, 9^? … band sequence extends into the Os nuclei.     

Description of high-spin isomers in theN=82 region

The high spin states and of those especially the yrast isomers are studied in the region near the rare earth nuclei. The deformation energy surface is calculated with a method modelled in principle according to a shape constrained cranked HF theory (CHF). In practice, the expectation value of the many-body Hamiltonian is calculated with cranked Nilsson functions. One finds rotation around a slightly deformed oblate or prolate symmetry axis in front or behind the rare earth region, respectively. Near the Hf isotopes strongly prolate deformed nuclei rotating around the symmetry axis are found in agreement with the knownK isomers in this region. These results are explained also qualitatively with the help of the MONA (Maximisation of theOverlap ofNucleonic wave functions byAlignment of single particle angular momenta) effect. In the second part high spin states and yrast isomers in theN=82 region are calculated in the cranked Hartree-Fock-Bogoliubov approach with four and six quasi-particle excitations. For the excitation energies in¹⁴⁶Gd and¹⁵²Dy and a measuredg factor one finds satisfactory agreement.