High-spin yrast and non-yrast bands in 176Os, 178Os and 180Os

High-spin yrast and non-yrast states have been identified in ¹⁷⁶Os, ¹⁷⁸Os and ¹⁸⁰Os using (¹⁶O, xn) reactions, and γ-ray techniques. Band crossing anomalies are observed in each of the positive-parity yrast bands. The magnitude of these anomalies decreases with decreasing neutron number, an effect attributed to the change in the moment of inertia of the ground state rotational bands. A 23 ns isomer, predominantly K^π = 7^?, is identified at 1930 keV in ¹⁸⁰Os. The configuration of this isomer is discussed on the basis of the properties of its rotational band. Negative parity, odd and even spin, sideband sequences are observed in each isotope. Their relationship to rotation-aligned octupole and 2-quasiparticle bands is discussed from their excitation energies, band spacings, and decay properties. Detailed calculations for Coriolis mixed bands are carried out for the likely 2-quasiproton and 2-quasineutron configurations. An anomaly observed at spin 17 in the odd-spin negative-parity sequence in ¹⁸⁰Os is attributed to a band crossing with a fourquasiparticle configuration.     

Static moments of 54Fem and perturbed angular distributions with combined dipole and quadrupole interactions

The static moments of a new I^π = (10⁺) isomer in ⁵⁴Fe (E_x = 6528keV; τ = 525 ± 10ns) were measured by the perturbed angular distribution method with the results g = +0.778 ± 0.010, |Q| = 72 ± 7efm². The isomer was then used to study the combined interaction produced by weak randomly-oriented electric field gradients in the presence of a strong magnetic field.     

Isomeric states in180Os

Two new isomers have been observed in¹⁸⁰Os. A high-K isomer withI, K ≧20 and a half-life ofT _1/2=12+4 ns have been established. It deexcites via two transitions into the 18⁺ level of the yrare band indicating an unusually smallK-hindrance factor. Evidence for an isomer withI, K>16 and a half-life ofT _1/2=41±10 ns was found. A half-life of 17±3 ns was measured for the previously known 7^? state at 1862 keV. The decay scheme of the previously known 7^? isomer at 1928 keV has been extended and a revised version is presented.     

Evidence for nuclear shape coexistence in180Hg

Theγ decay in the radiative fusion reaction⁹⁰Zr+⁹⁰Zr→¹⁸⁰Hg+xγ has been observed in an array of NaI detectors. States up to 6⁺ in the yrast sequence of¹⁸⁰Hg are tentatively assigned and suggest the coexistence of weakly oblate and strongly prolate nuclear shapes. The difference in potential energy between the two inferred shapes has dropped to about 200 keV, continuing the downward trend observed in the heavier even isotopes^188–182Hg.     

Search for missing predicted high-spin states in28Si

New shell model calculations have predicted several high-spin (I ^π=5⁺ and 6⁺) levels in²⁸Si near 10 MeV excitation energy which are missing from or ambiguous in existing experimental studies. Angular distributions, linear polarizations and Doppler-shifts ofγ-rays have been measured for theγ-decay of theE _p=1,911 and 2,073 KeV resonances of the²⁷Al(p, γ) reaction in an attempt to discover these missing states or confirm the discrepancies between experiment and theory. The excitation energies and spin-parities of the resonances were determined as 13,424.4±0.2 keV,I ^π=5⁺ and 13,582.3±0.5 keV,I ^π=6⁺. States populated in theγ-decay of these resonances were assigned spins and parities as follows: 11,777 keV,I ^π=5⁺; 11,331 keV,I ^π=6⁺; 10,417 keV,I ^π=5⁺; 9,417 keV,I ^π=4⁺ and 8,945 keV,I ^π=5⁺. On the basis ofγ-ray transition rates T=1 is assigned to the 13,424 keV level and T=0 to the 10,417 and 11,777 keV levels. With the new data excellent agreement is achieved between the experimental spectrum of²⁸Si and the new shell model predictions. These data provide evidence for aK ^π=3⁺ rotational band comprised by the 6,276, 6,889, 8,945 and 11,331 keV levels. This band emerges also from the shell model wave functions as do theK ^π=0⁺ bands based on the ground state and the 6,691 keV state.     

Coulomb excitation in 180Hf

Coulomb excitation studies have been performed to measure transition probabilities of collective quadrupole vibrational states in ¹⁸⁰Hf. The I = 2 level of the K^π = 2⁺ collective γ-band is established at 1200.5 keV with B(E2)_exc = (11.0 ± 1.1) × 10^?50e² · cm⁴ (3.6 ± 0.4 s.p.u.). The angular distribution of the de-exciting γ-rays from this level yields δ = 9.6⁺²²_?5.8 or, less likely, 0.7 ± 0.2 for the 1107.2 keV 2_γ⁺ → 2_g⁺ transition. The B(E2)_exc for any K^πI = 0⁺2 stateorother 2⁺ states up to 1500 keV is less than 5 × 10^?51e² · cm⁴ (< 0.2. s.p.u.).     

Decay of isomers stimulated by laser radiation

A theory is presented for the induced decay of isomer levels for nuclei exposed to a laser field, which couples the initial isomer level to intermediate higher lying levels with short lifetimes. We took into account the electric dipole transitions between these levels in a one-photon approximation. The time-dependent Schrödinger equation with a periodic Hamiltonian is solved by the method of the composite Hilbert space. A simple expression for the broadening of the isomer level ΔΓ, caused by the laser radiation, is derived. As an example, we considered the decay of the isomer level 970.17 keV with the spin I ^π = 23/2^? of ¹⁷⁷Lu, coupled by the laser wave to the virtual level 1352.33 keV with I ^π = 21/2⁺. The ratio of the broadening ΔΓ to the natural width of the isomer level Γ is found to be about 40% for a 10¹⁷ W/cm² laser. Any influence of the electronic environment is not taken into account.     

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.     

Structure of28Si from the spectroscopy of high-spin states

A search for high-spin states in²⁸Si has been performed byn?y coincidence measurements in the²⁵Mg(α,nγy) reaction atE _α=14 and 15.5 MeV. Spin-parity assignments of the observed levels were obtained fromn?γ angular correlation and lifetime measurements atE _α=14.5 MeV. Theγ-decay of the 9,164 keV level was investigated separately with the²⁷Al(p, γ) reaction at theE _p=2,160 and 2,312 keV resonances. Rotational bands withK ^π=3^? (comprising levels atE _x=6,879, 8,413, 10,188 and 12,204 keV),K ^π =5^? (comprising levels atE _x=9,702, 11,577 and 13,741 keV) andK ^π=0⁺ (comprising levels atE _x=6,691, 7,381, 9,164 and 11,509 keV) were observed. The finding of the latter band supports the idea of coexisting oblate and prolate shapes in²⁸Si. A level at 14,643 keV excitation energy has the properties of theI ^π=8⁺ member of the ground state band. There are additional positive-parity high-spin states which do not fit into rotational bands. All types of positive-parity states are well accounted for by shell model calculations.     

Investigation of dual decay of70Ga (21 min)

The weakβ ^?-branchings in⁷⁰Ga decay leading to excited states in⁷⁰Ge at 1039 keV, and 1215 keV were measured byβ ^?-ray and γ-ray detection in 4π geometry to be 0.36 ±0.02%, and 0.32±0.01%, respectively. The weak electron capture decay of⁷⁰Ga was confirmed and its intensity measured to be 0.41±0.05%. The energy of the γ-rays was remeasured to be 176.17±0.02 keV, and 1039.20±0.08 keV.     

Fragmentation of the yrast band in186Os atI π =18+ and disappearance of the collective minimum

High-spin states in¹⁸⁶Os have been populated by the¹⁸⁶W(⁴He,4n)-reaction at 55MeV. The emittedγ-radiation was detected with the OSIRIS spectrometer. The yrast band, for which the nucleus has a prolate shape, was found to terminate atI ^π =18⁺. Theγ-ray intensity is then distributed between several irregular sequences. Different to other cases of band termination, the minimum in the total routhian surface corresponding to a collective shape is calculated to disappear in this spin region, although the available spin of the valence nucleons is far from being exhausted. A different structure, which is dominated by non-collective states becomes yrast.     

Vibrational band structure in 184W populated in the decay of 184Re

The decay of ^184mRe has been investigated through γ-ray and conversion electron studies. The band head of the K^π = 2^? octupole band has been established at 1130.0 keV. The E2/M1 mixing ratios of three transitions from the γ-vibrational band to the ground state band have been determined by angular correlation measurements. A mixing of El, M2 and E3 multipolarity has been derived for the 921 keV transition combining angular correlation and conversion electron data. A value B(E3, 0⁺ → 3^? = (25 ± 5) × 10⁴e² · fm⁶ was obtained from the measured E2/M1 mixing of the 91 keV 3^? → → 2^? transition and γ-branchings. The data are discussed in terms of the collective model taking into account band mixing.     

Richtungskorrelationsuntersuchungen an den γ-Kaskaden des131Cs

The¹³¹Cs decay has been studied by angular correlation measurements of the (404-216), (832-216), and (247-373) keV γ-cascades using NaJ(Tl)-detectors in connection with a two parameter multi-channel analyzer. Unique spin assignments are obtained for the 216 (Jπ=3/2⁺) and 373 (Jπ=3/2⁺) keV levels. A previously discussed value ofJπ=5/2⁺ for the 1048 keV level is ruled out. The multipole character of the 404 keV transition is found to beE2 with less than 2%M1.     

Level structure of 184W from the 183W(n, γ) reaction

The level structure of ¹⁸⁴W has been studied from the prompt γ-rays emitted following the capture of both thermal and 2 keV neutrons by ¹⁸³W. Energies and intensities were measured for both the primary and the secondary (low-energy) prompt γ-rays. From these data, a level scheme is proposed for ¹⁸⁴W in which all the I^π = 0⁺, 1⁺ and 2⁺ states below ≈ 2.0 MeV are observed. Where possible, rotational-band assignments have been made to these and other levels. Additional evidence is presented which confirms the 1130 keV state as being the band head of a K^π = 2^? octupole vibrational band. Admixed K^π = 0⁺ and 2⁺ bands are established at 1322 and 1386 keV, respectively, with the I^π = 2⁺ states (at 1431 and 1386 keV) having a mutual admixture of ≈ 12%. In the energy region above 1.5 MeV, the following bands and band-head energies are identified: K^π = 1⁺, 1613 keV; K^π = 0⁺, 1614 keV; K^π = 1⁺, 1713 keV; K^π = 2⁺, 1877 keV. The neutron binding energy in ¹⁸⁴W has been determined to be 7411.1±0.6 keV. The band structure of the 1613 keV (1⁺) and 1614 keV (0⁺) bands is observed to be strongly distorted, the observed $\text{A (}\text{h}\text{?}{}^2\text{/2}\text{I}\text{)}$ values being ≈ 3.6 keV and ≈ 32 keV, respectively. This strong distortion is shown to be explainable in terms of Coriolis coupling of reasonable strength between the two bands. A similar explanation is shown to account for the somewhat less anomalous A-values (22.8 keV and 14.0 keV, respectively) of the 2⁺ band at 1386 keV and the 3⁺ band at 1425 keV. The results of a phenomenological fiveband-mixing analysis involving the K^π = 0⁺ and 2⁺ bands below ≈ 1.5 MeV are presented and discussed. These calculations indicate, among other things, that the direct E2 matrix element connecting the 1322 keV, K^π = 0⁺ band and the ground-state band is quite small, possibly zero. They also indicate that a nonzero E2 matrix element exists between this excited K^π = 0⁺ band and the γ-vibrational band and that the magnitude of this element is comparable with that between the γ-vibrational and ground-state bands. Arguments favoring and apparently refuting the interpretation of the 1322 keV, 0⁺ band as a “two-phonon γ-vibration” are presented.     

Isomeric state at 945.3 keV in237Np

A new isomeric state in²³⁷Np has been discovered at an excitation energy of (945.3±0.2) keV withT _1/2 =(711±40) ns. Six gamma-ray lines have been measured depopulating the isomeric state to known levels in²³⁷Np. The comparison of the measured absolute gamma-ray transition probabilities with empirical values yields as most probable spinI andK assignmentI=K=13/2 for this isomeric state. It is most likely a 1-proton-2-neutron quasiparticle state 13/2^?π5/2⁺ [642] +ν7/2^? [743] + ν 1/2⁺ [631]. However, a one-quasiparticle 11/2^? [505] configuration for the isomer cannot be excluded.     

Analysis of the properties of excited states in 165Er

An analysis of ¹⁶⁵Tm decay has shown that precision measurements of the γ-ray and internal conversion electron intensities, in combination with angular correlation measurements, provide additional data useful for the determination of the mixing and intensity ratios of γ-transitions with a small energy difference. The spin of the 589.868 keV state is found to be $\text{I}{}^{\mathrm{\pi }}\text{=}\text{1}\text{2}{}^{\mathrm{?}}$. Doublet opposite-parity transitions from states at 1103.495, 920.632 and 745.968 keV to levels near 590 keV with an energy difference ΔE = 108 ± 22 eV are identified.     

Possible evidence for shape isomeric γ-decay in μ− atoms of 238U

A search for the γ-decay of the shape isomer in muonic ²³⁸U excited by radiationless transitions has been performed. Seven delayed transitions in the energy region of 700 to 3200 keV have been observed with a large Ge(Li) detector. Two transitions with E_γ = 2215 and 3131 keV have been attributed to the decay of the shape isomeric state into levels in the first well. The isomeric shift of the second minimum E₁₁ ≈ 600 keV in the presence of the muon and the decrease of the lifetime of the shape isomer to τ = 12 ± 2 ns give arguments in favour of the connection of shape isomerism with large quadrupole deformations.     

Microscopic calculation of cross sections for the reactions 12C(p, 2p) and 16O(p, 2p): U. Wille and R. Lipperheide Nucl. Phys. A189 (1972) 113

Isomeric states of ⁶⁹Se were produced via the heavy-ion compound-nucleus process and from the positon decay of ⁶⁹Br, the former producing a high-spin state and the latter preferentially populating a low-spin state. Ge(Li) detectors as well as scintillation counting showed that the high-spin state decays with T_{$\text{1}\text{2}$} = 14.0 ± 1.0 min to levels in ⁶⁹As, giving rise to γ-rays of 116, 166 and 282 keV. The low-spin state decays predominantly to a state giving rise to a 145 keV γ-ray with T_{$\text{1}\text{2}$} = 1.8 ± 0.2 min. The beta end-point energy of both isomers is 5.2 ± 0.2 MeV. It is deduced that the high-spin state, assigned J^π = $\text{9}\text{2}$⁺, is the ground state; the isomeric state had J^π = $\text{3}\text{2}$^?. The angular momentum inversion as compared to neutron-excess Se isotopes is attributed to Coriolis coupling to a deformed core as described by Malik and Scholz.     

Evidence for band termination in118Xe

The yrast band of¹¹⁸Xe has been extended up to tentativelyI ^π = 34⁺ in a heavy-ion in-beam γ-spectroscopic study using the NORDBALL detector array. A band crossing is observed at the highest spins and interpreted within the Modified Oscillator Model as a transition to a terminating band.