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.     

Exotic clustering in 222,224,226Ra

We describe the J^π = 0⁺, 2⁺, 4⁺, … ground state bands of ^222,224,226Ra, using a model in which these bands are treated as a ¹⁴C cluster orbiting the appropriate Pb core. For each isotope, we obtain good agreement with the measured half-life for the ¹⁴C decay of the 0⁺ ground state, as well as with the excitation energies and in-band E2 decays of other states of the ground state band. We also propose extensions of the model to deal with the observed low-lying J^π = 1⁻, 3⁻, 5⁻, … bands, their in-band E2 decays, and the E1 and E3 transitions connecting to the ground state bands.     

Collective structures and smooth band termination in 109Sn

Six rotational bands up to energies E _x = 24.7 MeV and spins J^π=(79/2^?) have been identified in ¹⁰⁹Sn using the GAMMASPHERE γ-detector array. Four of the bands show smoothly decreasing dynamic moments of inertia at rotational frequencies ?ω > 0.6 MeV. The bands arise at medium spins from a coupling of a valence d_5/2, g_7/2 or h_11/2 neutron to the deformed 2p2h proton excitation of the Z=50 core ¹⁰⁸Sn. At very high ?ω these bands show the typical behaviour of smoothly terminating bands, i.e. a gradual alignment of the angular momenta of the valence particles and holes corresponding to a transition from high collectivity to noncollective states.     

Low-lying states of 109Sn from the 106Cd(α, nγ) reaction

Measurements of γ-ray, γγ-coincidence and internal conversion electron spectra from the ¹⁰⁶Cd(α, nγ)¹⁰⁹Sn reaction were carried out at 15–20 MeV α-particle bombarding energies with Ge(HP) γ-ray and superconducting magnetic lens plus Si(Li) electron spectrometers. The energies, relative intensities, internal conversion coefficients and coincidence relations of the ¹⁰⁹Sn transitions were determined, and a more complete, consistent level scheme has been deduced. Spin and parity values have been determined from the internal conversion coefficients, the bombarding-energy dependence of the side-feeding intensities of the states and the available γ-ray angular distribution data. The level scheme was interpreted in the framework of the quasi-particle shell model.     

Low-lying dipole excitations in the stable Cd isotopes: A systematics

Low-lying dipole excitations in the medium-weight vibrational nuclei of the Cd isotopic chain were investigated by means of nuclear resonance fluorescence experiments performed at the bremsstrahlung beam of the Stuttgart Dynamitron accelerator (endpoint energy 4.1 MeV). Detailed information has been obtained on excitation energies, spins, decay widths, and transition probabilities of numerous excited states in ^{110–114,116}Cd. Additionally, the use of two Compton polarimeters enabled model-independent parity assignments for excitations in the even-even isotopes. Strongly excited J ^π = 1^? states are found in all even-even Cd nuclei at excitation energies near the sumof the energies of the first 2⁺ and 3^? states. These excitations are interpreted as the 1^? member of the quadrupole-octupole coupled quintuplet (2+?3^?). The fragmented strength observed in the odd isotopes ^111,113Cd is compared with the strength distributions in the neighboring even-even Cd isotopes.     

Exotic clustering in 232,234,236,238U

We account for the energies and electromagnetic E2 transitions of the J^π = 0⁺, 2⁺, 4⁺ … ground state bands of ^{232,234,236,238}U, using a model in which these bands are treated in terms of a ²⁴Ne cluster orbiting the appropriate Pb core. The cluster-core potential parameters used are identical to those employed in a previous study of ^222,224,226Ra. In addition to the simultaneous fitting of energies and B(E2) strengths, we also obtain good agreement with the measured half-life for the ²⁴Ne decay of the 0⁺ ground state of ²³²U and ²³⁴U as well as with the measured B(E4 ↑; 0⁺ → 4⁺) transition strengths in ^234,236U. We find that a small parity dependence of the cluster-core potential is required in order to give a good account of the energies of the J^π = 1⁻, 3⁻, 5⁻,… states of the lowest negative parity band of ^{232,234,236,238}U, and of the B(E3 ↑; 0⁺ → 3⁻) strengths in ^234,236,238U.     

The brute-force polarization of 23Na and the 23Na(n, γ)24Na reaction

A Na target has been polarized by brute force to 22% and the γ-radiation produced by polarized thermal neutron capture has been investigated. The J^π = 2⁺ channel spin contribution has been determined model-independently and unambiguously for 22 primary transitions. The average J^π = 2⁺ channel contribution is 5.8(5)%. The data resulted in one spin assignment and two spin restrictions. The energies and lifetimes of positive-parity levels as well as branching ratios and the magnetic moment of the ground state are in agreement with a shell-model calculation in the complete sd shell.     

Scattering and resonance properties of the interaction of slow neutrons with the isotopes of silicon and sulfur

In¹¹⁷Te,¹¹⁹Te and¹²¹Te isomeric states withJ ^π=5/2⁺, 5/2⁺ and 7/2⁺ and half-lives of 19.1(9)ns, 2.2(2) ns and 86(6) ns, respectively, have been identified at low excitation energies using (α, 2n) reactions on enriched^115–119Sn targets. Positive parityΔJ=1 bands built on these isomeric states have been observed up to 17/2⁺. The states are interpreted as members of rotational bands built on deformation driving 5/2⁺ [402] and 7/2⁺ [404] Nilsson orbitals which overcome theN=64 subshell gap. The irregular level spacings and electromagnetic properties of the bands are well explained in Coriolis calculations. The moment of inertia parameter as function of collective angular momentum has been derived from the doubly even Te cores. The hindrance of the band head deexcitation may be caused by shape fluctuations of these transitional nuclei.     

A neutron decoupled from a rotating odd core in 114Sb and 116Sb

The ^{113, 115}In(α, 3nγ)^{114, 116}Sb reactions have been studied using γ-ray spectroscopic techniques. The experiments included γ-ray excitation function, γ-γ coincidence, lifetime, γ-ray angular distribution and conversion electron measurements. A ΔJ = 1 rotational band has been observed in either of the ^{114, 116}Sb final nuclei. Energy spacings and electromagnetic properties of the band show strong resemblances with those of rotational bands in the adjacent odd-mass Sb nuclei. In addition two-quasiparticle and two-quasiparticle core coupled states have been observed in these nuclei. One isomer was identified in ¹¹⁶Sb, i.e. a J^π = 11⁺ state at 1889 keV ($\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext><mtext>\; =\; 4.0\pm 0.1\; </mtext><mtext>ns</mtext>}}$). A simple model is proposed which explains the ΔJ = 1 band in terms of rotational alignment of the $\text{h}{}_{\mathrm{<mtext>11</mtext><mtext>2</mtext>}}$ neutron with the deformed rotating odd-A core.     

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.     

The level structure of 156Gd studied by means of the (α, 2nγ) reaction

A complete set of conventional γ-ray spectroscopic techniques has been applied to investigate the level structure of ¹⁵⁶Gd. A total of twenty-five new levels has been established; unambiguous spin assignments could be given for twelve of them on the basis of angular distributions and conversion electron measurements. The proposed level scheme contains 49 levels, which can be ordered in seven rotational bands. The ground-state band was excited up to J^π = 14⁺, the β-band up to 10⁺, the γ-band up to (11⁺), the second K^π = 0⁺ band tentatively up to (10⁺), the K^π = 4⁺ band up to (8⁺). Two negative-parity bands, one with even spins and one with odd spins, were excited to J^π = (12^?) and (13^?). An isomeric state was established with $\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 1.3 μs, J}{}^{\mathrm{\pi }}\text{= 7}{}^{\mathrm{?}}\text{, E}{}_{\mathrm{<mtext>x</mtext>}}\text{= 2137.7}\text{keV}$. The properties of the K^π = 4⁺ band and the isomeric state can be well explained by two-quasiparticle configurations. The negative-parity bands are interpreted as aligned octupole bands. Positive and negative-parity bands have been calculated in terms of the IBA model. Good agreement with the experimental results is obtained.     

Collective excitations in the 123, 125I nuclei

The odd-proton nuclei ¹²³I and ¹²⁵I have been studied in the reactions ¹²¹Sb(α, 2n)¹²³I and ¹²³Sb(α, 2n)¹²⁵I, respectively. The level schemes, spin and parity assignments are based on results obtained from singles γ-ray spectra (E_α = 27 MeV) and excitation functions, from measurements of delayed γ-rays, γ-γ coincidences, internal conversion electrons and γ-ray angular distributions. High-spin positive- and negative-parity bands with energies up to 2948 keV and spins up to $\text{23}\text{2}$ in ¹²³I and with energies up to 3775 keV and spins up to $\text{27}\text{2}\text{in}{}^{125}\text{I}$ have been established. In the decay schemes of both nuclei two separated structures of levels have been observed. One group of levels shows a strong ΔJ = 2 quasirotational pattern predicted by the particle-vibration coupling model. The ΔJ = 1 sequence on a $\text{9}\text{2}$⁺ state is assigned as a rotational structure built on the axially symmetric deformed state $\text{9}\text{2}{}^{\mathrm{+}}\text{[404]}$. In ¹²³I a 28 ns isomer at 2660.0 keV has been found.     

Spin assignment of the 8.555 MeV state in28Si

Angular correlations between the gamma rays following the decay of the 8.555 MeV state in²⁸Si, known to have natural parity, were measured. The spin assignment of this state, found in this experiment, isJ ^π=6⁺. Energy sequence of the lowestJ ^π=2⁺, 4⁺ and 6⁺ states in²⁸Si compares well with the energy sequence of the ground state rotational bands in²⁰Ne and²⁴Mg.     

The decay scheme of 228Fr and K = 0 BANDS in 228Ra

The γ-rays following the β^?-decay of ²²⁸Fr have been studied by means of γ-ray singles including multi-spectrum analysis and γ-γ coincidence measurements using Ge(Li) spectrometers. Most of the observed γ-transitions could be placed in the level scheme of ²²⁸Ra. The accuracy the energy of the first-excited state in ²²⁸Ra has been improved and 35 new excited levels have been established, 11 of them grouped into the ground-state band, the low-lying K^π = 0^? band with the head at 474.14 keV and two excited K^π = 0⁺ bands with heads at 721.17 and 1041.9 keV. Candidates for two close-lying K^π = 2⁺ bands have also been found. It is concluded that the ground state octupole deformation, if any, is less pronounced in ²²⁸Ra than in lighter radium isotopes.     

Collective and two-quasiparticle states in 158Gd observed through study of radiative neutron capture in 157Gd

The level structure of ¹⁵⁸Gd has been studied using the prompt γ-rays and conversion electrons emitted following neutron capture in ¹⁵⁷Gd. The γ-ray energy and intensity measurements were made using both Ge(Li) detectors and a curved-crystal spectrometer. Conversion-electron energy and intensity measurements were made using two separate magnetic spectrometers: one to measure the primary electron spectrum and the other to measure the lower energy secondary electron spectrum. Some γ-γ coincidence measurements were also made among the secondary γ-rays. From these data, a neutron separation energy of 7937.1 ± 0.5 keV has been determined for ¹⁵⁸Gd. A level scheme containing 59 excited states with energies < 2.25 MeV, for which de-excitation modes have been identified, is proposed for ¹⁵⁸Gd. Many of these states have been grouped into rotational bands. A total of thirteen excited rotational bands with band-head energies below 2.0 MeV are contained in the level scheme. Features of the proposed level scheme include: the K^π = 0^?, 1^? and 2^? octupole-vibrational bands with band-head energies of 1263, 977 and 1793 keV, respectively; the γ-vibrational band at 1187 keV; three excited K^π = 0⁺ bands with band-head energies of 1196, 1452 and 1743 keV; several two-quasiparticle bands with band-head energies in keV (and K^π assignments) of 1380 (4⁺), 1636 (4^?), 1847 (1⁺), 1856 (1^?), 1920 (4⁺) and 1930 (1⁺). An analysis of (d, p) reaction data is presented which permits definite two-quasiparticle configuration assignments to be made to most of these latter bands. Evidence is presented which suggests strong mixing of some two-neutron and two-proton bands. A phenomenological four-band mixing analysis is made of the energy and E2 transition-probability data for the ground-state band and the three lowest-lying excited collective positive-parity bands. Good agreement with experiment is obtained. A Coriolis-mixing analysis of the octupole bands has been carried out and good agreement with the data on level energies and E1 transition probabilities to the ground-state band has been achieved. Values of Z, the ratio of the E1 transition matrix element with ΔK = 1 to that with ΔK = 0, involving the octupole bands and the first four 0⁺ bands are derived. For three of these 0⁺ bands, absolute values of these matrix elements are deduced. An interesting alternation in the sign of Z is observed for these four 0⁺ bands.     

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.     

Electroexcitation of high spin states in 208Pb

The formfactors for the electroexcitation of the collective states of ²⁰⁸Pb have been calculated within the Model of Separable Residual Interaction (MSI); the results for the even parity states up to J^π = 10⁺ agree nicely with recent measurements. It turns out, that the energies of the lowest collective states of positive parity follow rather closely a J(J + 1) law.     

Study of5Li around 20 MeV excitation energy by a radiative capture reaction

The nucleus⁵Li has been investigated within an excitation range of 17.8 – 21.1 MeV by measuring γ-rays from the reaction D(τ, γ _o)⁵Li. In this range, excitation functions and angular distributions have been taken. The excitation functions exhibit broad maxima at 20 MeV which indicate, in agreement with other results, a level or a group of levels. Assuming that there is only one resonance, its properties are discussed. On the basis of the existing and the new experimental results, aJ ^π=1/2⁺ assignment is proposed besides the former resultJ ^π=(3/2, 5/2)⁺.     

Search for the parity-forbidden heavy-particle width Γ′dα of the Ex = 3.562 MeV state in 6Li

Alpha-particle capture by deuterium has been investigated at excitation energies in ⁶Li close to the J^π = 0⁺, T = 1, E_x, = 3.562 MeV state. A search for resonance γ-rays from this state has yielded a negative result and an upper limit for the heavy-particle width Γ′_dα ≦ 0.017 eV.