In-beam spectroscopy of thek π=0− bands in230–236U

TheK ^π=0^? bands in even uranium nuclei were studied in the compound reactions²³¹Pa(p, 2n)²³⁰U,^{230, 232}Th(α,2n)^{232, 234}U and²³⁶U(d, pn)²³⁶U. In-beamγ-rays were measured in coincidence with conversion-electrons, which were detected with an iron-free orange spectrometer. The negative-parity levels are observed up to intermediate spins (I<13^?). In addition, the 1^? and 3^? levels in²³⁰U were confirmed by a decay study with an isotope separated²³⁰Pa source. For the heavier isotopes (A≥232) the properties of theK ^π=0^? bands (energies andγ-branchings) are consistent with a vibrational character of these bands. For²³⁰U theK ^π=0^? band lies at rather low energy (E(1^?)=367 keV), and the level spacings within this band are very similar to those of the isotones²²⁸Th and²²⁶Ra, which might indicate the onset of a stable octupole deformation.     

States in95Mo and99Mo populated in the (3He, α) reaction

The (³He,α) reaction on⁹⁶Mo and¹⁰⁰Mo targets has been studied at a bombarding energy of 18 MeV. Thel _n transfer assignments have been made on the basis of angular distribution patterns and on an analysis of the ratios of the experimental and theoretical cross-sections of (³He,α) and (d, t) reactions data leading to the same final states. New states are observed in⁹⁵Mo at 3373 keV (9/2⁺); spin and parity assignments are made to levels in⁹⁹Mo at 1621 keV 7/2⁺ (9/2⁺), 1778 keV (5/2^?) and 2078 keV (11/2^?).     

Influence of collective rotational degrees of freedom in medium energy fission

In the present work rms angular momenta have been deduced for the fission fragments corresponding to¹³¹Te^m,g and¹³³Te^m,g in²³²Th(α _{40 MeV},f) and²³⁸U(α _{40 MeV},f) systems from the radiochemically determined independent isomeric yield ratios and statistical model based analysis. For¹³¹Te and¹³³Te the rms angular momenta deduced are 5.9±1.0 and 7.9±1.2 ? respectively in²³²Th(α _{40 MeV},f) and 7.2±0.6 and 8.0±0.8 ? respectively in²³⁸U(α ₄₀ MeV,f). Comparison of the present data with the literature data for these fragments in the same compound nuclei²³⁶U^* and²⁴²Pu^* at lower excitation energies shows increase in the fragment angular momentum with increasing excitation energy and angular momenta of the fissioning nuclei. Fragment angular momentum deduced theoretically for asymmetric and deformed fragments on the basis of thermal equilibration of the collective rotational degrees e.g., rigid rotation, wriggling, tilting, bending and twisting modes considering the effect of multichance fission, are in good agreement with the experimental observations.     

Structure of high-spin states in 232Th, 234U and 236U

Multiple Coulomb excitation of ²³²Th, ²³⁴U and ²³⁶U by 5.3 $\text{MeV}\text{u}$²⁰⁸Pb ions has been studied using γ-ray spectroscopy. Excitation of ground-band levels is observed up to spin I^π = 26⁺ (tentatively 28 ⁺) in ²³²Th and ²³⁴U and I^π = 28⁺ (tentatively 30⁺)in ²³⁶U. High-spin levels of the K^π = 0^? octupole-vibrational bands are also observed in these nuclei. The measured transition energies between ground-band levels suggest that at $\text{I≈ 28}\text{h}\text{?}$ several units of angular momentum are carried by single particles aligned with the rotation axis.This result can be understood in terms of a super band built on aligned two-quasiparticle configurations which crosses the ground-state rotational band at a rotational frequency of $\text{h}\text{?}\text{ω ? 0.25}\text{MeV}\text{(I ? 28}\text{h}\text{?}\text{)}$. The E2 transition-matrix elements deduced from the experimental γ-yields agree within their errors with the rigid-rotor predictions up to the highest spins observed.The experimental results are discussed using the concept of rotation alignment and are compared with predictions of the rotation-vibration model and the interacting-boson model.     

Proton states in193Ir and195Ir populated via the (α) reaction

The nuclear structures of¹⁹³Ir and¹⁹⁵Ir have been studied using the¹⁹⁴Pt(t, α) and¹⁹⁶Pt(t, α) reactions. Levels up to ～2MeV in excitation energy were studied with a resolution of ～13keV (FWHM) and spectroscopic strengths were extracted. The low-lying states in¹⁹³Ir have been interpreted in terms of both the Nilsson and the rotation-vibration models including Coriolis, particle-vibration and rotation-vibrational couplings. The previously assigned 3/2⁺ [402], 11/2^? [505], 1/2⁺[400] and 1/2⁺[411] bands were populated and candidates for the 5/2⁺ [402] and 7/2⁺ [404] bandheads were observed. TheK ₀ +2 gamma vibration based on the ground state was populated because it is mixed with the 7/2⁺ [404] state. Surprisingly, there was no significant difference between the stripping and pick-up strengths for the low-lying states in¹⁹³Ir, suggesting that the equilibrium nuclear shape of¹⁹³Ir has large overlaps with the shapes of both¹⁹²Os and¹⁹⁴Pt. The nuclear level structure of¹⁹⁵Ir appears to be similar to those of the lighter iridium isotopes.     

Spectroscopy of excited states in212Po,210Pb,and213At employing18O induced few-nucleon transfer reactions

γ-ray spectroscopic techniques have been applied to measure properties of excited states (E ^*<2MeV) in²¹⁰Pb,²¹²Po, and²¹³At after populating these neutron rich (N=128) nuclei via¹⁸O induced few-nucleon transfer reactions on²⁰⁸Pb and²⁰⁹Bi targets. In²¹²Po an isomeric state is located atE ^*=1,477 keV with a halflife oft _1/2=14.7±0.3 ns. This state is interpreted to be the 8⁺ yrast level which decays to the ground state via the measured $$8^ + \left( {121.1 keV} \right)6^ + \left( {223.3 keV} \right)4^ + \left( {405.1 keV} \right)2^ + \left( {727.8 keV} \right)0^ + $$ γ-cascade.α-decay (E _α≈10.2 MeV) of the isomer is also observed. In²¹⁰Pb the time spectra for members of the 4⁺ (297.8keV) 2⁺ (799.4keV)0⁺ γ-cascade show a delayed component with a halflife oft _1/2=152±13 ns which is attributed to the known 8⁺ yrast state atE ^*=1.27 MeV. For²¹³At first results on theγ-decay of excited states are presented.     

Investigation of196,198Pt and202,204Hg with the (d,pnγ) reaction

Gamma-ray and conversion electron spectra were measured in the reaction of 25 MeV deuterons on¹⁷⁴Yb,^196,198Pt,^202,204Hg, and²³²Th. The (d,pxn) deuteron break-up reactions were studied by the measurement ofpγ- andγγ-coincidences. Levels with spins up to approximately 10? are observed in the (d,pn) reactions, with a strong preference of the population of yrast states. In the Pt and Hg nuclei the ground bands are seen up to the 6⁺ states and in^196,198pt the semi-decoupled 5^?, 7^?, and 9^? yrast levels are populated most strongly. In^202,204Hg we observe fairly strongly new levels with tentative assignments of 5^? and 7^?. In addition a number of previously unknown levels are identified in the Pt and Hg nuclei, for which no spin-parity assignments could be obtained. A discussion of the level structure in terms of the interacting boson model (^196,198Pt) and the shell model (^202,204Hg) is given.     

A note on the structure of the 1518 keV isomeric state in174Yb

Studies of the decay of¹⁷⁴Tm and^174m Lu show that, in both cases, a level at 1517.6±0.8 keV in¹⁷⁴Yb is populated, which decays to the 4⁺, 6⁺ and 8⁺ members of the g.s. rotational band.     

Low- and medium-spin states in theN=124 isotones208Po and209At

Excited levels in²⁰⁸Po and²⁰⁹At were populated in the reactions (p, 2n) and (³He, 3n), and²⁰⁸Po also in the EC-decay of the²⁰⁸At ground state, which was produced in the²⁰⁹Bi(³He, 4n) reaction. The experimental procedures comprised gamma-ray excitation functions,γ- γ coincidences, as well as gamma-ray angular distribution and conversionelectron measurements, both on- and off-line. The properties of about twenty-five levels in each nuclide were studied. In²⁰⁸Po the energy of the 8⁺→6⁺ transition could be determined to be 4.0±0.2keV and the 8⁺ state thus has an excitation energy of 1,528 keV. The effective quadrupole charge of theh _9/2 proton can be derived ase _eff(τ h _9/2)=1.69(15)e. For the previously reported isomeric half-lives more accurate values are obtained, the systematics of electricL=1, 2 and 3 transition rates are discussed, and their microscopic structure is elucidated. Shell model considerations are used to support the configuration assignments, as are the logft values for the strongest²⁰⁸At(EC)²⁰⁸Po decay branches. The effect of core polarixation on the excitation energies of yrast states is studied in both²⁰⁸Po and²⁰⁹At. Many of the states populated in the²⁰⁸At EC decay are shown to be of core-excited character.     

Nuclear levels in228Th populated in the decay of228Pa

The electron-capture decay of²²⁸Pa to levels in²²⁸Th has been studied using mass-separated sources and high-resolutionγ-ray and conversion-electron spectroscopy. A level at 979.5 keV is assigned as 2⁺ member of a second excited K_π=0⁺ band, with the 0⁺ band head at 938.6 keV. The 2⁺ and 3⁺ members of a second excited K_π=2⁺ band at 1153.5 and 1200.5 keV, which decay by strongE0 transitions to the 969 keVγ-vibrational band, are confirmed. In addition we tentatively propose a K_π=1⁺ band at 944 keV. The K_π=0^?, 1^? and 2^? members of the octupole quadruplet are confirmed, and theγ decay of these levels is analysed in an approach, in which the mixing of the octupole bands by the Coriolis interaction is taken into account. It is suggested that octupole correlations might be important for theE1 transition moments. A total of 29 levels is observed between ～1.4 and ～2.0 MeV, for which the nuclear structure, and the possible assignment to rotational bands, is unclear.     

High-spin states in 232U investigated with the 232Th(α, 4nγ) reaction

The ground-state rotational band of ²³²U is established up to I^π = 16⁺ (tentatively 18⁺) through a study of the ²³²Th(α, 4nγ) reaction. Conversion electron spectroscopy is found to be especially useful in circumventing the difficulties caused by strong fission competition.     

Investigation of vibrational levels in226Ra by coulomb excitation and by the226Ra(d,pnγ) reaction

Vibrational bands in²²⁶Ra were studied by Coulomb excitation and by the²²⁶Ra(d,pnγ) reaction. The first-excitedK ^π = 0⁺ and 1^? bands with known band heads at 825 and 1049 keV, respectively, were extended up to the 8⁺ and 7^? levels. A new 2⁺ level at 1110 keV and the known 2⁺ level at 1156 keV were observed following Coulomb excitation and interpreted asγ vibration and possible member of a second-exitedK ^π = 0⁺ band, respectively. TheE1 andE2 branching ratios from these vibrational bands to the ground and first-excited 0^? band are explained within the rotational model including band mixings. No evidence was found for a 0⁺ level at 650 keV proposed earlier.     

High-spin levels in138Ce and140Nd Observed in the (α, 4n γ) reactions

Levels of¹³⁸Ce and¹⁴⁰Nd have been studied using the¹³⁸Ba(α,4nγ)¹³⁸Ce and¹⁴⁰Ce(α, 4nγ)¹⁴⁰Nd reactions. Singleγ-ray spectra,γ-γ coincidence spectra, angular and time distributions with respect to the beam bursts have been measured. A number of higher excited states with excitation energies up to about 5 MeV and with spin value up to 12 are populated in both nuclei. The lower states with spins and parities 7^?, 5^?, 6^? and 10⁺ can be explained by two-quasiparticle neutron configurations of the types (h _11/2 ^?1 ,d _3/2 ^?1 ) 7^? , (h _11/2 ^?1 ,S _1/2 ^?1 ) 5^?, 6^? and (h _11/2 ^?2 ) 10⁺. Several high-spin states observed in¹³⁸Ce and¹⁴⁰Nd can be explained qualitatively as four-quasiparticle states with two-proton-two-neutron configurations. The 3^? state at an energy of 2,137.4 keV is observed in¹³⁸Ce. The evidence for the existence of the low-lying 3^? states in¹⁴⁰Nd at 2,124.0 keV is discussed. Beside the known 9.6 ms (7^?) isomeric state in¹³⁸Ce another state at 3,538.5keV (10⁺) with a half life of about 200 ns has been observed. The observed levels in the¹³⁸Ce and¹⁴⁰Nd nuclei are compared with theoretical predictions using delta force interaction.     

Investigation of the ground state rotational bands in233U and239Pu in the (α, 3n) reactions

The ground state rotational bands in²³³U and²³⁹Pu were investigated in (α, 3n) reactions. Conversion electrons were measured with an iron free orange spectrometer in order to suppress the background from fission. Levels up toI ^π=33/2⁺ of theK=5/2 band in²³³U andI ^π=31/2⁺ of theK=1/2 band in²³⁹Pu were identified ine ^?γ coincidence measurements. The level energies of both rotational bands can be well described up to the highest observed spins by a two-parameter angular velocity expansion. The electromagnetic properties of theK=1/2 band in²³⁹Pu are discussed.     

Perturbed αγ-angular correlations in the decay of228Th

At external magnetic fields between 1.3 and 22.5 kG the integral αγ-angular correlations of theO ⁺(α)2⁺(γ)O ⁺ cascades from the ground states of²²⁸Th and²²⁴Ra respectively implanted into iron and aluminum lattices have been studied. The data were analyzed assuming different additional time dependent and static perturbations. The rotation of the angular correlation for Ra in Al proved independent of these assumptions. Therefore ag-factor of the 84.4 keV 2⁺ state in²²⁴Rag=0.46 (11) could be derived. Although static electric interactions seem the most probable cause for the attenuations observed for Ra and Rn implanted into Fe it was found that the two parameter Abragam and Pound theory better reproduces the data than the one parameter static perturbations. Therefore the hyperfine fields experienced by Ra and Rn in Fe were derived using Abragam and Pound theory to beH _HF(RaFe)=?127(31) kG andH _HF(RnFe)=1095 kG.     

On the yrast two proton-two neutron hole states in208Po

High-spin levels in²⁰⁸Po, populated in the²⁰⁸Pb(α,4n)-reaction, were studied usingα-particles in the energy region 41–51 MeV. The energies of levels above the 6⁺ level have an uncertainty of about 10 keV due to the fact that the 8⁺→6⁺ transition has not been observed so far, but this transition has previously been established to be converted neither in theK-shell nor in theL-shells. It was found that the yrast cascade ofγ-rays from a 19⁺ level at 5896+ε keV feeds levels of lower spin which all can be explained as originating from two proton-two neutron hole configurations. In the higher part of the cascade it is mainly the neutron holes which change their configuration, while the lower part of the cascade is dominated by changes in the proton configuration. The yrast levels in the angular momentum regionJ=8–19 vary practically linearly with energy in the region 1.5–6 MeV. No isomeric traps were found above the 11^? level at 2699+ε keV.     

Experimental consequences ofSU(3) symmetry in ans d g Boson Model

Energies of collective levels in¹⁷⁸Hf and²³⁴U are compared with predictions of theSU (3) limit of thesdg Interacting Boson Model. All known positive parity states of¹⁷⁸Hf below 1.8 MeV (with the exception of a 0⁺ band) have been satisfactorily reproduced. Most of the bands in²³⁴U are also described by the model. However, a few predicted states have no experimental counterpart. The introduction of theg-bosons strongly reduces the previously observed discrepancies between experimentalB(E2)'s in²³⁸U and thesd-IBM calculation.     

Lifetimes and magnetic moments of two states in62Cu

The lifetimes and the magnetic moments of the two excited⁶²Cu states at 40.84 and 390.19 keV are investigated by time differential perturbed angular distribution measurements of the delayedγ-rays following the nuclear reactions⁶¹Ni(d, n) and⁶²Ni(p, n). The results are:τ=6.7±0.5 nsec,μ=1.32±0.03 n. m. for the 2⁺ state at 40.84 keV, andτ=16.0±0.3 nsec,μ=2.00±0.01 n. m. for the 3⁺ state at 390.19 keV.     

Gamma-gamma angular correlation measurements of transitions in98Sr and98Zr and confirmation of shape coexistence in98Sr

γγ-coincidence and angular correlation measurements were performed for transitions in⁹⁸Sr and⁹⁸Zr at the on-line-separator facility OSTIS at the Institute Laue-Langevin (ILL) in Grenoble, France. These nuclei were populated byβ-decay from isotopically pure⁹⁸Rb produced by thermal fission of²³⁵U.γγ-angular correlations were measured allowing spin assignments for most levels up to 2 MeV in both nuclei. Listings of angular correlation coefficients and multipole mixing ratios are given. The appearance of shape coexistence is discussed for⁹⁸Sr and⁹⁸Zr in the framework of level systematics as well as neutron-separation energies.