Search for collectiveM3 excitations to low lying states in164Dy

We report on the search for mixed symmetricJ ^π=3⁺ states in the strongly deformed nucleus¹⁶⁴Dy and present an upper limit for theM3 transition strength. From the excitation of the symmetric 3⁺ state at 828 keV we deduce theF-scalar magnetic octupoleg-factor.     

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.     

States in 163Dy and 169Er populated in the (t,p) reaction

Angular distributions of protons from the ¹⁶¹Dy(t, p)¹⁶³Dy and ¹⁶⁷Er(t, p)¹⁶⁹Er reactions were studied, using 15 MeV and 17 MeV tritons from the McMaster University tandem Van de Graaff accelerator. The reaction products were analyzed with a magnetic spectrograph and detected with nuclear emulsions. Since the ¹⁶¹Dy target ground state is the $\text{5}\text{2}$⁺[642] orbital, a strong L = 0 transition was observed to the $\text{5}\text{2}$⁺[642] bandhead in ¹⁶³Dy, which was previously assigned at 251 keV. Also transitions to the $\text{7}\text{2}$, $\text{9}\text{2}$ and $\text{11}\text{2}$ band members were observed. Similarly, a strong L = 0 transition was observed to the $\text{7}\text{2}$⁺[633] bandhead at 244 keV in ¹⁶⁹Er, with the other band members only weakly populated. The angular distributions to the various members of these two bands can be described when higher-order reaction processes are taken into account. In ¹⁶³Dy, surprisingly strong L = 0 transitions were observed to levels at 1831 keV, 1937 keV and 2053 keV, with strengths of 23%, 30% and 37% of that for the $\text{5}\text{2}$⁺[642] bandhead. In ¹⁶⁹Er, the 905 keV level was populated with an L = 0 transition that had 31% of the strength observed for the strong L = 0 transition to the 244 keV level. The nature of these states is at present not understood.     

The (p,t) reactions on nuclei in the rare earth region

The (p, t) reactions on isotopic targets of ^{178, 180}Hf and all the stable isotopes of Yb and on natural targets of Gd, Dy, Er, Hf, Ta, W, Os and Au were studied at a beam energy of 19 MeV with an average resolution of 12 keV. A split-pole magnetic spectrometer was used to measure (p, t) Q-values and absolute differential cross sections. On the basis of angular distribution shapes definite 0⁺ and tentative 2⁺ assignments were made. Rotational bands were identified assuming an I(I+1) spacing. The (p, t) reaction populates excited 0⁺ states strongly in ¹⁷⁴Yb, ¹⁷⁶Hf, ¹⁶⁶Yb and several Gd, Dy and Er isotopes. The ¹⁷⁴Yb and ¹⁷⁶Hf 0⁺ states are discussed in terms of the pairing phase transition and in terms of Nilsson orbitals with unequal (p, t) reaction amplitudes. Members of gamma and octupole vibrational bands were observed in the even-N nuclei. The lowest L = 0 transfers to states in ^{169, 171}Yb were found to have less than 55% of the strength to ground states in adjacent even-N nuclei. A strong L = 0 transfer to a state at 1513 keV in ¹⁷¹Yb indicates the presence of a possible K = 0 core vibration coupled to the unpaired $\text{5}\text{2}$[512] neutron. The natural targets have furnished information on trends in cross sections for members of ground bands, gamma bandheads, 3^? octupole states, and strongly excited 0⁺ states.     

Fragmentation of the two-phonon γ-vibration in166Er

Two-phonon γ-vibrational states in¹⁶⁶Er have been populated using Coulomb excitation. The K^π=4⁺ component of the vibration appears to be fragmented over several states, whereas only one K^π=0⁺ state is observed.     

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.     

Magnetic moments in the backbending region of 158Dy

Yrast levels in the backbending region of ¹⁵⁸Dy were Coulomb excited with a 4.7 MeV/u ²⁰⁸Pb beam. Employing the transient field technique with a thin magnetized iron foil, the precessions of the angular correlations of decay γ-rays from levels up to spin I^π = 16⁺ were measured. The results show a clear reduction of the g-factor for states in the backbending region relative to that of the low-spin levels, thus demonstrating that the backbending in ¹⁵⁸Dy is mainly caused by the alignment of $\text{i}{}_{\mathrm{<mtext>13</mtext><mtext>2</mtext>}}$ neutrons. In a similar experiment, precession measurements on Coulomb excited low-spin levels of ¹⁶⁴Dy served to determine the static hyperfine field of Dy in Fe and the g-factor of the 6⁺ state in ¹⁶⁴Dy.     

Study of low-lying 0+ excitations in228Th and232,234,236U in the (p,t) reaction

The (p, t) reaction was studied at 22 MeV proton energy using targets of^230,232Th and^234,236,238U. In²²⁸Th we observed in addition to the 832 keV first-excited 0⁺ level a second-excited 0⁺ level at 939 keV populated with 5% of the ground-state strength. A 0⁺ level proposed recently in²³²U at 927 keV was not observed in the (p, t) reaction, with an upper limit for its excitation of 0.2% of the ground-state strength. In²³⁴U a level at 1045 keV, assigned in the literature as second-excited 0⁺ level, is weakly populated in the (p, t) reaction but does not show the angular distribution characteristic for single-stepL=0 transfer. In²³⁶U we observed a level at 1036 keV with an angular distribution suggesting that it might be a doublet composed of a known 3^? level and a new 0⁺ level. The properties of the first two excited 0⁺ bands in²²⁸Th,²³²U and²³⁴U are compared and discussed.     

The decay of174Ta

Levels in¹⁷⁴Hf excited in the decay of¹⁷⁴Ta have been studied. Measurements of gamma-rays, conversion electrons and gamma-gamma-time coincidences were performed. The ground state band, the beta-vibrational band, the gamma-vibrational band with its head at 1226.81 keV, aK=3⁺ band at 1303.42 keV and aK=2^? octupole band at 1308.67 keV were observed. Several 2⁺ levels withK=0 are excited. The mainβ ⁺-branches proceed through allowed or first-forbidden transitions to the 2⁺ and 4⁺ levels of the ground state band. The character of theI ^π=3⁽⁺⁾ ground state of¹⁷⁴Ta is discussed.     

Level structure of <Superscript>164</Superscript>Dy from the (<Emphasis Type="Italic">n</Emphasis>, <Emphasis Type="Italic">n</Emphasis>′γ) reaction

By employing a beam of reactor fast neutrons, gamma-ray spectra and angular distributions of gamma rays with respect to the neutron-beam axis were measured in the reaction ¹⁶⁴Dy(n, n′γ). The scheme of levels and gamma transitions in ¹⁶⁴Dy was composed on the basis of data published earlier and new levels, rotational bands, gamma transitions, and multipole-mixing parameters. It is concluded that the scheme of J = 0–4 levels is complete up to the excitation energy of 1.95 MeV. The commonly accepted rule in constructing K _n ^π = 0₂ ⁺ and 1₁ ⁺ rotational bands is found to be violated. The nature of the 2⁺ level at the energy of 1796.68 keV is discussed.     

On the 0<Superscript>+</Superscript> state with the energy of 681.3 keV in <Superscript>160</Superscript>Dy

A possibility that the 0⁺ state with the energy of 681.3 keV exists in the ¹⁶⁰Dy nucleus is discussed. Calculations based on the interacting vector boson model show that in addition to the known 0⁺ states with the number of bosons n = 2, 5, 6, and 7 there should exist other states with the number of bosons n = 1, 3, 4, and 8 in ¹⁶⁰Dy. It is shown that the peak at the energy 681.3 keV, which we experimentally observed in the ¹⁶⁰Dy internal conversion electron spectrum, can be ascribed to the 0⁺ state with the number of bosons n = 1 or n = 8.     

The decay scheme of the 3-s Isomer of100Nb and the properties of levels in100Mo

Theβ ^? decay of the longer-lived isomer in¹⁰⁰Nb has been studied at the fission-product separator JOSEF. Measurements ofγ-ray singles spectra, ofγ-γ coincidences and ofγ-γ angular correlations have been performed. A value oft _1/2=2.99(11)s has been determined for the isomer which probably hasI ^π=4⁺ or 5⁺. A scheme of the levels of¹⁰⁰Mo which are populated in the decay of this isomer has been established. Information on the spins of several states of¹⁰⁰Mo has been obtained. Thus,I=0 levels have been identified at 1,505, 2,038 and 2,087 keV. The mixing ratios have been determined for the 2₂ ⁺→2₁ ⁺ and 2₃ ⁺→2₁ ⁺ transitions. The results provide evidence for a vibrational structure of¹⁰⁰Mo with separate bands based on the ground state and on the first excited 0⁺ level.     

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.     

Quantum numbers of26Al levels above 6 MeV excitation energy

Measurements of resonance strengths and ofγ-ray angular distributions or anisotropies have been performed on selected resonances of the²⁵Mg(p, γ) reaction in the rangeE _p=2–4 MeV,E _x=8.2?10.1 MeV with an emphasis on high-spin andT=1 analog resonances. EightT=1 states are identified, among them high-spin states at 8747 keV (I=6), 9286 KeV (I=5), and 9986 keV (I ^π = 7⁺, 6⁺). Shell model calculations in thes-d basis space reproduce the branching ratios of these states and clarify the nature of final states. New high-spinT=0 states are observed at 9720 keV (I ^π = 7⁺), 8602 keV (I = 6), and 6695 keV (I ^π = 7⁺). TheI ^π assignments to severalE _x = 6–8 MeV states are revised and the role of two-particle excitations into thef-p shell is elucidated. A revised spectrum of 73 positive-parity,T = 0 states is compared to the predictions of shell-model calculations in thes-d basis space using the universals-d shell Hamiltonian.     

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.     

The structure of28Si above 10 MeV excitation energy III: Level scheme and shell model interpretation

²⁸Si level scheme up to 14.5 MeV excitation energy is reevaluated using information from two preceding papers. It consists of approximately 250 levels which are almost completely characterized according to the quantum numbersI, π, T of the levels. The properties of positive-parity states are compared to the predictions of shell model calculations within the completes-d basis space using the unifieds-d shell Hamiltonian. A spectrum of 48 experimentalT=1 states between 9.3 and 16 MeV is reproduced with a rms deviation of only 150 keV. A calculation of radiative widths and γ-decay modes which uses free-nucleong-factors yields excellent agreement with experiment and confirms that quenching of M1 transitions is only marginal in²⁸Si. The detailed shell model analysis of theT=0 spectrum is extended to the limiting energy whereT=1 wave function admixtures, not contained in the theory, become important experimentally. This happens at 6–8 MeV above the yrast state, depending on the spin value. Altogether it appears that a spectrum of 171 levels below 14.5 MeV, which have positive or unassigned parity, is almost completely accounted for by the model. Apparent intruder states from outside thes-d shell space are observed atE _x =10 945 keV (I ^π=4⁺) and 12 860 keV (I ^π=6⁺) and are interpreted as members of aK ^π=0⁺ rotational band.     

On the decay of116gIn

The decay of the 1⁺, 14 s ground state of¹¹⁶In produced by neutron capture has been reinvestigated using Ge(Li) detectors. It is found that besides the ground state of¹¹⁶Sn, excited levels at 1293.6 (2⁺), 1756.9 (0⁺), 2225.3 (2⁺), 2545.7 (0⁺, 1⁺, 2⁺), 2649.8(2⁺), 2790.7(2⁺) and 2844.6(2⁺) keV are populated, while no evidence could be obtained for the population of the 0⁺ and 2⁺ levels at 2030 and 2112.4 keV, respectively. The activation cross-section for^116m2In (2.16s,I ^π = 8^?) has been deduced as 83±8 b.     

Lifetimes and structures of some levels above 1.5 MeV in 168Yb, 164Er and 158, 160, 162Dy

The following lifetime measurements have been performed in ¹⁶⁸Yb: 62 ns (7^?, 2222.5 keV), 0.34ns (6^?, 2111.1 keV), 81.7 ns (5^?, 1998.7 keV), ≦ 0.14 ns (4⁺, 2203.8 keV); in ¹⁶⁴Er: 23.3 ns (7^?, 1985.0 keV), 0.22 ns (6^?, 1744.4 keV), ≦ 0.08 ns (5^?, 1664.2 keV); in ¹⁵⁸Dy: ≦ 0.11 ns (4⁺, 1895.3 keV); in ¹⁶⁰Dy: 0.18 ns (4⁺, 1694.0 keV); in ¹⁶²Dy: 1.93 ns (5^?, 1485.9 keV). The experimental reduced transition probabilities are discussed in the framework of current nuclear models.     

Levels of 234U and 236U excited by the (d,d') reaction

The energy levels of ²³⁴U and ²³⁶U have been studied through the inelastic scattering of 16 MeV douterons. A magnetic spectrograph was used to momentum-analyse the scattered deuterons at θ = 90° and 125°. Excited in both ²³⁴U and ²³⁶U were the ground state bands up to and including the 8⁺ members, the K^π = 0⁺β-vibrations, the K^π = 2⁺γ-vibrations, and the K^π = 0^? octupole vibrational bands. In ²³⁴U, additional levels at 1023 and 1126 keV are ascribed to a K^π = 2^? band, levels at 1238, 1312, and 1446 keV are identified as members of either a K^π = 0^? or 1^? configuration, and other tentative assignments are made for members of K^π = 1^? and 3^? configurations. Relative reduced transition probabilities, B(E2), to the 2⁺ rotational and γ-vibrational states are generally found to be in good agreement with Coulomb excitation measurements. Relative B(E3) values for the 3^? states excited are slightly higher than the predictions of a microscopic theory of octupole vibrations.