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.     

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.     

Rotational side bands in 160Dy

The ¹⁵⁸Gd(α, 2n)¹⁶⁰Dy reaction has been used to study rotational side bands in ¹⁶⁰Dy. Several negative-parity bands are observed, with several members of K^π = 1⁻, 2⁻ and 4⁻ bands and of a K^π = 8⁻ band only the bandhead. A pronounced odd-even staggering of moments of inertia is found for members of the K^π = 2⁻ band. Of the positive-parity bands the K^π = 2⁺ (γ) band, which is seen up to a possible spin of 12 and low-spin members of K^π = (0⁺), (S), and 4⁺ bands are observed. The mixing between most of the γ-band and S-band members is observed to be smaller than 2% and no evidence for possible anti-alignment of low-spin S-band members with rotation is found.     

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.     

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.     

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.     

Investigations of Properties of <Superscript>89</Superscript>Y Levels with Reactor Fast Neutrons

By employing a beam of reactor fast neutrons, the spectrum of gamma rays up to an energy of 4.6MeV and their angular distributions with respect to the neutron-beamaxis aremeasured in the reaction ⁸⁹Y(n, n'γ). The multipolarities and multipole-mixture parameters for 34 gamma transitions and the spin–parities J^π of states excited in this reaction are determined. The lifetimes of the lowest 32 levels of 89Y were measured by the Doppler shift attenuation method, and the reduced probabilities for the respective gamma transitions were calculated. Levels of the K^π = +5/2⁺ and K^π = ?7/2⁺ bands associated with, respectively, prolate and oblate deformation shapes are found in ⁸⁹Y at low excitation energies.     

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.     

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.     

Evidence for hexadecapole vibrations in A ≈ 170 deformed nuclei; New results for 168Yb

Dominating proton structure has been identified in the K^π=3⁺ band of ¹⁶⁸Yb, contrasting with neutron structure in the corresponding band of ¹⁷²Yb. A region of low-lying K^π=3⁺ bands is now established, with N=98?104, Z=68?72. For all stable nuclei in this region, the I^πK=4⁺3 state is excited by inelastic deutron scattering.     

Investigation of vibrational levels in 226Ra by Coulomb excitation and by the 226Ra (d,pnγ) reaction

Vibrational bands in ²²⁶Ra were studied by Coulomb excitation and by the ²²⁶Ra(d,pnγ) reaction. The first-excited K ^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 as γ vibration and possible member of a second-exited K ^π = 0⁺ band, respectively. The E1 and E2 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.     

Investigation of 228Th in the (α, α'2n) reaction

The ²³⁰Th(α, α'2n)²²⁸Th reaction at E_α = 56 MeV was used to investigate states of moderately high spin in ²²⁸Th. Conversion electron and e^?-γ coincidence measurements were carried out, where the electrons were detected with an iron-free orange spectrometer. The ground state and low-lying K^π = 0^? rotational bands were observed up to I^π = 14⁺ and 13^?, respectively. The data are interpreted in terms of an ω-expansion for the ground-state rotational band, and an octupole vibrational band distorted by the Coriolis coupling to the K^π ? 1^? excitations for the K^π = 0^? band.     

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.     

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.     

Rotational sidebands in 166Er

Rotational sidebands in ¹⁶⁶Er were observed using the 24 MeV ¹⁶⁴Dy(α, 2nγ) reactions. The ground-state band was observed up to spin 16⁺ and does not backbend. A strong backbend is, however, observed in a K^π = (O⁺) sideband, indicating that the 12⁺ state of the previously unknown S-band is at 2656 keV. The γ-band shows significant rotational alignment above I = 10⁺. Levels of at least two negative-parity bands, one of which is primarily the K^π = 2^? octupole vibration, are also observed.     

Collective bands in even mass Sn isotopes

Positive parity bands in ^{112, 114, 116, 118}Sn have been excited up to levels with spin and parity J^π = 12⁺ using Cd(α, 2nγ)Sn reactions. The experiments consisted of γ-ray excitation function, γ-γ coincidence, lifetime, γ-ray angular distribution, γ-ray linear polarization and conversion electron measurements. The observed bands show strong resemblances with ground-state bands of transitional nuclei in this mass region. It is pointed out that the J^π = 0⁺ band-heads originate from 2p-2h excitations in the Z = 50 proton shell. The excitation energies of the band-heads are calculated by means of the macroscopic-microscopic renormalization method. Pair correlations between the 2h and 2p configurations are included separately in a phenomenological way by taking into account the pairing energies of the Cd and Te ground states with respect to the Sn ground state.     

Clustering in neutron-rich nuclei

Clustering in nuclei is discussed putting emphasis on the investigation of the role of nuclear clustering in neutron-rich nuclei. The subjects we discuss include clustering in neutron-rich Be, B and C isotopes, clustering in the island of inversion around N = 20, and clustering in the region with A ≈ 40. Be isotopes present us typical examples of clustering in neutron-rich nuclei not only in their ground band states but also in their excited band states, for which we show the analyses based on antisymmetrized molecular dynamics (AMD). Clustering in Be isotopes near neutron dripline is intimately related to the breaking of the neutron magic number N = 8. In this connection we report our study about the possible relation of the clustering with the breaking of the neutron magic number N = 20 in the island of inversion including ³²Mg and ³⁰Ne. Our discussion is not only about the positive parity states but also about negative parity states. Recently in the latter half of sd shell and in the pf shell many excited rotational bands with large deformation have been found to exist. Since the first excited K ^π = 0⁺ and K ^π = 0^- bands in ⁴⁰Ca have been regarded as constituting inversion doublet bands having the ³⁶Ar + α structure, and since the first excited K ^π = 0^- band in ⁴⁴Ti has been concluded to have ⁴⁰Ca + α structure through the α transfer reaction and by using the unique α optical potential on ⁴⁰Ca, it is important to investigate the role of α clustering in these newly-found rotational bands with large deformation. We will report the AMD study about this problem.     

Identification of one-phonon and two-phonon γ-vibrational bands in odd-ZNb nucleus

Collective bands in the neutron-rich ¹⁰³Nb nucleus have been investigated by measuring prompt γ-rays following spontaneous fission of ²⁵²Cf. Three new bands have been identified. The band based on the 716.8 keV level is proposed as a candidate for the Kπ=9/²⁺$K^{\pi }=9/2^{+}$ one-phonon γ  -vibrational band, and the band built on the 1282.1 keV level is proposed as a candidate for the Kπ=13/²⁺$K^{\pi }=13/2^{+}$ two-phonon γ-vibrational band. The two-phonon γ-vibrational band is the first such band identified in odd-Z nuclei.     

Experimental search for radiative decays of the pentaquark baryon Θ+(1540)

The data on the reactions K ⁺Xe → K ⁰ γX and K ⁺Xe → K ⁺ γX, obtained with the bubble chamber DIANA, have been analyzed for possible radiative decays of the Θ⁺(1540) baryon: Θ⁺ → K ⁰ pγ and Θ⁺ → K ⁺ nγ. No signals have been observed, and we derive the upper limits Γ(Θ⁺ → K ⁰ pγ)/Γ(Θ⁺ → K ⁰ p) < 0.032 and Γ(Θ⁺ → K ⁺ nγ)/Γ(Θ⁺ → K ⁺ nγ) < 0.041 which, using our previous measurement of Γ(Θ⁺ → KN) = 0.39 ± 0.10 MeV, translate to Γ(Θ⁺ → K ⁰ pγ) < 8 keV and Γ(Θ⁺ → K ⁺ nγ) < 11 keV at 90% confidence level. We have also measured the cross sections of K ⁺-induced reactions involving emission of a neutral pion: σ(K ⁺ n → K ⁰ pπ ⁰) = 68 ± 18 µb and σ(K ⁺ N → K ⁺ Nπ ⁰) = 30 ± 8 µb for incident K ⁺ momentum of 640 MeV.     

The direct decay K+ → π+π0γ and its relation to other weak radiative decays

A model for weak nonleptonic transitions, previously shown to explain K₂⁰ → γγ and other weak radiative K-meson decays, is applied here to K⁺ → π⁺π⁰γ. It reproduces correctly the recently determined dominance of the magnetic transitions and allow to make firm predictions for the related K⁺ → π⁺γγ and K₂⁰ → π⁺π^?γ transitions.