Study of the level structure of239U using the thermal neutron capture reaction

For the investigation of vibrational states in odd A nuclei we have studied the level structure of²³⁹U by the²³⁸U (n _thermal,γ) reaction. Various complementary measurement techniques as curved-crystal-, anti-Compton-,γ-γ coincidence- and conversion electron spectroscopy have been applied. The resulting data have been used to establish the deexcitation scheme of²³⁹U up to ～1.3 MeV and to make spin assignments. Most of the levels are interpreted in terms of the Nilsson model. The data also indicate the presence of the Nilsson states ¦501?¦ and ¦750¦. Three members of theΒ-vibrational band built on the ¦631?¦ state and one member of theΒ-vibration built on the ¦622↑¦ ground state have been identified through transitions with strongE0 admixtures. The octupole vibrational state built on the ¦631?¦ band is proposed at 815 keV.     

Structure of the doubly odd nucleus 180Ta: Description of 23 bands

The structure of the doubly-odd nucleus ¹⁸⁰Ta has been studied by γ–γ coincidence measurements with a DC beam at 52 and 57 MeV and time-correlated γ–γ coincidence measurements with a pulsed beam at 55 MeV via the ¹⁷⁶Yb(¹¹B, α3n)¹⁸⁰Ta reaction. In all measurements, γ-rays were detected in coincidence with charged particles. In the time-correlated γ–γ coincidence measurements with a pulsed ¹¹B beam, three rotational bands and one octupole vibrational band have been identified above the I^π=15⁻ T_1/2=30 μs isomer. The configuration of three bands built on 8⁺ states has been discussed by means of three-band mixing calculations. BCS calculations with blocking have been used in support of configuration assignment of four- and six-quasiparticle structures. Totally, 19 rotational bands, one β-, one γ- and two octupole-vibrational bands, plus one intrinsic state have been identified with two-, four- and six-quasiparticle configurations. The K values of these bands range from 0 to 19. The K-forbidden transition rates are discussed on the basis of mixing between states with widely different K-values. The BBCS calculations predict a K^π=22⁻ isomer not identified experimentally in this nor in previous works.A search for specific intermediate states which could explain the transformation from K^π=9⁻ to 1⁺ during the astrophysical s- and r- processes was negative.     

Gamma decay of the giant dipole resonance and feeding of superdeformed states in 143Eu

The γ decay of the giant dipole resonance (GDR) built on excited nuclear states has been measured in coincidence with the low-energy γ discrete transitions for the nucleus ¹⁴³Eu. The reaction used was ¹¹⁰Pd(³⁷Cl, 4n)¹⁴³Eu at a beam energy of 165 MeV. The EUROBALL spectrometer (for the measurement of discrete γ transitions) coupled with the HECTOR array (for high-energy γ-ray detection) has been used. The high-energy γ-ray spectrum in coincidence with superdeformed (SD) discrete transitions of ¹⁴³Eu shows an “excess” between 9–12 MeV if compared with the one associated to cascades which do not pass through the SD configurations. Such an “excess” is in the energy region where one expects the low-energy component of the GDR strength function built on a SD state. The measured intensity can be reproduced by the statistical model assuming that the superdeformation survives only few MeV above the yrast line. A similar and consistent scenario has also been obtained by comparing the high-energy γ-ray spectra of ¹⁴³Eu in coincidence with its spherical (which is fed by the SD configuration) and its triaxial configuration (which is bypassed by the decay of the SD states).     

High-spin states in26Mg

Theγ-decays of levels in²⁶Mg have been investigated up to 12.5 MeV excitation energy by proton-γ-ray coincidence measurements in the²³Na(α, pγ) reaction at 14.2 and 16 MeV bombarding energy. Lifetime-measurements, made with the Doppler-shift attenuation method, and proton-γ-ray angular-correlation measurements were performed at E_α=14.2 MeV. Many high-spin states were observed, among them levels at 6,978 (5⁺), 7,283(4^?), 7,395(5⁺), 7,953(5^?), 8,202(6⁺), 8,472(6⁺), 9,065(5), 9,112(6⁺), 9,169(6^?), 9,383 (6⁺), 9,542(5), 9,829(7⁺), 9,989(6⁺) and 12,479(8⁺, 7^?) keV excitation energy. The spectrum of positive-parity states and their electromagnetic properties are reproduced with good accuracy by shell-model calculations which employ a unifieds-d shell Hamiltonian and the unrestricted configuration space of the 0d _5/2 1s _1/2 0d _3/2 shell. Members of five inferred rotational bands, withK ^π=0⁺, 2⁺, 3⁺, 0⁺ and 3^? have been observed up to at leastI=6. TheK ^π=2⁺ band shows strong anomalies of excitation energies andE2 transition rates near theI=6 state. The static intrinsic quadrupole moments calculated from the shell model wave functions indicate transitions from prolate to oblate deformation within theK ^π=2⁺ band and also the ground state band. The lowest lyingI ^π=4⁺ state appears to be “spherical” and cannot be associated with a rotational band.     

Rotations and intrinsic Kπ = 1+ excitations in deformed rare-earth nuclei

The K^π = 1⁺ intrinsic excitations in deformed rare-earth nuclei are studied in the framework of the random-phase approximation, which excludes the spurious state (zero-energy excitation) related with the rotation of the whole nucleus. We take the Nilsson-plus-pairing, quadrupole and spin-dependent (magnetic dipole or spin-quadrupole) force model. Cranking-model formulae for the moment of inertia and the collective gyromagnetic ratio are derived as the zero-energy limit of the lowest K^π = 1⁺ excitation. They are modified from the usual formulae due to the spin-dependent force. The M1 and E2 transitions from the K^π = 1⁺ excited states to the ground state of ¹⁷⁰Yb are also calculated. The M1 transition strengths are concentrated almost below 6 MeV, while the E2 ones are scattered even up to 12 MeV. Several levels around 12 MeV show rather strong E2 transitions of ΔN = 2 character.     

Vibrational resonances below 5 MeV in the fission of 240Pu

Using the ²³⁹Pu(d, pf) reaction, the next lower K^π = 0⁺ vibrational resonance below 5 MeV in the deformed potential of ²⁴⁰Pu was located at 4.55 MeV. The resultant spacing of fission vibrations near 5 MeV in the second minimum is 500 keV. A resonance at 4.70 MeV is interpreted as the K^π = 0^? vibration coupled to the 4.55 MeV K^π = 0⁺ state, which yields E(K^π = 0^?) = 150 keV relative to the ground state of the second minimum.     

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.     

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.     

Coulomb excitation of the 242m Am isomer

The ^242m Am isomer, a well-known candidate for photodepopulation research, has been studied in this first ever Coulomb excitation of a nearly pure (≈98%) isomer target. Thirty new states, including a new rotational band built on a K ^π = 6^? state, have been identified. Strong K-mixing results in nearly equal populations of the K ^π=5^? and 6^? states. Newly identified states have been assigned to the K ^π=3^? rotational band, the lowest states of which are known to decay into the ground-state band. Implications regarding K-mixing and Coulomb excitation paths to the ground state are discussed.     

Low-lying states in112In

Low-lying states below 500 keV excitation in¹¹²In have been investigated via the¹¹²Cd(p, nγ) reaction. New levels have been established atE ^x=206.5keV and 456.1 keV from the measuredγ-ray excitation functions,γ?γ coincidences and the precision measurements of the (p, n) threshold energy of the ground state and of the 206.5 keV state of¹¹²In. Spins and parities of the 206.5 keV state (2⁺) and the 456.1 keV state (3⁺) and multipolarities and mixing ratios of the deexcitationγ-rays have been determined from the angular distributions and linear polarizations of the deexcitation γ-rays as well as the excitation functions of the residual levels. Possible configurations of the newly-found levels are discussed. Half-lives of two states have been remeasured:T _1/2=15.2±0.1 min for the ground state andT _1/2=20.9±0.1 min for the 156.4 keV (4⁺) state. The ground stateQ-value for the¹¹²Cd(p, n)¹¹² In reaction has been measured to be ?3.376±0.006 MeV.     

Resonant and intrastate transitions to the 2+ state in8Be

We have studied resonant, intrastate and non-resonant bremsstrahlungγ-ray transitions leading to the 2⁺ state in⁸Be atE _x=2.94 MeV. We find aγ-width of Γ _γ =0.45 eV for the 4⁺ state in⁸Be, corresponding to anE 2 strength of 19 W.u. For the intrastateγ-ray transition within the 2⁺ resonance we predict a maximum cross section of 2.5 nb atE≈3.3 MeV. An experimental observation of this novel type ofγ-ray emission, however, appears to be difficult due to the dominance of competing resonant and direct capture processes.     

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.     

Spectroscopy of highly excited states in29Si

Particleγ-ray coincidences have been measured in the²⁸Si (d,pγ) reaction at 6.5 and 7 MeV bombarding energy, in the²⁶Mg (α,nγ) reaction at 12, 14 and 15 MeV, and in the²⁷A1 (τ,pγ) reaction at 9 MeV. Theγ-decay has been observed for all bound states of²⁹Si and for 56 unbound states up to 12,960 KeV excitation energy. Particleγ-ray angular correlations were measured in the²⁸Si (d,pγ) reaction at 6.5 MeV and in the²⁶Mg (α,nγ) reaction at 12 MeV. Spin (-parity) assignments or restrictions were obtained for nearly all bound states and some high-spin states above the binding energy. The assignment of mirror levels in²⁹Si and²⁹P has been extended to 8.2 MeV excitation energy. The excitation energies of 41 positive-parity states are reproduced by shell model calculations. The possible existence of aK ^π=5/2⁺ band with prolate deformation is discussed.     

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.     

Shape coexistence and other aspects of nuclear structure in40Ar

Theγ-decay of⁴⁰Ar has been studied by particle-γ-ray coincidence measurements in the³⁷Cl(α, pγ) reaction at 12 and 13 MeV bombarding energy. Particle-γ-ray angular correlations and linear polarizations ofγ-rays were measured at 12 MeV. A lifetime measurement using the Doppler-shift attenuation method was performed at 11 MeV. The coexistence of spherical and deformed states in⁴⁰Ar could be concluded from the observation of aK ^π=0⁺ rotational band which has itsI ^π=0⁺ through 6⁺ members at 2,121, 2,524, 3,515 and 4,959 KeV excitation energy. The intrinsic quadrupole moment derived fromB(E2) values is ∥Q ₀∥=1,320 _?120 ⁺⁶⁰ mb. Negative-parity states with high spin were observed at 4,858(5^?), 4,494(5^?), 4,226(4^?) and 4,991 KeV(4^?) excitation energy. A complete account of all levels below 5 MeV excitation energy is obtained by a model in which twod _3/2 proton holes couple weakly to the⁴²Ca levels below 4.75 MeV excitation energy.     

Collective structures in185Hg

Excited states of¹⁸⁵Hg have been investigated via the¹⁶¹Dy(²⁸Si, 4n) reaction at 145 MeV. In-beamγ-ray spectroscopic studies have been performed with the “Château de Cristal” 4π-multidetector array. A level scheme of¹⁸⁵Hg has been established. Shape coexistence, still present in¹⁸⁵Hg like in the neighbouring Hg isotopes, manifests itself through a weakly populated decoupled band built on the 13/2⁺ isomer and three strongly-coupled bands built on the prolate 1/2^? [521], 7/2^? [514], and 9/2⁺ [624] Nilsson states.     

Experimental investigation on the possible complexity in the potential energy surface of232Th

The reaction²³²Th(γ,f) is studied using bremsstrahlungγ-beams in the (5.45–6.85) MeV end point energy range. Fragments angular distributions are measured and yield components with opposite parities are separated. The unfolded cross sections have resonant behaviour which indicates the existence of low damped vibrational states. Furthermore, a near degeneracy forK ^π =0⁺ andK ^π = 0^? resonances at ～5.5 MeV and ～6 MeV is observed. Data are analysed in terms of a double humped barrier configuration within the doorway state model for fission. For each component maxima with almost equal heights and a shallow intermediate minimum are determined. Moreover almost equal potential barriers withK=0 and opposite parities are found. This is a signature for mass-asymmetric shapes and seems a clear indication for a threehumped barrier configuration with a low inner maximum and a third mass asymmetric shallow minimum due to the fragmentation of the normal mass-asymmetric second maximum.     

The parity of the 3134 keV,J=1 State in18F

The¹⁶O(³He,?γ)¹⁸F reaction atE(³He)=2.95 MeV was used to study the low-lying states in¹⁸F belowE _x =3.2 MeV. A combination of γ-ray angular distribution and linear polarization measurements using a single planar Ge(Li) polarimeter resulted in aJ ^π =1^? assignment to the 3134 keV state. In addition, the positive parity assignments to the 937(3⁺) and 1702(1⁺) keV states have been verified. Information on branching and mixing ratios as well as on lifetimes are reported. The results are discussed in terms of the weak-coupling and strong-coupling models.     

Response of cooled PWO scintillators to low-energy gamma-rays and its importance in studies of the h c →η c + γ transition in charmonium

Suppression of non-resonant background due to emission of π ⁰ increases signal to background ratio in γ-ray emission studies between the discrete states of charmonium. We demonstrate with Monte-Carlo simulations how π ⁰ identification capability increases with decreasing threshold for γ-ray detection in π ⁰ decay. An apparatus has been designed and measurements of the FWHM resolution performed for the three energies 6.13, 12.7 and 16.9 MeV. The trend established in the 50–650 MeV energy range is corroborated.