Search

The quasiparticle-random-phase approximation is used to study the properties of the wobbling bands in $^{163}\mathrm{Lu}$. Assuming that the wobbling mode represents pure isoscalar orientation oscillations results in too low wobbling frequencies and transition probabilities between the one- and zero-phonon wobbling bands that are strongly collective but yet too weak for $B{(E2)}_{\mathrm{out}}$ and too strong for $B{(M1)}_{\mathrm{out}}$. The inclusion of an LL interaction, which couples the wobbling mode to the scissors mode, generates the right upshift of the wobbling frequencies and the right suppression of the $B{(M1)}_{\mathrm{out}}$ values toward the experimental values, but does not change the $B{(E2)}_{\mathrm{out}}$ values. In analogy to the quenching of low-energy $E1$ transition by coupling to the isovector giant dipole resonance, a general reduction of the $M1$ transitions between quasiparticle configurations caused by coupling to the scissors mode is suggested. The small $\mathrm{B}{(E2)}_{\mathrm{out}}$ values are related to small triaxiality of the density distribution, which is found by all mean field calculations for the triaxial strongly deformed nuclei in the mass 160 region.

A pair of transverse wobbling bands is observed in the nucleus ^{135}Pr. The wobbling is characterized by ΔI=1, E2 transitions between the bands, and a decrease in the wobbling energy confirms its transverse nature. Additionally, a transition from transverse wobbling to a three-quasiparticle band comprised of strong magnetic dipole transitions is observed. These observations conform well to results from calculations with the tilted axis cranking model and the quasiparticle rotor model.

A new facility for the production of polarised bremsstrahlung has been built at the superconducting electron accelerator ELBE of the Forschungszentrum Rossendorf. The bremsstrahlung facility and the setup for photon-scattering experiments are designed such that the background radiation caused by the scattering of photons and the production of neutrons is minimised. The sensitive setup in connection with electron energies up to 20 MeV and average currents up to 1 mA delivered by the ELBE accelerator enables novel experiments using photon-induced reactions. First results of photon-scattering experiments are presented.

A new facility for the production of polarised bremsstrahlung has been built at the superconducting electron accelerator ELBE of the Forschungszentrum Rossendorf. The bremsstrahlung facility and the detector setup are designed such that the background radiation due to scattering of photons and production of neutrons is minimised allowing for experiments close to and above particle separation energies in nuclei. First results of photon-scattering and photo-dissociation experiments on {92,98,100}Mo are presented. The results are compared to recent cross-section calculations for astrophysical networks.

The contribution of quantum shape fluctuations to inertial properties of rotating nuclei has been analyzed for QQ-nuclear interaction using the random phase approximation (RPA). The different recipes to treat the cranking mean field plus RPA problem are considered. The effects of the dN=2 quadrupole matrix elements and the role of the volume conservation condition are discussed., Comment: 14 pages, 7 figures, To be published in J. Phys. G: Nucl. Phys

The nucleus Se-70 was studied using the Ca-40(Ca-40, 2alpha2p) reaction at a beam energy of 185 MeV. Gamma rays were measured with the EUROBALL III spectrometer. The known positive-parity bands have been extended and one new band of positive parity and two of negative parity have been identified. These bands are interpreted in terms of the cranked Nilsson-Strutinsky approach. Calculations suggest that the two negative-parity bands, which have the same signature, are both based on a configuration with two protons and three neutrons lifted from the fp shell to the g(9/2) orbital, but at different nuclear shapes. This represents a shape coexistence at high spin.

The magnetic dipole strength in the energy region of the spin-flip resonance is investigated in $^{128}\mathrm{Xe}$ and $^{134}\mathrm{Xe}$ using quasimonoenergetic and linearly polarized $\ensuremath{\gamma}$-ray beams at the High-Intensity $\ensuremath{\gamma}$-Ray Source facility in Durham, North Carolina, USA. Absorption cross sections were deduced for the magnetic and electric and dipole strength distributions separately for various intervals of excitation energy, including the strength of states in the unresolved quasicontinuum. The magnetic dipole strength distributions show structures resembling a resonance in the spin-flip region around an excitation energy of 8 MeV. The electric dipole strength distributions obtained from the present experiments are in agreement with the ones deduced from an earlier experiment using broad-band bremsstrahlung instead of a quasimonoenergetic beam. The experimental magnetic and electric dipole strength distributions are compared with phenomenological approximations and with predictions of a quasiparticle random phase approximation in a deformed basis.

The photoabsorption cross section of $^{181}\mathrm{Ta}$ up to the neutron-separation energy is deduced using bremsstrahlung produced with an electron beam of 9.6 MeV energy. The analysis of the measured $\ensuremath{\gamma}$-ray spectra includes the quasicontinuum of levels at high energy. Simulations of $\ensuremath{\gamma}$-ray cascades are performed to estimate intensities of inelastic transitions and branching ratios of the ground-state transitions. The resulting photoabsorption cross section shows enhanced dipole strength in the energy range from 5 to 8 MeV, which may be related to a pygmy dipole resonance. The results of the present experiment are compared with predictions of a quasiparticle-random-phase approximation in a deformed basis. A combination of the present experimental data and $(\ensuremath{\gamma},n)$ data is used as an input to the statistical code talys applied to calculate cross sections and reaction rates of photonuclear reactions that are important for the nucleosynthesis of $^{180}\mathrm{Ta}$.

Using the Hamiltonian that consists of the separable quadrupole + pairing forces and the cranking term, we analyze the correlations associated with shape, orientation, and particle-number fluctuations in rotating nuclei. Quantum fluctuations around mean field solutions are treated in the random phase approximation (RPA), with special emphasis on the restoration of rotational symmetry and particle number conservation. The mean field calculations have been made within the self-consistent cranking model. The effect of the RPA correlation energy for the moment of inertia is studied with the integral representation method proposed.

High-spin states in 72Br were studied with the EUROBALL III spectrometer using the 40Ca(40Ca,alpha3p1n) reaction. The negative-parity band observed in this experiment displays a signature inversion around spin I = 16. The interpretation within the cranked Nilsson-Strutinsky approach shows that this signature pattern is a signal of a substantial triaxial shape change with increasing spin where the nucleus evolves from a triaxial shape with rotation about the intermediate axis at low spin through a collective prolate shape to a triaxial shape but with rotation about the shortest principal axis at high spin.

We have studied the isotopes 82Rb45, 83Rb46, and 84Rb47 to search for magnetic rotation which is predicted in the tilted-axis cranking model for a certain mass region around A = 80. Excited states in these nuclei were populated via the reaction 11B + 76Ge with E = 50 MeV at the XTU tandem accelerator of the LNL Legnaro. Based on a -coincidence experiment using the spectrometer GASP we have found magnetic dipole bands in each studied nuclide. The regular M1 bands observed in the odd-odd nuclei 82Rb and 84Rb include B(M1)/B(E2) ratios decreasing smoothly with increasing spin in a range of 13-≤Jπ≤16-. These bands are interpreted in the tilted-axis cranking model on the basis of four-quasiparticle configurations of the type π(fp) πg9/22 vg9/2. This is the first evidence of magnetic rotation in the A ≈ 80 region. In contrast, the M1 sequences in the odd-even nucleus 83Rb are not regular, and the B(M1)/B(E2) ratios show a pronounced staggering.

A nuclear resonance fluorescence experiment on 88Sr has been performed with bremsstrahlung of 6.7 MeV endpoint energy. The γ-ray linear polarisation has been measured with a EUROBALL CLUSTER detector used as a Compton polarimeter. The results indicate positive parity for the J= 1 state at 4.742 MeV in 88Sr, in contrast to the previous interpretation as a 1− two-phonon (2+1⊗ 3−1) state and in conflict with the predictions of the quasiparticle-phonon model. On the basis of such calculations the 1+ state at 3.486 MeV may be considered as the 1+1 one-phonon state and the very strong 1+1→ 0+1 deexcitation as proton spin-flip 2p1/2→ 2p3/2 transition.

Excited states in the T z= 0 nucleus 70Br have been investigated using the reaction 58Ni(16O,1p3n). γ rays were detected with one EUROBALL CLUSTER detector and three single HPGe detectors. Charged particles and neutrons were registered with the Rossendorf silicon ball and six modules of the EUROBALL neutron wall, respectively. The identification of γ transitions in 70Br is based on the analysis of γγ-proton-neutron coincidences. A level scheme of 70Br has been established for the first time. It shows a multiplet-like structure of probably isospin T= 0 while T= 1 isobaric analogue states are not observed.

Rotational bands with strong magnetic dipole transitions have been observed in the doubly odd nuclei ${}^{82}\mathrm{Rb}$ and ${}^{84}\mathrm{Rb}$. These bands show the characteristic features of magnetic rotation. They are the first evidence of this new kind of nuclear excitation in the $A\ensuremath{\approx}80$ region. The results are well reproduced within the framework of the tilted axis cranking model on the basis of four-quasiparticle configurations of the type $\ensuremath{\pi}(\mathrm{fp})\ensuremath{-}\ensuremath{\pi}({g}_{9/2}^{2})\ensuremath{-}\ensuremath{\nu}({g}_{9/2})$.

The calculation of complicated transition matrix elements between multiquasiparticle states can be essentially simplified by transforming them in a canonical form. This method allows the extension of the basis space of generator coordinate studies aiming to include orthogonal quasiparticle excitations into the commonly considered basis set of collective states. Furthermore, it is shown that the neglect of the exchange contribution of multipole forces may lead to dangerous pole terms in nondiagonal matrix elements.

Using a microscopic self-consistent model, we analyse the excitation built upon the rotational band in 163Lu, which has been identified as a wobbling excitation on top of the rotation of a triaxial, strongly deformed shape. We find that the presence of pairing correlations substantially affects the energy of the excitation. Our calculations predict the onset of tilted rotation at a critical rotational frequency where the energy of the excitation approaches zero.

The wobbling motion of a triaxial rotor coupled to a high-j quasiparticle is treated semiclassi- cally. Longitudinal and transverse coupling regimes can be distinguished depending on, respectively, whether the quasiparticle angular momentum is oriented parallel or perpendicular to the rotor axis with the largest moment of inertia. Simple analytical expressions for the wobbling frequency and the electromagnetic E2 and M1 transition probabilities are derived assuming rigid alignment of the quasiparticle with one of the rotor axes and harmonic oscillations (HFA). Transverse wobbling is characterized by a decrease of the wobbling frequency with increasing angular momentum. Two examples for transverse wobbling, 163Lu and 135Pr, are studied in the framework of the full triax- ial particle-rotor model and the HFA. The signature of transverse wobbling, decreasing wobbling frequency and enhanced E2 inter-band transitions, is found in agreement with experiment., Phys. Rev. C revised version, submitted

The electromagnetic dipole strength below the neutron-separation energy has been studied for the xenon isotopes with mass numbers A = 124, 128, 132 and 134 in nuclear resonance fluorescence experiments using the ELBE bremsstrahlung facility at Helmholtz-Zentrum Dresden-Rossendorf and the HIgS facility at Triangle Universities Nuclear Laboratory Durham. The systematic study gained new information about the influence of the neutron excess as well as of nuclear deformation on the strength in the region of the pygmy dipole resonance. The results are compared with those obtained for the chain of molybdenum isotopes and with predictions of a random-phase approximation in a deformed basis. It turned out that the effect of nuclear deformation plays a minor role compared with the one caused by neutron excess. A global parametrization of the strength in terms of neutron and proton numbers allowed us to derive a formula capable of predicting the summed E1 strengths in the pygmy region for a wide mass range of nuclides.

Negative-parity bands in deformed nuclei have traditionally been interpreted in terms of octupole excitations, but interest in these bands has been revived recently by predictions of tetrahedral shape. The level schemes of numerous nuclei in the mass 160 and 230 regions have been studied at iThemba LABS. In particular we look at the results for 158 Dy. B(E1)/B(E2) ratios for transitions from the odd and even-spin negative parity bands to the ground bands were measured and are compared with Cranked Random Phase Approximation calculations. The results support an interpretation in terms of aligned octupole vibrations.

The possibility is investigated that a C 4ν -symmetric hexadecapole deformation causes the observed ΔI = 2 staggering of rotational bands in even and odd- A nuclei. Calculating the energy as a function of intrinsic angular momentum components by means of the tilted cranking model, a rotor Hamiltonian with C 4ν -symmetry is derived, but the magnitude of the fourth order terms is found to be too small to generate a ΔI = 2 staggering of the experimental size. Considering an odd- A system composed of a rotor and a high j -quasiparticle, the ΔI = 2 staggering emerges only when the rotor core contains a C 4ν -symmetric fourth order angular momentum term.

The M1 excitations in the nuclide 90Zr have been studied in a photon-scattering experiment with monoenergetic and linearly polarized beams from 7 to 11 MeV. More than 40 J(π)=1+ states have been identified from observed ground-state transitions, revealing the fine structure of the giant M1 resonance with a centroid energy of 9 MeV and a sum strength of 4.17(56) μ(N)(2). The result for the total M1 strength and its fragmentation are discussed in the framework of the three-phonon quasiparticle-phonon model.

Rotational band structures with distinct properties found in several even-even Xe and Ba nuclei are investigated by means of the conventional cranked shell model and by the newly developed tilted axis cranking model. Analysing the band energies and electromagnetic branching ratios a nearly oblate configuration ( π h 11 2 d 5 2 ) ( ν h 11 2 2 ) with a tilted i.e. non-principal axis spin orientation is found to give a fair description of the experimental features.

The neutron-deficient nucleus Kr-75 has been studied in two EUROBALL experiments. The analysis yielded a considerably extended level scheme including two newly observed excited high spin bands. The results are interpreted in the framework of the cranked Nilsson-Strutinsky approach. The calculations compare well to the experimentally established level scheme and predict the nucleus to be mainly prolate or triaxially deformed at high spin. Evidence for an oblate-prolate shape coexistence could not be found at high spin. ©2012 American Physical Society, Supported by the German BMBF under Contract No. 06K167.

The M1 excitations in the nuclide 90Zr have been studied in a photon-scattering experiment with monoenergetic and linearly polarized beams from 7 to 11 MeV. More than 40 J(π)=1+ states have been identified from observed ground-state transitions, revealing the fine structure of the giant M1 resonance with a centroid energy of 9 MeV and a sum strength of 4.17(56) μ(N)(2). The result for the total M1 strength and its fragmentation are discussed in the framework of the three-phonon quasiparticle-phonon model.

Gamma-ray transitions in $^{235}\mathrm{U}$ were measured using the ($\ensuremath{\gamma},{\ensuremath{\gamma}}^{'}$) reaction below 3 MeV. The nuclear-resonance-fluorescence experiment was carried out at the High-Intensity $\ensuremath{\gamma}$-ray Source facility using nearly monoenergetic and circularly polarized photon beams. More than 20 transitions corresponding to deexcitations to the ground state and low-lying levels in $^{235}\mathrm{U}$ were observed. The integrated cross sections to the excited levels and intensities of branching transitions were deduced. The experimental results are compared with predictions from a quasiparticle random-phase approximation in a deformed basis.

The influence of the quadrupole shape fluctuations on the dipole vibrations in transitional nuclei is investigated in the framework of the instantaneous-Shape Sampling (ISS) Model, which combines the Interacting Boson Model for the slow collective quadrupole motion with the Random Phase Approximation for the rapid dipole vibrations. Coupling to the complex background configurations is taken into account by folding the results with a Lorentzian with energy dependent width (collisional damping CD). The low-energy energy portion of the gamma absorption cross section, which is important for photo-nuclear processes, is studied for the isotop series of Kr, Xe, Ba, and Sm. The experimental cross sections are well reproduced. The low-energy dipole strength is determined by the Landau fragmentation of the dipole strength and its redistribution caused by the shape fluctuations. CD only wipes out fluctuations of the absorption cross section, generating the smooth energy dependence observed in experiment. In the case of semi magical nuclei, shallow pygmy resonances are found in correspondence with experiment.

The routhian diagrams calculated by means of the cranked shell model are known to be a rather successful tool for describing the experimental alignment pattern observed for rotational band structures. However, this mean-field approach seems to fail when applied to band structures built on intruder configurations. It is investigated whether the observed anomalies are affected by the neutron-proton interaction which is only partly and indirectly included in the standard mean-field approach.

Experimental data on the angular momentum dependence of the parity splitting of the yrast band in actinide nuclei are interpreted in the framework of a one-dimensional model of octupole vibrations with axial symmetry. The tunnelling effect as a main source of the parity splitting is calculated in the quasiclassical approximation.

A photon scattering experiment has been performed on the odd-[ital A] nucleus [sup 143]Nd. Strong dipole excitations have been detected in the energy region around 3.2 MeV. This is near the expected position of the two-phonon (2[sup +][direct product]3[sup [minus]]) multiplet in the even-even neighboring nucleus [sup 142]Nd. The summed dipole strength between 2.8 and 3.6 MeV in [sup 143]Nd is in agreement with the sum rule for weak particle coupling to the core nucleus [sup 142]Nd. Model calculations in a harmonic approximation are in a very good agreement with the experiment and they suggest a two-phonon (2[sup +][direct product]3[sup [minus]]) particle structure of the excitations in [sup 143]Nd.

The $\ensuremath{\gamma}$-ray strength function is an important input quantity for the determination of the photoreaction rate and the neutron capture rate for astrophysics as well as for nuclear technologies. To test model predictions, the photoabsorption cross section of $^{139}\mathrm{La}$ up to the neutron-separation energy was measured using bremsstrahlung produced at the electron accelerator ELBE of Forschungszentrum Dresden-Rossendorf with an electron beam of $11.5$ MeV kinetic energy. The experimental data were analyzed by applying Monte Carlo simulations of $\ensuremath{\gamma}$-ray cascades to obtain the intensities of the ground-state transitions and their branching ratios. We found a significant enhancement of electric dipole strength in the energy range from $6$ to $10$ MeV that may be related with a pygmy dipole resonance. The present data are combined with photoneutron cross sections for $^{139}\mathrm{La}$ and compared with results of calculations on the basis of a quasiparticle-random-phase approximation using an instantaneous-shape sampling.

The dipole response of the doubly magic nuclide $^{208}\mathrm{Pb}$ was studied in photon-scattering experiments at the electron linear accelerator ELBE with bremsstrahlung produced at kinetic electron energies of 9.0 and 15.0 MeV. The present $(\ensuremath{\gamma},{\ensuremath{\gamma}}^{\ensuremath{'}})$ data combined with $(\ensuremath{\gamma},n)$ data from the literature are compared with results of shell-model calculations and calculations using a quasiparticle random-phase approximation. The shell-model calculations including (2p-2h) excitations describe the experimental $E1$ strength well and reproduce the spreading of the giant dipole resonance by applying a small smearing width only.

High-spin states in Kr73 were studied at the XTU tandem accelerator of the Laboratori Nazionali di Legnaro using the reaction Ca40(Ca40,α2pn) at a beam energy of 185 MeV. Gamma rays were detected with the EUROBALL spectrometer. Particle detection enabled us to identify the reaction channel leading to Kr73. The yrast bands of positive and negative parity were established up to probable spins of 61/2+ and 63/2-, respectively. The energies of the two identified bands agree well with the predictions of the configuration-dependent Cranked Nilsson-Strutinsky approximation and indicate that the nucleus remains collective up to the maximum spins of the respective high-spin configurations. The positive-parity band represents one of the rare cases where band non-termination at the highest spin of the configuration could be established experimentally. © 2010 The American Physical Society., Supported by the German BMBF under Contract No. 06K167.

The γ‐ray strength function is an important input quantity for the determination of the photoreaction rate and the neutron capture rate for astrophysics as well as for nuclear technologies. Recent studies show that extra γ‐ray strength near the neutron separation energy Sn (pygmy resonance) affects the stellar reaction rate strongly. In this work, the photoabsorption cross section for 139La below Sn was measured using bremsstrahlung produced at the electron accelerator ELBE of Eorschungszentrum Dresden‐Rossendorf with an electron beam of 11.5 MeV kinetic energy. Experimental result of 139La is presented.

A set of deformed states generated with the VAMPIR approach is used for a generator coordinate method description of the 0 + states in 28 Si. As model space the 1s0d shell was used, and as effective hamiltonian the Chung-Wildenthal interaction. Thus a comparison with the “exact” shell-model solutions was possible. Potential-energy functions, overlaps between different deformation points and the hamiltonian matrices are discussed for three different types of VAMPIR solutions: with full symmetry projection, with particle-number projection only and without any projection. The validity of the “horizontal expansion” of the hamiltonian matrix is examined.

High-spin states have been populated in odd-odd {sup 186,188}Au via the heavy-ion {sup 171,173}Yb({sup 19}F,4{ital n}) reactions at 93--94 MeV. Rotational bands built on 11{sup {minus}} oblate {nu}{ital i}{sub 13/2}{sup {minus}1}{direct product}{pi}{ital h}{sub 11/2}{sup {minus}1} structures have been observed in both nuclei, similar to those reported in {sup 190{minus}194}Au. Two new strongly coupled bands in {sup 186}Au are interpreted as prolate {pi}{ital h}{sub 9/2}{direct product}{nu}{ital i}{sub 13}2 and {pi}{ital i}{sub 13/2}{direct product}{nu}{ital i}{sub 13/2} configurations. Based on blocking arguments a band crossing in the latter structure is attributed to the alignment of {ital h}{sub 9/2} protons. Measured {ital B}({ital M}1;{ital I}{r arrow}{ital I}{minus}1)/{ital B}({ital E}2;{ital I}{r arrow}{ital I}{minus}2) ratios are compared to predictions of the Doenau and Frauendorf geometrical formalism extended to odd-odd cases.

High-spin states in {sup 83}Rb were populated in the reaction {sup 11}B+{sup 76}Ge at beam energies of 45 and 50 MeV. {gamma} rays were detected with the spectrometer GASP. The level scheme of {sup 83}Rb was extended up to 13.9 MeV. Mean lifetimes of 23 levels were determined using the Doppler-shift-attenuation method. Among the bands newly established is a sequence comprising intense M1 transitions and crossover E2 transitions. This sequence turns out to be irregular and thus shows that magnetic rotation as observed in the neighboring odd-odd isotopes is not realized in this odd-even nuclide. Excited states in {sup 83}Rb were interpreted in terms of the shell model using the model space {pi}(0f{sub 5/2},1p{sub 3/2},1p{sub 1/2},0g{sub 9/2}) {nu}(1p{sub 1/2},0g{sub 9/2}). The configurations predicted for the negative-parity M1 sequence reproduce the M1 transition strengths fairly well.

Photoexcitation of the N=50 nucleus {sup 89}Y has been performed at the bremsstrahlung facility at the superconducting electron accelerator ELBE at electron energies of E{sub e}{sup kin}=9.5 and 13.2 MeV. About 250 levels up to the neutron-separation energy were identified. Statistical methods were applied to estimate intensities of inelastic transitions and to correct the intensities of the ground-state transitions for their branching ratios. The photoabsorption cross section derived in this way up to the neutron-separation energy is combined with the photoabsorption cross section obtained from ({gamma}, n) data and provides information about the extension of the giant dipole resonance toward energies below the neutron-separation energy. An enhancement of E1 strength has been found in the range from about 6 to 11 MeV. The experimental photoabsorption cross sections of {sup 89}Y and of the neighboring N=50 isotones {sup 88}Sr and {sup 90}Zr are compared with predictions of the quasiparticle-random-phase approximation.

Dipole‐strength distributions in the stable even‐mass molybdenum isotopes up to the neutron‐separation energies have been studied in photon‐scattering experiments with bremsstrahlung at the superconducting electron accelerator ELBE at the Research Center Dresden‐Rossendorf, Germany, and with mono‐energetic photon beams at the High Intensity Gamma‐ray Source facility at Triangle Universities Nuclear Laboratory. In order to determine the dipole‐strength distribution, statistical methods were developed for the analysis of the measured spectra. The data obtained for the stable even‐mass molybdenum isotopes from the present (γ,γ’) experiments are combined with (γ,n) cross sections from the literature resulting in a photoabsorption cross section covering the full range from about 4 to 15 MeV, which is of interest for nuclear structure as well as for nuclear astrophysics network calculations. Novel information about the low‐energy tail of the Giant Dipole Resonance and the energy spreading of its strength is der...

Electromagnetic dipole absorption cross‐sections of transitional nuclei with large‐amplitude shape fluctuations are calculated in a microscopic way by introducing the concept of instantaneous‐shape sampling. The concept bases on the slow shape dynamics as compared to the dipole vibrations. The dipole strength is calculated by means of RPA for the instantaneous shapes, the probability of which is obtained by means of IBA‐1. Very good agreement with the experimental absorption cross sections near the nucleon emission threshold is obtained.

Photoabsorption cross sections ${\ensuremath{\sigma}}_{\ensuremath{\gamma}}$ up to the neutron-separation energy ${S}_{n}$ were measured for the stable even-mass isotopes $^{92\ensuremath{-}100}\mathrm{Mo}$ in photon-scattering experiments. The photon-scattering data were analyzed in a novel way by taking into account the intensity of unresolved levels at high excitation energy and high level density. Simulations of $\ensuremath{\gamma}$-ray cascades were performed to estimate the intensity distribution of inelastic transitions to low-lying levels and, hence, to deduce intensities and branching ratios of the ground-state transitions needed for the determination of ${\ensuremath{\sigma}}_{\ensuremath{\gamma}}$. The present $(\ensuremath{\gamma},{\ensuremath{\gamma}}^{'})$ data can be combined for the first time with $(\ensuremath{\gamma},n)$ data, which allows us to obtain ${\ensuremath{\sigma}}_{\ensuremath{\gamma}}$ in the energy range from about 4 MeV up to the giant dipole resonance for a series of isotopes. The ${\ensuremath{\sigma}}_{\ensuremath{\gamma}}$ values below ${S}_{n}$ increase with the number of neutrons above the neutron shell closure at $N=50$. Calculations using a quasiparticle random-phase approximation in a deformed Woods-Saxon potential describe this effect as a consequence of the increasing nuclear deformation.

Dipole-strength distributions in the nuclides $^{98}\mathrm{Mo}$ and $^{100}\mathrm{Mo}$ up to the neutron-separation energies have been studied in photon-scattering experiments at the bremsstrahlung facility of the Forschungszentrum Dresden-Rossendorf. To determine the dipole-strength distributions up to the neutron-emission thresholds, statistical methods were developed for the analysis of the measured spectra. The measured spectra of scattered photons were corrected for detector response and atomic background by simulations using the code GEANT3. Simulations of $\ensuremath{\gamma}$-ray cascades were performed to correct the intensities of the transitions to the ground state for feeding from higher-lying levels and to determine their branching ratios. The photoabsorption cross sections obtained for $^{98}\mathrm{Mo}$ and $^{100}\mathrm{Mo}$ from the present $(\ensuremath{\gamma},{\ensuremath{\gamma}}^{'})$ experiments are combined with $(\ensuremath{\gamma},n)$ data from literature, resulting in a photoabsorption cross section covering the range from 4 to about 15 MeV of interest for network calculations in nuclear astrophysics. Novel information about the low-energy tail of the giant dipole resonance and its energy dependence is derived. The photoabsorption cross sections deduced from the present photon-scattering experiments are compared with existing data from neutron capture and $^{3}\mathrm{He}$-induced reactions.

Recently much effort has been spent to make the generator coordinator method applicable to problems concerning the dynamics of collective motion at high spin, which goes beyond the possibilities of standard mean field approaches. Using the so-called horizontal expansion, we report in this paper on first calculations about how fluctuations in the triaxial shape can affect the band crossing process among high-spin states in deformed nuclei. For the yrast band in $^{184}\mathrm{Pt}$, we find that the neutron ${\mathit{i}}_{13/2}$ pair first starts to align smoothly, but is followed by a sharp alignment of a proton ${\mathit{h}}_{9/2}$ pair. These two rotational alignment processes take place in a frequency range where there is a strong mixture of shapes with different \ensuremath{\gamma} values ranging from -15 to 4\ifmmode^\circ\else\textdegree\fi{}. The second \ensuremath{\nu}${\mathit{i}}_{13/2}$ pair alignment occurs at a higher frequency (\ensuremath{\Elzxh}\ensuremath{\omega}\ensuremath{\ge}0.45 MeV). Such theoretical results can explain nicely the double crossing observed in the yrast band of $^{184}\mathrm{Pt}$ and the relevant blocking experiments in the neighboring odd-A nuclei. The result on $^{186}\mathrm{Pt}$ is very different, as only the \ensuremath{\pi}${\mathit{h}}_{9/2}$ crossing is observed in the calculations up to a frequency of 0.40 MeV. This compares well with the data, which indicate one sharp band crossing in $^{186}\mathrm{Pt}$ at low frequency.

In the three-dimensional cranking approach the deformed nucleus is rotated about any chosen axis, i.e. contrary to the ordinary cranking this axis does not necessarily coincide with one of the principal axes of the nuclear deformation. The inclusion of the three-dimensional cranking term ω · I = ω x I x + ω y I y + ω z I z into the Hartree-Fock-Bogoliubov theory enables one to bui quasiparticle states with non-standard “spin vectors” 〈 I 〉 , thus denoting for simplicity the corresponding cranking expectation value of the angular momentum vector operator I, that may get also non-parallel to the principal axes of deformation. This freedom in the orientation of the spin vector provides us in fact with two Euler angles necessary for transforming cranking states from the body-fixed into the laboratory system. The generator coordinate (GCM) method is applied in order to quantize the system of cranked quasiparticles. As generator basis, a particular family of cranking states is constructed where all members have a common spin vector 〈 I 〉 , but each belonging to a distinct orientation of the deformed mean field in space. Accordingly, the collective variables characterizing the generator basis are two Euler angles and the phase angle of the rotation about the spin vector. Eigenstates of good angular momentum (a.m.) can be found by solving the Hill-Wheeler equation. The physical meaning of this procedure which is different from the usual a.m. projection, becomes transparent by applying of the so-called horizontal expansion (HEX) of Donau et al. onto the GCM hamiltonian kernel. This expansion finally leads to a practical scheme for extending the cranked Strutinsky mean field approach aiming to recover the broken rotational symmetry. The resulting method allows us to take into account the collective modes analogous to the classical precessional and wobbling motion and to treat the coupling of configurations in the band crossing region. Furthermore, electromagnetic transition amplitudes can be straightforwardly calculated, which is of great importance for interpreting high-spin experiments.

The role of nuclear deformation in the photoabsorption cross section in the tail region of the electric giant dipole resonance (GDR) is studied in terms of a deformed oscillator model and a Nilsson-plus-random-phase-approximation model. It is found within the framework of these approaches that extra electric dipole strength is generated at energies below the GDR maximum if the nuclear system is spatially deformed. This is important in the prediction of stellar photodisintegration rates, knowing that an extra strength can affect these rates even below the particle separation energies through the so-called \ensuremath{\gamma} process. Because the nuclear deformation is governed by shell effects, this extra strength does not directly correlate with the neutron excess.