Search Results (34 total)

Dipole-strength distributions in the nuclides ⁹⁸Mo and ¹⁰⁰Mo 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 γ-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 ⁹⁸Mo and ¹⁰⁰Mo from the present (γ,γ^') experiments are combined with (γ,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 ³He-induced reactions.

Lifetimes of exited states in ¹³⁴Pr were measured by means of the recoil distance Doppler-shift and Doppler-shift attenuation techniques. The branching ratios and the electric or magnetic character of the transitions were also investigated. The experiments were performed at IReS, Strasbourg, using the EUROBALL IV spectrometer, in conjunction with the inner bismuth germanate ball and the Cologne coincidence plunger apparatus. Exited states in ¹³⁴Pr were populated in the fusion-evaporation reaction ¹¹⁹Sn(¹⁹F, 4n)¹³⁴Pr. The possible chiral interpretation of twin bands was investigated in the two-quasiparticle triaxial rotor and interacting boson-fermion-fermion models. The analysis of the wave functions has shown that the possibility for the angular momenta of the proton, neutron, and core to find themselves in the favorable, almost orthogonal geometry, is present but is far from being dominant. The structure is characterized by large β and γ fluctuations. The existence of doublets of bands in ¹³⁴Pr can be attributed to weak chirality dominated by shape fluctuations.

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 γ process. Because the nuclear deformation is governed by shell effects, this extra strength does not directly correlate with the neutron excess.

The nuclides ⁹²Mo, ⁹⁸Mo, and ¹⁰⁰Mo have been studied in photon-scattering experiments by using bremsstrahlung produced at an electron energy of 6 MeV at the ELBE accelerator of the Forschungszentrum Rossendorf and at electron energies from 3.2 to 3.8 MeV at the Dynamitron accelerator at the University of Stuttgart. Six dipole transitions in ⁹⁸Mo and 19 in ¹⁰⁰Mo were observed for the first time in the energy range from 2 to 4 MeV. The experimental results are compared with predictions of the shell model and with predictions of the quasiparticle random-phase approximation (QRPA) in a deformed basis. The latter show significant contributions of isovector-orbital and isovector-spin vibrations. The change of the magnetic dipole strength in the isotopic chain of the even-mass isotopes from ⁹²Mo to ¹⁰⁰Mo is discussed. The calculations within the QRPA are extrapolated to the particle-separation energies to estimate the possible influence of M1 strength on the stability of the nuclides against photodissociation in cosmic scenarios.

Three rotational bands in ⁷⁴Kr were studied up to (in one case one transition short of) the maximum spin I_max⁡ of their respective single-particle configurations. Their lifetimes have been determined using the Doppler-shift attenuation method. The deduced transition quadrupole moments reveal a modest decrease, but far from a complete loss of collectivity at the maximum spin I_max⁡. This feature, together with the results of mean field calculations, indicates that the observed bands do not terminate at I=I_max⁡.

The nuclides ⁹⁸Mo and ¹⁰⁰Mo have been studied in photon-scattering experiments by using bremsstrahlung produced from electron beams with kinetic energies from 3.2 to 3.8 MeV. Six electromagnetic dipole transitions in ⁹⁸Mo and 19 in ¹⁰⁰Mo were observed for the first time in the energy range from 2 to 4 MeV. A specific feature in the two nuclides is the deexcitation of one state with spin J=1 to the 0⁺ ground state as well as to the first excited 0⁺ state, which cannot be explained in standard models. We present a model that allows us to deduce the mixing coefficients for the two 0⁺ shape-isomeric states from the experimental ratio of the transition strengths from the J=1 state to the 0⁺ ground state and to the 0⁺ excited state.

A general but simple method is proposed to eliminate the quantum fluctuations generated by selected one-body operators in the excitation spectrum of a discrete random phase approximation (RPA) Hamiltonian. This method provides an outstanding tool for the removal of the contaminating spurious effects originated from symmetry violations. It can be also applied as a mode filter for analyzing RPA response functions.

Dipole and quadrupole excitations in the semimagic N=50 nucleus ⁸⁸Sr were investigated at the superconducting Darmstadt electron linear accelerator S-DALINAC with bremsstrahlung of an end-point energy of 6.8  MeV. Many new dipole excitations could be identified, and their reduced excitation probabilities were determined. The experimental findings are discussed in the context of quasiparticle-phonon-model and shell-model calculations. A breaking of the N=50 core is essential to describe the structure of the observed excitations. The two-phonon quadrupole-octupole J^π=1⁻ state exhibits unusual features which are presently not understood.

High-spin states in ⁸²Rb and ⁸⁴Rb were populated in the reaction ¹¹B +⁷⁶Ge at beam energies of 45 and 50 MeV. γ rays were detected with the spectrometer GASP. The level schemes of ⁸²Rb and ⁸⁴Rb were extended up to 6.0 and 7.4 MeV, respectively. Mean lifetimes of five levels in ⁸²Rb and 11 levels in ⁸⁴Rb were determined using the Doppler-shift-attenuation method. Regular magnetic dipole bands including strong M1 and weak E2 transitions observed in both nuclei show the characteristic features of magnetic rotation. These bands have been successfully described in the tilted-axis cranking model on the basis of the four-quasiparticle configuration π(fp) π(g_9/2²) ν(g_9/2). The calculations reproduce the band-like properties as well as absolute B(M1) and B(E2) transition strengths in both nuclei, which supports the concept of magnetic rotation. Excited states in ⁸⁴Rb were also interpreted in terms of the shell model using the model space π(0f_5/2,1p_3/2,1p_1/2,0g_9/2) ν(1p_1/2,0g_9/2). The predictions for low-lying states agree in general with the experiment. Moreover, calculated states with the main configuration π(0f_5/2^-21p_3/2^-10g_9/2²) ν(0g_9/2^-3) can be combined into M1 sequences which reproduce roughly the experimental transition strengths. However, these sequences do not show the features of magnetic rotation such as regular level spacings and B(M1) values which decrease with increasing rotational frequency.

High-spin states in ¹²³Xe were populated in the ¹¹⁰Pd(¹⁸O,5n) reaction at 75 MeV and gamma-ray coincidences were measured with the GASP spectrometer. A new rotational sequence of enhanced dipole transitions was established. This band, as well as a similar band in ¹²⁴Xe, may be described within the framework of the tilted axis cranking model as bands for which comparable amounts of angular momentum are generated by magnetic and collective rotation, respectively.

Dipole excitations in the semimagic N=50 nucleus ⁸⁷Rb were investigated at the Stuttgart Dynamitron facility using bremsstrahlung with an end-point energy of 4.0 MeV. The widths Γ or the reduced excitation probabilities B(Π1)↑ of 18 states were determined for the first time. The magnetic dipole excitations are well reproduced in the framework of the shell model, however, these calculations cannot describe the observed electric dipole excitations. The 1/2⁺ state at 3060 keV is proposed to be the weak coupling of an f_5/2 proton hole to the 3^- octupole vibrational state in the N=50 core ⁸⁸Sr. The relatively strong E1 transition from that state to the ground state is explained as mainly the neutron h_11/2→g_9/2 transition. The breakup of the N=50 core and neutron excitations into the h_11/2 shell are essential to describe electric dipole excitations, but neutron-core excitations do not play an important role for the structure of magnetic dipole excitations.

Excited states of the nucleus ⁷⁹Br were investigated via the reaction ⁷⁶Ge(⁷Li,4n) at a beam energy of 35 MeV. Coincidence data of emitted γ rays were measured with an arrangement of six EUROBALL CLUSTER detectors. The E2 bands built on the 9/2⁺ and 3/2^- states were extended up to J=37/2 at E≈8.8 MeV. The M1 band starting with a 15/2^- state at 2.6 MeV was observed up to J=(29/2) at E=6.4 MeV. Crossover E2 transitions within this band were observed for the first time. Mean lifetimes of 17 levels were deduced using the Doppler-shift-attenuation method. The M1 band can be described within the tilted-axis-cranking model on the basis of the tilted three-quasiparticle configuration π(g_9/2) ν(g_9/2) ν(fp) which has a triaxial shape. This band appears as a mixed case including contributions of both magnetic and collective rotation.

The contribution of quantum shape fluctuations to inertial properties of rotating nuclei has been analyzed within the self-consistent one-dimensional cranking oscillator model. It is numerically proven that for even-even nuclei the dynamical moment of inertia calculated in a mean field approximation in the rotating frame is equivalent to the Thouless-Valatin moment of inertia. If the contribution of the quantum fluctuations to the total energy is taken into account, the dynamical moment of inertia differs from the Thouless-Valatin value.

The chiral-twin candidate bands recently observed in ¹³⁶Pm have been extended to high spins [I=(21)] using the Gammasphere γ-ray spectrometer and the Microball charged-particle detector array. A more-detailed spectroscopy of the bands was possible, where the rotational alignments and B(M1)/B(E2) ratios confirm that both sequences have the πh_11/2νh_11/2 configuration. Particle-rotor calculations of intraband and interband transition strength ratios of the chiral-twin bands are compared with experimental values for the first time. Good agreement was found between the predicted transition strength ratios and the experimental values, thus supporting the possible chiral nature of the πh_11/2νh_11/2 configuration in ¹³⁶Pm.

High-spin states of ⁸⁹Sr and ⁹⁰Sr were studied via the reactions ⁸²Se(¹¹B, p3n) and ⁸²Se(¹¹B, p2n), respectively, at a beam energy of 37 MeV. Gamma rays were detected with the GASP spectrometer. The level schemes of ⁸⁹Sr and ⁹⁰Sr were extended up to E≈8 MeV and E≈10 MeV, respectively. Level structures in ⁸⁹Sr and ⁹⁰Sr were interpreted in terms of the spherical shell model. The calculations were performed in the configuration space (0f_5/2,1p_3/2,1p_1/2,0g_9/2) for the protons and (1p_1/2,0g_9/2,1d_5/2) for the neutrons. High-spin level sequences in ⁸⁹Sr are characterized by coupling the unpaired d_5/2 neutron to proton excitations of the core nucleus ⁸⁸Sr. An equidistant level sequence with ΔJ=2 found in ⁹⁰Sr is well described by the configuration π[(0f_5/2^-2)(0g_9/2²)]ν(1d_5/2²) favoring even spins.

High-spin states in the doubly odd N=75 nuclei ¹³⁶Pm and ¹³⁸Eu were populated following the ¹¹⁶Sn(²⁴Mg,p3n) and ¹⁰⁶Cd(³⁵Cl,2pn) reactions, respectively. A new ΔI=1 band is reported in ¹³⁸Eu and new data are presented for the recently reported band in ¹³⁶Pm. Polarization and angular correlation measurements have been performed to establish the relative spin and parity assignments for these bands. Both bands have been assigned the same πh_11/2⊗νh_11/2 structure as the yrast band and are suggested as candidates for chiral twin bands.

The tilted axis cranking model is used in combination with the random phase approximation and particle number projection to analyze the influence of dynamical pair correlations in the high-K bands of ¹⁷⁸W and their effect on relative energy and angular momentum. The calculations show the importance of dynamical pair correlations to describe the experiment as well as advantages and problems with the different models in the superfluid and normal state regions.

High-spin states of the nucleus ⁸⁸Sr have been studied via the reaction ⁸⁰Se(¹¹B,p2n) at a beam energy of 45 MeV. Gamma rays were detected with the six-detector array OSIRIS CUBE. The level scheme of ⁸⁸Sr has been extended up to E≈11 MeV and J=17. Mean lifetimes of three levels have been determined using the Doppler-shift-attenuation method. The level structures in ⁸⁸Sr have been interpreted in terms of the shell model. The calculations were performed in the configuration space (0f_5/2,1p_3/2,1p_1/2,0g_9/2) for the protons and (1p_1/2,0g_9/2,1d_5/2) for the neutrons. These calculations describe the high-spin level sequences linked by M1 transitions with strengths of B(M1)≈0.3 to 1.4 W.u. as multiplets of seniority υ=4 and 6 states including proton configurations and neutron-core excitations.

High-spin states in ⁷²Br were studied with the EUROBALL III spectrometer using the ⁴⁰Ca(⁴⁰Ca,α3p1n) 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.

A hybrid version the deformed nuclear potential is suggested, which combines a spherical Woods-Saxon potential with a deformed Nilsson potential. It removes the problems of the conventional Nilsson potential in the mass 130 region. Based on the hybrid potential, tilted axis cranking calculations are carried out for the magnetic dipole band in ¹²⁸Ba.

Rotational bands in ⁷³Br have been investigated up to spins of I=65/2 using the EUROBALL III spectrometer. One of the negative-parity bands displays the highest rotational frequency ħω=1.85 MeV reported to date in nuclei with A>~25. At high frequencies, the experimental J⁽²⁾ dynamic moment of inertia for all bands decreases to very low values, J⁽²⁾<~10ħ² MeV^-1. The bands are described in the configuration-dependent cranked Nilsson–Strutinsky model. The calculations indicate that one of the negative-parity bands is observed up to its terminating single-particle state at spin 63/2. This result establishes the first band termination in the A≈70 mass region.

It is shown that the rotating mean field of triaxial nuclei can break the chiral symmetry. Two nearly degenerate ΔI  =  1 rotational bands originate from the left-handed and right-handed solutions.

Using the spectral function φ^′(z)/φ(z), the random-phase approximation correlation energy and other properties of a finite system can be written as a contour integral in a compact way. This yields a transparent expression and reduces drastically the numerical efforts for obtaining reliable values. For illustration the method is applied to pairing vibrations in rotating nuclei.

Rotational bands with strong magnetic dipole transitions have been observed in the doubly odd nuclei ⁸²Rb and ⁸⁴Rb. These bands show the characteristic features of magnetic rotation. They are the first evidence of this new kind of nuclear excitation in the A≈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 π(fp)-π(g_9/2²)-ν(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.