The dipole strength of the nuclide 59Co was studied in photon-scattering experiments using bremsstrahlung produced with electron beams of energies of 8.6 and 12.2 MeV at the bremsstrahlung facility 𝛾⁢ELBE. We identified 130 levels up to an excitation energy of 10.5 MeV. The quasicontinuum of unresolved transitions was included in the analysis of the spectra and the intensities of branching transitions were estimated on the basis of simulations of statistical 𝛾-ray cascades. The photoabsorption cross section up to the neutron-separation energy was determined and is compared with predictions of the statistical reaction model as well as data for even-mass nuclides in this mass region. The experimental dipole strength function is compared with an 𝑀⁢1 strength function obtained from shell-model calculations. locked icon locked icon locked icon locked icon locked icon locked icon locked icon locked icon locked icon locked icon locked icon locked icon locked icon locked icon Physics Subject Headings (PhySH)Electromagnetic transitionsEnergy levelsLifetimes & widthsNuclear structure & decays59 ≤ A ≤ 89

The dipole strength of the nuclide 59Co was studied in photon-scattering experiments using bremsstrahlung produced with electron beams of energies of 8.6 and 12.2 MeV at the bremsstrahlung facility gamma ELBE. We identified 130 levels up to an excitation energy of 10.5 MeV. The quasicontinuum of unresolved transitions was included in the analysis of the spectra and the intensities of branching transitions were estimated on the basis of simulations of statistical gamma -ray cascades. The photoabsorption cross section up to the neutron-separation energy was determined and is compared with predictions of the statistical reaction model as well as data for even-mass nuclides in this mass region. The experimental dipole strength function is compared with an M1 strength function obtained from shell-model calculations.

. - The dipole response of the open-shell nuclide 70Ge has been investigated in high-resolution (gamma, gamma') experiments using bremsstrahlung produced with electron beams of energies of 8.5 and 14.7 MeV at the linear accelerator ELBE. A resonance-like structure of levels mostly with spin J = 1 has been identified, distributed between 5 MeV up to neutron separation energy Sn as in the case of 76Ge and in contast to 74Ge where the level density is lower and ceases abruptly at about 1 MeV below Sn. The distibution strength was complemented by the unresolved levels using simulations of statistical gamma-ray cascades, corrected by estimations of branching transitions. The summed strength in 70Ge, completed by the data from 74,76Ge do not fit with a linear trend as function of the neutron excess. Such unexpected behaviour might be related to the nuclear deformation which seems to play the major role in the moderately deformed Germanium isotopic chain.

The dipole strength of the nuclide Zn-66 was studied in photon-scattering experiments using bremsstrahlung produced with electron beams of energies of 7.5 and 13.4 MeV at the gamma ELBE facility as well as using quasimonoenergetic and linearly polarized photon beams of 30 energies within the range of 4.3 to 9.9 MeV at the HI gamma S facility. A total of 128 J = 1 states were identified, among them 9 with 1(+) and 86 with 1(-) assignments. The quasicontinuum of unresolved transitions was included in the analysis of the spectra and the intensities of branching transitions were estimated on the basis of simulations of statistical gamma-ray cascades. As a result, the photoabsorption cross section up to the neutron-separation energy was determined and compared with predictions of the statistical reaction model. The experimental M1 strengths from resolved 1(+) states are compared with results of large-scale shell-model calculations.

Schwengner, R.; Massarczyk, R.; Scheck, M.; Tornow, W.; Battaglia, G.; Beck, T.; Bemmerer, D.; Benouaret, N.; Beyer, R.; Butterling, M.; Fiedler, F.; Finch, S. W.; Fransen, C.; FrimanGayer, U.; Frotscher, A.; Gonzalez, R.; Grieger, M.; Hartmann, A.; Hensell, T.; Hoemann, E.; Hoffmann, H.; Janssens, R. V. F.; Johnson, S.; Jones, M. D.; Junghans, A. R.; Kelly, N.; Kleemann, J.; Krishichayan; Little, D. R.; Ludwig, F.; Müller, S. E.; O'Donnell, D.; Papst, O.; Pirovano, E.; Sinclair, J.; Takács, M. P.; Turkat, S.; Urlaß, S.; Wagner, A.; Werner, V.; Wieland, O.; Wilhelmy, J.

The low-lying dipole strength of the deformed nucleus 156Gd was investigated in the energy region from 3.1 MeV to 6.2 MeV using the method of nuclear resonance fluorescence (NRF). The NRF experiments were performed at the Darmstadt High Intensity Photon Setup (DHIPS) at Technische Universität Darmstadt using unpolarized continuous-energy bremsstrahlung and at the High-Intensity γ-ray Source (HIγS) at Duke University using quasi-monoenergetic and linearly-polarized photon beams. The combination of both experiments allows to separate electric and magnetic contributions and to determine absolute transition strengths for individual excited states as well as averaged quantities over narrow excitation energy regions. The investigated energy regions cover the region of the scissors mode as well as the low-energy part of the Pygmy Dipole Resonance. This is the first experiment where both of these excitation modes as well as the region in between has been successfully studied in a deformed heavy nucleus using the NRF method.

We present a general scheme of a shell-model analysis of a β-delayed proton emission. We show that the experimental proton to γ-ray branching ratio for the isobaric analog state (IAS) populated in β decay of a precursor, supplemented by theoretical proton and γ-ray widths, can be used to extract spectroscopic factors for isospin-forbidden proton emission. In the case of a well-justified two-level mixing approximation and a relatively well known spectroscopic factor of the admixed state, the proposed scheme provides a new way to determine the amount of the isospin mixing in the IAS. This conjecture is illustrated by the theoretical analysis of Cr44 and Fe48 decay.

We present a general scheme of a shell-model analysis of a $\ensuremath{\beta}$-delayed proton emission. We show that the experimental proton to $\ensuremath{\gamma}$-ray branching ratio for the isobaric analog state (IAS) populated in $\ensuremath{\beta}$ decay of a precursor, supplemented by theoretical proton and $\ensuremath{\gamma}$-ray widths, can be used to extract spectroscopic factors for isospin-forbidden proton emission. In the case of a well-justified two-level mixing approximation and a relatively well known spectroscopic factor of the admixed state, the proposed scheme provides a new way to determine the amount of the isospin mixing in the IAS. This conjecture is illustrated by the theoretical analysis of $^{44}\mathrm{Cr}$ and $^{48}\mathrm{Fe}$ decay.

We present a shell-model analysis of the $\ensuremath{\beta}$ decay of $^{56}\mathrm{Zn}$. The calculations are performed using isospin-nonconserving Hamiltonians constructed on the basis of the GXPF1A and KB3G interactions. Our theoretical results reproduce the essential features of the decay of $^{56}\mathrm{Zn}$ and explain the surprising competition between $\ensuremath{\beta}$-delayed proton and $\ensuremath{\gamma}$-ray emission from the isobaric analog state.

The low-lying electromagnetic dipole strength of the odd-proton nuclide 205Tl has been investigated up to the neutron separation energy exploiting the method of nuclear resonance fluorescence. In total, 61 levels of 205Tl have been identified. The measured strength distribution of 205Tl is discussed and compared to those of even–even and even–odd mass nuclei in the same mass region as well as to calculations that have been performed within the quasi-particle phonon model.

We present a shell-model analysis of the beta decay of Zn-56. The calculations are performed using isospinnonconserving Hamiltonians constructed on the basis of the GXPF1A and KB3G interactions. Our theoretical results reproduce the essential features of the decay of Zn-56 and explain the surprising competition between beta-delayed proton and. -ray emission from the isobaric analog state.

The low-lying electromagnetic dipole strength of the odd-proton nuclide Tl has been investigated up to the neutron separation energy exploiting the method of nuclear resonance fluorescence. In total, 61 levels of Tl have been identified. The measured strength distribution of Tl is discussed and compared to those of even–even and even–odd mass nuclei in the same mass region as well as to calculations that have been performed within the quasi-particle phonon model.

We analysed our experimental recent findings of the dipole response of the odd-mass stable nucleus 205Tl within the quasi-particle phonon model. Using the phonon basis constructed for the neighbouring 204Hg and wave function configurations for 205Tl consisting of a mixture of quasiparticle ⊗ N-phonon configurations (N=0,1,2), only one group of fragmented dipole excited states has been reproduced at 5.5 MeV in comparison to the experimental distribution which shows a second group at about 5 MeV. The computed dipole transition strengths are mainly of E1 character which could be associated to the pygmy dipole resonance.

Although the systematics of the Pygmy Dipole Resonance was established in several stable even even nuclei, the collectivity of this mode, explained in a macroscopic picture as an oscillation of a neutron skin against an isospin symmetric proton neutron core, is still not well understood. In order to guide the theoretical models, we investigated the heavy odd mass, stable isotopes (203,205)T1 using unpolarized bremsstrahlung photon beams at the S-DALINAC facility at TU Darmstadt. The NRF experiments were complemented by measurements at the High Intensity gamma-ray Source (HITS) at the Triangle Universities Nuclear Laboratory (TUNL) in Durham, NC, USA, with a fully linearly polarized quasi-monoenergetic photon beam. Between 4 and 7 MeV, a concentration of dipole strength is observed in the odd-mass nuclide (205)T1. For the (203)T1 isotope, two ground-state transitions around 5 MeV could be resolved in the spectra. In this report, the obtained results will be presented. The results for the proton-odd nuclide 205T1 will be compared to the ones for the neighbouring even even nucleus Pb-206.

The low-lying dipole strength of the N = 28 closed-shell nucleus Cr-52 was studied with nuclear resonance fluorescence up to 9.9 MeV, using bremsstrahlung at the superconducting Darmstadt linear electron accelerator S-DALINAC. Twenty-eight spin-1 states were observed between 5.0 and 9.5 MeV excitation energy, 14 of which for the first time. Both electric dipole excitations (E1, around 8 MeV) and magnetic dipole excitations (M1, around 9 MeV) were detected. Microscopic calculations within the quasiparticle-phonon nuclear model were performed and show good agreement with experimental results. The structure of E1 and M1 excitations, respectively, is discussed.

Photoexcitation of the N = 50 nucleus Y-89 has been performed at the bremsstrahlung facility at the superconducting electron accelerator ELBE at electron energies of E-e(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 Y-89 and of the neighboring N = 50 isotones Sr-88 and Zr-90 are compared with predictions of the quasiparticle-random-phase approximation.

The dipole response of the $N=50$ nucleus $^{90}\mathrm{Zr}$ was studied in photon-scattering experiments at the electron linear accelerator ELBE with bremsstrahlung produced at kinetic electron energies of 7.9, 9.0, and 13.2 MeV. We identified 189 levels up to an excitation energy of 12.9 MeV. Statistical methods were applied to estimate intensities of inelastic transitions and to correct the intensities of the ground-state transitions for their branching ratios. In this way we derived the photoabsorption cross section up to the neutron-separation energy. This cross section matches well the photoabsorption cross section obtained from ($\ensuremath{\gamma}$, $n$) data and thus provides information about the extension of the dipole-strength distribution toward energies below the neutron-separation energy. An enhancement of $E1$ strength has been found in the range of 6 to 11 MeV. Calculations within the framework of the quasiparticle-phonon model ascribe this strength to a vibration of the excessive neutrons against the $N=Z$ neutron-proton core, giving rise to a pygmy dipole resonance.