Euthria boavistensis Cosel, 1982

Catálogo do Museo de Historia Natural USC. n. inventario 10023

Euthria rolani Cosel, 1982

Catálogo do Museo de Historia Natural USC. n. inventario 10023

Shell-model test of the rotational-model relation between static quadrupole moments Q(2^+_1), B(E2)'s, and orbital M1 transitions

In this work, we examine critically the relation between orbital magnetic dipole (scissors mode) strength and quadrupole deformation properties. Assuming a simple K=0 ground state band in an even-even nucleus, the quantities Q(2^+_1) (i.e., the static quadrupole moment) and B(E2)_{0_1 \to 2_1} both are described by a single parameter--the intrinsic quadrupole moment Q_0. In the shell model, we can operationally define Q_0(Static) and Q_0(BE2) and see if they are the same. Following a brief excursion to the sd shell, we perform calculations in the fp shell. The nuclei we consider ({44,46,48}Ti and {48,50}Cr) are far from being perfect rotors, but we find that the calculated ratio Q_0(Static)/Q_0(BE2) is in many cases surprisingly close to one. We also discuss the collectivity of orbital magnetic dipole transitions. We find that the large orbital B(M1) strength in {44}Ti relative to {46}Ti and {48}Ti cannot be explained by simple deformation arguments.Comment: 12 pages, RevTeX4. Sections II (Quadrupole properties in the sd-shell) and V (Random interaction studies) added. Minor changes throughout the text and 48Cr added to present Table IV, as well as results for the lowest 100 state

Study of the pd→3HeK+K−pd\to ^3\textrm{He} K^+K^- and pd→3Heϕpd\to ^3\textrm{He} \phi reactions close to threshold

Two--kaon production in proton--deuteron collisions has been studied at three energies close to threshold using a calibrated magnetic spectrograph to measure the final $^3$He and a vertex detector to measure the $K^+K^-$ pair. Differential and total cross sections are presented for the production of $\phi$--mesons, decaying through $\phi\to K^+K^-$, as well as for prompt $K^+K^-$ production. The prompt production seems to follow phase space in both its differential distributions and in its energy dependence. The amplitude for the $pd\to ^3${He}$\phi$ reaction varies little for excess energies below 22 MeV and the value is consistent with that obtained from a threshold measurement. The angular distribution of the $K^+K^-$ decay pair shows that near threshold the $\phi$--mesons are dominantly produced with polarization $m=0$ along the initial proton direction. No conclusive evidence for $f_0(980)$ production is found in the data.Comment: 13 figure

Strong fragmentation of low-energy electromagnetic excitation strength in 117^{117}Sn

Results of nuclear resonance fluorescence experiments on $^{117}$Sn are reported. More than 50 $\gamma$ transitions with $E_{\gamma} < 4$ MeV were detected indicating a strong fragmentation of the electromagnetic excitation strength. For the first time microscopic calculations making use of a complete configuration space for low-lying states are performed in heavy odd-mass spherical nuclei. The theoretical predictions are in good agreement with the data. It is concluded that although the E1 transitions are the strongest ones also M1 and E2 decays contribute substantially to the observed spectra. In contrast to the neighboring even $^{116-124}$Sn, in $^{117}$Sn the $1^-$ component of the two-phonon $[2^+_1 \otimes 3^-_1]$ quintuplet built on top of the 1/2$^+$ ground state is proved to be strongly fragmented.Comment: 4 pages, 3 figure

Electric and magnetic dipole strength in ⁵⁸Ni from forward-angle proton scattering

Background: Electric and magnetic dipole strengths in nuclei at excitation energies well below the giant resonance region are of interest for a variety of nuclear structure problems including a possible electric dipole toroidal mode or the quenching of spin-isospin flip modes. Purpose: The aim of the present work is a state-by-state analysis of possible ⁢1 and ⁢1 transitions in 58Ni with a high-resolution (,′) experiment at 295 MeV and very forward angles including 0∘ and a comparison to results from studies of the dipole strength with the (,′) and (,′) reactions. Methods: The ⁢1 and ⁢1 cross sections of individual peaks in the spectra are deduced with a multipole decomposition analysis (MDA). They are converted to reduced ⁢1 and spin ⁢1 transition strengths using the virtual photon method of relativistic Coulomb excitation and the unit cross-section method, respectively. The experimental ⁢1 strength distribution is compared to large-scale shell-model calculations with the effective GXPF1A and KB3G interactions. Results: In total, 11 ⁢1 and 26 ⁢1 transitions could be uniquely identified in the excitation energy region 6–13 MeV. In addition, 22 dipole transitions with preference for either ⁢1 or ⁢1 multipolarity and 57 transitions with uncertain multipolarity were found. Despite the high level density good agreement is obtained for the deduced excitation energies of =1 states in the three types of experiments indicating that the same states are excited. The ⁡(⁢1) and ⁡(⁢1) strengths deduced in the (,′) experiments are systematically smaller than in the present work because of the lack of information on branching ratios to lower-lying excited states and the competition of particle emission. Fair agreement with the ⁡(⁢1) strengths extracted from the (,′) data is obtained after removal of ⁢1 transitions uniquely assigned in the present work belonging to a low-energy toroidal mode with unusual properties mimicking ⁢1 excitations in electron scattering. The shell-model calculations provide a good description of the isospin splitting and the running sum of the ⁢1 strength. A quenching factor 0.74 for the spin-isospin part of the ⁢1 operator is needed to attain quantitative agreement with the data. Conclusions: High-resolution forward-angle inelastic proton scattering experiments at beam energies of about 300 MeV are a highly selective tool for an extraction of resolved ⁢1 and ⁢1 strength distributions in medium-mass nuclei. Fair agreement with results from electron scattering experiments is obtained indicating a dominance of spin contributions to the ⁢1 strength. Shell-model calculations are in good agreement with gross properties of the ⁢1 strength distribution when a quenching factor for the spin-isospin part comparable to the one needed for a description of Gamow-Teller (GT) strength is included