The electric dipole strength distribution in 48 Ca between 5 and 25 MeV has been determined at RCNP, Osaka, from proton inelastic scattering experiments at forward angles. Combined with photoabsorption data at higher excitation energy, this enables the first extraction of the electric dipole polarizability αD( 48 Ca) = 2.07(22) fm 3 . Remarkably, the dipole response of 48 Ca is found to be very similar to that of 40 Ca, consistent with a small neutron skin in 48 Ca. The experimental results are in good agreement with ab initio calculations based on chiral effective field theory interactions and with state-of-the-art density-functional calculations, implying a neutron skin in 48 Ca of 0.14 − 0.20 fm.Introduction.-The equation of state (EOS) of neutronrich matter governs the properties of neutron-rich nuclei, the structure of neutron stars, and the dynamics of corecollapse supernovae [1,2]. The largest uncertainty of the EOS at nuclear densities for neutron-rich conditions stems from the limited knowledge of the symmetry energy J, which is the difference of the energies of neutron and nuclear matter at saturation density, and the slope of the symmetry energy L, which is related to the pressure of neutron matter. The symmetry energy also plays an important role in nuclei, where it contributes to the formation of neutron skins in the presence of a neutron excess. Calculations based on energy density functionals (EDFs) pointed out that J and L can be correlated with isovector collective excitations of the nucleus such as pygmy dipole resonances [3] and giant dipole resonances (GDRs) [4], thus suggesting that the neutron skin thickness, the difference of the neutron and proton root-mean-square radii, could be constrained by studying properties of collective isovector observables at low energy [5]. One such observable is the nuclear electric dipole polarizability α D , which represents a viable tool to constrain the EOS of neutron matter and the physics of neutron stars [6][7][8][9][10][11].While correlations among α D , the neutron skin and the symmetry energy parameters have been studied extensively with EDFs [12][13][14][15][16], only recently have ab initio calculations based on chiral effective field theory (χEFT) interactions successfully studied such correlations in medium-mass nuclei [17,18]. By using a set of chiral two-plus three-nucleon interactions [19,20] and

Inelastic proton scattering at energies of a few hundred MeV and extreme forward angles selectively excites the isovector spin-flip M1 (IVSM1) resonance. A method based on isospin symmetry is presented to extract its electromagnetic transition strength from the (p, p ) cross sections. It is applied to 48 Ca, a key case for an interpretation of the quenching phenomenon of the spin-isospin response, and leads to a M1 strength consistent with an older (e, e ) experiment excluding the almost two times larger value from a recent (γ, n) experiment. Good agreement with electromagnetic probes is observed in 208 Pb suggesting the possibility to extract systematic information on the IVSM1 resonance in heavy nuclei.PACS numbers: 21.10. Re, 24.30.Cz, 25.40.Fq, 25.40.Kv Introduction.-The isovector spin-flip M1 (IVSM1) resonance is a fundamental excitation mode of nuclei [1]. Its properties impact on diverse fields like the description of neutral-current neutrino interactions in supernovae [2,3], γ strength functions utilized for physics of reactor design [4] or for modeling of reaction cross sections in largescale nucleosynthesis network calculations [5], and the evolution of single-particle properties leading to new shell closures in neutron-rich nuclei [6]. It also contributes to the long-standing problem of quenching of the spinisopin response in nuclei [7], whose understanding is, e.g., a prerequisite for reliable calculations of nuclear matrix elements needed to determine absolute neutrino masses from a positive neutrinoless double β decay experiment [8].The strength distributions of the IVSM1 resonance in light and medium-mass (f p-shell) nuclei have been studied extensively using electromagnetic probes like electron scattering and nuclear resonance fluorescence (NRF). However, information in heavy nuclei is limited to a few magic nuclei [9][10][11][12][13], and it is questionable whether the full strength has been observed since NRF is typically applicable only up to the neutron threshold. Furthermore, there is no model-independent sum rule for the IVSM1 resonance like in the case of electric or Gamow-Teller (GT) giant resonances. One exception is 208 Pb, where additional information from neutron resonance studies above threshold is available [14] and observation of the complete M1 strength distribution is claimed [15].The J π = 1 + states forming the IVSM1 resonance in even-even nuclei can also be excited in small-angle inelastic proton scattering at energies of a few hundred MeV because angular momentum transfer ∆L = 0 is favored in these kinematics and the spin-isospin part dominates the proton-nucleus interaction leading to the population

E 2 decay strength of the M 1 scissors mode of The E2/M 1 multipole mixing ratio δ1→2 of the 1 + sc → 2 + 1 γ-ray decay in 156 Gd and hence the isovector E2 transition rate of the scissors mode of a well-deformed rotational nucleus has been measured for the first time. It has been obtained from the angular distribution of an artificial quasimonochromatic linearly polarized γ-ray beam of energy 3.07(6) MeV scattered inelastically off an isotopically highly-enriched 156 Gd target. The data yield first direct support for the deformation dependence of effective proton and neutron quadrupole boson charges in the framework of algebraic nuclear models. First evidence for a low-lying J π = 2 + member of the rotational band of states on top of the 1 + band head is obtained, too, indicating a significant signature splitting in the K = 1 scissors mode rotational band.Introduction. -Orbital out-of-phase oscillations of a coupled two-component many-body quantum system are generally called Scissors Modes (ScMs). A ScM has been discovered in deformed atomic nuclei [1]. It has later been identified in Bose-Einstein condensed gases [2,3] and is expected to occur in Fermi gases [4], in metallic clusters [5][6][7], and in deformed quantum dots [8]. ScMs are interesting quantum modes because their properties are sensitive to the restoring forces between the many-body subsystems. They inevitably break spherical symmetry and hence lead to a sequence of quantum states of the many-body system that form a rotational band.

The technique of self absorption has been applied for the first time to study the decay pattern of low-lying dipole states of 140 Ce. In particular, ground-state transition widths 0 and branching ratios 0/ to the ground state have been investigated in the energy domain of the pygmy dipole resonance. Relative self-absorption measurements allow for a model-independent determination of 0 . Without the need to perform a full spectroscopy of all decay channels, also the branching ratio to the ground state can be determined. The experiment on 140 Ce was conducted at the bremsstrahlung facility of the superconducting Darmstadt electron linear accelerator S-DALINAC. In total, the self-absorption and, thus, 0 were determined for 104 excited states of 140 Ce. The obtained results are presented and discussed with respect to simulations of γ cascades using the DICEBOX code.

Background: The quenching of spin-isospin modes in nuclei is an important field of research in nuclear structure. It has an impact on astrophysical reaction rates and on fundamental processes like neutrinoless double-β decay. Gamow-Teller (GT) and spinflip M 1 strengths are quenched. Concerning the latter, the J π = 1 + resonance in the doubly magic nucleus 48 Ca, dominated by a single transition, serves as a reference case.Purpose: The aim of the present work is a search for weak M 1 transitions in 48 Ca with a high-resolution (p, p ) experiment at 295 MeV and forward angles including 0 • and a comparison to results from a similar study using backward-angle electron scattering at low momentum transfers in order to estimate their contribution to the total B(M 1) strength in 48 Ca. Methods: The spin-M 1 cross sections of individual peaks in the spectra are deduced with a multipole decomposition analysis (MDA) and converted to reduced spin-M 1 transition strengths using the unit cross section method. For a comparison with electron scattering results, corresponding reduced B(M 1) transition strengths are extracted following the approach outlined in J. Birkhan et al., Phys. Rev. C 93, 041302(R) (2016). Results: In total, 30 peaks containing a M 1 contribution are found in the excitation energy region 7 − 13 MeV. The resulting B(M 1) strength distribution compares well to the electron scattering results considering different factors limiting the sensitivity in both experiments and the enhanced importance of mechanisms breaking the proportionality of nuclear cross sections and electromagnetic matrix elements for weak transitions as studied here. The total strength of 1.14(7) µ 2 N deduced assuming a non-quenched isoscalar part of the (p, p ) cross sections agrees with the (e, e ) result of 1.21(13) µ 2 N . A binwise analysis above 10 MeV provides an upper limit of 1.51(17) µ 2 N . Conclusions: The present results confirm the previous electron scattering work that weak transitions contribute about 25% to the total B(M 1) strength in 48 Ca and the quenching factors of GT and spin-M 1 strength are then comparable in f p-shell nuclei. Thus, the role of meson-exchange currents (MECs) seems to be negligible in 48 Ca, in contrast to sd-shell nuclei.