Search for weak M1 transitions in 48^{48}Ca with inelastic proton scattering

The spinflip M1 resonance in the doubly magic nucleus $^{48}$Ca, dominated by a single transition, serves as a reference case for the quenching of spin-isospin modes in nuclei. The aim of the present work is a search for weak M1 transitions in $^{48}$Ca with a high-resolution (p,p') experiment at 295 MeV and forward angles including 0 degree 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(M1) strength. M1 cross sections of individual peaks in the spectra are deduced with a multipole decomposition analysis. The corresponding reduced B(M1) transition strengths are extracted following the approach outlined in J. Birkhan et al., Phys. Rev. C 93, 041302(R) (2016). In total, 29 peaks containing a M1 contribution are found in the excitation energy region 7 - 13 MeV. The resulting B(M1) 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 1.19(6) $\mu_N^2$ deduced assuming a non-quenched isoscalar part of the (p,p') cross sections agrees with the (e,e') result of 1.21(13) $\mu_N^2$. A binwise analysis above 10 MeV provides an upper limit of 1.62(23) $\mu_N^2$. The present results confirm that weak transitions contribute about 25% to the total B(M1) strength in $^{48}$Ca and the quenching factors of GT and spin-M1 strength are comparable in fp-shell nuclei. Thus, the role of of meson exchange currents seems to be neglible, in contrast to sd-shell nuclei.Comment: 11 pages, 9 figures, revised analysis with oxygen contamination remove

Electromagnetic M1 transition strengths from inelastic proton scattering: The cases of 48Ca and 208Pb

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 48Ca, 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 (\gamma,n) experiment. Good agreement with electromagnetic probes is observed in 208Pb suggesting the possibility to extract systematic information on the IVSM1 resonance in heavy nuclei.Comment: 6 pages, 4 figure

Electric dipole polarizability of 40^{40}Ca

The electric dipole strength distribution in $^{40}$Ca between 5 and 25 MeV has been determined at RCNP, Osaka, from proton inelastic scattering experiments at very forward angles. Combined with total photoabsorption data at higher excitation energy, this enables an extraction of the electric dipole polarizability $\alpha_\mathrm{D}$($^{40}$Ca) = 1.92(17) fm$^3$. Together with the measured $\alpha_{\rm D}$ in $^{48}$Ca, it provides a stringent test of modern theoretical approaches, including coupled cluster calculations with chiral effective field theory interactions and state-of-the art energy density functionals. The emerging picture is that for this medium-mass region dipole polarizabilities are well described theoretically, with important constraints for the neutron skin in $^{48}$Ca and related equation of state quantities.Comment: 6 pages, 3 figure