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

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 spin-flip M1 strengths are quenched. Concerning the latter, the Jπ=1+ resonance in the doubly magic nucleus Ca48, dominated by a single transition, serves as a reference case. Purpose: The aim of the present work is to search for weak M1 transitions in Ca48 with a high-resolution (p,p′) experiment at 295 MeV and forward angles including 0° and a comparison with 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 in Ca48. Methods: The spin-M1 cross sections of individual peaks in the spectra are deduced with a multipole decomposition analysis (MDA) and converted to reduced spin-M1 transition strengths by using the unit cross-section method. For a comparison with electron-scattering results, corresponding reduced B(M1) transition strengths are extracted following the approach outlined in Birkhan et al. [Phys. Rev. C 93, 041302(R) (2016)2469-998510.1103/PhysRevC.93.041302]. Results: In total, 30 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 1.14(7) μN2 deduced assuming a nonquenched isoscalar part of the (p,p′) cross sections agrees with the (e,e′) result of 1.21(13) μN2. A bin-wise analysis above 10 MeV provides an upper limit of 1.51(17) μN2. Conclusions: The present results confirm the previous electron-scattering work that weak transitions contribute about 25% to the total B(M1) strength in Ca48 and the quenching factors of GT and spin-M1 strength are then comparable in fp-shell nuclei. Thus, the role of meson-exchange currents seems to be negligible in Ca48, in contrast to sd-shell nuclei.