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

Abstract

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.

11 pages, 9 figures, revised analysis with oxygen contamination removed

References in corpus (3)

• Dipole polarizability of 120Sn and nuclear energy density functionals
• On the extraction of weak transition strengths via the (3He,t) reaction at 420 MeV
• Low-energy electric dipole response in 120Sn

Cited by in corpus (10)

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• Reference Database for Photon Strength Functions
• Electric and magnetic dipole modes in high-resolution inelastic proton scattering at $0^{\circ }$
• Test of the Brink-Axel hypothesis for the pygmy dipole resonance
• Electric and magnetic dipole strength in 112,114,116,118,120,124Sn
• Studies of the Giant Dipole Resonance in ${}^{27}$Al, ${}^{40}$Ca, ${}^{56}$Fe, ${}^{58}$Ni and ${}^{208}$Pb with high energy-resolution inelastic proton scattering under 0${}^{\circ }$
• Role of residual interaction in the relativistic description of M1 excitation
• Magnetic dipole transition in ${}^{48}$Ca
• Electric and magnetic dipole strength in ${}^{58}$Ni from forward-angle inelastic proton scattering
• Recent applications of the subtracted second RPA method