52 research outputs found
Search for weak M1 transitions in Ca with inelastic proton scattering
The spinflip M1 resonance in the doubly magic nucleus 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 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)
deduced assuming a non-quenched isoscalar part of the (p,p') cross sections
agrees with the (e,e') result of 1.21(13) . A binwise analysis above
10 MeV provides an upper limit of 1.62(23) . The present results
confirm that weak transitions contribute about 25% to the total B(M1) strength
in 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
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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 Ca
The electric dipole strength distribution in 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 (Ca) = 1.92(17) fm. Together with
the measured in 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 Ca and related equation of state quantities.Comment: 6 pages, 3 figure
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