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Search for weak M1 transitions in ⁴⁸\mathrmCa with inelastic proton scattering

Mathy, M. and Birkhan, J. and Matsubara, H. and von Neumann-Cosel, P. and Pietralla, N. and Ponomarev, V. Yu. and Richter, A. and Tamii, A. (2017):
Search for weak M1 transitions in ⁴⁸\mathrmCa with inelastic proton scattering.
In: Phys. Rev. C, American Physical Society, pp. 054316, 95, DOI: 10.1103/PhysRevC.95.054316,
[Online-Edition: https://link.aps.org/doi/10.1103/PhysRevC.95.054316],
[Article]

Abstract

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 48Ca, 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 48Ca 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 48Ca.

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)].

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) μ2N deduced assuming a nonquenched isoscalar part of the (p,p′) cross sections agrees with the (e,e′) result of 1.21(13) μ2N. A bin-wise analysis above 10 MeV provides an upper limit of 1.51(17) μ2N.

Conclusions: The present results confirm the previous electron-scattering work that weak transitions contribute about 25% to the total B(M1) strength in 48Ca 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 48Ca, in contrast to sd-shell nuclei.

Item Type: Article
Erschienen: 2017
Creators: Mathy, M. and Birkhan, J. and Matsubara, H. and von Neumann-Cosel, P. and Pietralla, N. and Ponomarev, V. Yu. and Richter, A. and Tamii, A.
Title: Search for weak M1 transitions in ⁴⁸\mathrmCa with inelastic proton scattering
Language: German
Abstract:

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 48Ca, 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 48Ca 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 48Ca.

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)].

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) μ2N deduced assuming a nonquenched isoscalar part of the (p,p′) cross sections agrees with the (e,e′) result of 1.21(13) μ2N. A bin-wise analysis above 10 MeV provides an upper limit of 1.51(17) μ2N.

Conclusions: The present results confirm the previous electron-scattering work that weak transitions contribute about 25% to the total B(M1) strength in 48Ca 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 48Ca, in contrast to sd-shell nuclei.

Journal or Publication Title: Phys. Rev. C
Volume: 95
Publisher: American Physical Society
Divisions: 05 Department of Physics > Institute of Nuclear Physics
05 Department of Physics
Date Deposited: 31 May 2017 07:24
DOI: 10.1103/PhysRevC.95.054316
Official URL: https://link.aps.org/doi/10.1103/PhysRevC.95.054316
Projects: Teilprojekt A01 des SFB 1245, Teilprojekt A07 des SFB 1245, Teilprojekt B02 des SFB 1245
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