C. Loelius | Searcharxiv

arXiv2025

Extraction of the non-spin- and spin-transfer isovector responses via the $^{12}\mathrm{C}(^{10}\mathrm{Be},{}^{10}\mathrm{B}+\gamma)^{12}\mathrm{B}$ reaction

The isovector response in $^{12}$B was investigated via the $^{12}$C($^{10}\mathrm{Be}$,$^{10}\mathrm{B}$+$\gamma$)$^{12}$B$^\ast$ reaction at $100 A \, \mathrm{MeV}$. By utilizing the $\gamma$-decay properties of the 1.74 MeV $0^{+}$ and 0.718 MeV $1^{+}$ states in $^{10}\mathrm{B}$, the separate extraction of the non-spin-transfer ($\Delta S=0$) and spin-transfer ($\Delta S=1$) isovector responses up to an excitation energy of 50 MeV in $^{12}$B in a single measurement is demonstrated. The experimental setup employed the S800 spectrometer to detect and analyze the $^{10}\mathrm{B}$ ejectiles and the Gamma-Ray Energy Tracking In-beam Nuclear Array (GRETINA) for obtaining the Doppler-reconstructed spectrum for $\gamma$-rays emitted in-flight by $^{10}\mathrm{B}$. A $^{12}$C foil was placed at the pivot point of the spectrograph. The $^{12}$B reaction product was not detected. Contributions from transitions associated with the transfer of different units of angular momentum in the non-spin- and spin-transfer responses were analyzed using a multipole decomposition analysis. The extracted non-spin-dipole ($\Delta S=0$, $\Delta L=1$) and spin-dipole ($\Delta S=1$, $\Delta L=1$) responses were found to be consistent with available data from other charge-exchange probes, validating the non-spin- and spin-transfer filters used. While statistical uncertainties and experimental resolutions were relatively large due to the modest intensity of the $^{10}\mathrm{Be}$ secondary beam, the results show that, with the much higher intensities that will be available at new rare-isotope beam facilities, the ($^{10}\mathrm{Be}$,$^{10}\mathrm{B}$+$\gamma$) reaction and its $\Delta T_{z}=-1$ partner, the ($^{10}$C,$^{10}\mathrm{B}$+$\gamma$) reaction, are powerful tools for elucidating the isovector non-spin- and spin-transfer responses in nuclei.

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arXiv2019

Electromagnetic properties of $^{21}$O for benchmarking nuclear Hamiltonians

The structure of exotic nuclei provides valuable tests for state-of-the-art nuclear theory. In particular electromagnetic transition rates are more sensitive to aspects of nuclear forces and many-body physics than excitation energies alone. We report the first lifetime measurement of excited states in $^{21}$O, finding $\tau_{1/2^+}=420^{+35}_{-32}\text{(stat)}^{+34}_{-12}\text{(sys)}$\,ps. This result together with the deduced level scheme and branching ratio of several $\gamma$-ray decays are compared to both phenomenological shell-model and ab initio calculations based on two- and three-nucleon forces derived from chiral effective field theory. We find that the electric quadrupole reduced transition probability of $\rm B(E2;1/2^+ \rightarrow 5/2^+_{g.s.}) = 0.71^{+0.07\ +0.02}_{-0.06\ -0.06}$~e$^2$fm$^4$, derived from the lifetime of the $1/2^+$ state, is smaller than the phenomenological result where standard effective charges are employed, suggesting the need for modifications of the latter in neutron-rich oxygen isotopes. We compare this result to both large-space and valence-space ab initio calculations, and by using multiple input interactions we explore the sensitivity of this observable to underlying details of nuclear forces.

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