6533b826fe1ef96bd1284f3a

RESEARCH PRODUCT

Testing refined shell-model interactions in thesdshell: Coulomb excitation ofNa26

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description

Background: Shell-model calculations crucially depend on the residual interaction used to approximate the nucleon-nucleon interaction. Recent improvements to the empirical universal $sd$ interaction (USD) describing nuclei within the $sd$ shell yielded two new interactions---USDA and USDB---causing changes in the theoretical description of these nuclei.Purpose: Transition matrix elements between excited states provide an excellent probe to examine the underlying shell structure. These observables provide a stringent test for the newly derived interactions. The nucleus $^{26}\mathrm{Na}$ with 7 valence neutrons and 3 valence protons outside the doubly-magic ${}^{16}\mathrm{O}$ core is used as a test case.Method: A radioactive beam experiment with $^{26}\mathrm{Na}$ (${T}_{1/2}=1,07s$) was performed at the REX-ISOLDE facility (CERN) using Coulomb excitation at safe energies below the Coulomb barrier. Scattered particles were detected with an annular Si detector in coincidence with $\ensuremath{\gamma}$ rays observed by the segmented MINIBALL array. Coulomb excitation cross sections of the beam have been obtained by normalization to the well known Coulomb excitation cross sections of the $^{104}\mathrm{Pd}$ target.Results: The observation of three $\ensuremath{\gamma}$-ray transitions in $^{26}\mathrm{Na}$ together with available spectroscopic data allows us to determine $E2$- and $M1$-transitional matrix elements. Results are compared to theoretical predictions.Conclusion: The improved theoretical description of $^{26}\mathrm{Na}$ could be validated. Remaining discrepancies between experimental data and theoretical predictions indicate the need for future experiments and possibly further theoretical improvements.