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: Transitionmatrix 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 26Na
with 7 valence neutrons and 3 valence protons outside the doubly-magic 16O core is used as a test case.
Method: A radioactive beam experiment with 26Na (T1/2 = 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 γ 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 104Pd target.
Results: The observation of three γ -ray transitions in 26Na 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 26Na could be validated. Remaining discrepancies between
experimental data and theoretical predictions indicate the need for future experiments and possibly further