Direct measurement of the L/K ratio in 7Be electron capture -: art. no. 012501

The ratio of L- to K-shell electron captures in light nuclei is particularly sensitive to electron overlap and exchange effects. Calculations of these effects in Be-7 disagree by more than 20%. We report a measurement of the L/K ratio in Be-7, using a cryogenic microcalorimeter which clearly separates Land K-shell captures. The obtained L/K ratio of 0.040(6) is less than half that of existing predictions for free Be-7. The discrepancy is likely due to in-medium effects distorting the L-shell electron orbitals.