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

Using a cryogenic micro-calorimeter originally developed for x-ray astrophysics, we have the performed the first direct investigation of the L/K capture ratio in 7 Be electron capture decay. The low background and excellent energy resolution clearly resolve capture from the individual shells, resulting in a direct determination of the L/K ratio.