  A study of 3D pixel sensors of cell size 50 {\mu}m x 50 {\mu}m fabricated at
IMB-CNM using double-sided n-on-p 3D technology is presented. Sensors were
bump-bonded to the ROC4SENS readout chip. For the first time in such a
small-pitch hybrid assembly, the sensor response to ionizing radiation in a
test beam of 5.6 GeV electrons was studied. Results for non-irradiated sensors
are presented, including efficiency, charge sharing, signal-to-noise, and
resolution for different incidence angles.
