a) 3DEES on its GSE with the Sr-90 source holder aligned in front of it. The distance between the holder’s collimator exit hole and the 3DEES aperture was set to ~15 mm. The GSE allows for precise rotation of the aperture under test by a given angle with respect to a rotation axis passing through the center of the selected aperture. b) Cut view through the source holder, highlighting the elements (absorbing materials) that affect the electron spectrum.