Electron build-up patterns were recorded with the mask centered on the double-slit. The
electron source’s intensity was reduced so that the electron detection rate in the pattern was
about 1 Hz. At this rate and kinetic energy, the average distance between consecutive electrons
was 2.3×106 m. This ensures that only one electron is present in the 1m long system at any
one time, thus eliminating electron–electron interactions. The electrostatic quadrupole lens was
set to zoom in on the central five diffraction orders. In figure 3 the build-up of the diffraction
pattern is shown. In figures 3(a)–(c), the electron hits appear to be completely random and only
after many electrons are accumulated can a pattern be discerned, figure 3(d). In figure 3(e) the
New Journal of Physics 15 (2013) 033018 (http://www.njp.org/)