The electron flow in a planar‐anode diode having an extended anode‐cathode gap operating on the HERMES III accelerator is characterized and compared with predictions of a computational model. The model combines a particle‐in‐cell code with Monte Carlo radiation transport. The comparisons confirm the model and show that the diode provides both a matched load and a versatile large‐area source of &ggr; rays for the study of nuclear radiation effects. Electrical and spatial parameters of the beam at the diode and the downstream radiation fields from a graphite target are presented as a function of the anode‐cathode gap.