Secondary electron transmission measurements are used to examine the electrondiffusion and thermalization processes in B-doped nanocrystalline diamondmembranes of thickness 2.3 μm and 650 nm. Specifically, transmittedenergy spectra are measured following impact by an electron beam that penetratesdeeper in the membrane as the beam energy Eo increases. Afully-thermalized emission peak is observed for Eo ≤ 16keV and Eo ≤ 7 keV from the 2.3-μm-thick and650-nm-thick films, respectively, with a small high-energy tail beginning toemerge at higher Eo. These measurements are analyzed using MonteCarlo simulations that generate constant-energy contour lines (down to 0.05Eo) as a function of beam depth into the film. From thesesimulations, we deduce that secondary electrons have a minimum thermalizationlength of ∼ 630 nm and ∼ 260 nm in the2.3-μm-thick and 650-nm-thick films, respectively. Further insightinto the secondary-electron transport behavior is gained from the analysis, andthis understanding is applied to the design of a transmission electron-currentamplifier device.