A probe assembly is described which permits the use of an electron microscope to irradiate materials with low energy (50–150 keV) electrons. The electron beam current produced by the microscope can be monitored quickly and accurately with this unit so that the irradiation rate can be closely controlled. Although developed for use in the Hitachi HU‐11 electron microscope, the device can, with but minor modifications, be utilized in other microscopes as well.