Abstract: The electron applicator incorporates a stationary support tube and an insert tube. The support tube and the insert tube each have a first end portion and a second end portion. The first end portion of the insert tube slideably extends into the second end portion of the support tube. Electrons of high energy are transmitted through the assembly of both tubes. The surface of the insert tube is tapered inwardly toward the end thereof in the travelling direction of the high energy electrons. A device is provided for preventing the insert tube from sliding out of the support tube. A first engaging device engages the insert tube in a first working position. This device is such that upon a predetermined axial force which may inadvertently be exercised by a patient under treatment, the insert tube will slide into the support tube.An adapter for small field or cavetory treatment may be connected to the output end of the insert tube.