1. An apparatus, comprising:an accelerator guide comprising an electron source at a first end, a target at a second end, and a plurality of accelerating cavities coupled in series along a longitudinal axis between the first end and the second end, the accelerator guide having a contour as viewed in the longitudinal axis;
a shielding structure enclosing at least the electron source and the plurality of accelerating cavities of the accelerator guide, the shielding structure having an inner wall surface defining a contour as viewed in the longitudinal axis generally conformal to the contour of the accelerator guide.