A well logging instrument includes an instrument housing to traverse a wellbore penetrating subsurface formations. An electrically operated energy source that emits ionizing radiation is disposed inside the housing. An insulating sleeve is disposed between the energy source and an interior wall of the housing. The insulating sleeve comprises a thin dielectric film arranged in a plurality of tightly fitting layers of dielectric material disposed adjacent to each other and successively. A thickness of each layer and a number of layers is selected to provide a dielectric strength sufficient to electrically insulate the energy source from the housing and to provide a selected resistance to dielectric failure resulting from the ionizing radiation.