Abstract: A method and electron linac system for production of radioisotopes is provided. The electron linac is an energy recovery linac (ERL) with an electron beam transmitted through a thin bremsstrahlung radiator. Isotopes are produced through bremsstrahlung photon interactions in an isotope production target that is spatially separated from the bremsstrahlung radiator. The electron beam does not pass through the isotope production target. The electron beam energy is recollected and reinjected into the linac accelerating structure. The reduction of material in the beam by removing the isotope production target and making the radiator thin is the essential aspect of the invention because large spreads in energy and transverse scattering angles caused by material in the beam preclude efficient energy recovery. The method described here can reduce the cost of energy to produce a quantity of radioisotope by more than a factor of 3 compared to a non-ERL bremsstrahlung method.