The design, construction and calibration of a convenient irradiator is described that provides controlled exposure to a uniform external source of well-characterized alpha particles at a dose-rate of 0.99 cGy min-1. The use of a precis- ion photographic shutter allows the accurate delivery of doses as low as 0.01 mGy (1 mrad) to cultured cells. Special features also include a helium environment and reciprocal collimator that permits the use of collimators with different transmission factors to achieve differing dose-rates. A simple method is described to characterize the energy spectrum of alpha particles by use of particle range as measured by track etch.