A source container for a radiation source includes a vessel having an external wall defining a space within which is located a shield formed from a radiation absorbing material and defining a cavity for receiving a radiation source, the shield including a window extending from the cavity through the radiation absorbing material, and at least two shutters, each shutter being movable between a closed position in which the shutter covers the window and an open position in which the shutter does not cover the window. The provision of two or more shutters provides a way to emit radiation of different intensities from the same source and container.