A stepper motor.
DC motors always have some sort of encoder or tachometer that feeds back position information to a controller that commands the DC motors position. Encoders all contain semiconductors of one sort or another and any semiconductors that have been place close to the beam have always failed within a couple of weeks. A tachometer that uses magnetic elements to produce pulses could work- a Hall sensor would be trouble.
Stepper motors on the other hand are open loop and rely on precision manufacturing to obtain their accuracy per step and have no feedback for position. Hybrid stepper motors also contain magnets but at this point magnetic failure due to radiation has not been a problem. In a high radiation environment, if your experiment can work with the stepper motor error specifications it is best to go with the stepper.