Closed-beam security screening devices must have a mechanism to ensure operator presence at the control area in a location that enables surveillance of the openings and doors of the control area during generation of radiation. The following apply:

1. (1) During an exposure or preset succession of exposures of 0.5 second or greater duration, the closed-beam radiation-producing device must have a mechanism to enable the operator to terminate the exposure or preset succession of exposures at any time.
2. (2) During an exposure or preset succession of exposures of less than 0.5 second duration, the closed-beam radiation-producing device must have a mechanism to allow completion of the exposure in progress but must enable the operator to prevent additional exposures.