1. A system for detecting concealed threats in an object by simultaneously performing active and passive radiation detection, the system comprising: an X-ray source with a modulating device to produce a pencil beam of radiation for scanning the object, wherein said modulating device comprises a controller and a cylindrical structure with at least two helical slits extending across its outer surface and wherein said cylindrical structure is adapted to be rotated to produce the pencil beam that is blocked at regular intervals and wherein said system does not illuminate the object with radiation when the pencil beam is blocked and wherein said X-ray source remains switched on during both active and passive radiation detection; and a detector module for detecting both X-ray radiation backscattered by the object when scanned with the pencil beam of radiation and passive radiation emitted from threats within said object when the pencil beam of radiation is blocked, wherein said detector module comprises at least one detector adapted to detect both X-ray backscatter radiation and passive radiation, integrating electronics for generating X-ray backscatter data from the detected X-ray backscatter radiation, and spectroscopic electronics for generating passive radiation data from the detected passive radiation, both of said integrating electronics and spectroscopic electronics being gated to, and actuated by, the controller of the modulating device.