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 cylindrical structure, with at least two helical slits extending across its outer surface and wherein said cylindrical structure is 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; a detector module for detecting both 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; and a controller to measure backscattered radiation only when the X-ray pencil beam is on and to measure only passive radiation when the X ray pencil beam is blocked.