the present invention may relate to methods and systems for inspecting objects by means of penetrating radiation that use multiple x ray sources which may be individually activated as described in u s patent application publication no 2007 0258562 issued as u s pat no 7 505 562 hereby incorporated by reference herein in its entirety x ray sources may be based on field emission cathodes offering advantages in both spatial and temporal resolution when compared with thermionic sources because field emission of electrons is produced by a high electric field no heating is necessary whence such electron emitters are commonly referred to as cold cathodes the electron beams emitted by such devices may have low divergence and thus provide ease of focusing moreover the virtually instantaneous response of the source offers time gating capabilities comparable with the time resolution of the control circuit and may be as fast as nanoseconds using current technology zhang et al a multi beam x ray imaging system based on carbon nanotube field emitters in medical imaging 2006 proceedings of spie vol 6142 mar 2 2006 reported the fabrication by xintek inc of research triangle park n c of a linear array of 5 x ray sources each with a focal spot between 200 and 300 u03bcm based on the use of carbon nanotube cnt electrodes electron currents in the range of 0 1 1 ma were reported at an accelerating voltage of 40 60 kvp the lifetime of the cold cathode was estimated to exceed 2000 hours for an accelerating voltage of 200 kv a beam current of 13 ma.