per second the vertical and horizontal accelerators are synchronized so that the x ray beams alternate every 2 5 ms the lower part of fig4 shows the 34 on time 34 structure of the detectors which are easily gated on and off with microsecond timing the time line for the vertical detectors is shown in line 92 that for the horizontal transmission detectors in line 94 and that for the backscatter detectors in line 96 the horizontal and vertical x ray detectors are gated on only during the times when the respective x rays are on in this way each of the detectors in insensitive to any radiation or detector noise that is present when their respective x ray beams are off the timing introduces no loss of signal for the transmission detectors and only an 0 2 loss of signal for the backscatter detectors which are off for 5 u03bcs every 2 5 ms referring again to fig1 the only cross talk between the detectors is between the two backscatter detectors 28 and 29 that cross talk is minimized by spatially separating the backscatter systems 12 and 14 a 15 feet separation which is quite practical when scanning a 40 foot container reduces the cross talk to insignificance since pencil beams 70 are relatively weak the placement of linacs 36 and 18 is a matter of design choice and the linacs may be placed for example in an intermediate position between the backscatter detectors as shown in fig1 the use of alternately gated transmission and scattering functionalities in accordance with the present invention is not limited to the embodiments heretofore described referring now to fig5 a further embodiment of the invention is shown schematically from the side wherein two independent.