| 摘要 |
<p>A method and apparatus for detecting the mean atomic number of a mass of material, for example freight in a vehicle comprises: subjecting the mass to X-rays and determining the mean number NA passing therethrough; subjecting the mass to higher energy X-rays and determining the new mean number NB; and determining the mean atomic number of the mass from look-up tables against the computed ratio NA/NB. The mass (170), such as a railway waggon, may be scanned by two linear accelerators (162, 166) arranged perpendicular to each other, detector arrays (164, 168) being disposed respectively opposite the accelerators, so that a three dimensional image can be built up of the mass.</p> |
