| 摘要 |
<p>An apparatus 10 for scanning a structure (pipe) 100 to detect density differences comprises a source of gamma radiation 20, a plurality of first detectors 30 arranged to detect radiation along a plurality of first paths 32, a plurality of second detectors 40 arranged to detect radiation along a plurality of second paths 42, each path being substantially aligned with a respective radius of a circle centred on the source, and an angular separation between neighbouring first paths is S. The first and second detectors and the source are arranged for rotation in a fixed relationship with respect to each other, about an axis 60 located between the source and detectors and substantially orthogonal to said circle. There are 2N+1 detectors, the first paths are located on a first side of a plane containing the source and said axis of rotation and the positions DL of the first detectors are DL=nS, where n=0,1,2...N, and the second paths are located on a second side of said plane and the positions DR of the second detectors are DR=(n+f)S n=0,1,2...N-1, and where 0<f<1.</p> |
