Diamond detection

The invention concerns a method of detecting the presence of diamond in a body. In method the body is irradiated with a fast neutron beam modulated between first and second, distinct energy levels which are respectively resonant and non-resonant energy levels for diamond. Corresponding first and second absorption images are obtained for the body and from these images, a third absorption image is derived in which absorption effects attributable to presence of non-diamond material in the body are eliminated or at least reduced. The resulting image is then analysed for the presence of diamond in the body. The energy modulated fast neutron beam is produced by nuclear reaction between an energy modulated particle beam and a target. The energy modulated particle beam itself is produced by a procedure in which a particle beam is passed sequentially through first and second particle accelerators in series. Modulation is achieved by operating the second particle accelerator alternately in first and second modes to produce, from the particle beam delivered by the first particle accelerator, a particle beam which is modulated between relatively high and relatively low energy levels.