A collimator, a radiological imaging apparatus and a nuclear medicine diagnosis apparatus which are able to improve the sensitivity are provided. The absorbent assembly includes an absorbent core that may contain superabsorbent particles, which may be contained inside pockets. The outer peripheries of the rollers and the roller guideways make angular contact with each other. The apparatus according to the invention is furthermore very simple in construction and requires few components. It is characteristic that the particles have one or more recesses or throughgoing holes, or that the particles provided with an unbroken surface layer are hollow.