It is described a logging tool 20 for underground formations surrounding a borehole 14, comprising an elongated body 21 along a major axis; a collar 22 disposed peripherally around said body 21 having a collar wall defined by an inner and an outer surface; a radiation emitting source 201 arranged to illuminate the earth formation 16 surrounding the borehole; at least one radiation detector 211 arranged to detect radiation reflected by the earth formation resulting from illumination by the source 201; at least one source collimation—window 202 and one detector collimation window 212 through which the earth formation is illuminated and radiation is detected; and characterized in that it further comprises at least one radiation shield 30 located between said inner collar surface and the outer surface of the tool, said radiation shield positioned so as to eliminate parasitic radiation that has not traversed the outer collar.