Patent ID: 7026627

Claim:
A survey instrument comprising: (a) a moderator; (b) a neutron detector, wherein: (i) said moderator is substantially rectangular, and (ii) dimensions of said moderator and material of said moderator and position of said neutron detector within said moderator are selected so that said neutron detector exhibits a neutron response that is about the same for fast and thermal neutrons; (c) at least four gamma ray detectors wherein (i) each gamma ray detector comprises a scintillator and a light collecting device optically coupled to said scintillator, and (ii) said gamma ray detectors are disposed within said moderator symmetrically around said neutron detector and with each at a corner of said moderator; and (d) a processor in which gamma ray responses from said gamma ray detectors are combined to yield (i) intensity of gamma radiation impinging upon said survey instrument from a source, and (ii) an indication of azimuthal direction of said source with respect to a reference on said survey instrument, wherein; (e) said indication of azimuthal direction comprises a ratio of responses of pairs of said gamma ray detectors; (f) intensity of gamma radiation comprises a sum of said responses of said gamma ray detectors; (g) major axes of said gamma ray detectors are parallel to the major axes of said neutron detector; and (h) said ratio and said sum are used to determine said azimuthal direction.