Patent Number: 
Section: claims

1. A downhole tool, comprising:a photon source;a photon detector having a plurality of discrete detector pixels in a cylindrical row and column arrangement, each detector pixel coupled to an adjacent detector pixel within a given cylindrical row;a radial collimator having at least two concentric frustoconical collimators circumferentially arranged about the photon detector, and at least two azimuthal collimators radially arranged with respect to the photon detector, wherein one of the azimuthal collimators is on a first side of a detector pixel and a second azimuthal collimator is on a second side of the detector pixel opposite the first side. 2. The downhole tool of claim 1, wherein the radial collimator further comprises a plurality of concentric frustoconical radial collimators that extend radially from the photon detector at a predefined angle, each adjacent pair of the frustoconical radial collimators being separated at a predetermined distance to create a photon pathway that corresponds to a row of detector pixels. 3. The downhole tool of claim 2, further comprising a plurality of blade-shaped azimuthal collimators arranged in the longitudinal direction of the photon detector, wherein the blade-shaped azimuthal collimators extend radially from the photon detector to subtend an angle formed by a pair of the azimuthal collimators corresponding to a photon detector column. 4. The downhole tool of claim 3, wherein the radial collimator is coupled to the photon detector so that each photon detector pixel is correspondent to a region defined—by—a pair of the azimuthal collimators and a pair of the concentric frustoconical radial collimators. 5. The downhole tool of claim 4, wherein the radial collimator is made of a high-Z material. 6. The downhole tool of claim 4, wherein the radial collimator is made of lead, tantalum or tungsten. 7. The downhole tool of claim 4, wherein the photon detector comprises a gamma-ray scintillator or a solid state detector. 8. A collimator, comprising:a plurality of concentric frustoconical radial collimators that extend radially from the longitudinal axis of the radial collimators at a predefined angle, wherein adjacent pairs of the frustoconical radial collimators are separated by a predetermined distance; anda plurality of blade-shaped azimuthal collimators arranged along the longitudinal axis of the concentric frustoconical radial collimators, the blade-shaped azimuthal collimators extending radially outward from the longitudinal axis of the frustoconical radial collimators with a subtended angle of 2π/M, where M is the number of azimuthal collimators, formed by each adjacent pair of the blade-shaped azimuthal collimators;wherein a photon pathway is created by adjacent azimuthal collimators and adjacent frustoconical radial collimators, each photon pathway corresponding to one detector pixel in a plurality of discrete detector pixels. 9. The collimator of claim 8, wherein the collimator is made of a high-Z material. 10. The collimator of claim 9, wherein the collimator is made of lead, tantalum or tungsten. 11. A system for detecting defects in the cement surrounding a wellbore, the system comprising:an apparatus for lowering a tool into the wellbore and receiving signals from the tool indicating a photon count, the tool further comprising:a photon source;a photon detector having a plurality of detector pixels in a cylindrical row and column arrangement, each detector pixel coupled to an adjacent detector pixel within a given cylindrical row;a radial collimator having at least two concentric frustoconical collimators circumferentially arranged about the photon detector, and at least two azimuthal collimators radially arranged with respect to the photon detector, wherein one of the azimuthal collimators is on a first side of a detector pixel and a second azimuthal collimator is on a second side of the detector pixel opposite the first side. 12. The system of claim 11, wherein the radial collimator further comprises a plurality of concentric frustoconical radial collimators that extend radially from the photon detector at a predefined angle, each adjacent pair of the frustoconical radial collimators being separated at a predetermined distance to create a photon pathway that corresponds to a row of detector pixels. 13. The system of claim 12, further comprising a plurality of blade-shaped azimuthal collimators arranged in the longitudinal direction of the photon detector, wherein the blade-shaped azimuthal collimators extend radially from the photon detector to subtend an angle formed by a pair of the azimuthal collimators corresponding to a photon detector column. 14. The system of claim 13, wherein the radial collimator is coupled to the photon detector so that each photon detector pixel is correspondent to a region defined—by—a pair of the azimuthal collimators and a pair of the concentric frustoconical radial collimators. 15. The system of claim 14, wherein the radial collimator is made of a high-Z material. 16. The system of claim 14, wherein the radial collimator is made of lead, tantalum or tungsten. 17. The system of claim 14, wherein the photon detector comprises a gamma-ray scintillator or a solid state detector.