Patent Number: 
Section: claims

1. A method of constructing a radiation shield, comprising:determining an angular distribution of radiation propagating from a source of x-ray, wherein the source of x-ray comprises a target producing x-rays upon impingement of electrons;determining an angular function of thickness of a material in attenuating the radiation to a certain level of its original value, wherein the determining of an angular function of thickness of a material in attenuating the radiation comprises determining an angular function of tenth-value-layer of the material; andcalculating thicknesses of the material at a plurality of angular locations around the source and distances with respect to the source for attenuating the radiation to or less than a predetermined threshold value based on the angular distribution of radiation from the source and the angular function of tenth-value-layer of the material;constructing a radiation shield around the source based on the calculated thicknesses of the material, wherein radiation passing through the radiation shield is below the predetermined threshold value. 2. The method of claim 1 wherein the angular distribution of radiation propagating from the source comprises an angular distribution of radiation dose. 3. The method of claim 1 wherein the angular distribution of radiation propagating from the source is determined using a Monte Carlo method. 4. The method of claim 3 wherein the angular distribution of radiation propagating from the source comprises an angular distribution of radiation dose (D(θj)) determined according to following equation:                                          D            ⁡                          (                              θ                j                            )                                =                                                    1.6                ⨯                                  10                                      -                    8                                                                              R                2                                      ⁢                          Σ              0              Emax                        ⁢                          E              k                        ⁢                          S              ⁡                              (                                                      E                    k                                    ,                                      θ                    j                                                  )                                      ⁢                                                            μ                  en                                ⁡                                  (                                      E                    k                                    )                                            ρ                                      ,                            [        2        ]            whereS(Ek, θj) represents the probability to find a particle with energy Ek∈[Ek, Ek+dE] and angle θj∈[θj, θj+dθ], and the sum is over all the detected particles with energy below Emax;            μ      en        ⁡          (              E        k            )        ρ is the energy-mass absorption coefficient for water (in cm2/g);R is the distance from the source. 5. The method of claim 1 wherein the thicknesses of the material at a plurality of angular locations around the source (x(θj)) are calculated according to the following equation:x(θj)=TVL(θj)*[log10 D(θj)−log10(Ispec)].  [5]whereIspec represents the predetermined threshold value;TVL(θj) represents an angular function of tenth-value-layer of the material; andD(θj) represents the angular distribution of radiation dose generated and propagating from the source obtained according to the following equation:                                          D            ⁡                          (                              θ                j                            )                                =                                                    1.6                ⨯                                  10                                      -                    8                                                                              R                2                                      ⁢                          Σ              0              Emax                        ⁢                          E              k                        ⁢                          S              ⁡                              (                                                      E                    k                                    ,                                      θ                    j                                                  )                                      ⁢                                                            μ                  en                                ⁡                                  (                                      E                    k                                    )                                            ρ                                      ,                            [        2        ]            whereS(Ek, θj) represents the probability to find a particle with energy Ek∈[Ek, Ek+dE] and angle θj∈[θj, θj+dθ], and the sum is over all the detected particles with energy below Emax;            μ      en        ⁡          (              E        k            )        ρ is the energy-mass absorption coefficient for water (in cm2/g);R is the distance from the source. 6. The method of claim 5 wherein the angular function of tenth-value-layer of the material (TVL(θj)) comprises an average of the angular function of a first, second, third, and fourth TVL of the material. 7. The method of claim 1 wherein the source comprises a target configured to produce photons upon impingement of electrons. 8. The method of claim 1 wherein a portion of the shield comprises components including a primary collimator, secondary, and tertiary collimator. 9. The method of claim 1 further comprising determining an angular distribution of radiation outside the constructed shield, modifying the calculated thickness of the material based on the angular distribution of radiation outside the constructed shield, and constructing the shield based on the modified calculated thickness using an iterative procedure. 10. The method of claim 9 wherein the calculated thicknesses of the material is modified according to the following equation:{tilde over (x)}(θj)=L(θj)*[log10 D(θj)−log10(Ispec)],  [6]where:{tilde over (x)}(θj) represents the modified calculated thicknesses;L(θj) represents a modified angular function of tenth-value-layer of the material;D(θj) represents the angular distribution of radiation generated and propagating from the source; andIspec represents the threshold value. 11. The method of claim 10 wherein the modified angular function of tenth-value-layer of the material is obtained according to the following equation:                                        ⁢                      (                          θ              j                        )                          =                                            x              ⁡                              (                                  θ                  j                                )                                                                                      log                  10                                ⁢                                  D                  ⁡                                      (                                          θ                      j                                        )                                                              -                                                log                  10                                ⁢                                                      D                    coll                                    ⁡                                      (                                          θ                      j                                        )                                                                                .                                    [        7        ]            wherein:x(θj) is obtained according to the following equation:x(θj)=TVL(θj)*[log10 D(θj)−log10(Ispec)].  [5]TVL(θj) represents an angular function of tenth-value-layer of the material, andDcoll(θj) represents an angular distribution of radiation outside the constructed shield. 12. A radiation apparatus, comprising:a target producing radiation of x-ray upon impingement by an electron beam, wherein the electron beam impinging the target has a central axis in a forward direction; anda shield around the target to confine radiation produced by the target to certain directions and reduce radiation passing through the shield, wherein the shield has a generally ovate shape in cross-section viewed in a direction perpendicular to the central axis, provides increased shielding at locations that have decreased angles with respect to the forward direction to allow radiation passing through the shield to be substantially uniform across entire angular directions other than the forward direction and having intensities no greater than a threshold value. 13. The radiation apparatus of claim 12, wherein the threshold value is 400 particles per million particles (ppm) or less. 14. The radiation apparatus of claim 12, wherein the threshold value is 100 particles per million particles (ppm) or less.