X-ray inspection using spatially and spectrally tailored beams

#The present invention relates to systems and methods for inspecting objects and, more particularly, the invention relates to systems and methods for inspecting objects with radiation beams tailored to provide optimized cross-sectional profiles.

#X-ray inspection systems, such as those used to characterize the contents of concealing enclosures such as baggage or cargo containers, typically employ an irradiating beam of specified cross-section that is swept relative to an object while portions of the beam that are either transmitted through the object or scattered by it are detected. Cross-sectional shapes of beams typically employed include fan beams, otherwise referred to as ‘fan-shaped’ beams, and pencil beams, where the characteristic dimension of the beam governs the spatial resolution of the system. The irradiating beam is characterized by an energy distribution of x-rays that is governed by the nature of the x-ray source and is invariant across the entire cross-section of the beam.

#For a specified set of beam characteristics, the total photon flux through an object scales with the area of the beam. Thus, higher resolution, achieved by virtue of a tighter beam, is achieved at the expense of photon flux. Therefore, the thickness of the object through which radiation can be detected with a useful signal-to-noise ratio is also limited unless other parameters are changed. In the prior art, this trade-off is part of the design of the system that is performed prior to its operation in the field.

#The foregoing features of the invention will be more readily understood by reference to the following detailed description, taken with reference to the accompanying drawings, in which:

#FIG. 1 is a schematic cross-sectional view of an exemplary embodiment of an inspection system using a collimated pencil beam in accordance with the invention; and

#FIG. 2 is a ray trace of x-rays collimated by an aperture in accordance with the invention.

#As discussed in U.S. patent application Ser. No. 09/502,093, the design of an x-ray inspection system to examine heterogeneous cargo requires joint consideration of conflicting requirements for penetration, radiation dosage, and sensitivity. For example, the high-energy x-ray components of a radiation beam from a 3 MeV x-ray accelerator penetrate approximately 3 times farther through iron than do the high-energy x-ray components of a radiation beam from a 450 keV x-ray accelerator. However, radiation dosage, that is, the integrated radiated energy, increases as the electron energy from an x-ray accelerator is raised. For example, the radiation dose from a 3 MeV x-ray accelerator operating at 100 microamps is about 5 times greater than the radiation dose from a 450 keV x-ray accelerator operating at 10 mA. In addition, the high-energy x-ray components of a radiation beam are not as “sensitive” for distinguishing among materials as the low-energy x-ray components of a radiation beam, in the sense in which sensitivity is the detected change in transmitted countrate per unit thickness of a specified material.

#One measure of the ability of an x-ray inspection system to detect “contraband” is the minimum thickness of material that can be detected. In determining that minimum thickness, consider a mono-energetic beam of photons penetrating an object having thickness T. The object has a linear absorption co-efficient λ(E,Z), which is a function of the material and the energy of the photons that penetrate the object. If NO(E) is the number of x-ray photons incident on the object, then N(E), the number of x-ray photons emerging from the object, is given by: N(E)=NO(E)e−λT  (Eqn. 1) To determine the minimum thickness, ΔT, that can be detected, differentiate Eqn. 1: Δ ⁢ ⁢ N Δ ⁢ ⁢ T = - N O ⁢ λⅇ - λ ⁢ ⁢ T ( Eqn . ⁢ 2 ) The relative change in count rate per thickness is: Δ ⁢ ⁢ N N ⁢ ⁢ Δ ⁢ ⁢ T = - λ ( Eqn . ⁢ 3 ) The minimal detectable signal may be taken to be 3 times the standard deviation of the signal (or 6 times the standard deviation, with Eqns. 4 and 5 changed mutatis mutandis): ΔN=3√{square root over (N)}  (Eqn. 4) Substituting Eqn. 4 into Eqn. 3 yields the minimum thickness that can be detected for a given number of detected counts:  Δ ⁢ ⁢ T  = 3 λ ⁢ N ( Eqn . ⁢ 5 ) Thus, the minimum detectable thickness, for a given pixel, varies inversely with the square root of the counts in the detector and inversely with the linear attenuation coefficient λ.

#The linear attenuation coefficient for iron is 8.8 cm−1 at 60 keV, the energy of the strong, characteristic x-ray beams from a tungsten anode. As the energy of the photon increases, λ(Fe) drops rapidly, for example, at 200 keV, λ(Fe) is 1.1 cm−1 and at 1 MeV, λ(Fe) is 0.47 cm−1. Thus, for the same counts in the detector, a 60 keV photon beam can detect 1/20th the thickness that can be detected by a 1 MeV photon, all other parameters being equal.