Development of a reconstruction algorithm for material determination using 90-degree Compton scattering of Co-57 gamma rays

A modified reconstruction algorithm has been developed for an X-ray luggage inspection modality based on 90° Compton scattering imaging. Incident photon energy E ₀ of 122.1 keV was used in simulated experiments with MCNP for 5×5×5 samples and the 90° Compton scattered photon energy E ₁ is 98.6 keV. In the reconstruction calculation for the total attenuation _μt(E ₁) the least-squares solution was computed for the over-determined system equations of the side detector responses in the simulated experiments. In the reconstruction calculation for photon attenuations at energy E ₀, _μt(E ₀) and _μt(E ₀), instead of _μt(E ₀) = _μc(E ₀) approximation which does not hold true for the source energy, the semi-empirical formula _μt(E ₀) = a* _μt(E ₁) + b* _μc(E ₀) was used and the least-squares solution was computed for _μt(E ₀) and _μc(E ₀) system equations of the detector responses and the semi-empirical formula. Self attenuation correction factors in a simple form were applied in an iterative manner until the factors and reconstruction output are converged. The maximum errors of the reconstruction calculation for a sample with aluminum core and polyethylene matrix were 2.9% for _μt(E ₀), 1.0 % for _μt(E ₁), and 1.9 % for _μc(E ₀), showing good agreements with theoretical values. The maximum errors for the case with steel core and polyethylene matrix were 10.2 % for _μt(E ₀), 3.3% for _μt(E ₁), and 20.3% for _μc(E ₀).