Monte Carlo analysis of megavoltage x-ray interaction-induced signal and noise in detectors for container inspection

In a scanner system, a scintillation crystal is the first stage in the cascaded imaging chain transferring x-ray interaction information in cargo to be investigated to the final user who investigates x-ray images. On the other hand, the signal and noise is irreversibly transferred through the cascaded imaging chain. Therefore, the imaging performance of the first stage scintillator mainly governs the ultimate imaging performance of the system. In MV imaging, it is generally accepted that high-density scintillators, because of their sufficient optical yield, and low optical self-absorption and scattering coefficients. We chose the CdWO₄ as the scintillation material. CdWO₄ has a high density (7.9 g/cm³), high atomic number (64), resistance to radiation, high optical yield, and low optical self-absorption. For the given MV spectrum, the improvement of QE from a detector with a thickness of 10 mm to 30 mm is 27% whereas the improvement from 30 mm to 50 mm is only 7%. On the other hand, the Swank noise is almost independent of the detector thickness. Consequently, the improvement of DQE from a detector with a thickness of 10 mm to 30 mm is 46% whereas the improvement from 30 mm to 50 mm is only 11%. In conclusion, the detector thickness of 30 mm would be the best for x-ray interaction-induced signal and noise performance as well as cost