Determination of Optimal Energy Window for Measurement of Prompt Gammas from Proton Beam by Monte Carlo Simulations

The range of a proton beam in a patient can be determined by scanning the distribution of the prompt gammas emitted from the beam passage. However, this method suffers from a high level of background gammas, especially in the case of high energy proton beams. The present study determined the optimal energy window for selective measurement of the prompt gammas, effectively discriminating background gammas, for a prompt gamma scanning system. To that end, the energy spectra of the prompt and background gammas were calculated by transporting the protons and other secondary particles with MCNPX. A detailed analysis of these spectra revealed that the optimal energy window is 4-10 MeV. The application of the energy window to simulated and measured data confirmed that the range of a proton beam in a patient can be determined much more accurately by employing the optimal energy window.