Design study of the absorber detector of a compton camera for on-line control in ion beam therapy

In order to take advantage of the assets of ion irradiation, the position of the Bragg peak has to be monitored accurately. Here we propose a technique relying on the detection of the prompt γ with a Compton camera. We reconstruct analytically the prompt-γ emission points by intersecting the ion trajectories given by a beam hodoscope and the Compton cones reconstructed with the camera. For the Compton camera, we investigate the use of a stack of 2 mm thick silicon strip detectors as a scatterer and one scintillator as an absorber. The response of the Compton camera to a photon point source with a realistic spectrum (corresponding to the prompt-γ spectrum emitted during the carbon irradiation of a water phantom) is studied. The goal of this study is to tune the design of the absorber detector. First the absorber detector dimensions are optimized. Then we compare the performances of three scintillators : LYSO, NaI and LaBr₃. Finally, we study the influence of the spatial and energy resolutions of the absorber detector.