pCT calorimeter energy calibration and verification
Abstract
Purpose: The experimental procedure followed to calibrate the direct measurement of the proton stopping power relative to water (SPR) using a proton Computed Tomography (pCT) apparatus is described. Measurements performed on water phantoms are used to validate the tuning of the experimental system.Methods: The pCT system is made of a silicon microstrip tracker followed by a segmented YAG:Ce scintillating crystals calorimeter. To perform the energy calibration, the apparatus has been exposed to proton beams of ten different known energies ranging from 83 to 211 MeV measuring, for each proton, the trajectory and the signal generated by the particle when entering the calorimeter. As the first step of the calibration procedure, the tracker is aligned minimizing the proton trajectories residuals, then the calorimeter signal is converted to energy fitting the ten experimental points to polynomial functions. Using the tracker information, a position dependent calibration procedure has been developed to keep the energy response uniform through the segmented calorimeter surface. Finally, to verify the calibration procedure, demineralized water phantoms with different diameters have been imaged with the pCT system, reconstructing the three-dimensional SPR maps of the water volumes and checking for the presence of possible artifacts.
Results: The measured energy resolution of the pCT YAG:Ce calorimeter, after the calibration, resulted to be at an incoming proton energy of 196 MeV. Once the beam energy spread, mainly due to the tantalum scattering plate, is taken into account the intrinsic calorimeter resolution at 196 MeV is below 0.8% (figure 1). The averaged values of the water SPR in the phantoms have been measured to be 0.994±0.003. The SPR images non-uniformity within the water phantoms are below 1.3%.
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