A Monte Carlo feasibility study on quantitative laser-driven proton radiography.

Laser-accelerated proton bunches with kinetic energies up to several tens of MeV and at repetition rates in the order of Hz are nowadays achievable at several research centres housing high-power laser system. The unique features of such ultra-short bunches are also arousing interest in the field of radiological and biomedical applications. For many of these applications, accurate positioning of the biological target is crucial, raising the need for on-site imaging. One convenient option is proton radiography, which can exploit the polyenergetic spectrum of laser-accelerated proton bunches. We present a Monte Carlo (MC) feasibility study to assess the applicability and potential of laser-driven proton radiography of millimetre to centimetre sized objects. Our radiography setup consists of a thin time-of-flight spectrometer operated in transmission prior to the object and a pixelated silicon detector for imaging. Proton bunches with kinetic energies up to 20MeV and up to 100MeV were investigated. The water equivalent thickness (WET) of the traversed material is calculated from the energy deposition inside an imaging detector, using an online generated calibration curve that is based on a MC generated look-up table and the reconstructed proton energy distribution. With a dose of 43mGy for a 1mm thin object imaged with protons up to 20MeV, the reconstructed WET of defined regions-of-interest was within 1.5% of the ground truth values. The spatial resolution, which strongly depends on the gap between object and imaging detector, was 2.5lpmm for a realistic distance of 5mm. Due to this relatively high imaging dose, our proposed setup for laser-driven proton radiography is currently limited to objects with low radio-sensitivity, but possibilities for further dose reduction are presented and discussed.