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Laser-accelerated proton bunches with kinetic energies of several tens of MeV can be produced with 100-TW to PW laser pulses. The increasing availability of high-power laser systems operating at repetition rates in the order of Hz, along with the unique features of laser-accelerated proton bunches and the meanwhile repeatably achievable kinetic energies are arousing interest in the field of radiological and biomedical applications. Accurate positioning and characterization of the biological specimen is crucial for such applications, raising the need for on-site imaging. Proton radiography could be promising, as it can make use of the intrinsic energy spread of laser-accelerated proton bunches [1]. The proton energy distribution obtained in the target-normal-sheath acceleration (TNSA) regime results in a monotonically decreasing depth-dose distribution in matter, resembling the attenuation of X-rays. With knowledge of the energy distribution of individual bunches and a pixelated semiconductor detector downstream of the imaged object, quantitative information on the object in terms of water-equivalent thickness (WET) can be obtained online.
We present a detailed Monte Carlo (MC) study assessing the feasibility of laser-driven proton radiography [2] for object sizes relevant for small-animal experiments. The simulation setup (fig. 1, left) consists of a thin and coarsely pixelated transmission time-of-flight spectrometer prior to the object and a model of a CMOS sensor for imaging. Two scenarios with TNSA-like proton bunches of energies up to 20 MeV and up to 100 MeV were studied. Phantoms of different geometry and material with a thickness of 0.3 mm to 23 mm were placed at distances from 1 mm to 20 mm from the detector. The bunch energy distribution was determined from the time-dependent energy deposition in the spectrometer. Together with a MC generated look-up table, it was used to create a bunch-specific conversion from energy deposition per pixel of the imaging detector to WET. Requirements on the spectrometer performance were examined by variation of its temporal and spatial resolution.
Exemplary, the reconstructed WET distribution of a 2 cm thick phantom is shown in fig. 1 (right). With an imaging dose of a few cGy, corresponding to a proton fluence in the detector plane of around 106 cm-2, reconstructed WET values are in good agreement with ground truth (differences < 1.5%). Due to Coulomb scattering in the object, spatial resolution strongly depends on the distance between object and detector. For a gap of 5 mm, which is realistic for an experimental setup, a spatial resolution of 2.5 lp/mm and 1.9 lp/mm was found for the low and the high-energy scenario, respectively. A temporal resolution of the spectrometer of 0.2 ns or better is required for a drift space of 1 m and 3 m for the two scenarios, respectively.
We have shown the feasibility and the potential of a setup for online and quantitative laser-driven proton radiography. The relatively high imaging dose may be reduced by tailoring the initial proton energy distribution. Experimental radiographies are foreseen at the Centre for Advanced Laser Applications (CALA), in Garching near Munich, Germany.
[1] J. Schreiber, et al., “Hands-on” laser-driven ion acceleration: A primer for laser-driven source development and potential applications, Rev Sci Instrum, 87, (2016), 071101.
[2] M. Würl, et al., A Monte Carlo feasibility study on quantitative laser-driven proton radiography, Z Med Phys, In Press, (2020).
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