Development of laser-driven quasi-monoenergetic proton beam line for radiobiology

We report the development of a laser-driven quasi-monoenergetic proton beamline for the purpose of cell samples irradiation. Laser pulses are focused to an intensity of 5×1019W/cm2 onto a polyimide foil target of 7.5−μm7.5−μm thickness. The emitted proton spectrum is continuous up to a maximum energy of 4 MeV. Energy selection for transport to cancer cells is determined by four pairs of dipole magnets, which consists of a pair of permanent magnets generating a central magnetic field of 0.78 T. Protons are steered by the first magnetic field, and again by the second one, such that transmitted proton trajectories in the middle of the four dipoles are shifted laterally from the target normal axis by an energy-dependent displacement. Proton energy is selected by a pinhole in this middle plane and subsequently steered downstream by the other two dipole magnets. We have obtained 2.25 MeV proton beams with an energy spread of 0.66 MeV (FWHM) and single bunch duration of 20 ns. The dose given in a single proton bunch was 0.2 Gy, hence, the single bunch dose rate is estimated to be 107 Gy/s. At the 1 Hz repetition-rate cell samples were irradiated with successive proton bunches with integrated dose levels up to 8 Gy.