Radiation protection modelling for 2.5 Petawatt-laser production of ultrashort x-ray, proton and ion bunches: Monte Carlo model of the Munich CALA facility

The 'Centre for Advanced Laser Applications' (CALA) is a new research institute for laser-based acceleration of electron beams for brilliant x-ray generation, laser-driven sub-nanosecond bunches of protons and heavy ions for biomedical applications like imaging and tumour therapy as well as for nuclear and high-field physics.

The radiation sources emerging from experiments using the up to 2.5 petawatt laser pulses with 25 femtosecond duration will be mixed particle-species of high intensity, high energy and pulsed, thus posing new challenges compared to conventional radiation protection. Such worldwide pioneering laser experiments result in source characteristics that require careful a-priori radiation safety simulations.

The FLUKA Monte-Carlo code was used to model the five CALA experimental caves, including the corridors, halls and air spaces surrounding the caves. Beams of electrons ($<5GeV$), protons ($<200MeV$), ¹²C ($<400MeV/u$) and ¹⁹⁷Au ($<10MeV/u$) ions were simulated using spectra, divergences and bunch-charges based on expectations from recent scientific progress.

Simulated dose rates locally can exceed 1.5 kSv h⁻¹ inside beam dumps. Vacuum pipes in the cave walls for laser transport and extraction channels for the generated x-rays result in small dose leakage to neighboring areas. Secondary neutrons contribute to most of the prompt dose rate outside caves into which the beam is delivered. This secondary radiation component causes non-negligible dose rates to occur behind walls to which large fluences of secondary particles are directed.

By employing adequate beam dumps matched to beam-divergence, magnets, passive shielding and laser pulse repetition limits, average dose rates in- and outside the experimental building stay below design specifications ($<0.5\mu Sv{h}^{-1}$) for unclassified areas, $<2.5\mu Sv{h}^{-1}$ for supervised areas, $<7.5\mu Sv{h}^{-1}$ maximum local dose rate) and regulatory limits ($<1mSv{a}^{-1}$ for unclassified areas). (paper)