Numerical investigation of spallation neutrons generated from petawatt-scale laser-driven proton beams

Abstract

Laser-driven neutron sources could offer a promising alternative to those based on conventional accelerator technologies in delivering compact beams of high brightness and short duration. We examine this through particle-in-cell and Monte Carlo simulations that model, respectively, the laser acceleration of protons from thin-foil targets and their subsequent conversion into neutrons in secondary lead targets. Laser parameters relevant to the 0.5 PW LMJ-PETAL and 0.6–6 PW Apollon systems are considered. Owing to its high intensity, the 20-fs-duration 0.6 PW Apollon laser is expected to accelerate protons up to above 100 MeV, thereby unlocking efficient neutron generation via spallation reactions. As a result, despite a 30-fold lower pulse energy than the LMJ-PETAL laser, the 0.6 PW Apollon laser should perform comparably well both in terms of neutron yield and flux. Notably, we predict that very compact neutron pulses, of ∼10 ps duration and ∼100 μm spot size, can be released provided the lead convertor target is thin enough (∼100 μm). These sources are characterized by extreme fluxes, of the order of 1023 n cm−2 s−1, and even ten times higher when using the 6 PW Apollon laser. Such values surpass those currently achievable at large-scale accelerator-based neutron sources (∼1016 n cm−2 s−1), or reported from previous laser experiments using low-Z converters (∼1018 n cm−2 s−1). By showing that such laser systems can produce neutron pulses significantly brighter than existing sources, our findings open a path toward attractive novel applications, such as flash neutron radiography and laboratory studies of heavy-ion nucleosynthesis.