Production of alpha particles sources through proton-boron nuclear reactions initiated by relativistic lasers*

The landscape of attainable radio-isotopes with medical cyclotrons is limited due to the low energy of accelerated particles. A few radio-isotopes are produced with low energy protons at hospitals. In France, only the ARRONAX cyclotron in Nantes is able to produce several isotopes with energetic protons but also alpha particles, for a broad range of medical applications. A new way of producing those radio-isotopes has been studied, that is, using secondary alpha particles sources as a way to generate those relevant isotopes. Proton-Boron nuclear reactions have been actively studied these last few years as a possible way of producing secondary alpha particles sources. Proton acceleration by interaction of ultra-high intense lasers with hydrogenated targets is the preferred way to initiate those type of reactions. [1] The versability of such laser systems is the preferred way to complement conventionally used medical cyclotrons.

The two main mechanisms of proton acceleration studied for this nuclear scheme are the Target

Normal Sheath Acceleration (TNSA) and the Hole-Boring (HB) process. In the first case, protons

are accelerated at the rear side of the target via the electrostatic field induced by laser driven electrons escaping from the target. The exponential shape of the proton energy spectrum induces a

great number of nuclear reactions throughout a Boron secondary target despite a decrease of the

cross-section above the main resonance at 675 keV.

For the Hole-Boring process, protons are accelerated at the front side thanks to the electric field

induced by the electrons pushed by the radiation pressure of these high laser intensities. Accelerated

protons interact directly with boron atoms contained within the same target [2]. Different types of targets have been studied both numerically and experimentally for Hole-Boring based alpha production.

Particle-in-Cell (PIC) and Monte-Carlo (FLUKA) simulations have been conducted to better

understand experimental campaigns done on the VEGA-III laser at CLPU, Salamanca, Spain in

november 2022 and march 2023. This laser is characterized by a short pulse duration, 30fs and a

high-repetition rate of 1Hz. The two proton acceleration schemes have been studied numerically to better understand the experimental data and to deepen the analysis. PIC simulations for TNSA protons could directly be compared with experimental diagnostics and gave confidence for Hole-Boring protons results. Monte-Carlo simulations for both schemes were then directly compared to experimental data and confirmed the results. Simulations for scattered ions also gave confidence in the interpretation of the diagnostic and helped discriminate particles obtained on the detectors

[1] Margarone Daniele, Alessio Morace, Julien Bonvalet et al. « Generation of α-Particle Beams With a Multi-KJ, Peta-Watt Class Laser System ». Frontiers in Physics 8 (9 septembre 2020): 343.

https://doi.org/10.3389/fphy.2020.00343.

[2] Margarone, Daniele, Julien Bonvalet, Lorenzo Giuffrida, et al. « In-Target Proton–Boron Nuclear Fusion Using a PW-Class Laser ». Applied Sciences 12, no 3 (28 janvier 2022): 1444. https://doi.org/10.3390/app12031444.