Introducing the fission-fusion reaction process: using a laser-accelerated Th beam to produce neutron-rich nuclei towards the N = 126 waiting point of the r -process

摘要

We propose to produce neutron-rich nuclei in the range of the astrophysical r-process (the rapid neutron-capture process) around the waiting point N = 126 (Kratz et al. in Prog. Part. Nucl. Phys. 59:147, 2007; Arnould et al. in Phys. Rep. 450:97, 2007; Panov and Janka in Astron. Astro-phys. 494:829,2009) by fissioning a dense laser-accelerated thorium ion bunch in a thorium target (covered by a polyethylene layer, CH_2), where the light fission fragments of the beam fuse with the light fission fragments of the target. Using the 'hole-boring' (HB) mode of laser radiation pressure acceleration (RPA) (Robinson et al. in Plasma Phys. Control. Fusion 51:024004, 2009; Henig et al. in Phys. Rev. Lett. 103:245003, 2009; Tajima et al. in Rev. Accel. Sci. Technol. 2:221, 2009) using a high-intensity, short pulse laser, bunches of ~(232)Th with solid-state density can be generated very efficiently from a Th layer (ca. 560 nm thick), placed beneath a deuterated polyethylene foil (CD_2 with ca. 520 nm), both forming the production target. Th ions laser-accelerated to about 7 MeV/u will pass through a thin CH_2 layer placed in front of a thicker second Th foil (both forming the reaction target) closely behind the production target and disintegrate into light and heavy fission fragments. In addition, light ions (d,C) from the CD_2 production target will be accelerated as well to about 7 MeV/u, also inducing the fission process of ~(232)Th in the second Th layer. The laser-accelerated ion bunches with solid-state density, which are about 10~(14) times more dense than classically accelerated ion bunches, allow for a high probability that generated fission products can fuse again when the fragments from the thorium beam strike the Th layer of the reaction target. In contrast to classical radioactive beam facilities, where intense but low-density radioactive beams of one ion species are merged with stable targets, the novel fission-fusion process draws on the fusion between neutron-rich, shortlived, light fission fragments from both beam and target. Moreover, the high ion beam density may lead to a strong collective modification of the stopping power in the target by 'snowplough-like' removal of target electrons, leading to significant range enhancement, thus allowing us to use rather thick targets. Using a high-intensity laser with two beams with a total energy of 300 J, 32 fs pulse length and 3 μm focal diameter, as, e.g. envisaged for the ELI-Nuclear Physics project in Bucharest (ELI-NP) (http://www.eli-np.ro, 2010), order-of-magnitude estimates promise a fusion yield of about 10~3 ions per laser pulse in the mass range of A = 180-190, thus enabling us to approach the r-process waiting point at N - 126. First studies on ion acceleration, collective modifications of the stopping behaviour and the production of neutron-rich nuclei can also be performed at the upcoming new laser facility CALA (Center for Advanced Laser Applications) in Garching.