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

Abstract

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 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. Via the _s14hole-boring_s14 (HB) mode of laser Radiation Pressure Acceleration (RPA) using a high-intensity, short pulse laser, very efficiently bunches of ^232Th with solid-state density can be generated 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, inducing the fission process of ^232Th also 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, short-lived, light fission fragments both from beam and target. Moreover, the high ion beam density may lead to a strong collective modification of the stopping power in the target by _s14snowplough-like_s14 removal of target electrons, leading to significant range enhancement, thus allowing 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 micron focal diameter, as, e.g., envisaged for the ELI-Nuclear Physics project in Bucharest (ELI-NP), order-of-ma-gni-tude estimates promise a fusion yield of about 10^3 ions per laser pulse in the mass range of A=180-190, thus enabling 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.