Quantitative feasibility study of sequential neutron captures using intense lasers

Deciphering the conditions under which neutron captures occur in the Universe to synthesize heavy elements is an endeavor pursued since the 1950s, but has proved elusive up to now due to the experimental difficulty of generating the extreme neutron fluxes required. It has been evoked that laser-driven (pulsed) neutron sources could produce neutron beams with characteristics suitable to achieve nucleosynthesis in the laboratory. In this scheme, the laser first generates an ultra-high-current, high-energy proton beam, which is subsequently converted into a dense neutron beam. Here we model, in a self-consistent manner, the transport of laser-accelerated protons through the neutron converter, the subsequent neutron generation and propagation, and finally the neutron capture reactions in gold (${}^{197}\mathrm{Au}$), chosen as an illustrative example. Using the parameters of present-day available lasers, as well as of those foreseeable in the near future, we find that the final yield of the isotopes containing two more neutrons than the seed nuclei is negligible. Our investigation highlights that the areal density of the laser-driven neutron source is a critical quantity and that it would have to be increased by several orders of magnitude over the current state of the art in order to offer realistic prospects for laser-based generation of neutron-rich isotopes.

Figure 1

Neutron capture cross section dependence on energy for ${}^{197}\mathrm{Au}$ and ${}^{198}\mathrm{Au}$ isotopes, from the TENDL-2019 library [24].

Figure 2

Example scheme for neutron generation by a laser-driven proton beam [22]. A PW-class, ultraintense ($>{10}^{21}\mathrm{W}{\mathrm{cm}}^{-2}$), ultrashort (20 fs) laser pulse interacts with a thin (submicron) solid foil (green rectangle), from which protons are accelerated. To boost the on-target laser intensity, and thus proton acceleration, a foam layer (pink box), of $\approx 20\mu \mathrm{m}$ thickness and acting, once ionized, as a near-critical-density plasma lens, can be attached to the solid foil. The neutron converter, made of a high-$Z$ material (the brown box), is placed behind the laser target.

Figure 3

Simulated neutron energy spectra (in orange and green) for the scheme shown in Fig. 2, using a 1 PW, $2\times {10}^{21}{\mathrm{cm}}^{-2}$, 20 fs laser pulse, according to Ref. [22]. Neutrons are generated in a Pb converter by protons (blue spectrum) accelerated from a double-layer target, composed of a 20 $\mu \mathrm{m}$ thick, $1n_c$ density carbon layer followed by a 115 nm thick, $200n_c$ density ${\mathrm{CH}}_2$ foil. The neutron converter has a thickness of either $100\mu \mathrm{m}$ (orange) or 2 cm (green).

Figure 4

Setup of Monte Carlo simulations of proton transport, neutron generation, and neutron transport. The protons are injected through the left-hand side of the ${}^{197}\mathrm{Au}$ target. The spatial grid over which neutron captures are computed is shown as white dashed lines.

Figure 5

Proton [(a), (b)] and neutron [(c), (d)] transport through the ${}^{197}\mathrm{Au}$ converter (located between the two white dashed lines) and final abundances of ${}^{198}\mathrm{Au}$ [(e), (f)] and ${}^{199}\mathrm{Au}$ [(g), (h)] for cases C [(a), (c), (e), (g)] and G from Table 2, respectively. The inset in panel (b) displays the energy-angle spectrum of the incident protons in case G.