Charged particle-induced Reactions-in-Flight observed in a Discovery Science implosion at National Ignition Facility

We observe production of the unstable ⁴²Ar nuclide in a Discovery Science implosion of a DT filled capsule doped with 0.1% ^natAr in a symcap configuration at National Ignition Facility. The experiment investigated neutron-induced reactions on ⁴⁰Ar and specifically searched for ³⁹Ar(t_1/2= 268 y) produced by the fast-neutron ⁴⁰Ar(n,2n) reaction and for ⁴¹Ar(110 min) and ⁴²Ar(33 y), produced by one- and two-neutron capture. ⁴²Ar is considered a candidate for the observation of rapid captures similar to the astrophysical r-process. The Radiochemical Analysis of Gaseous Samples (RAGS) apparatus was used to collect the Ar reaction products, using Ar enriched to 33% in ³⁸Ar (terrestrial abundance of 0.063%) as tracer. Detection of ⁴¹Ar (2×10⁷ atoms) was performed by γ spectrometry after collection. The two isotopes ³⁹Ar and ⁴²Ar were analyzed in the same sample at Argonne National Laboratory by Noble Gas Accelerator Mass Spectrometry (NOGAMS). The measured abundances ³⁹Ar/³⁸Ar = (1.76±0.06)×10^-10 and ⁴²Ar/³⁸Ar= (1.0 ± 0.3)×10^-14 together with the volume of the ³⁸Ar tracer provide a background-free determination of the total production yields (2.8±0.1)×10^{10 39}Ar and (1.6±0.5)×10^{6 42}Ar atoms. HYDRA hydrodynamical simulations show that the measured ⁴²Ar yield is incompatible with two-neutron capture reactions but inclusion of tertiary chains of reactions in flight (RIF’s) creating an energetic triton produce a ⁴²Ar yield via a ⁴⁰Ar(t,p)⁴²Ar charged-particle reaction, roughly consistent with the experiment. The energetic triton is produced by knock-out of either a 14 MeV neutron or possibly also by a 3.5 MeV α particle from DT fusion before its slow-down in the dense medium. The calculated ⁴²Ar yield is strongly dependent on the adopted stopping power of the dense medium. The results are under analysis and an additional Discovery Science shot is scheduled to study the dependence of the ⁴²Ar yield on the D:T ratio.