Astrophysics with Radioactive Isotopes

The $^{39,41,42}$Ar nuclides as probes of neutron-induced reactions in a high-density plasma at the National Ignition Facility: a proposed experiment and calibration measurements

Not scheduled

20m

Oral Presentation

Speaker

Prof. Michael Paul (The Hebrew University of Jerusalem)

Description

Inertial fusion laser-induced implosions at the National Ignition Facility (NIF) are a unique environment to reproduce astrophysical conditions in the laboratory. The laser energy is used to compress and heat a capsule filled with deuterium-tritium fuel to conditions (density, temperature, and pressure) comparable to or exceeding those in the center of stars. Recent experiments at NIF first passed the burning-plasma threshold [1,2], where self-heating exceeded the external heating applied to the fuel and produced record fusion yields of $\approx$1 MJ . Neutrons are produced in a volume with a radius of ~50 $\mu$m within ~100 ps, representing a uniquely high neutron density approaching 10$^{22}$ cm$^{-3}$ close to those of the astrophysical $r$ process and fluxes of 10$^{31}$ cm$^{-2}$s$^{-1}$. In a dedicated NIF high-power laser shot, we plan to investigate the following neutron-induced reactions on $^{40}$Ar incorporated in the capsule gas; the chemical inertness of noble gas Ar allows for reliable collection of reaction products. The $^{40}$Ar($n,2n$)$^{39}$Ar reaction is a direct monitor of the fast-neutron flux and the $^{40}$Ar($n,\gamma$)$^{41}$Ar and $^{40}$Ar($2n,\gamma$)$^{42}$Ar reactions are sensitive to energy downgraded neutrons. The latter reaction is a monitor of extreme neutron densities produced in the process and may provide an indication of the feasibility to study the important $^{58}$Fe($2n,\gamma$)$^{60}$Fe reaction [3] in the laboratory. The long-lived $^{39}$Ar (t$_{1/2}=$ 268 y) and $^{42}$Ar (33 y) nuclides are detected via noble-gas accelerator mass spectrometry at Argonne National Laboratory. We report here on calibration measurements of the total yield of the $^{40}$Ar($n,2n$)$^{39}$Ar reaction in a 14 MeV neutron activation, investigated for the first time. The neutron activation was performed with the DT neutron generator of Technical University Dresden located at Helmholtz-Zentrum Dresden-Rossendorf. Direct detection of the $^{42}$Ar nuclide in a $^{40}$Ar sample activated by the slow double-neutron capture reaction $^{40}$Ar($n,\gamma$)$^{41}$Ar($n,\gamma$)$^{42}$Ar at the high flux reactor of Institut Laue-Langevin was successfully demonstrated for the first time. Preliminary results of these calibration experiments are presented.

Support from the Pazy Foundation (Israel) and USA-Israel Binational Science Foundation is gratefully acknowledged. This work was supported by the U.S. Department of Energy, Office of Nuclear Physics, under Contract No. DE-AC02-06CH11357. This research used resources of ANL’s ATLAS facility, which is a DOE Office of Science User Facility.
[1] A. B. Zylstra et al., Nature 93, 542 (2022).
[2] A. L. Kritcher et al., Nature Phys. 18, 251 (2022).
[3] W. Wang et al., Astrophys. J. 889, 169 (2020)