⁴⁰Ar proposed as probe of neutron-induced reactions in a high-density stellar-like plasma at the National Ignition Facility

The thermodynamical conditions of plasma density, temperature, pressure and the neutron density produced in a laser-induced implosion of a deuterium-tritium (DT) filled capsule at the National Ignition Facility (NIF) are the closest laboratory analog of stellar conditions. We plan to investigate neutron-induced reactions on 40Ar, namely the 40Ar(n, 2n)39Ar(t1/2 =268 y), the 40Ar(n,gamma)41Ar(110 min) and the potential rapid two-neutron capture reaction 40Ar(2n,gamma)42Ar(33 y) in the same implosion on an Ar-loaded DT capsule. The chemical inertness of noble gas Ar enables reliable collection of the reaction products. We describe here the technique of Noble-Gas Accelerator Mass Spectrometry (NOGAMS) to be used for ultra-sensitive detection of the long-lived 39Ar and 42Ar at Argonne National Laboratory. A 42Ar(33 y) sample was produced via the slow two-neutron capture reaction 40Ar(2n, gamma)42Ar
in a high thermal-neutron flux irradiation. 39Ar and for the first time 42Ar atoms were directly detected in the abundance range 10^-12 -- 10^-13 .
The present sensitivity of 42Ar detection is of the order of 104 atoms.