Ar with Accelerator Mass Spectrometry*

ATLAS at Argonne National Laboratory, composed of an Electron Cyclotron Resonance ion source and a superconducting linear accelerator, hosts the sole Accelerator Mass Spectrometry (AMS) setup based on positive ion acceleration to a few MeV/u. A new multi-anode ionization chamber, MONICA, was built to enhance the ion identification for noble gases and medium to heavy mass isotopes. After undergoing commissioning at the University of Notre Dame, MONICA was used to study the ⁴⁰Ar(n,2n)³⁹Ar (t_1/2=268 y) reaction and, for the first time, detect ⁴²Ar (33 y) by AMS. The AMS detection of 39Ar leads to a first measurement of the ⁴⁰Ar(n,2n)³⁹Ar total reaction cross section at 14 MeV. These isotopes were explored as part of a proposed experiment to probe neutron-induced reactions in a high-density plasma at the National Ignition Facility.