Radioactive Decay Measurements of 41Ar for SLIC

The short lived isotope-counting system (SLIC) being built for the OMEGA laser facility at LLE requires gaseous radioisotopes for calibration purposes. Using a Plutonium-Beryllium (Pu-Be) source at SUNY Geneseo, 41Ar was made by capturing thermal neutrons via the 40Ar(n,gamma) reaction. Once activated, 41Ar beta decays to produce an electron with an endpoint energy of 1.198 MeV. The daughter product is found to be in the second excited state of 41K 99.1% of the time. This promptly decays to the ground state and emits a 1.293 MeV gamma ray. To accurately measure the beta activity of 41Ar for SLIC calibrations, the Gamma-X counting system at Geneseo has been repurposed as a high precision counting station. Gamma-X is composed of 3 orthogonal pairs of Thallium doped NaI detectors surrounding a central cubic counting region with sides of 8 cm. Each of the six detectors is shielded in 11.5 cm of lead and clad in aluminum to reduce background radiation. The calibration was performed by integrating the counts in the 1.293 MeV gamma peak. Measuring the decay curve of the peak counts confirms the origin of the peak. The radioactive 41Ar gas will then be used as an electron calibration source for the SLIC system used in ICF and HEDP nuclear reaction studies.