Sensitivity and Detection Limits for Measuring Low-Levels of Argon Radioisotopes (Argon 37 and Argon 39) using Ultra-Low-Background Proportional Counters

Pacific Northwest National Laboratory (PNNL) has developed ultra-low-background detec- tors that enable the measurement of low-levels of radioactivity for various applications such as ³⁹Ar for age dating groundwater aquifers and ³⁷Ar for detection of underground nuclear testing. These low background detectors can achieve even greater levels of sensitivity when paired with the low-background counting system located in PNNL’s Shallow Underground Laboratory (at ∼35 meters water equivalent).

Argon-37 is a challenging isotope to measure due to the radioactive decay via Auger electrons and x-rays at very low energies (mean peak energy of 2.82-keV) and benefits from the use of an internal-source proportional counter for detection and quantification. Argon-39 is less of a challenge to measure since ³⁹Ar is created in the atmosphere in abundance; it undergoes beta decay with an end-point energy at 565-keV. The greater challenge is to quantify a depletion of ³⁹Ar relative to the modern atmospheric abundance.

This talk will review the current PNNL low-background capabilities that enable the detection of low-levels of radioactivity from argon samples (³⁷Ar and ³⁹Ar). We also present a methodology for using ³⁹Ar to age-date groundwater aquifers and discuss the current detection limits and sensitivity to measure ³⁷Ar and ³⁹Ar using low-background proportional counters at PNNL.

This presentation was used for the Low-Radioactivity Underground Argon Workshop held at Pacific Northwest National Laboratory in Richland, Washington on March 19 - 20, 2018.