Radioargon production at The University of Texas at Austin

Abstract

The interest in the detection of radioargon isotopes--³⁷Ar, ³⁹Ar, and ⁴²Ar--is increasing important for on-site inspections within the Comprehensive Nuclear-Test-Ban Treaty verification regime. In an underground nuclear explosion ³⁷Ar is produced by ⁴⁰Ca(n,[alpha])³⁷Ar reaction in surrounding soil and rock. With a half-life of 35 days, ³⁷Ar provides a signal useful for confirming the location of an underground nuclear event. The development of detector systems is underway. This work produced radioargon isotopes by three methods for the development and testing of radioargon detection systems. The irradiation of argon gas at natural enrichment in the 3L facility within the Mark II TRIGA reactor facility at The University of Texas at Austin provides a source of ³⁷Ar for the calibration of the ULBPC in development at PNNL. The ⁴¹Ar activity is measured by the gamma activity using an HPGe detector after the sample is removed from the core. Using the ⁴¹Ar/³⁷Ar production ratio and the ⁴¹Ar activity, the amount of ³⁷Ar created is calculated. The ⁴¹Ar decays quickly (half-life of 109.34 minutes) leaving a radioactive sample of high purity ³⁷Ar and only trace levels of ³⁹Ar. The second method was the irradiation of a calcium-containing compound. This option is not the best match for the TRIGA reactor type due to the thermal neutron flux. Therefore, the use of the Cd-lined 3L irradiation canister minimized the thermal activation of impurities while still allowing the majority of the ⁴⁰Ca(n,[alpha])³⁷Ar reactions occur. The third and last irradiation technique was a large volume, in-core gas facility developed at The University of Texas at Austin MARK II TRIGA reactor to produce a sample of ⁴²Ar with an activity above 1 mBq. The method requires a large volume, 1.4 L, of natural argon gas (99.6003% ⁴⁰Ar) at about 1 atm and three-12 hour irradiation periods. The production of ⁴²Ar requires a double capture to be produced from the stable 40Ar isotope. This method produced 940 kBq of ³⁹Ar, 3.08 MBq ³⁷Ar, 114 GBq ⁴¹Ar, and 0.311 Bq ⁴²Ar at the end of the final irradiation period.