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Abstract:
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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 . |