NEUTRON ACTIVATED 80KR++ INTERFERING 40AR/39AR GEOCHRONOLOGY

Various interfering reactions producing Ar isotopes during neutron irradiation from Cl, Ar, K, and Ca have been previously detailed and are routinely corrected for. Interference from doubly charged ⁸⁰Kr (⁸⁰Kr⁺⁺) on ⁴⁰Ar (mass/charge ratio difference of 0.0042 Da) is generally not considered. We show that significant amounts of ⁸⁰Kr are produced through the activation reaction ⁷⁹Br(n,β)⁸⁰Kr. While the computed fission spectrum averaged cross section of the reactions is ~48 mb—compared to ~113 mb for ³⁹K(n,p)³⁹Ar—we observe ~10 times higher production rates of ⁸⁰Kr compared to ³⁹Ar in the Oregon State University fission reactor. For the herein used MAP-215 we determined an ⁸⁰Kr⁺/⁸⁰Kr⁺⁺ ionization ratio of ~3.3. We show possible influence of ⁸⁰Kr⁺⁺ interference on ⁴⁰Ar/³⁹Ar dates for samples with elevated—but realistic—Br concentrations in the percent range. Most Br in samples occurs in fluid inclusions. Traditionally, strong ⁸⁰Kr⁺⁺ interference from decrepitating fluid inclusions would be erroneously interpreted as ⁴⁰Ar_E. The herein newly described interference can be corrected by using the parallel ⁸¹Br(n,β)⁸²Kr reaction.