The new CLOCIT irradiation facility for 40Ar/39Ar geochronology: Characterization, comparison with CLICIT, and implications for high-precision geochronology

The Cadmium-Lined Outer-Core Irradiation Tube (CLOCIT) is a new irradiation facility for 40Ar/39Ar geochronology at the Oregon State University TRIGA® reactor. We report fluence parameters from the first four CLOCIT irradiations and compare them to the existing Cadmium-Lined Inner-Core Irradiation Tube (CLICIT). CLOCIT provides an average neutron flux equivalent of 1.45–1.53 ×10-4 J/h; about 55% of CLICIT. Radial fluence gradients are on the order of 0.2–2.4 %/cm. A planar fit of J-values results in residuals in the range of uncertainty in the J-value, but systematic deviations resolve a non-planar component of the neutron flux field which has also been observed in CLICIT. Axial neutron fluence gradients are 0.6–1 %/cm, compared to 0.7–1.6 %/cm for the CLICIT. Production rate ratios of interfering reactions are (40Ar/39Ar)K = (4±6) ×10-4 and (38Ar/39Ar)K = (1.208±0.002) ×10-2, (36Ar/37Ar)Ca = (2.649±0.014) ×10-4, (38Ar/37Ar)Ca = (3.33±0.12) ×10-5, and (39Ar/37Ar)Ca = (9.1±0.28) ×10-4, similar to the CLICIT values.