The New CLOCIT Irradiation Facility for 40Ar/39Ar Geochronology: Characterisation, 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((R)) reactor. We report fluence (i.e., time-integrated flux) parameters from the first four CLOCIT irradiations and compare them with the existing Cadmium-Lined Inner-Core Irradiation Tube (CLICIT). CLOCIT provides an average neutron flux equivalent of 1.45-1.53x10(-4)Jh(-1); about 55% of CLICIT. Radial fluence gradients were on the order of 0.2-4.2% cm(-1). 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 were 0.6-1% cm(-1), compared with 0.7-1.6% cm(-1) for the CLICIT. Production rate ratios of interfering reactions were (40Ar/39Ar)(K)=(4 +/- 6)x10(-4) and (Ar-38/39Ar)(K)=(1.208 +/- 0.002)x10(-2), (Ar-36/Ar-37)(Ca)= (2.649 +/- 0.014) x 10(-4), (Ar-38/Ar-37)(Ca)= (3.33 +/- 0.12) x 10(-5) and (39Ar/Ar-37)(Ca)= (9.1 +/- 0.28) x 10(-4), similar to the CLICIT values.