The New CLOCIT Irradiation Facility for 40 Ar/ 39 Ar 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 40 Ar/ 39 Ar geochronology at the Oregon State University TRIGA ® 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 1.45–1.53 × 10 −4 J  h −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 ( 40 Ar/ 39 Ar) K  = (4 ± 6) × 10 −4 and ( 38 Ar/ 39 Ar) K  = (1.208 ± 0.002) × 10 −2 , ( 36 Ar/ 37 Ar) Ca  =  (2.649 ± 0.014) × 10 −4 , ( 38 Ar/ 37 Ar) Ca  =  (3.33 ± 0.12) × 10 −5 and ( 39 Ar/ 37 Ar) Ca  =  (9.1 ± 0.28) × 10 −4 , similar to the CLICIT values.