Radial fast‐neutron fluence gradients during rotating⁴⁰Ar/³⁹Ar sample irradiation recorded with metallic fluence monitors and geological age standards

Abstract

Characterizing the neutron-irradiation parameter J is one of the major uncertainties in 40 Ar/ 39 Ar dating. The associated uncertainty of the individual J-value for a sample of unknown age depends on the accuracy of the age of the geological standards, the fast-neutron fluence distribution in the reactor, and the distances between standards and samples during irradiation. While it is generally assumed that rotating irradiation evens out radial neutron fluence gradients, we observed axial and radial variations of the J-values in sample irradiations in the rotating channels of two reactors. To quantify them, we included three-dimensionally distributed metallic fast (Ni) and thermal-(Co) neutron fluence monitors in three irradiations and geological age standards in three more. Two irradiations were carried out under Cd shielding in the FRG1 reactor in Geesthacht, Germany, and four without Cd shielding in the LVR-15 reactor in Re z, Czech Republic. The 58 Ni(n f ,p) 58 Co activation reaction and c-spectrometry of the 811 keV peak associated with the subsequent decay of 58 Co to 58 Fe allow one to calculate the fast-neutron fluence. The fast-neutron fluences at known positions in the irradiation container correlate with the J-values determined by massspectrometric 40 Ar/ 39 Ar measurements of the geological age standards. Radial neutron fluence gradients are up to 1.8 %/cm in FRG1 and up to 2.2 %/cm in LVR-15; the corresponding axial gradients are up to 5.9 and 2.1 %/cm. We conclude that sample rotation might not always suffice to meet the needs of highprecision dating and gradient monitoring can be crucial.

Building the Pamir-Tibetan Plateau-Crustal stacking, extensional collapse, and lateral extrusion in the Central Pamir: 2. Timing and rates

“…[] detailed the analytical procedures for the ALF laboratory, and Rutte et al . [] detailed the irradiation geometry and reactor specifics used for the ALF samples, which comprise the majority of the reported ages; additional to age standards, metallic fluence monitors were used to correct for horizontal fluence gradients in some irradiations (Table S2). We used the revised age‐standard values of Renne et al .…”

Interpreting and reporting 40Ar/39Ar geochronologic data

“…(9) Irradiation parameter (J): The parameter J, which quantifies the production of 39 Ar from 39 K in the age equation, is determined by analyzing a co-irradiated fluence monitor with accurately known age. The J value varies horizontally and vertically in an irradiation stack due to neutron flux gradients in the reactor (e.g., Rutte et al, 2015), which can be quantified by analyzing numerous fluence monitors interspersed with the samples at known positions relative to each other.…”

Intercalibration and age of the Alder Creek sanidine 40Ar/39Ar standard

“…However, given the finite spatial extent of the pits, the fluence gradient adds to the variance of the F values within each pit. Rutte et al (2015) showed that the three--dimensional variation of neutron fluence in a reactor core is expected to be the result of superposing inverse--square and exponential decay functions. However, over the small extent of the irradiation discs a planar fit is a good first order approximation with residuals of <0.3%, and mostly <0.1%.…”