Radial fast‐neutron fluence gradients during rotating 40 Ar/ 39 Ar sample irradiation recorded with metallic fluence monitors and geological age standards

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 Řež, Czech Republic. The 58 Ni(n f ,p) 58 Co activation reaction and γ‐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 mass‐spectrometric 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 high‐precision dating and gradient monitoring can be crucial.