Direct comparison of zeta calibration constants for fission-track dating by double-checking of two irradiation facilities with different degrees of neutron flux thermalization

Zeta (ζ) constants were determined using zircon, apatite and sphene age standards for a systematic calibration of fission-track dating. Identical crystals were double-checked in two facilities having significantly different degrees of neutron flux thermalization: Irradiation Pit-6 (IP-6) of the Korea Atomic Energy Research Institute (KAERI) TRIGA III reactor and the Graphite Pneumatic Tube (GrPn) of the Kyoto University Research Reactor (KUR-1) which have Cd ratios of 12.5 and ∼200, respectively, for Au. The final single weighted mean ζ baselines are given with errors (2σ) as: 324.5 ± 7.9 for SRM612, 110.9±2.7 for CN1 and 118.8±2.9 for CN2 from KAERI IP-6; and 333.7±8.5 for SRM612, 109.0±2.8 for CN1 and 119.5 ± 3.1 for CN2 from KUR GrPn. The final results from both facilities show anexcellent consistency in ζs for CN1 and CN2 and a discrepancy ( ∼2.8%) in SRM612-ζ values despite of a rough agreement within the error (2σ). Lower ratios (∼2.5–4%) of ϱ_d^CN1/ϱ_d^SRM612, ϱ_d^CN2/ϱ_d^SRM612 and ϱ_i^mineral/ϱ_d^SRM612 in KAERI IP-6 strongly imply such a decrease of ζ_SRM6l2.

The consistency in ζ_CN1 and ζ_CN2 in both facilities implies that the large difference in ²³²Th/²³⁵5U ratio between the mineral and CN1 or CN2 may not significantly affect thermal neutron dosimetry, even for apatite which has the largest difference of the ratio against dosimeter glasses. The larger (n,f ) reaction cross-section of ²³⁸U than that of ²³²2Th in epithermal and fast neutron energy regions also suggests that the difference of ²³⁸U /²³⁵U ratio as seen in glass SRM612, which has depleted U, may affect the thermal neutron dosimetry more significantly than that of ²³²Th/²³⁵U ratio regardless of Th/U ratios of the three minerals. Therefore, the discrepancy in ζ_SRM612 results mainly from the larger contribution of ²³⁸U fission induced by non-thermal neutrons to the thermal neutron dosimetry in the less thermalized KAERI IP-6. In consequence, the final ζ_SRM6l1 from KAERI IP-6 is significantly less precise. The final ζ baselines for CN1 and CN2, obtained independently from both facilities, are to be recommended for accurate fission-track analyses in preference to the SRM612-ζ baseline, even in the well-thermalized facility, e.g. KUR GrPn.