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

Abstract

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 SRM 612, 110.9±2.7 for CN 1 and 118.8±2.9 for CN 2 from KAERI IP- 6; and 333.7±8.5 for SRM 612, 109.0±2.8 for CN 1 and 119.5 ± 3.1 for CN 2 from KUR GrPn. The final results from both facilities show anexcellent consistency in ζs for CN 1 and CN 2 and a discrepancy ( ∼2.8%) in SRM 612-ζ values despite of a rough agreement within the error (2σ). Lower ratios (∼2.5–4%) of ϱ d CN1/ ϱ d SRM 612, ϱ d CN2/ ϱ d SRM612 and ϱ i mineral/ ϱ d SRM612 in KAERI IP- 6 strongly imply such a decrease of ζ SRM6 l2 . The consistency in ζ CN1 and ζ CN2 in both facilities implies that the large difference in 232Th/ 2355U ratio between the mineral and CN 1 or CN 2 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 SRM 612, 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 SRM6 l1 from KAERI IP- 6 is significantly less precise. The final ζ baselines for CN 1 and CN 2, obtained independently from both facilities, are to be recommended for accurate fission-track analyses in preference to the SRM 612-ζ baseline, even in the well-thermalized facility, e.g. KUR GrPn.