ABSTRACT

Fast neutron flux gradient in the hydraulic rabbit irradiation facility of the JMTR reactor (core portion of the reactor) was determined using a laser-heating ⁴⁰Ar/³⁹Ar dating system. The gradient was obtained as a J value gradient determined by measuring age-known standard minerals closely spaced in an irradiation capsule. A vertical flux gradient was 1.5-1.7%/cm and relatively small compared to other reactors. On the other hand, a horizontal flux gradient was much larger than the vertical gradient being up to 13%/cm. This gradient can cause as much as 10.5% variation in J values across a single sample packet (8 mm × 8 mm in size) at a given vertical position in an irradiation capsule. This possible dispersion far exceeds the analytical error for each isotope analysis and causes large error and poor accuracy of obtained ages without correction for horizontal flux gradient. This study indicates that both unknown samples and monitor minerals should be irradiated at the same horizontal position, or relative positions among the unknown samples and monitor minerals should be measured in both vertical and horizontal directions and the 3D variation of fast neutron flux should be determined.