VERTICAL AND HORIZONTAL VARIATIONS OF THE FAST-NEUTRON FLUX IN A SINGLE IRRADIATION CAPSULE AND THEIR SIGNIFICANCE IN THE LASER-HEATING AR-40 AR-39 ANALYSIS - CASE-STUDY FOR THE HYDRAULIC RABBIT FACILITY OF THE JMTR REACTOR, JAPAN/

Fast neutron flux gradient in the hydraulic rabbit irradiation facilit y of the JMTR reactor (core portion of the reactor) was determined usi ng a laser-heating Ar-40/Ar-39 dating system. The gradient was obtaine d as a J value gradient determined by measuring age-known standard min erals closely spaced in an irradiation capsule. A vertical flux gradie nt 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 v ertical gradient being up to 13%/cm. This gradient can cause as much a s 10.5% variation in J values across a single sample packet (8 mm x 8 mm in size) at a given vertical position in an irradiation capsule. Th is possible dispersion far exceeds the analytical error for each isoto pe 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.