Age Measurements in ThO₂-D₂O-H₂O Lattices

The ages to the indium 1.46-eV resonance, of “point source” ²³⁵U thermal-fission neutrons in mixtures of ThO₂-D₂O-H₂O have been measured in a lattice in which the volume ratio of moderator-to-aluminum-clad thoria pellets was 1.00 and the square pitch was 0.548 in. The results are

D2O Concentration, mol%	Age, cm2
0.016 (ordinary water)	49.7 ± 2.5
49.77	67.4 ± 3.5
69.19	81.6 ± 1.5
79.19	97.6 ± 3.4
90.00	118.8 ±4.5

Within the experimental uncertainty, no difference was found between measurements made parallel rather than perpendicular to the thoria axis, nor were differences found when the detector foils were located inside the aluminum tubes among the pellets rather than in the moderator surrounding the tubes. However, it was found that the size and composition of the stringer which held and positioned the foils in the moderator could perturb the results. Analysis indicated that corrections ranging from +0.4% for 0% D₂O to −0.4% for 90% D₂O arise from considerations of absorption in other than the main indium resonance, detector foil shielding, and the lattice discontinuity required for source irradiation.