Some Calculations on the Age of Neutrons in D₂O

The moments method has been used to calculate the flux age at 1.44 ev in D₂O and D₂O-H₂O mixtures of neutrons from various point isotropic sources. For the neutrons from a D-D source averaged overall solid angle and operating at a deuteron energy of 200 kev, the age in 99.8% D₂O was computed to be 118.6 ± 1.2 cm², in good agreement with the experimental value of Spiegel and Richardson. The rate of change of age for this source with very small admixtures of H₂O was found to be —4.5% per 1% H₂O, which agrees with the results of experiment and other calculations. Flux ages to 1.44 ev were also calculated for seven monoenergetic point sources from 2.00 to 2.98 Mev in energy. The approximate linearity of these ages with source energy is used to show that uncertainties in the angular distribution of the D-D source neutrons have a negligible effect on the averaged age. It is also shown that the 2.4 Mev antiresonance in oxygen is manifested in the age in D₂O only as a correction to the first flight term.