PHOTONEUTRONS IN A HEAVY WATER PILE

Abstract

Decay curves of the neutron intensity in the low power heavy water pile (ZEEP) at Chalk River as a function of time from shutdown with cadmium rods were obtained with three proportional counters of different sensitivities. The range of counting rates spanned, including operation at 50 w., was 8 × 105. The durations of steady operation before shutdown were 10, 50, and 230 min. The mean lifetimes and absolute yields of delayed and photoneutron groups were found by analyzing each decay curve into a sum of decreasing exponentials and using the theory of pile kinetics. The yields were corrected to infinite irradiation time. These results are compared with data obtained by others. The comparison shows that the energetic γ-rays of fission products which produce photoneutrons in the heavy water are absorbed to the extent of about 26% in the heavy water and 74% in the uranium rods. A table of recommended mean lifetimes and absolute yields of delayed and photoneutrons is given for use in the kinetics of heavy water piles such as ZEEP. Some quantitative data on the effects of the control plates and shutoff rods are included.