Delayed Neutrons From The Neutron Irradiation Of 235u

A series of experiments was performed with the Texas A & M University Nuclear Science Center Reactor (NSCR) to verify 235U delayed neutron emission rates. A custom device was created to accurately measure a sample's pneumatic flight time and the Nuclear Science Center's (NSC's) pneumatic transfer system (PTS) was redesigned to reduce a sample's pneumatic flight time from over 1,600 milliseconds to less than 450 milliseconds. Four saturation irradiations were performed at reactor powers of 100 and 200 kW for 300 seconds and one burst irradiation was performed using a $1.61 pulse producing 19.11 MW-s of energy. Experimental results agreed extremely well with those of Keepin. By comparing the first ten seconds of collected data, the first saturation irradiation deviated ~1.869% with a dead time of 2 microseconds, while the burst irradiation deviated ~0.303% with a dead time of 5 microseconds. Saturation irradiations one, three and four were normalized to the initial count rate of saturation irradiation two to determine the system reproducibility, and deviated ~0.449%, ~0.343% and ~0.389%, respectively.

Experiments have been designed and conducted to measure the periods and yields of delayed neutrons from fast fission of 235U and 237Np. These measurements were performed in a pool type reactor using a fast flux in-core irradiation device. The energy dependent neutron flux spectrum within the irradiation device was characterized using a foil activation technique and the SAND-II unfolding code. Five delayed neutron groups were measured. The total yield (sum of the five group yields) for 235U was found to be 0.0141 ± 0. 0009. The total yield for 237Np was found to be 0.0102 ± 0. 0008. The total delayed neutron yield data were found to be in good agreement with previous measurements. The individual group yields reported here are preliminary and are being further refined.

A comprehensive review is given of existing measurements as of October 1955 on the properties of delayed neutrons from various fissionable isotopes. The relation of these measurements to current theory and practice is discussed.