Direct and absolute measurements of average yield of neutrons in thermal fission of U²³⁵ and spontaneous fission of Cf²⁵²
De Volpi, Alexander
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Absolute measurements, using the manganese bath technique and prompt-neutron coincidence calibration of fission counters, have been carried out to obtain independent evaluations the total yield of neutrons in thermal fission of U²³⁵[ν̅(U²³⁵)] and spontaneous fission of Cf²⁵²[ν̅(Cf²⁵²)]. In this experiment, the neutron-fission coincidence phase was used to calibrate only the fission counter efficiency; this calibrated fission counter was then inserted in a large manganese bath where the fission rate and neutron rate were simultaneously monitored. The absolute efficiency of the manganese bath was determined with the aid of a series of supplementary experiments, including frequent comparison of absolute 4π beta-gamma coincidence counting with the liquid radio-assay system. The over-all neutron detection efficiency relies heavily on the published manganese/hydrogen cross-section ratio, now known to about 0.3%. Corroboration of the neutron efficiency is found by obtaining a source rate of (1.185 ± 0.007) x 10⁶ neutrons/ sec for the U.S. National Bureau of Standards secondary source (NBS-II), which has a most recent best value (1.180 ± 0.012) x 10⁶ neutrons/sec. It is believed that this method is free of certain possible systematic errors, namely (1) a priori with regard to the fission neutron spectrum; (2) calibration of a detector whose sensitivity is comparatively neutron-energy dependent; and (3) angular anisotropy of neutron emission in flat-foil fission chambers. Examination of the literature and of data obtained from specific experiments indicates that this latter problem may be responsible for the relatively large scatter compared to the reported precision of published values of the U²³⁵ fission cross section and v̅. The results of this experiment, ν̅(U²³⁵) = 2.407 ± 0.03 neutrons/fission and ν̅(Cf²⁵²) = 3.750 ± 0.028 neutrons/fission, are highly consistent with weighted averages determined by international data compilation centers.
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