The need for accurate measurements of the total yield of neutrons ($\nu$) in spontaneous fission of ${}_{}{}^{252}{}_{}{}^{}\mathrm{Cf}$ has continued. Such a measurement, incorporating advances over previous work, has been accomplished with a high degree of authentication. The yield was determined by separate direct evaluations of the neutron rate and fission rate of three fission-fragment counters plated with ${}_{}{}^{252}{}_{}{}^{}\mathrm{Cf}$. The neutron emission-rate factor in the yield was found with the aid of a manganese-bath facility independently calibrated with reference to absolute $\beta -\gamma$ coincidence counting of ${}_{}{}^{56}{}_{}{}^{}\mathrm{Mn}$. Special measurements were also made of other subsidiary parameters, while precise support in the accuracy of neutron assay was obtained from comparisons with international standard sources. The fission rate was derived primarily from absolute fission-neutron coincidence data obtained from the fission counters, with special attention to angular-correlation effects. The resulting value of $\nu \left({}_{}{}^{252}{}_{}{}^{}\mathrm{Cf}\right)=3.725\mathrm{}\pm 0.015$ (total yield) is generally independent of cross sections, neutron spectrum assumptions, counting statistics, and fission-fragment self-absorption. The error assigned is a combination of statistical and systematic effects at a confidence level of 68%. Two unverified correction factors have been isolated.

• Received 2 June 1969

DOI:https://doi.org/10.1103/PhysRevC.1.683