The method for measuring effective delayed neutron fraction β
eff proposed by E.F. Bennett, which is based on measuring a covariance of neutron counts and an absolute fission rate, was applied to thermal neutron systems in Kyoto University Critical Assembly. The successful results were already published in this journal. At that time, an only skeleton of the derivation of theoretical formula shown by E.F. Bennett was introduced, because his derivation was so complicated that the detail description was impossible in the limited space.
We found that the formula of the covariance method could be derived easily by using the compound detection probability technique which was the standard technique to derive the formulae of various neutron correlation experiments. As a result, it became clear that the present formula was different by a factor (1-β
eff)
2 from the original one derived by E.F. Bennett.
In this paper, we show the process of derivation based on the compound detection probability technique, and discuss the difference between the original formula and our formula.
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