The Bethe–Feynman efficiency formula, a simple method for calculating the yield of a fission bomb,[1] was first derived in 1943 after development in 1942.
Aspects of the formula are speculated to be secret restricted data.
A numerical coefficient would then be included to create the Bethe–Feynman formula—increasing accuracy by more than an order of magnitude.
{\displaystyle E_{f}f=\left({\frac {E_{2}}{\gamma -1}}\right)\cdot \alpha _{max}^{2}\cdot R_{crit}^{2}\cdot \left({\frac {\delta }{1-\delta }}\right)\cdot \left({\frac {2+3\delta }{2}}\right)}
where γ is the thermodynamic exponent of a photon gas, E2 is the prompt energy density of the fuel, α is Vn (neutron velocity) / λmfptot (total reaction mean free path), Rcrit is the critical radius and 𝛿 is the excess supercritical radius (Rcore - Rcrit) / Rcrit.
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