Bethe–Feynman formula

The Bethe–Feynman efficiency formula, a simple method for calculating the yield of a fission bomb, was first derived in 1943 after development in 1942. Aspects of the formula are speculated to be secret restricted data.

Related formula

 * a = internal energy per gram
 * b = growth rate
 * c = sphere radius

$$a \approx (bc)^2 f$$

A numerical coefficient would then be included to create the Bethe–Feynman formula—increasing accuracy by more than an order of magnitude.

$$E_ff = \left( \frac{E_2}{\gamma-1} \right) \cdot \alpha_{max}^2 \cdot R_{crit}^2 \cdot \left(\frac{\delta}{1-\delta}\right) \cdot \left(\frac{2 + 3\delta}{2} \right)$$

where γ is the thermodynamic exponent of a photon gas, $E2$ is the prompt energy density of the fuel, α is V$n$ (neutron velocity) / λ$mfptot$ (total reaction mean free path), R$crit$ is the critical radius and 𝛿 is the excess supercritical radius $(Rcore - Rcrit) / Rcrit$.