Neutron Yield from the Nuclear Photoeffect

The yields of neutrons produced in photonuclear reactions by a bremsstrahlung-photon-spectrum are analyzed in terms of the multiplicity of neutron production. Approximating the neutron multiplicity as proportional to the photon energy, we derive the integrated photonuclear cross section from the experimental neutron yields. The integrated cross section from copper to bismuth is $\int 0^{\infty }\sigma \mathrm{dW}=0.14\mathrm{}\frac{\mathrm{NZ}}{A}$. (Uranium has a neutron yield 35 percent higher than given by this relation, probably due to photofission.) This relation has the correct form, but a somewhat higher absolute value than the theoretical relation $\int 0^{\infty }\sigma \mathrm{dW}=0.060\left(\frac{\mathrm{NZ}}{A}\right)(1+0.8x)\mathrm{}$, where $x$ is the fraction of exchange force.