Neutron-Proton Capture Cross Section

The mean lifetime of thermal neutrons in water is measured with a large moderator (radius ∼7 diffusion lengths), for which the correction for escape is only about 5%. The geometry is such that the perturbing effect of the immersed B${\mathrm{F}}_3$ neutron detector can be almost rigorously calculated. The value obtained is $\tau =206.3\mathrm{}\pm 5.0$ μsec, giving the neutron-proton capture cross section as (0.330±0.008)×${10}^{-24\mathrm{}}$ ${\mathrm{cm}}^2$. A possible systematic error from higher modes in the neutron distribution is discussed.