  The measurement of the neutron capture cross-section as a function of energy
in the thermal range requires a precise knowledge of the absolute neutron flux.
In this paper a new method of calibrating a thermal neutron beam using the
controlled activation of sodium is described. The method is applied to the
FP-14 Time Of Flight neutron beam line at the Los Alamos Neutron Science Center
to calibrate the beam to a precision of $\pm5$%.
