  Activation analysis is used in this work to measure the flux of a fast
neutron beamline at a spallation source over a wide energy spectrum, extending
from thermal to hundreds of MeV. The experimental method is based on the
irradiation of multiple elements and measurements of activation {\gamma}-lines
using a High Purity Germanium detector. The method for data analysis is then
described in detail, with particular attention to the evaluation of
uncertainties. The reactions have been chosen so to cover the whole energy
range, using mainly (n,{\gamma}) for thermal and epithermal neutrons, and
threshold reactions for the fast region. The variety of these reactions allowed
for the unfolding of the neutron spectrum, using an algorithm based on a
Bayesian statistical model, and limited correlations have been found between
the energy groups.
