  An accurate characterization of the neutron fields at spallation sources is
crucial for many applications based on neutron irradiations, such as radiation
damage tests that need a precise dose estimate. In this work we present the
neutron flux measurements performed with the multi-foil activation technique in
the ROTAX and ChipIr beamlines of the ISIS spallation source, characterized by
moderated and unmoderated spectra, respectively. We selected many different
activation reactions to cover a wide energy range, from thermal to very fast
neutrons up to about 100 MeV. By applying a Bayesian unfolding algorithm, we
demonstrate the effectiveness of this technique in measuring the neutron flux
intensity and energy spectrum with precision and accuracy.
