The excitation function is the variation of cross
section with energy. It can be considered as having three
regions: the 'one over v' (1/v), resonance, and fast neutron regions.
Resonance region - in the range of 0.1 to 1000 eV. There
are often sharp peaks in the structure. These are characteristic of
1/v region - in the low energy region it is often found that the
cross section decreases steadily with increasing neutron energy.
As f = nv, then s µ
1/v µ 1/En it appears as a
straight line on the usual log-log graph of s