  Diffractive scattering of "fast" or "high energy" neutrons, can give low
energy nuclear recoils in the signal region for dark matter searches. We
present a discussion using the 'black disc' model. This permits a simple and
general, although approximate, description of this possible background. We note
a number of its features. In particular there are mass number A dependent
aspects which can be studied in setups where events on different nuclei are
observable at the same time. These include the recoil energy distributions, and
the A behavior of the cross section. We define a parameter $E^o_R$ which
characterizes the recoil energy to be expected due to fast neutrons. It ranges
from 100 keV on light nuclei to a few keV on heavy nuclei, and a general
treatment is possible in terms of it, within the 'black disc' approximation. In
addition, the presence of inelastic processes would be characteristic of fast
neutrons.
