  The radiation hardness of silicon charged particle sensors is compared with
single crystal and polycrystalline diamond sensors, both experimentally and
theoretically. It is shown that for Si- and C-sensors, the NIEL hypothesis,
which states that the signal loss is proportional to the Non-Ionizing Energy
Loss, is a good approximation to the present data. At incident proton and
neutron energies well above 0.1 GeV the radiation damage is dominated by the
inelastic cross section, while at non-relativistic energies the elastic cross
section prevails. The smaller inelastic nucleon-Carbon cross section and the
light nuclear fragments imply that at high energies diamond is an order of
magnitude more radiation hard than silicon, while at energies below 0.1 GeV the
difference becomes significantly smaller.
