  Data available in the literature of lattice parameter changes in Ni, Cu and
MgO single crystals irradiated in neutron reactors are tentatively reproduced
using ad hoc ion irradiation experiments. The nature and energy of the ions
were selected so that their weighted average recoil spectrum matches at best
that of the neutron flux inside the reactor. The ion fluence was calculated to
obtain similar, average displacements per atom (dpa) levels. The lattice
parameters of the irradiated samples were determined using X-ray diffraction.
We show that, for the metals, it is not possible to reproduce the neutron data,
essentially because the dpa levels are low and the irradiated, damaged volume
is small. On the contrary, for MgO, a very close lattice parameter change is
obtained, providing that the irradiation temperature is significantly increased
(1000 K) with respect to that in the reactor (300 K).
