  The effect of a diluted Planckian radiation field on a Xe gas at the electron
temperature of 100 eV is investigated within the framework of a Collisional
Radiative Model, using the HULLAC code. The atomic model spans 19 charge
states, includes 20 375 configurations and contains more than 2 10^6 levels. We
have simulated detailed spectra comprising more than 10^9 transitions with the
Mixed UTA model. The radiation temperature Tr is varied from 0 to 1.5 Te. The
dilution factor, D, applied to decrease the radiation field, is varied
independently from 0 to 3 at fixed Tr = Te. In both cases, the average charge
state Z* increases from 15 to 27, but in different ways. It is shown that even
a dilution D = 0.01 changes Z* by more than 1.5. Different combinations of Tr
and D yielding exactly the same Z*, may give line ratios sufficiently different
to be observed. This fact is explained by the interplay of the shape of the
radiation field and the atomic structure.
