  Recently, a peak in the light fermion pair spectrum at invariant q^2
approximately equal to 16.7 Me^2. has been observed in the bombardment of Li-7
by protons. This peak has been interpreted in terms of a protophobic
interaction of fermions with a gauge boson (X boson) of invariant mass of
approximately 16.7 MeV which couples mainly to neutrons. High-precision atomic
physics experiments aimed at observing the protophobic interaction need to
separate the X boson effect from the nuclear-size effect, which is a problem
because of the short range of the interaction 11.8 fm, which is commensurate
with a "nuclear halo". Here, we analyze the X boson in terms of its
consequences for both electronic atoms as well as muonic hydrogen and
deuterium. We find that the most promising atomic systems where the X boson has
an appreciable effect, distinguishable from a finite-nuclear-size effect, are
muonic atoms of low and intermediate nuclear charge numbers.
