  The cosmic-ray ionization rate (zeta) of dense molecular clouds is a key
parameter for their dynamics and chemistry. Variations of zeta are well
established, but it is unclear if these are related to source column density or
to Galactic location. Using the APEX telescope, we have mapped the 364 GHz line
of H3O+ in the Sgr B2 region and observed the 307 GHz line at selected
positions. With the IRAM 30-m telescope we have observed the 203 GHz line of
H2O-18 at the same positions. Strong H3O+ emission is detected over a ~3x2 pc
region, indicating H3O+ column densities of 10^15 - 10^16 cm^-2 in an 18" beam.
The H3O+ abundance of ~3 x 10^-9 and H3O+/H2O ratio of ~1/50 in the Sgr B2
envelope are consistent with models with zeta ~4 x 10^-16 s^-1, 3x lower than
derived from H3+ observations toward Sgr A, but 10x that of local dense clouds.
The ionization rates of interstellar clouds thus seem to be to first order
determined by the ambient cosmic-ray flux, while propagation effects cause a
factor of ~3 decrease from diffuse to dense clouds.
