  Recent ammonia (1,1) inversion line data on the Galactic star forming region
Sgr B2 show that the column density is consistent with a radial Gaussian
density profile with a standard deviation of 2.75 pc. Deriving a formula for
the virial mass of spherical Gaussian clouds, we obtain a virial mass of 1.9
million solar masses for Sgr B2. For this matter distribution, a reasonable
magnetic field and an impinging flux of cosmic rays of solar neighbourhood
intensity, we predict the expected synchrotron emission from the Sgr B2 giant
molecular cloud due to secondary electrons and positrons resulting from cosmic
ray interactions, including effects of losses due to pion production collisions
during diffusive propagation into the cloud complex.
  We assemble radio continuum data at frequencies between 330 MHz and 230 GHz.
  From the spectral energy distribution the emission appears to be thermal at
all frequencies. Before using these data to constrain the predicted synchrotron
flux, we first model the spectrum as free-free emission from the known ultra
compact HII regions plus emission from an envelope or wind with a radial
density gradient. This severely constrains the possible synchrotron emission by
secondary electrons to quite low flux levels. The absence of a significant
contribution by secondary electrons is almost certainly due to multi-GeV energy
cosmic rays being unable to penetrate far into giant molecular clouds. This
would also explain why 100 MeV--GeV gamma-rays (from neutral pion decay or
bremsstrahlung by secondary electrons) were not observed from Sgr B2 by EGRET,
while TeV energy gamma-rays were observed, being produced by higher energy
cosmic rays which more readily penetrate giant molecular clouds.
