  The diffuse continuum emission from the Galactic plane in the energy range
18-1000 keV has been studied using 16 Ms of data from the SPI instrument on
INTEGRAL. With such an exposure we can exploit the imaging properties of SPI to
achieve a good separation of point sources from the various diffuse components.
Using a candidate-source catalogue derived with IBIS on INTEGRAL and a number
of sky distribution models we obtained spectra resolved in Galactic longitude.
We can identify spectral components of a diffuse continuum of power law shape
with index about 1.7, a positron annihilation component with a continuum from
positronium and the line at 511 keV, and a second, roughly power-law component
from detected point sources. Our analysis confirms the concentration of
positron annihilation emission in the inner region (|l|<10), the disk
(10<|l|<30) being at least a factor 7 weaker in this emission. The power-law
component in contrast drops by only a factor 2, showing a quite different
longitude distribution and spatial origin. Detectable sources constitute about
90% of the total Galactic emission between 20 and 60 keV, but have a steeper
spectrum than the diffuse emission, their contribution to the total emission
dropping rapidly to a small fraction at higher energies. In the SPI energy
range the flux is lower than found by OSSE, probably due to the more complete
accounting for sources by SPI. The power-law emission is difficult to explain
as of interstellar origin, inverse Compton giving at most 10%, and instead a
population of unresolved point sources is proposed as a possible origin, AXPs
with their spectra hardening above 100 keV being plausible candidates. We
present a broadband spectrum of the emission.
