Gamma-ray observations of the galactic plane at energies E > 500 GeV

In 1998 and 1999 the Whipple Observatory 10 m telescope was used to search for diffuse gamma-ray emission from the Galactic plane. At this time, the t elescope was equipped with a large (4"8) held of view camera, well suited t o detect diffuse gamma-ray emission. No significant evidence of emission wa s found. Assuming the TeV emission profile matches EGRET observations above 1 GeV with a differential spectral index of 2.4, we derive an upper limit of 3.0 x 10(-8) cm(-2) s(-1) sr(-1) for the average diffuse emission above 500 GeV in the Galactic latitude range from -2 degrees to +2 degrees at Gal actic longitude 40 degrees. Comparisons with EGRET observations provide a l ower limit of 2.31 for the differential spectral index of the diffuse emiss ion, assuming there is no break in the spectrum between 30 and 500 GeV. Thi s constrains models for diffuse emission with a significant inverse Compton contribution.