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We have converted the former solar electrical plant THEMIS (French Pyrenees) into an atmosphericČerenkov detector called CELESTE, which records gamma rays above 30 GeV (7 × 10 24 Hz). Here we present the first sub-100 GeV -2detection by a ground based telescope of a gamma ray source, the Crab nebula, in the energy region between satellite measurements and imaging atmospherič Cerenkov telescopes. At our analysis threshold energy of 60 ± 20 GeV we measure a gamma ray rate of 6.1 ± 0.8 per minute. Allowing for 30% systematic uncertainties and a 30% error on the energy scale yields an integral gamma ray flux of I(E > 60 GeV) = 6.2 +5.3 −2.3 × 10 −6 photons m −2 s −1 . The analysis methods used to obtain the gamma ray signal from the raw data are detailed. In addition, we determine the upper limit for pulsed emission to be ¡12% of the Crab flux at the 99% confidence level, in the same energy range. Our result indicates that if the power law observed by EGRET is attenuated by a cutoff of form e −E/E 0 then E 0 < 26 GeV. This is the lowest energy probed by ǎ Cerenkov detector and leaves only a narrow range unexplored beyond the energy range studied by EGRET.

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