Possible γ-ray line from the Crab Nebula

It is stated that during a balloon-borne γ-ray telescope search for celestial γ-ray lines in the energy range 100 keV to 5 MeV, flown in May 1976 from Alamogordo, New Mexico, evidence was found for a line from the Crab Nebula at 400 +- 1 keV. Although the counting statistics did not allow making an incontestable claim, in all respects it appeared to be a real astronomical line. The design of the telescope is described, together with the adopted technique. A careful search was made for potential sharp line features from the Crab Nebula, but definitive proof of the reality of the observed feature must await further experimentation. The feature corresponded to a line flux of 2.24 +- 0.65 x 10-3 photons/sec/sq.cm., incident on the Earth with a width <= 3 keV. A possible physical origin for the feature is discussed. (U.K.)