Two alternatives have been suggested to explain the approximately exponential decrease in the light output of Type I supernovae. The Californium 254 hypothesis [Burbidge et al., 1957; Hoyle and Fowler, 1960] is that the exponential decay in light intensity is due to the nuclear energy released by the spontaneous fission of Cf254; no efficient mechanism for converting fission energy into visible light was presented. A second hypothesis [Hoyle et al., 1964] is based on the damping of an assumed relativistic oscillation in gravitating masses. Theoretically it has not been possible to specify either radioactivity or gravitation as the sole energy source of the initial light output. The detector system described here will provide a means of experimentally verifying or refuting the Cf254 hypothesis; however, this thesis will not resolve this controversy. Assuming the Cf-hypothesis to be valid and Type I supernovae to be the site of r-process nucleosynthesis, the gamma-ray line spectrum from radioactive decay of trans-bismuth nuclei can be calculated for any time in the history of a Type I supernovae. These calculations have been carried out for the Crab Nebula [Clayton and Craddock, 1965] and the resulting spectrum was used as the design criterion for the detector discussed here. This spectrum sets the requirement that the detector must be able to detect line fluxes of the order of 1 x 10-4 to 1 x 10-5 photons cm 2 sec-1 above a background [Arnold et al., 1962; Metzger et al., 1964] given approximately by dN/dE = 1.2 x 10 -5 E6 -7/4 photons cm 2 sec-1 kev-1 sterad-1 , where E6 is the photon energy in Mev. For photons in the energy range of 30-450 kev, a detector with an energy resolution of approximately 20 kev and a viewing angle of 0.1 sterad or less will give an observable signal. A sodium iodide (thallium activated) scintillator four inches in diameter and two inches thick is used as the detector. The signal from this crystal from photons with energies between 30 and 455 kev is analyzed by a 128 channel pulse height analyzer. Collimation is obtained by a mixture of shielding and anticoincidence techniques. The detector is recessed eight inches into the end of a cylinder of NaI(T1) nine and one half inches in diameter by twelve inches long. Additional collimation is provided by the use of three toroidal shaped plastic scintillators. The detector and the surrounding scintillators are optically decoupled and their signals are connected in anticoincidence. These scintillators reduce the effective half angle of the detector to one and one half to two degrees 3 x 10-3 sterad). A thin plastic scintillator is used to reject charged particles within this viewing angle. This instrument, along with its associated orientation system, is capable of observing the predicted gamma-ray spectrum from the Crab Nebula at altitudes of approximately 130,000 feet. Experimental data are given to demonstrate the capabilities of the system.