The Cosmic y-RAY Spectrum Between 0.3 and 25 MeV Measured on Apollo 15

The spectrum of the total (diffuse and discrete sources) cosmic y-ray background over the 0.3- 27 MeV range has been measured with an uncollimated NaI(Tl) scintillation counter 7.0 cm in diameter x 7.0 cm long located on a boom 7.6 m from the Apollo 15 service module. Data on cosmic y-rays were taken during Trans-Earth coast at various boom extensions, detector gains, and with the plastic anticoincidence scintillator enabled and disabled. The energy-loss spectrum at full extension with the anticoincidence enabled, corrected for monochromatic spacecraft 7-rays, is 0.18 counts (cm2 s MeV) 1 at 1 MeV, in agreement with previous measurements on the Ranger 3 and the ERS-l8. At 5 MeV, the Apollo measurement of 7 >c l0- counts (cm2 s MeV)-1 is about a factor of 5 below that determined from the ERS-18. Corrections for line contributions and spacecraft continuum at higher energies give a steep equivalent photon spectrum with a broad feature in the 1-3 MeV region. This flattening can be reduced (but not eliminated) with an estimated correction for spallation effects in the detector, leaving a spectrum which we interpret as a maximum estimate to the true cosmic photon spectrum in the 0.3-27 MeV range. At 3 MeV this limit is a factor of 3 and at 27 MeV a factor of 10 above a power law which extrapolates from measurements below 1 MeV. We have also measured a flux of 0.03 i 0.015 photons (cm2 s)-' at 0.51 MeV which may be mostly of local origin. Subject headings: cosmic rays - gamma rays