Spectrum of atmospheric gamma rays to 10 Mev at λ = 40°

We report measurements of the differential counting rate spectra due to atmospheric γ rays as a function of altitude to 3.6 g/cm² over Texas. Two gain settings and a 128-channel pulse height analyzer were used to cover the range 0.2–10 Mev. The detector was a 7.6 cm × 7.6 cm NaI crystal, which was surrounded on five sides by a 2-cm-thick plastic anticoincidence shield for charged particle rejection. The system had a nearly isotropic response to photons above 0.2 Mev. The spectrum at ceiling appeared as a steep continuum with a power law index of about 1.4. The only obvious feature was the 0.51-Mev positron annihilation line. The spectral shape was independent for depths less than 20 g/cm², the absolute intensity varying in proportion to the intensity of the cosmic ray secondary charged particles. Also, at depths less than 30 g/cm² the observed flux variation with altitude can be described in terms of an empirical depth-dependent source function. At 3.6 g/cm² the continuum flux had a value of 0.4 count/cm² sec Mev near 1 Mev, and the deduced strength of the 0.51-Mev line was 0.10 ± 0.005 photon/cm² sec.