The 0.5 MeV gamma ray flux and the energy loss spectrum in CsI(Tl) at 4 g/cm 2

Abstract

This paper describes one of a series of experiments designed to study the time variation of the 0.5 MeV gamma ray line at small atmospheric depths. The experiment was flown from Palestine, Texas on April 6, 1966 and yielded an equivalent omnidirectional flux of 0.29 ± 0.04 photons cm −2 sec −1 at an atmospheric depth of 4.0 ± 0.3 g/cm 2 for the line at 0.5 MeV as well as the continuous energy loss spectrum in a CsI gamma ray spectrometer from 0.2 to 0.9 MeV. An additional feature nominally at ~0.70 → 0.75 MeV was significantly present in the spectrum and corresponds to an equivalent omnidirectional flux of 0.10 ± 0.02 photons cm −2 sec −1, at 4 g/cm 2. The results are compared with a similar measurement in 1961 and it is found that the total flux of 0.5 MeV gamma rays in 1966 is about a factor of two larger than expected if one assumes this radiation is solely a result of interactions of charged primary cosmic rays in the atmosphere and follows the modulation of ion chamber intensity. Possible explanations of this discrepancy are discussed. Since atmospheric nuclear detonations may contribute to the observed features of the spectrum, it is necessary to consider seriously the possibility that the 0.5 MeV line, sometimes attributed to e +-- e − annihilation radiation, may have a ignificant time varying contribution from other sources. These include nuclear debris gamma rays from Ru 106 → Rh 106 → Pd 106 with energies of 0.51 MeV and 0.62 MeV; and Ru 103 → Rh 103 with an energy of 0.49 MeV. Gamma rays with an energy of 0.48 MeV from naturally occurring Be 7, which is produced by cosmic ray interactions in the atmosphere, may also contribute. Further experiments and improved instrumentation are required before definitive knowledge can be obtained concerning the high altitude flux of annihilation photons or the detailed nature of the gamma ray spectrum in the energy region from 0.2 MeV to 0.9 MeV.