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The 0.07-2 5 mev scintillation pulse height spectra of several NaI crystals, heavily shielded by Fe and Hg, have been analyzed in coincidence and anticoincidence with cosmic ray counters surrounding the crystals. The residual spectra have been analyzed further and the following sources have been identified: 1) Ra226 in Al casing, 2) K40 in phototube, crystal, and canning window, and 3) low pulse height spectrum due to the ionizing components of cosmic rays. The above sources still leave 0.05 c/m/g of NaI unexplained. Acitivities in stainless steel, quartz, and MO reflectors are too low to account for the bulk activity. Moreover, activation analysis of crystal fragments has eliminated the presence of Rb87 as a natural contaminant, and decay studies in neutron free caves preclude the presence of I126 from cosmic ray neutron activation. A possible source of contamination, consistent with the none too well defined shape of the residual spectrum, is Cs137, a fission product, in weight concentrations of about 5 Ã 10-1. Studies aimed at proving or disproving its existence are currently being pursued.