DOSIMETRIC INVESTIGATIONS OF ENVIRONMENTAL GAMMA RADIATION FROM DEPOSITED FISSION PRODUCTS WAYNE M. LOWDER, HAROLD L. BECK, and WILLIAM J. CONDON Health and Safety Laboratory, U. S. Atomic Energy Commission, New York, New York ABSTRACT The presence of significant quantities of gamma-emitting fission products in the environment as a result of nuclear-weapons testing has made necessary the development of techniques for differentiating be- tween the natural- and the fallout-gamma components of the total environmental radiation field. This has been achieved by the use in the field of a gamma-spectrometer system with a 5- by 3-in. Nal(T1) detector. A combination of theoretical calculations and laboratory calibration experiments has enabled natural-gamma-radiation dose rates to be obtained from estimated total-absorption peak areas in the Spectra or from the total counts in bands including these peaks. The peak method can be similarly applied to the main fallout-gamma emitters (*Zr—*Nb, '%cs, ‘Rh, and !%Ru) to obtain their respective dose-rate contributions. The total fallout dose rate can be obtained from the difference between the total gamma dose rates and the natural-~gamma contribution. It is estimated that the total dose rate can be measured to +0.5 ur/hr (standard deviation) and the individual components to +10%. Over 300 gamma-energy spectra have been obtained at more than 200 different locations throughout the United States. Most of the inferred natural-gamma dose rates were in the range of 5 to 9 ur/hr. Open-field-fallout dose rates between July 1962 and September 1963 ranged between 2 and 6ur/hr, except in areas having very little rain233