exhibit large fallout peaks. The calibration factor obtained in the laboratory using a standard radium source was consistent with that inferred from the field ionization chamber values.* Dose rates can also be inferred from the portable scintillation counters used to survey the site area, These instruments are equipped with count rate meters and the dependence of their response on environmental conditions and incident gamma ray energy is such that they are not suitable for direct dose rate calibration. The instruments can, however, be “field calibrated" against the ion chamber by plotting the readings of each instrument in a given area against the corresponding terrestrial dose rates obtained from the ionization chamber data. The calibration curve thus obtained may be used to interpret readings of the portable scintillation counter in terms of dose rate for any location in the area as well as to detect any spurious ionization chamber readings. For each location a "best" value of terrestrial dose rate was calculated by averaging the ionization chamber and total spectrum energy values, The dose rates inferred from the portable instruments were usually used only as a check since these values are generally not as precise as the values obtained from the other two methods, Natural Gamma Two methods: of spectrum analysis are used to obtain estimates of the dose rate contributions from the three major natural contributors to the terrestrial gamma radiation field. The first method is based on the assumption that the estimated area under a total absorption peak characteristic of the u238 series (1.76 Mev), Th232 series (2.62 MeV), and K40 (1.46 Mev) is directly proportional to the total dose rate contribution from all the gamma rays of the parent emitter or series of emitters. Laboratory studies of the variation of estimated peak counts as a function of primary flux and angle were carried out for various standard monoenergetic sources using the laboratory background spectrum to simulate the field situation. The information obtained from these experiments was combined with calculations of the angular distribution of primary flux and the total dose rate per unit primary flux for the various emit- ters, assuming known decay schemes and uniform source distributions in the ground, to arrive at a conversion factor from i 4