Instrumentation and Field Measurements The instruments used in a typical survey included portable scintillation counters, the NaI(Tl) spectrometric system, a high pressure ionization chamber, and an altimeter (Fig. 1). The portable scintillation detectors were used to survey the area to assure that the dose rate was relatively uniform through- out. The large NaI(Tl1) detector was placed near the center of the area, one meter above the ground, and connected by a 50 foot coaxial cable to a multichannel analyzer system in the HASL Corvan vehicle, Power was supplied by the car battery through a 12 volt DC~AC converter, The gain of the system was adjusted to obtain a spectrum extending from zero to about 3.5 Mev. A twenty minute live time was found to provide adequate statistics. Two typical spectra are shown in Figure 2. The high pressure ionization chamber filled with argon was used to measure the total dose rate and was placed a few feet from the crystal. The altimeter provided the elevation data necessary to infer the cosmic ray contribution to the ionization current measured by the pressure chamber. In addition to the instrument measurements, notes were made of any unusual conditions prevalent such as recent rainfall, very wet soil, unusual instrumental fluctuations, and ground depressions. A complete set of measurements usually required about 35 minutes from time of arrival to time of departure. DOSE RATE DETERMINATIONS Total Terrestrial The field reading of the ionization chamber (current) can be converted directly into terrestrial and cosmic dose rate values since the chamber has been calibrated in the laboratory for gamma rays*® and its cosmic ray response and the variation of the cosmic radiation dose rate as a function of altitude have been determined fairly accurately.* The total terrestrial dose rate can also be calculated from the gamma ray spectrum obtained from the NaI(Tl) detector. The total “energy" (counts per channel multiplied by the gamma ray energy represented by that channel) in the spectrum from 0.15 to 3.4 MeV has been found to correlate very well with the ionization chamber terrestrial dose rate values, even for locations whose spectra