samples was taken so that the variation of isotcpic concentrations with depth might later be investigated. en y A. Instrumentation Spectrometer System The HASL spectrometer system consists of a4 in. x 4 in. NaI detector with preamplifier and high voltage battery pack, a 400 channel multichannel analyzer and parallel printer, a DC~AC converter and a 12 volt storage battery. The calibration of the detector is described in detail in previous HASL reports’’*. In brief, the exposure rate due to a particular isotope in the ground is inferred from the number of counts under a spectrum total absorption peak characteristic of that isotope using calibration factors determined from a combination of laboratory experiments and theoretical caiculations. The prominent total absorption peaks observed in all the field spectra (see Figure 2 for a typical field spectrum) were at 1.17 and 1.33 Mev (°°co), .10 Mev (°°* Bu). (**®sb), and .662 Mev (~°’Cs}. .44 Mev These isotopes appeared to be responsible for almost all of the total expcsure rate at the spectrometer locations. Individual exposure rates for 37 a5, ®°co and ***°Sb were inferred for each field location. The *®° gu exposure rate was assumed to be of minor impcrtance due to its very low source energy. For those locations where the ionization chamber was not used or the other total exposure data were inconsistent, the spectrcmetric data were analyzed to obtain total exposure directly*. .Except for the two Eneu locations, ay taken in ten minute runs. all spectra were During this time there was no appreciable gain shift due to temperature changes. In almost all cases we were able to obtain excellent field spectra with only minor equipment malfunctions even though the equipment had to be carried by hand through heavy brush, loaded and unloaded into small boats in fairly rough surf, and operated and stored in ambient temperatures of approximately 85 - 95°F and relative humidities of 70 - 80%. Ionization Chamber - Our high pressure ionization chamber has a 5.6 liter sensitive volume and is filled to a pressure of about 700 psi with pure argon gas*. The chamber wall is