0.135 in. stainless steel and effectively discriminates against all beta radiation. The ionization current is read out on a Victoreen Model #475A Dynamic Capacitor Electrometer. This chamber has been shown to have a flat energy response over all y-ray energies of importance in environmental radiation studies and to allow determination of total y-ray exposure rates from about 1 ur/hr to 200 ur/hr with a precision usually better than 2% and an accuracy of better than 5%°. A careful calibration was done in the laboratory with standard NBS calibrated *°°Ra and °°co sources both before and after the survey and periodic checks for consistency were made in the field with a small #2°Ra check source. A correction for the contribution to the ionizaticn current from cosmic rays was determined by measuring the cosmic ray component alone on top of the bridge deck of the ship in the middle of the Lagoon. The value obtained (3.4 ur/hr) was consistent with the value of the cosmic ray exposure rate for this latitude inferred from our previcus extensive cosmic ray measurements®*. Total exposure rates were measured with the ionization chamber at almost all the field spectrometer locations, as well as at several other sites. These total exposure rates were later compared with the sum of the individzal component exposure rates inferred from the field spectrometer data. Portable Scintillation Detector - The HASL portable scintil- lation detector consists of al5in. x 1 in. NaT(T1} crystal and 1 in. photomultiplier tube attached to a very stable count rate circuit. This instrument is relatively insensitive to beta and cosmic radiation. It is calibrated for particular field conditions by comparing readings with y-ray exposure rates determined from the ionization chamber at a number of locations. The "field calibration" was done both on Bikini and Nam with identical results. This type of calibration allowed us to use the instrument to extend the results of a few very accurate and precise ionization chamber and spectrometer measurements over a wide area. We thus quickly obtained a large amount of data which, “ . AR ne