- C Techniques Used to Measure Gamma-Ray Exposure Rates Since the external dose is expected to be almost entirely due to gamma-emitting radionuclides, with only minor contributions from alpha and beta emitters, it was essential to obtain the best possible description of the geographical variability of the gamma-ray exposure rates on Bikini and Eneu Islands. Several techniques were used to measure these exposure rates, since each technique has its own set of limitations (i.e., nonlinear energy response, portability of equipment, and extent of geographical coverage). These techniques included making measurements with the use of portable, hand-held Nal scintillation detectors, a commercially available pressurized ion chamber, and two types of thermoluminescent dosimeters (TLDs}. The portable scintillation detectors consisted of a 2.5-cm-diam x 3,8-cm-long Nal crystal with ratemeter readout. The instruments were calibrated with a 13765 point source on the primary calibration range of the National Environmental Research Center, Las Vegas, Nevada. Since the response of this instrument is energy-dependent, it overresponds when the gamma flux is due to scattering from a buried area source rather than from a point source as used in the calibration. Therefore it was necessary to normalize these measurements to those obtained by the pressurized ion chamber. This instrument utilizes a stainless steel sphere filled with high-pressure ultrapure argon.. The current produced by the radiation-induced ionization within the chamber is measured by a sensitive electrometer with digital readout. The instrument exhibits an éssentially flat energy response over all gamma-ray energies of interest to this survey. It was calibrated by the manu- facturer and verified by several ERDA laboratories. Measurements of the exposure rates at 1 m above. the ground were made with the NaI scintillators at about 2500 locations on a.30-m rectangular grid over the entire surface of Bikini Island and at about 200 locations on a 120-m grid on Eneu Island. Comparison measurements between the pressurized ion Ghamber and the Nal scintillators were made at.” roughly 200 locations selected from within the interior portions of the islands, the . village areas, and along the beaches. In addition, the gamma exposure rates are currently being measured by means of LiF and CaF, :Dy TLD chips that. were placed at some 80 locations on the two islands. The LiF chip displays an essentially flat energy response and excellent thermal stability. - Our extensive experience with this chip in a variety of environmental radiation measurement programs at Livermore as well as the Enewetak survey indicated that the results obtained by this detector may also serve as an excellent reference to which measurements obtained by other techniques can be compared. The CaF, TLDs have an enhanced energy response at low energies and may be used to detect possible low-energy radiation fields. An attempt is also being made to assess the contribution of the beta radiation to the total exposure rate by placing absorbers of various thicknesses over arrays of TLDs at three selected ‘locations on Bikini Island. The ‘beta radiation is believed to be principally due to. sr-9°y activities in the soil. =m