Techniques Used to Measure Gamma-Ray Exposure Rates Because the external dose is at about 2500 locations on a 30-m primarily due to gamma-emitting rectangular grid over radionuclides, with only minor face of Bikini Island, contributions from alpha and beta 200 locations on a 120-m grid on emitters, we had to obtain the best Eneu Island. possible description of the geo- the detectors was energy-dependent graphical variability of the and they were calibrated with a point ‘gamma-ray exposure rates on Bikini source, and Eneu Islands. respond to the gamma flux on the Any technique the entire surand at about Since the response of they were expected to over- for measuring gamma exposure rates atoll because the flux is depth dis- has its own set of Limitations tributed and has a higher scatter (e.g., component — and, nonlinear energy response, portability of equipment, therefore, a lower energy — than the point source. and The extent of geographical coverage). detectors could be carried easily, We therefore used four different which allowed us to make measurements techniques to obtain the detailed at many locations on a uniform grid geographical coverage and accuracy of the islands. we desired: portable, hand-held Nal insensitive to cosmic radiation. scintillation detectors, a commercially They are virtually The response of the detector and was compared with that of the pres- two types of thermoluminescent dosime- surized ion chamber over the entire ters (TLD's). range of observed exposure rates. available pressurized ion chamber, The portable scintillation The ion chamber consists of a detectors consisted of a 2.5-cm-diam stainless steel sphere filled with x 3.8-cm-long Nal crystal with rate high-pressure ultra-pure argon. meter readout. The current produced by the radiation- The detectors were calibrated in microroentgens per induced ionization within the hour (UR/h) against a 1376g point chamber is measured by a sensitive source on the primary calibration electrometer with digital readout. range of the National Environmental The detector was calibrated by the Research Center, Las Vegas, Nevada. manufacturer and verified by several Calibration was repeated on selected ERDA laboratories. instruments following the survey. relatively flat energy response The detectors measured the exposure rates at 1] m above the ground It exhibits a over the gamma~ray energies of interest in a typica:t environmental