radiation field. Therefore, its stakes at a height of 1 m above the response is often used as a ground. reference to which other measure- chosen to obtain exposures over the ments may be compared. full range of gamma exposure rates The chamber The locations were carefully walls are sufficiently thick to observed by the portable instrument render the detector insensitive survey. to the beta radiation present in period, fallout fields. and handcarried The instrument is, however, sensitive to cosmic After the 3-month exposure the dosimeters were retrieved (by air) in a lead container to Livermore for readout. radiation. We studied calibration and signal Further gamma exposure rate fading by exposing separate sets of 137 comparisons were made by means of TLD's to a LiF and CaF: Dy thermoluminescent and after the exposure period. dosimeters (TLD's) placed at 80 special low-scatter calibration locations. fixture was constructed for field use The LiF chip displays Cs point source before A an essentially flat energy response which aided in obtaining uniform, and excellent thermal stability. reproducible exposures. The response of LiF is within of the 13765 calibration source was approximately 1% of being air determined by equivalent for a typical environmental radiation field. e The CaF,: Dy TLD's have an enhanced energy The intensity Using a NBS calibrated Radocon* chamber e Comparing the response of a set response at low energies, and of TLD's exposed to a NBS- were used to detect possible low- calibrated 600, source to that energy radiation fields by comparison obtained from the calibration with the LiF readings. source CaF, The LiF and chips were matched to 5% and 4% respectively within each batch. The TLD's were annealed on the atoll + The calibration is known within + 3% at one standard deviation. We stored a set of control TLD's immediately before being placed on in a lead pig on a "clean" island the two islands. in the Marshalls during the Two Lawrence Livermore Laboratory (LLL) plastic personnel badges containing three LiF and three CaF, 2 chips were placed at each field location. The TLD packets were attached to trees by nylon straps or placed on wooden *Reference to a company or product name does not imply approval or recommendation of the product by the University of California or the U.S. Energy Research & Development Administration to the exclusion of others that may be suitable.