effective window thickness of 1.5 to 2 mg/em= (mica) and diameter of 1.75 inches. The window thickness was increased to 7 me/em” by applying plastic tape to the probe face. This instrument was calibrated for gamma radiation using Heo and for beta emissions using Msgr, 3. Gamma Detectors. Again, two instrumonts were utilized for gamma radiation measurements, the Baird-Atomic Model NE-148 Scintillation Monitor 904-148, for low level gamma field measurement, and the FIDLER plutonium-americium gamma probe used only on the island of YVONNE for "hot spot"location. a. NE-148 Scintillation Monitor 904-148. This was the “Baird-Atomic" portable survey instrument which was the mainstay of the surface terrestrial radiation exposure rate measurement program. The model 904-148 Scintillation Monitor is a highly sensitive instrument capable of measuring extremely fine variations of gamma radiation in three ranges from 0-30 uR/hr to’ 3 mR/hr. The detecting element is of the same nature as that used in the aerial radiation survey, only smaller, being a 1 x 1 inch Sodium Iodide, NalI(T1l), - erystal scintillator. The instrument was calibrated using 137s, for gamma fields. It proved to be an extremely reliable device which was very rugged for use in the field. FIDLER probe. N be.