FALL-OUT GAMMA RAY INTENSITY 4 1 INTRODUCTION i Knowledge of the fall-out pattern of a large thermonuclear device is essential for proper evaluation by military as well as test planners and personnel. In order to learn something of the fall-out characteristics of the first thermonuclear device an experiment was desigaed to measure the fall-out gamma ray intensity as a function of time and spatial disposition with respect to ground zero. i , Implementation of this experiment was achieved by placing completely self-contained measuring instruments at a large number of locations dis- tributed around the shot island and varying in distance from approximately one mile to several hundred miles, 2 MEASUREMENT LOCATIONS Instruments were installed on the following islands of Eniwetck Atoll: Bogallua, Ruchi, Teiteiripucchi, Engebi, Bijiri, Aniyaanii, Parry, Eniwetok and Rigili, Additional stations were established at the following sitess Roi, Kwajalein, Majuro, Ponape, Bikini, Ujelang and Kusaie. These latter locations were selected as a compromise between the most desirable distribution of sites and logistic accessibility for installation and servicing of the instruments. 3 EQUIPMENT In order that the equipment be as versatile as possible with respect to both gammaintensity and measurement locations the following features of the equipment were considered to be essential: (1) A lower limit of 5 mr/hbr sensitivity with a dynamic razge af six orders of magnitude since no previous data were avaiable an a guide to the intensities to be expscted, (2) Complete self-containment and self-calitcation se that the eauip~ ment could operate in the open without attendance for an extended period, The manner in which thece requirements were satisfied will be given below with a description of the equipment. 4 ELECTROMETER The central feature governing the cesign of the equipment was that of the extended dynamic range, This immediately suggested that the record~ ing instrument plot the logarithm of the radiation intensity. The electrometer employed was based on a circuit developed by C, W. Johnson of LASL. A circuit diagram of this device is given in Fig. 1. The circuit consists of a differential amplifier the output of which is proportional to the logarithm of the ionization chamber current, The limiting sensitivity of the circuit was approximately 107/° amperes. Th? output of the unit was linearly pro- A i © .’ ‘N ed 5 En