FALL-OUT GAMMA RAY INTENSITY
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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.
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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-
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