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CHAPTER 2
BACKGROUND
2.1
PRINCIPLE OF OPERATION
A detailed explanation of the working principle and descriptions of the working parts of
the densitometer appear in previous reports.!:?
Briefly, the densitometer consists of a fixed source of beta rays, a detector which re-
sponds to changes in beta-ray intensity, and a recording instrument which converts the detector output into an oscilloscope-beam deflection that ig photographed by a strip camera for
permanent record. The attenuation of the beta rays, and hence the detector output, is a func-
tion only of the density of material between source and detector. Densitometer components
are shown in Figs. 2.1 to 2.3.
Three 1-curie amounts of Sr®* were used as beta sources in each station, except at Station
690.02, where two 1-curie amounts were used.
2.2
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2.2.1
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PROBLEMS IN DENSITOMETER DESIGN
Greenhouse Model
The Greenhouse densitometer used 6 curies of Sr® at 50 cm from the detector. The
amplifier was a balanced d-c type, designed to receive the total-gamma andtotal-beta signal
from one RCA 5819 multiplier phototube at one grid input and the total-gammasignal only from
a second phototube at another grid input. The subtracted outputs of the two plates of the
amplifier were to give only the beta signal with all gamme or background signal canceled out.
Disregarding for the momentthe difficulty of balancing a d-c amplifier and the variation of
each photomultiplier gain with fatigue, the limitation of the design itself presented a formidable
obstacle against obtaining a beta record in the presence of any appreciable background (mainly
gamma) radiation. This obetacle arises from the fact that the grid input signal must be limited
between the saturation valng (ugually a value of plus 4 or 5 volts) andthe cutoff value (usually
a value of minus 10 to 15 volte}. Ifdisparity of amplifier outpiewith5grid input is desired, an
even smaller spread of grid input must be adhered to. Consequently, to get any usable betasignal componentin the output, the combined gammaand beta signal must be restricted to the
usable portion of the amplifier characteristics. For practical reasons this means that the
magnitude of the gamma signal must be comparable to the magnitude of the beta signal. Large
gamma background will saturate both halves of an amplifier designed to accomodate a relatively
sma)beta signal. With such an input system, therefore, only weak gamma backgrounds will be
subtracted. Moreover, because of the difficulty of maintaining a balanced output, subtraction
may not necessarily give zero gammasignal.
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RESTRICTED DATA -Q— SECURITY INFORMATION
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