Page 578 - Instrumentation Reference Book 3E

560 Measurements employing nuclear techniques
23.3.2.2 The ‘)lug” method gamma ray source falls on the material and the
transmitted radiation is measured by a suitable
This involves injecting a radioactive liquid into detector. In the nucleonic belt weigher shown in
the flow line in a single pulse. By measuring the Figure 23.14, designed to measure the mass flow
time this “plug” of radioactive liquid takes to
pass two positions a known distance apart, the rate of granular material such as iron ore, lime-
stone, coke. cement, fertilizers, etc., the absorption
flow can be calculated. across the total width of the belt is measured. The
A variation of the “plug” method again uses a
single pulse of radioactive liquid injected into the signal representing the total radiation falling on
the detector is processed with a signal representing
stream, but the measurement consists in taking a the belt speed by a solid-state electronic module
sample at a constant rate from the line at a pos- and displayed as a mass flow rate and a total mass.
ition beyond which full mixing has been com- The complete equipment comprises a C frame
pleted. Here the flow rate can be calculated by assembly housing the source. consisting of 137Cs
measuring the average concentration of the con- enclosed in a welded steel capsule mounted in a
tinuous sample over a known time.
shielding container with a radiation shutter, and
the detector, a scintillation counter whose sensitive
23.3.3 Mass and thickness length matches the belt width, housed in a cylin-
drical flameproof enclosure suitable for Groups 11
Since the quantitative reaction of particles and A and B gases, with the preamplifier. A calibration
photons depends essentially on the concentration plate is incorporated with the source to permit a
and mass of the particles with which they are spot check at a suitable point within the span. In
reacting it is to be expected that nuclear tech- addition, there is a dust- and moisture-proof hous-
niques can provide means for measuring such ing for the electronics which may be mounted
things as mass. We have already referred to the locally or up to 300 m from the detector.
measurement of density of the material near a The precision of the measurement is better than
borehole. We now describe some other tech- *I percent, and the operating temperature of the
niques having industrial uses. detector and electronics is -10 to +40”C. The
detector and preamplifier may be serviced by
23.3.3. I Measzrwnerit of niass, mass per ziiiit unclassified staff, as the maximum dose rate is less
clrecr, aid thickness than 7.5 pGyIh.
The techniques employed in these measurements Similar equipment may be used to measure mass
are basically the same. The radiation from a per unit area by restricting the area over which the

radiation beam
radiation beam

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Figure 23.14 Nucleonic belt weigher. Courtesy Nuclear Enterprises Ltd.