Page 572 - Instrumentation Reference Book 3E

554 Measurements employing nuclear techniques

the actual content of hydrogen and the other The slow neutron detector is a standard, com-
elements is kept constant. mercially available detector and is accessible
Since the BF3 or 3He counter is insensitive to without dismantling the measuring head. The
fast neutrons it is possible to mount the radio- electronics, housed in a sealed case with the meas-
active fast neutron source close to the thermal uring head, consist of an amplifier, pulse-height
neutron detector. Then any scattered thermal analyzer, and a line-driver module, which feed pre-
neutron from the slowing down of the fast neu- shaped 5-V pulses to the operator unit.
trons, which enters the counter, will be recorded. The pulses from the head electronic unit are
This type of equipment can be used to measure processed digitally in the operator’s unit to give
continuously the moisture content of granular an analog indication of moisture content on a
materials in hoppers, bins, and similar vessels. 100-mm meter mounted on the front panel, or
The measuring head is mounted external to the an analog or digital signal to other equipment.
vessel, and the radiation enters by a replaceable The instrument has a range of 0-20 percent
ceramic window (Figure 23.5). moisture and a precision of +0.5 percent mois-
The transducer comprises a radioisotope source ture, with a response time of 5-25 s. The sample
of fast neutrons and a slow neutron detector volume is between 100 and 300 1, and the operat-
assembly, mounted within a shielding measuring ing temperature of the detector is 5-70°C and of
head. The source is mounted on a rotatable disc. the electronics 5-30 “C. Figure 23.6 shows such a
It may thus be positioned by an electropneumatic moisture gauge in use on a coke hopper.
actuator either in the center of the shield or adja- A similar arrangement of detector and source is
cent to the slow neutron detector and ceramic used in borehole neutron moisture meters. The
radiation “window” at one end of the measuring fast neutron source is formed into an annulus
head, which is mounted externally on the vessel around the usually cylindrical thermal neutron
wall. Fast neutrons emitted from the source are detector and the two components are mounted
slowed down, mainly by collisions with atoms of in a strong cylinder of steel, which can be let
hydrogen in the moisture of material situated near down a suitable borehole and will measure the
the measuring head. The count rate in the detector distribution of water around the borehole as the
increases with increasing hydrogen content and is instrument descends. This system is only slightly
used as an indication of moisture content. sensitive to the total soil density (provided no

0 Manual actuator and
source indicator

Hopper wall
Figure 23.5 Moisture meter for granular material in hoppers, surface mounting. Courtesy, Nuclear Enterprises , Ltd