Using Neutrons 
for Remote 
Liquid-Level 
Gaging 
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FIG. 1. Experimental set-up for determining liquid-level by 
neutron radiation uses rectangular brass tank, 30-cm square 
by 50-cm high, and movable counter with neutron source 
attached. Inset shows shield for B!°F3; counter, consisting of 
two concentric Cd tubes, each 0.4-mm thick, filled with 2.5-mm 
of boron carbide 
First tests of liquid-level measurement using 
a fast-neutron source and a new slow- Rene 
neutron detector show that method works = | 
well for hydrogenous and other light liquids. | 
Less shielding is needed than with a gamma | 
gage; advantage of completely external 
measuring is retained. Accuracy of new 
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~ 
Cadmium 
strip 
"Neutron phosphor" 
method is — 2 mm 
By S. BARNARTT and K. H. SUN 
Westinghouse Research Laboratories 
East Pittsburgh, Pennsylvania 
FIG. 2. Slow-neutron detector is made of “neutron- 
phosphor" strip placed against face of DuMont type K 
1198 photomultiplier tube. Neutron phosphor is made by 
embedding silver-activated zinc sulfide particles in glyceryl- 
borate polyester. Strip is 11-cm long by 0.15-cm wide by 
0.12-cm thick. Cadmium sheet 1.2-cm wide and 0.12-cm 
thick surrounds phosphor. Entire assembly is encased in 
light-tight aluminum housing 
THE FEASIBILITY OF USING neutron 
radiation in a remote liquid-level gage 
is the subject of this article. The 
equipment used for the study and re- 
sults for different geometries are ex- 
plained. Measuring liquid level in a 
container by nuclear radiation depends 
on the fact that radiation is trans- 
mitted with less loss through gas than 
through liquid, or on the fact that back- 
scattering occurs to a greater degree in 
liquid. Several level gages based on 
differential transmission of gamma rays 
have been described (1-8). Other 
gages utilize back-scattering of y-rays 
(1, 9, 10). However, the paucity of 
actual data published prevents com- 
paring the precision of the various 
y-ray devices. 
Using neutron radiation for measur- 
ing liquid level has received much less 
attention than using y-radiation, al- 
though neutron gaging appears to be 
more attractive because less and conse- 
quently lighter shielding is required. 
Neutron gaging is especially attractive 
for liquids containing hydrogen or 
other low-atomic-weight elements, since 
collisions with these elements quickly 
slow down neutrons, and slow neutrons 
can be detected much more efficiently 
than fast ones. Thus a convenient 
remote liquid-level gage can be made 
by placing a fast-neutron source near 
a slow-neutron counter, and moving 
the assembly vertically just outside the 
liquid container. The fast neutrons 
that enter the liquid are slowed down, 
and some of them are scattered back 
and detected by the counter. As the 
detector is moved from just above the 
liquid level to just below it, a sudden 
increase in count is obtained. This 
method of determining level was sug- 
gested in 1945 by Hare (11), although 
no data have appeared to indicate what 
positioning of source and counter is 
most effective or how accurately the 
level may be measured. These factors 
were investigated in the following ex- 
periments. The results indicate that 
the neutron-radiation method may be 
suitable for practical applications. 
How Experiment Was Done 
Essential parts of the apparatus used 
in this study are shown schematically 
in Fig. 1. The liquid (tap water) was 
in a rectangular brass tank with 0.16- 
cm walls. The source was in a nickel 
tube 2.5-cm long X 0.6-cm diameter. 
It produced 8 X 10® fast neutrons/ 
sec from 400 me of polonium with 
155 
