Modern techniques and applications in Cherenkov counting Page: 4 of 19
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LIQUID SCINTILLATION COUNTING
tive index of the generator is changed, the detection
threshold also changes. It has also been demonstrated that
the use of highly refractive liquids will increase the pho-
ton yield for a given particle energy (10). Occasionally,
use is made of these characteristics to select a solvent to
enhance the counting .efficiency of :a given nuclide -or :tO -
change the detection energy threshold.
The most widely used solvent for Cerenkov counting is
water. It is highly transparent to the Cerenkov emission
and can act as an efficient solvent for a wide variety of
inorganic and organic materials. Other peripheral consid-
erations are its low cost, and lack of toxicity and flam-
mability. However, for many biological materials, other
solvents are preferred. Table II lists experimental rela-
tive counting efficiencies for 32P in various solvents
(8,9). The change in the relative counting efficiency in
the above table is a direct reflection of the variability
of the index of refraction combined with any optical .
absorption properties of the solvents. As the index of
refraction increases, the energy threshold is lowered and
thus, more of the beta spectrum is included in the measure-
ment. The photon yield is enhanced also.- In some cases,
small inconsistancies can be seen; this is probably the
result of experimental error. There are many practical
applications for the use of these special solvents. For
example, it has been noted (8) that chloroform is used to
extract lipids (a severe quenching situation in organic
scintillators) and formic acid is used as a solvent for de-
fatted tissue. Acetone (5%) in chloroform has been used
for counting organic radiobromides (11). Additionally,
a solvent can be selected to count a nuclide that cannot
even be detected in water. The low energy emitter 99Tc has
been counted with very low efficiency in'a 95% glycerol
solution (1) and 14C has been measured in a-bromonaphtha-
lene (12). These techniques are, however, rather limited
in their scope of application.
The effect of solvent volume has been adequately discus-
sed in the literature (1,2,3,9,13,14) and will not be
repeated in detail here. Most investigators agree in gen-
eral, if not in detail, that counting volume is a parameter
that must be controlled for reproducible results. In all
of the counting systems tested that use a "standard" size.
liquid scintillation Trial, the volume-efficiency curve ap-
pears to go through a maximum at about 10 ml. The main
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Ross, H.H. & Rasmussen, G.T. Modern techniques and applications in Cherenkov counting, article, January 1, 1972; Tennessee. (https://digital.library.unt.edu/ark:/67531/metadc1030287/m1/4/: accessed May 23, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.