Tritium or Carbon-14? 
Since most organics are made up of hydrogen, carbon, and oxygen, and since 
there is no usable radioactive oxygen, organic tracers must be tagged with tritium 
ore Git 
and assay is applicable to both. 
Both are weak beta emitters so the same instrumentation for detection 
F. Marott Sinex, chairman of the recent symposium, made a comparison of 
tritium and C' for tracer purposes. 
Carbon-14 
Maximum beta energies 
0.158 
Mev 
Here are some of his figures: 
Tritium 
0.018 
Dose from 1 me uniformly distributed in a 70-kg man: 
0.039 
Cost of the isotope. 
up to 80 mc/gm. 
28 
Cost of labeled compounds. 
leucine and C'*-labeled amino acids: 
800 
Specific activities. 
rep/day 
C"4 is sold at Oak Ridge in solid BaCO; at specific activities 
Tritium is sold as the pure gas except for the He’ decay product. 
$/me 
For example, here are the price of tritium-labeled 
0.0044 
0.002 
$/me 35 
Tritium’s much shorter half-life (12.3 yr compared with 
5,600 yr) indicates that it takes much fewer atoms to make a millicurie than is the 
case for C!4, 
Thus maximal specific activities for the two are 
5 curies/mole 
700 
into filter paper that contains the 
scintillator (13). 
In the other method one introduces 
tritium-labeled gases including tritium 
gas itself into gas counters to reduce 
absorption. This leads to two types of 
counting: conventional G-M counting 
with the tritium in the counter initi- 
ating the discharges and gaseous 
scintillation counting (14, 15). (See 
Table 3, page 67.) 
Liquid Counting 
Today’s techniques for liquid-scintil- 
lation counting require a solution that 
has five components aside from the 
sample that is to be counted. This 
solution is viewed by two photo- 
multipliers operated in coincidence to 
reduce background. Further back- 
ground reduction is accomplished with 
pulse-height discrimination. 
Scintillating solutions. The five 
components of a good scintillating so- 
lution are (a) an aromatic solvent, 
(b) a fluorescent dye, (c) a wavelength 
shifter, (d) a diluent in which the 
sample can be dissolved, and (e) 
naphthalene as a light restorer. 
Solvents that are frequently used to 
hold the dye are toluene, xylene, tri- 
ethylbenzene, ethylhexene, ete. Pref- 
erably one uses a solvent that will not 
attack Lucite, since Lucite is the most 
convenient container because of its 
high transparency. Terphenyl and 
diphenyloxazole (PPO) are the most 
useful dyes. 
188 
The purpose of the wavelength 
shifter is to absorb the light emitted by 
the dye at 3,500—-4,000 angstroms and 
re-emit it at wavelengths closer to the 
phototube peak of ~4,600 angstroms. 
Most commonly used is POPOP, stand- 
ing for phenyl-oxyzol-phenyl-oxyzol- 
phenyl. 
Samples that do not dissolve readily 
in the basic three-component mixture 
can be introduced after introduction of 
an appropriate diluent. Water can be 
introduced at the cost of increased 
light absorption, and quartenary amine 
bases make it possible to dissolve 
carbon dioxide and proteins. Some of 
the light lost upon introduction of 
diluents is restored to the detection 
system when napthalene is added. 
Electronics. Even when most of the 
energy of a tritium beta is efficiently 
converted into light in a scintillator it 
still produces a relatively weak pulse at 
the output of a photomultiplier. A 
counting system that is sensitive enough 
to detect these pulses is sensitive to 
many background pulses. The two 
devices that are used to reduce back- 
ground are coincidence counting of two 
phototubes looking at the same scintil- 
lator and pulse-height discrimination. 
Coincidence counting eliminates es- 
sentially all of the thermal-noise pulses, 
which have energies comparable to 
those that originate from tritium 
scintillations. Discrimination against 
high-energy pulses eliminates most of 
the pulses that originate from non- 
tritium radiation absorbed in the 
scintillator. Two-channel pulse height 
counting makes possible simultaneous 
counting of the two most useful organic 
tracer nuclides, tritium and C4. An 
upper channel counts only C!4, and a 
lower one counts tritium and some 
C4 pulses as well. 
Improvements in liquid-counting 
equipment are reducing technical diffi- 
culties. Figure 2 shows one example, 
an automatic sample changer that 
enables long counting runs without any 
exposure of sensitive elements to light. 
This saves time since extensive dark 
adaption is required for low-noise 
counting. 
Other Assay Techniques 
Most non-scintillation methods for 
tritium assay depend on conversion of 
the labeled compound into a suitable 
gas. Methods that have been found 
useful for conversion and assay are 
described by Fig. 3 and Table 8, and 
the bibliography (p. 67). Charles V. 
Robinson described these methods at 
FIG. 2. Automatic sample changer for 
liquid scintillation counting permits long 
runs without interruption by exposure to 
external light 
the symposium and prepared the figure, 
table, and bibliography. 
Radioautography. Photographic 
detection is also used with tritium 
betas, principally in making radioauto- 
graphs. An example is the figure 
shown in the box on page 64. The low 
energy of the particles is at once an 
advantage and a disadvantage. The 
short range in emulsions leads to ex- 
tremely fine resolution. On the other 
hand, exposures of several months are 
