Simultaneous-Tracer Trio 
These characteristics make these radiotracers appropriate for simultaneoous use: 
Isotope 
Ce! 44/ Pr'** 
Cr 
Sc** 
Ce'™ emits gammas in only about 40% of its disintegrations. 
daughter, 17-min Pr‘, emits essentially only 3.0-Mey beta particles. 
Half-life (days) 
Gamma energy (Mev) 
285 0.14 
0.32 
0.88/1.12 
Its radioactive 
Many of these 
latter have sufficient energy to penetrate the glass wall of the sample vial and the 
aluminum-MgO housing of the Nal crystal, producing a long high-energy tail to 
the observed Ce'*/Pr'** gamma-ray spectrum. To minimize this a steel insert about 
1 mm thick was used as a liner in the crystal well, improving the spectrum. 
Cr™ emits 0.32-Mev. gammas in 8% of its disintegrations and emits no other 
radiation other than very soft V"' X-rays resulting from K-capture of Cr’’. 
Sc“ emits essentially only 0.36-Mev betas and then two gammas in cascade. 
separated for adequate resolution with 
a scintillation counter and pulse-height 
analyzer, and they possess suitable 
half-lives. (See box above.) 
Samples removed from the commer- 
cial unit after 1.5, 6, and 39 hours, 10 
and 30 days were separated by sink- 
float methods into fractions of dif- 
ferent skeletal density, pore volume, 
and surface area (5). Only small frac- 
tionated samples were available (a few 
grams) so a 2-in. Nal scintillation well 
counter was employed—in conjunction 
with a Tracerlab window analyzer. 
Figure 1 presents the observed gamma- 
ray spectra obtained with this crystal 
and three standard samples, using the 
five-window analyzer as a_point-by- 
point spectrum analyzer; that is, meas- 
uring five equal channels simultane- 
ously (0-5, 5-10, 10-15, 15-20, 20-25 
volts, then 25-30, 30-35, etc.). From 
those spectra it was decided to count 
all of the samples in the following three 
channels: A, 10-40 volts; B, 40-80 
volts; C, 80-250 volts. 
FIG. 4. Patch panel chassis contains plug-and-jack arrangement 
to position windows and choose their widths 
Sharper spectra could have been ob- 
tained, especially for Sce*®, if a larger 
crystal thickness had been used. Ours 
was the largest well crystal available at 
the time. However, the 2-in. crystal 
gave adequate results. 
For calibration purposes pure sam- 
ples of each isotope were counted. The 
following list shows the percentage of 
the net counts falling in each of the 
selected channels in each of the three 
counts: 
A B C 
Cel*4 90.9 5.5 3.6 
Cre 30.0 69.7 0.3 
Gas 41.7 1929 38.4 
Each of the three channels contains 
the photoelectric peak of one of the 
three isotopes (A, Ce!4; B, Cr®!; C, 
Se**). Channel A also includes Comp- 
ton scattering pulses from Cr*! and Sc**, 
Channel B includes Compton pulses 
from Se*. The fraction of the pulses 
falling in the photopeak decreases with 
increasing gamma-ray energy (6). 
Counts 
1,000 
From the above distributions, the 
following equations were derived: 
Total Ce!#4epmin sample = 1.170 A 
— 0.499 B — 1.010 C 
Total Cr°' cpm in sample = —0.060 
A + 1.464 B + 0.694 C 
Total Sc*® cpm in sample = —0.110 
A + 0.035 B + 2.704 C 
In these equations A denotes the net 
sample cpm in Channel A, etc. The 
standard samples had total counting 
rates of about 10,000 epm, and were 
The 
time-weighted averages of the six de- 
terminations shown in the table were 
the values shown in the table above. 
The degree of reproducibility is indi- 
cated by the fact that six determina- 
tions were made at various times over 
a total period of five weeks. 
The data coded for the Electrodata 
computer were the sample and back- 
ground data (gross counts and counting 
time) from the five-window analyzer, 
date of counting, and weight of sample. 
The computer performed all necessary 
calculations for some 200 samples. It 
provided decay-corrected cpm/gm of 
each isotope with its standard devia- 
tion. The computer was not necessary, 
but it saved considerable time. 
Three-gram samples of our unknowns 
were counted in a standard Harshaw 
Nal well-counter crystal for 10-30 min. 
The over-all gamma-counting efficien- 
cies were approximately as follows: 
Ce144, 80%; Cr®!, 60%; Sc**, 40% for 
0.89 Mev, 30% for 1.12 Mev. 
The channels were selected to-mini- 
mize any effect of slight gain drifts and 
with 2-in. lead shielding had typical 
backgrounds of 110 epm, 55 cpm, 65 
cpm, respectively. 
counted for periods of 5-20 min. 
| 
Ccel4! 
145 kev 
: 
| 
20) oe 20. S02) 750. 
Channel Number 
40. 45 
Sass Side cas ee} 
FIG. 5. Gamma spectrum of Ce!*! plotted from automatically 
typed data of stepping analyzer (5 min per channel) 
17 
