” 
= 
c 
=) 
> 
a 
° 
iS 
= 
-) 
a 
- 
> 
= 
= 
= 
° 
o ¢ 
100 200 300 400 
500 
600 700 800 1400 1500 
Absorber Thickness (mg/cm?) 
Activity (Arbitrary Units ) 
1 
35 40 45 50 55 60 65 70 75 80 
Time After Removal from Reactor (hr) 
selves. Similarly, the reactions hav- 
ing the highest cross section for 
thermal neutrons are of the (n,¥) 
type, but under actual irradiation 
conditions nonthermal neutrons may 
cause transmutation reactions [such 
as (n,a) and (n,p)]; producing radio- 
isotopes of the element in question 
from other elements originally present 
in the sample. 
Experimental Details 
All the samples analyzed were cut 
from existing spectrochemical stand- 
DECAY (left) and absorption 
(above) properties of Na” sepa- 
rated from irradiated aluminum 
alloys and pure sodium salts. 
Close correspondence (parallel to 
within 1 %) shows effectiveness of 
purification procedure in eliminat- 
ing all radioisotopes other than 
Na?4, Curves 1 and 2 show pure 
sodium salts, curves 3-7 show so- 
dium separated from aluminum 
alloys. Decay measurements are 
followed for three sodium half- 
lives 
ards. One-gram disks machined from 
the sparking area were leached with 
nitric acid, then wrapped in aluminum 
foil for irradiation. Sodium bicarbon- 
ate and sodium carbonate (10-30 mg) 
were the primary standards irradiated 
with the alloy samples. 
Packaging. Three types of pack- 
aging for the sodium salts were in- 
vestigated with respect to neutron at- 
tenuation by the container. A quartz 
vial (1-mm wall) gave 5% lower flux 
than an aluminum vial. Use of Vycor 
resulted in a 16% lower flux on the 
sodium salt than did aluminum. A 2S 
aluminum tube 5-mm i.d. and J-in. 
long, crimped at each end and cleaned 
with Alcoa R-5 Bright Dip (U.S. pat- 
ent 2,650,157) was adopted. 
Samples were irradiated in a stand- 
ard aluminum irradiation can (0.75-in. 
id. X 2.875-in. long) for one week at 
the Oak Ridge X-10 reactor. 
Irradiation conditions. Irradiation 
of aluminum alloys leads, in general, to 
the production of Na?‘ from sodium 
(n,y), aluminum (n,qa), and magnesium 
(n,p). Specific activity of the final 
salt is given by (K X Na+ K’ X Al + 
K" X Mg)/(Na + carrier Na), where 
the K’s are activation constants, and 
the element symbol stands for the 
weight of the element involved. 
In the first work, the contribution of 
Na* activity from transmutation re- 
actions put a serious limitation on the 
sensitivity and precision. By acti- 
vating samples of super-purity alumi- 
num and magnesium, positions in the 
reactor that reduce K’ and K” to very 
small values were found. The appar- 
ent sodium content of the super-purity 
aluminum as determined after expo- 
sure in several positions was: 
Apparent content from 
Reactor position Al?? (n,«) Na*4 reaction 
14-A-14 0.014% Na 
13-H-7 0.005% Na 
14-Z-20 0.00019% Na 
All further activations were carried 
out in positions 14-Z-19 through 
14-Z-21 in which super-purity alumi- 
num gave from 0.00014-0.00020% 
sodium. The lower flux in these posi- 
tions (as compared to the maximum 
flux available in the reactor) was of no 
serious consequence. 
After irradiating the samples, the 
sodium formed is chemically separated 
and its activity determined. The pro- 
cedures for these steps will be presented 
after first considering the irradiation 
conditions in more detail. Specifi- 
cally, the flux received by samples in 
different positions in the container will 
be examined. 
Flux variations. Two experiments 
were used to detect and evaluate flux 
variations over the can dimensions 
that might occur from flux inhomoge- 
neities in the irradiation can’s immedi- 
ate vicinity. 
Liners of tin were placed inside the 
cans, the cans irradiated, and the 
specific activity at various points on 
the liner determined. There was a 
197 
