TABLE 3—Counting Efficiencies vs Thickness for Billet No. 2 
Gamma Relative 
Nominal Surface Total counting counting 
thickness Weight density activity rate Efficiency efficiency 
(in.) (gm) (gm/cm?) (curies) (cpm) (%) (%) 
0.050 30:7, 1.14 0.614 X 1077 12,275 9.0 100 
0.100 59.7 2.03 1219 < 10>? 20,544 (25 94 
0.250 153.1 §.21 3.03 X 1077 44,712 6.65 73 
0.500 311 10.6 6.22 X 1077 71,732 5.2 63 
1.00 567 19.3 134 x< 1057 92,778 3.7 45 
25, 720 24.5 14.4 xX 107 98,665 aol 37 
125 878 29.9 17.56 X 1077 ~—: 102,962 2.6 32 
1.75 1,031 35.1 20.62 X 1077 =: 107,575 2.35 28 
2.00 1,184 40.3 25505) 6 LO? 23 (est) 
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For steel containing 2.00 x 10° curies/gm of Ta 
182 
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0.75 
1.0 125 
Nominal Thickness (in.) 
RELATIVE counting efficiency vs thickness 
shield and other effects; however, it is 
more than sufficiently accurate for the 
purpose at hand. 
Dose-rate factor. The preceding 
activity measurements have all in- 
volved counting rates determined with 
a scintillation counter and are not 
readily converted into dose rates. To 
obtain a factor for converting activity 
in curies into dose rate, the following 
simple experiment was carried out: 
The dose rate was measured at vari- 
ous distances from a nominal 1-me 
Ta!® source obtained from Oak Ridge 
National Laboratory. A 47 count of 
an aliquot of the solution established 
the total absolute beta activity of the 
source as 0.76 me. The instrument 
used in the dose-rate measurement was 
a recently calibrated ‘Cutie Pie” 
gamma survey instrument.* 
The results for the range of interest 
were: 0.2 mr/hr at 2.0 meters; 0.4 mr/ 
hr at 1.5 meters; 0.9 mr/hr at 1.0 
164 
meters, and 3.8 mr/hr at 0.5 meters. 
Note that the inverse square law is 
obeyed to a first approximation. 
Conclusions 
The experiments show that steels 
containing amounts of Ta!*? of the 
order of 1 X 10~° curies/gm (1 mc/ 
metric ton) can be readily identified 
with counting equipment commonly 
available in radiochemical laboratories. 
For large pieces—1 lb and greater—a 
beta-gamma survey meter is sufficient. 
Smaller parts require the use of a 
scintillation counter. 
What remains is to calculate the 
minimum specific activity that meets 
our detection condition, viz., ability to 
detect the activity in 1 gm of metal 
after 1 yr, and, using this, to estimate 
the maximum probable dose rates. 
* For example, radiation survey meter, 
model SU-1E, Tracerlab, Boston, Mass. 
With a sample-to-background ac- 
tivity ratio of unity and a background 
of 50 cpm, the required final specific 
activity is 0.31 X 10° curies/gm. 
Thus the initial specific activity is 
2.5 X 107° curies/gm or 2.5 mc/ton. 
In estimating the radiation dose rate 
in the neighborhood of a large accumu- 
lation of such steel, we assume the 
particularly severe case of a hollow 
sphere 1-in. thick and 1 meter in radius. 
The total mass of such a sphere would 
be about 2.5 X 10° gm, the total ac- 
tivity about 6.3 mc. At the center of 
the sphere, the dose rate would be 
6.3 X (0.9/0.76) X 0.45 = 3.4 mr/hr, 
where the second factor is the dose-rate 
and the third is the relative efficiency 
for metal 1-in. thick. This dose rate is 
only about half that deemed safe for 
day in and day out exposure (4). An 
inspection of the relative efficiencies in 
Table 3 shows that significantly higher 
dose rates would not be obtained by 
increasing the thickness of the sphere. 
Since an inverse-square law relates dose 
rate to distance from the source, de- 
creasing the sphere radius, while main- 
taining the same mass, does not in- 
crease dose rate. 
The conclusion is that it is entirely 
safe, insofar as personnel are con- 
cerned, to employ radioactive tracers 
in amounts of the order of 1 mc/ton 
for tagging special melts of ferrous 
or other alloys. However, it should 
be realized that this activity is 220 
times the maximum activity of con- 
taminated scrap steel (10 dpm/gm) re- 
leased by the AEC and considered as 
technically nonhazardous to sensitive 
industries. 
* * * 
The assistance of the following individuals 
and departments of the Knolls Atomic Power 
Laboratory is gratefully acknowledged: Mr. 
F. D. Nicol and other members of the Manu- 
facturing Unit for many helpful discussions 
pointing up the importance of the problem; 
Dr. D. W. White and Mr. R. V. Gray of the 
Metallurgy Section for preparing the steel 
billets; Special Material Shop for machining 
the billets; and Mrs. A. C. Mewherter of the 
Chemistry Section for assistance in carrying 
out the radiochemical work. 
BIBLIOGRAPHY 
1. K. Way, etal., Nuclear data, NBS Circular 499 
(National Bureau of Standards, Washington, 
D. C., 1950) 
2. J. M. Hollander, D. Perlman, G. T. Seaborg, 
Revs. Mod. Phys. 25, 469 (1953) 
8. C. J. Borkowski, Conference on Absolute 
Beta-Counting, Preliminary Report No. 8, 
Nuclear Science Series, National Research 
Council, Washington, D. C. (Oct. 1950); Anal. 
Chem. 21, 348 (1949) 
4. “Radiological Health Handbook," issued by 
the Federal Security Agency, Public Health 
Service, Environmental Health Center, Cin- 
cinnati, Ohio 
