NUCLEONICS DATA SHEET No. 20 
Shielding 
Gamma-Ray Streaming Through a Duct 
By DAVID G. CHAPPELL 
Knolls Atomic Power Laboratory, General Electric Co., Schenectady, New York 
PRACTICAL DESIGN of biological shields 
for radiation sources often involves 
accounting for the gamma flux that 
streams through a cylindrical duct that 
penetrates the shield. 
This nomogram permits rapid deter- 
mination of the geometrical attenua- 
tion of the gamma flux streaming 
through the duct from an emitting sur- 
face upon which the duct abuts per- 
pendicularly. Scattering effects are 
not accounted for. ; 
The formula for the attenuation* is 
where K is ¥4, 1, 1.268 for a spherical, 
cosine and Fermi emitter, respectively; 
D is the diameter of the duct; and L 
is the duct length in identical units. 
The formula is valid if L>D, i.e., 
long, thin, straight cylindrical ducts 
for which (D/2)? is negligible compared 
with L?. 
Example: 0.8-in.-diameter duct pene- 
trates 2l-in.-thick shield and abuts 
perpendicularly onto a cosine emitting 
surface. As shown, the nomogram 
gives A = 3.5 X10‘ for a cosine 
emitter, 4.6 X 10-4for a Fermi emitter. 
Attenuation for a spherical emitter is 
half that of the cosine emitter, or 
1.8, >< 10:4. 
* * * 
1 wish to acknowledge the assistance of Miss 
Joyce Berger, who drew the nomogram. 
Knolls Atomic Power Laboratory is operated 
by the General Electric Co. for the U.S. Atomic 
Energy Commission under contract No. 
W-31-109 Eng. 52. 
* “Reactor Shielding Design Manual,” 
T. Rockwell III, editor, p. 290 (McGraw- 
Hill Book Co., New York, 1956). 
228 
Duct diometer 
D L 
0 D 
_ 
ik 
$o —>? 
' 
' 
oO 
10° a 
- 107! 
10 
s 1072 
= j07% 
§ 
\ E & 
o © 
10 
0.9 Fe z 
08 ee : 
O7 ae g 
06 s. = 8 
$ 
05 ra 20 
rs) 1074 
04 5 
fa) 
30 
03 
1i0°° 40 
0.2 50 
60 
108 70 
80 
90 
0.1 100 
Duct length 
