NUCLEONICS DATA SHEET No. 27 
Gamma-Ray Shielding 
50 Glass A 
P=27gm/cm3 
Buildup Factor 
So 
oa 
5 50 
10 
Thickness (Hor) 
FIG. 1. 
Glass B 
P=3.3 gm/cm3 
Buildup Factor 
5 10 50 
Thickness (or } 
Shielding-Glass Buildup Factors 
By JOHN W. LINDNER,* Pittsburgh Plate Glass Co., Pittsburgh, Pennsylvania 
Gamma-ray buildup factors for three 
shielding glasses are presented in the 
accompanying figures. The computa- 
tions have been based on standard 
methods (1-4). These data enable 
the design of a shielding window 
giving the desired protection at a 
minimum cost. Previously it has been 
customary to correct for buildup by 
adding ~15% to the computed thick- 
ness. The data of Fig. 1-3 show that 
any rule of thumb like this is valid 
only for a single photon energy. 
Computations. To compute build- 
up factors for a homogeneous mixture 
such as glass, it is only necessary to 
compute an effective atomic number 
for the glass (1). This can be done by 
(a) calculating the mean absorption 
coefficient per electron for the mixture 
and then (b) finding the element having 
the same absorption coefficient in the 
energy range over which Compton 
scattering is important. 
The absorption coefficients of Fig. 2 
* PRESENT ADDRESS: Space Technology 
Laboratories, Los Angeles, Calif. 
were calculated from the data of 
Gladys White Grodstein (5). For 
energies of 0.1-2.0 Mev, the calculated 
absorption coefficients of the glasses of 
densities 2.7, 3.3, and 6.2 gm/cm? agree 
to within +3% with those of elements 
having atomic numbers 11, 54, and 
73, respectively. The difference at 
4 Mev is still less than 10%. 
The dose buildup factors for the 
three glasses (Fig. 1) are plotted as 
a function of glass thickness expressed 
in relaxation lengths. 
Glasses. The compositions of the 
glasses for which the computations 
were carried out are given in the table. 
Glasses A and B correspond to Pitts- 
burgh Plate Glasses No. 6740 and No. 
4966, respectively. Glasses of cor- 
responding densities produced else- 
where can be considered identical 
for calculating buildup factors. Dif- 
ferences in composition do not have 
significant effect on effective atomic 
number. Glass C is an experimental 
glass that corresponds to commercially 
available 6.2-gm/cm! glass. 
Compositions (wt%) of Three Shielding Glasses 
Density 
Glass (gm/cm) Si02 PbO 203 Na:0 BaO K.0 CeO2 
A Piet 64.8 = 3.0 6.5 15 9.0 Ths?A 
B 3.3 49 34.8 — 0.5 —_ 14 Levi 
Cc 6.2 5.5 82 12.5 — — — 
250 
Glass C 
P=6.2 gm/cm3 
10 50 
Thickness (por) 
Computed buildup factors for three glass compositions and six gamma-ray energies 
Linear Absorption Coefficient (in-!) 
05 Glass C 
“01 05 10 5 
Eo(Mev) 
FIG. 2. Calculated coefficients 
Accuracy. Goldstein and Wilkins 
claim an accuracy of +5% for their 
calculations. After making allowance 
for probable errors in the choice of 
effective atomic numbers and graphical 
interpolation, the probable errors in the 
final results should be < +15%. 
However, for shield thickness greater 
than 20 relaxation lengths, the error 
may be as great as +20%. 
* * * 
The author wishes to express his gratitude 
to Mr. Carlo Del Carlo, who prepared the 
curves. 
BIBLIOGRAPHY 
1. H. Goldstein, J. E. Wilkins, Jr.. NYO-3075 
(1954) 
2. L. V. Spencer, U. Fano, J. Research Nat. Bur. 
Standards 46, 446 (1951) 
3. L. V. Spencer, U. Fano, Phys. Rev. 81, 464 
(1951) 
4. U. Fano, nucueonics 11, No. 8, 8 (1953) and 
11, No. 9, 55 (1953) 
6. G. W. Grodstein, National Bureau of Stand- 
ards Circular 583 (1957) 
