The absorption coefficient yp in Equation 1.2 is applicable for narrow-beam geometry,
and a correction shoulc be made for field conditions where the detector is approximately
a 2m sensing element. This is done by adding a buildup factor B to Equation 1.2 to ac-
count for the scattered radiation that will be detected.
Buildup factors for different
energies and distances have been calculated (Reference 6), and some values are shown
in Table 1.2. For omni-directional detectors, the expression is:
Ip = Ip Bet /anD?
(1.3)
1.3.4 Hydrodynamic Effect. As shown in Section 1.3.3, the attenuation of gamma
radiation is highly dependent on the amount of absorber between the source and the detector. For devices of less than 100-kt yield, essentially all of the initial-gamma radiation is emitted before the shock front can produce an appreciable change in the effective
TABLE 1.2
CALCULATED BUILDUP FACTORS
The buildup factor (B) given here is the factor B,. (Hp D, Eg) as
computed by Nuclear Development Associates for AFSWP (Reference 6).
Energy (o)
7000
500
3,000
Mev
yds
yds
yds
1
3
4
10
16.2
3.85
2.97
1.70
29.3
5.35
4.00
2.01
85.0
10.2
7.00
2.90
absorption of the air between source and detector.
For high-yield devices, the velocity
of the shock front is sufficiently high to produce a strong enhancement of a large percentage of the initial-gammaradiation (Reference 7). The higher the yield, the larger
is this percentage. A simplified treatment of the hydrodynamic effect follows.
Assume a sphere that has a volume Vj and radius R, andis filled with a gas of density
Py and mass M. Then,
M = Vo po = 47R° 94/3
(1.4)
Let the gas be compressed into a shell with thickness AR (R remaining constant).
The new gas volume is expressed as V; (V; = 47K? AR) with a density of p,.
The mass
has not changed; thus,
M = Vopq = 47R? AR,
(AR «R)
47R3¥p,/3 = 47R? ARp;
(1.5)
16