for qualitatively by ignoring scatter contribution (Assumption 3).
‘The
assumption of uniform distribution is necessary for simplicity and is
justified in estimating an average dose.
The other three assumptions will
be treated in the development.
Derivation of Area Function
It is required to calculate the radiation incident on an elemental
volume dAdl three feet above an infinite plane of uniform gamma activity.
If we consider an infinitesimal plane source, ds, (Fig. 1) having
a length rar/ 72 _ g and width 7729 af, its area is
ds
=
rdr
*
VY re -9
dg
=
rdrdg
V re - 9
The area of the ring at a distance r from the elemental volume is,
Area
=
2 TY rdr
The fraction of the radiation (dI) from the area 2 q rdr adsorbed in the
elemental volume is
=
i
=
acos
(2 VY rdr)(e~a¥)(a
Q) (L a2)
hfyr-
cos8
where the four parenthetical terms are,
1.
The area of the ring.
2e
The attenuation in air (fa = air absorption coefficient).
3.
The fraction of the spherical radiation front intercepted by dA.
ls
—
oR
s
.
eee
.
.
a
=<
The fraction c2 the incident radiation absorbed in the elemental
volume.
7
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