DNA 1240H-2
energy spectrum of the radioactive sources; (2) specification of the
major ship characteristics, particularly the shielding configuration
for the point considered and the nature of the shielding materials; (3)
development of methods for computing the interaction of the radiations
with the ship.
Basically, the method presented of calculating ship-shielding factore
for the transit dose (from a volume radioactive source surrounding the
ship) is a point-by-point calculation. The radioactive source region is
considered to be made up of an aggregate of point iatropic sources. The
dose rate from each source is calculated at a given location by commiting
the radiation attenuation along the entire path length, and the total
dose rate is found by summing over all sources.
All the energies in
each source spectrum as well as all the sources mst be sumed.
In
practice, the summation process is replaced, to whatever extent poss-
ible, vy integration.
The theoretical development of ship-shielding calculations is based on
ean idealized concept of the interaction of radiations with e ship.
The
expression of the shielding factor for the transit dose was developed
from the expression of dose rate due to a point source. For a point
isotropic source emitting 1 photon/second of energy Ey (Mev/photon), the
exposure dose rate d, (r/hr) at a distance x (cm) from that source ina
homogeneous medium can be expressed by:
dy =
K Hy, Ey By ed® r/hr
Luscx
(17-13)
vhere k = a factor to convert Mev absorbed in a cm or gm of the
medium per second to r/pr.
3
If up te in units of em,» k = 5.086 x 10° r/hr per Mev/cm>-eec
If 4A is in units of cm°/em, k = 6.6 x 10-9 r/nr per Mev/gm-sec
Hai
«= the energy-absorption coefficient for air corresponding to the
quantum energy E,
By = the infinite-medium dose buildup factor®
Ky
= the linear total absorption coefficient for the medium.
Then the exposure dose rate d (r/hr) at the same distance x (cm)
from a point isotropic source emitting ny (photone/sec) quanta of energy
E,
(Mev/photon) in a homogeneous medium can be expressed as the eum of
the exposure dose rates due to all the emitted energies:
*This buildup factor is defined as the ratio of the dose from both
unscattered and scattered radiation to the dose from unscattered
radiations only.
17-56
ee ee
eee: