energy region.
For most weapons and soils, this is the most important source of ther-
mal neutrons which cause most of the induced activity.
Properties of soil elements
pertaining to this most important effect are ignored. Elements are weighted strictly
on the basis of the relative number of thermal neutrons they absorb.
2. No allowance is made for the distribution of activity with depth in soil nor for
properties of the soil constituents which influence this distribution.
3.
type.
The neutron spectrum is assumed to be invariant with slant range and weapon
In other respects, too, this method was developed from unrealistic assumptions.
From a practical standpoint, however, the shortcomings above are the most important.
1.4
PREDICTION METHOD
A prediction method (Reference 4) has been developed which requires the following:
1.
Soil Data.
2.
Neutron Data.
Soil density (gm/cm') and percentage by weight of Na, Mn, Al.
Gold neutrons versus slant range and total number of 14-Mev
neutrons per kt produced external to the case.
1.4.1 Basie Theory.
Briefly, the theory for this method is developed as follows:
Ny = time-integrated neutron density from an isotropic source in air through
N
a thin layer of soi] at the surface
_
= time-integrated neutron density through a thin layer at depth X below the
surface
Then,
N = £(X)Ng
(1.1)
where f{(X) depends on the soil and the incident neutron spectrum.
The number of neutrons absorbed by a particular isotope in a layer of unit area dx
thick at depth X is
where 0; is the number of atoms per unit volumeof an isotope (considered constant with
depth), and k;(X) depends on the capture cross section for this isotope and the neutron
spectrum at X~ The contribution made by the radioactive daughter of this isotope to the residual
gamma-radiation field above the surface is
di = gi(X)dmje
.
-A\
it
(1.3)
The gamma-ray attenuation function, gj(X), depends primarily on the density of the
soil and the energy of gamma radiations from the radioactive daughter. When there
is no gamma-emitting daughter, B(X) = 0.
Radiation of the soil by neutrons is considered to be instantaneous att = 0.
No
daughter-daughter gamma-emitting products are considered since the neutron-induced
10