Neutron-induced activity in the soil has been a subject of considerable study.
In sandy soil, radiosilicon may be formed with a 170-
minute half-life, emitting a 1.8 mev beta but no gamma, decaying to
stable phosphorous.
In clay soil, radioaluminum may be formed with a
2.4 minute half-life, emitting 2.7 to 3.3 mev betas and a 1.8 mev
gamma, and decaying to stable silicon.
In the coral sand of the
Pacific Proving Ground a considerable quantity of calcium-45 may be
formed.
Its relatively long half-life of 180 days, a 0.26 mev beta
with no gamma, and its position of importance in the biological system,
make it interesting.
However, the quantity formed by detonation of a
weapon over average soil would not result in a sufficient amount of
calcium-45 to constitute a hazard. Radiosodium may be formed in areas
where the concentration of sodium is high, and this may become importent
in detonations over and in sea water.
It is the considered opinion of
most investigators that forornear-surface bursts, induced
radioactivity in elements of th
1dwater is of minor importance
ee
to the over-all fall-out hazard, particularly so after the first day
following the detonation.
The area where ground induced activity may
importantto the area around ground zero.
The amount of
activity formed depends upon the type of bomb, height of burst, and
type of soil over which the detonation occurred.
Neutron-induced
activity in the ground is apt to be of greatest importance when the
weapon is burst in such a position that the fireball is just clear of
the ground.
In this case, there would be very little fission product
activity deposited locally, as will be discussed later; however, the
induced activity at ground zero might be as much as 2000 r/hrat one
hour after burst time for certain high neutron flux weapons such as
"
"Tt 4s important to note that the t~/°= redioactive decay
‘factor discussed previously would not be applicable in this case.
The
early decay would be apt to be slower but the later decay would be
faster then the t/°= formula would indicate. For example, high sodium
ccutent in the soil would probably give an effective half-life of about
10