203Principles of Nuclear Explosive Devices and Debris Analysis
231
Pa in undisturbed soil is dened by its secular equilibrium with
235
U (Chapter 2) and
i
s
r
oughly
p
roportional
t
o
t
he
r
atio
o
f
t
he
t
wo
h
alf-lives
(
33,000/7.0
×
1
0
8
= 4.7 × 10
−5
).
I
f
1
%
o
f
t
he
235
U inventory in a debris sample arising from a device with 20-year-old
H
EU
f
uel
i
s
d
ue
t
o
t
he
c
ontribution
o
f
s
oil,
t
hen
o
nly
4
%
o
f
t
he
231
Pa content of the
d
ebris originated from the weapon.
During
n
uclear
t
esting,
t
he
p
roblem
o
f
d
etermining
t
hermonuclear
(
or
f
usion)
e
fciency
r
eceived
at
l
east
a
s
m
uch
at
tention
a
s
d
id
t
he
i
ssue
o
f
d
educing
ssion
e
fciency.
H
owever,
o
ur
s
uccesses
i
n
f
usion
d
iagnostics
d
o
n
ot
t
ranslate
t
o
a
f
oren-
sic
method as readily as those involving ssion diagnostics. ...