112 Nuclear Forensic Analysis
species
235
UX,
238
UX,
235
UZ, and
238
UZ. If the U occupies a chemically active site
i
n
e
ach
m
olecule,
t
hen
i
sotopic
e
xchange
c
an
o
ccur:
235
UX +
238
UZ ⇄
235
UZ +
238
UX.
An
e
quilibrium
i
s
e
stablished
i
n
w
hich
t
he
r
elationship
b
etween
t
he
c
oncentrations
o
f the four species is dened by an equilibrium constant K:
K = × ×([ ] [ ]) / ([ ] [ ]).
235 238 235 238
UZ UX UX UZ
Statistical
me
chanics
w
ould
r
equire
t
hat
K
=
1
f
or
a
ny
s
uch
r
eaction,
b
ut
q
uantum
mechanics allows different isotopic afnities among the compounds, based on
e
nergy
l
evels
o
f
v
ibrational
s
tates,
t
ending
t
o
m
ake
l
ighter
i
sotopes
c
oncentrate
i
n
l
ess
t
ightly
b
ound
m
olecules
[
17].
T
he
F
rench
C
hemex
p
rocess,
w
hich
e
xploited
a
v
ery
s
light
d
ifference
i
n
t
he
I