
257
7
Techniques forSmall
Signatures
The last section discussed chronometry and how the concentrations of nuclides that
a
re
l
inked
b
y
d
ecay
ca
n
b
e
us
ed
t
o
d
etermine
t
he
t
ime
e
lapsed
s
ince
t
he
l
ast
c
hemical
p
urication.
I
n
m
any
i
nstances,
p
articularly
f
or
s
hort
t
imes
o
r
m
ore
d
istant
g
enetic
r
elationships,
t
he
at
om
r
atios
o
f
t
he
r
elated
s
pecies
ca
n
b
e
q
uite
s
mall,
a
pproaching
z
ero.
I
n
t
hose
ca
ses,
p
articularly
i
f
s
ample
s
izes
a
re
l
imited,
a
r
adionuclide
c
on-
centration
needs to be determined from very few atoms. This can be done by atom
c
ounting
o
r
b
y
d
ecay
c
ounting
a
nd
m
ay
r
equire
a
c
hemical
s
eparation
t
o
r
emove
a
n
o
verwhelming background activity ...