Geoscience Reference
In-Depth Information
a trail of grains of silver, but in the case of, say, a nucleus of iron (charge 26),
one sees a hairy sausage-like cylinder penetrating the emulsion.
Another way of detecting particles uses solid-state track detectors. Tracks
can be registered in many important mineral crystals and in a number of com-
mercially available plastic sheets. A highly charged particle that passes through
the plastic sheet produces sufficient damage for later etching (usually in NaOH)
to reveal the damaged zone, which is dissolved more rapidly than the undam-
aged material. Under the microscope, one sees two cones, one on each side of
the plastic, marking the entry and exit points of the particle. An important fea-
ture of all solid-state track detectors is that they have a threshold damage level
below which no track is produced. This enables one to detect a minute number
of, say, fission particles among a very large number of particles that leave no
tracks. This is relevant in detection of spontaneous fission in uranium-containing
mineral crystals. One of the most noteworthy results of this technique was the
discovery of tracks of spontaneous fission of plutonium-244 in crystals in certain
meteorites.
14
When a surface of a rock or mineral is cut and polished and then etched
in a suitable solvent, tracks of these fission products of
238
U, or
fission tracks
,
are visible under a microscope because the very numerous
particles do not
register. Thus, it is possible to use fission tracks to date geological samples.
For dating meteorite samples, one has to be concerned about the now-extinct
244
Pu; in principle, a correction could be needed for Archaean terrestrial samples
also.
Consider a small polished sample of a mineral and assume that at present it has
[
238
U]
now
atoms of
238
U distributed evenly throughout its volume. The number
of radioactive decays of uranium 238,
D
R
, during time
t
is given by Eq. (6.11)as
[
238
U
]
now
(e
λ
t
D
R
=
−
1)
(6.56)
is the decay constant for
238
U decay.
The number of decays of uranium-238 by spontaneous fission,
D
S
, occurring
in time
t
is then
where
λ
λ
S
λ
[
238
U]
now
(e
λ
t
D
S
=
−
1)
(6.57)
λ
S
is the decay constant for the spontaneous fission of
238
U. To use
Eq. (6.57)tocalculate a date
t
,wemust count the visible fission tracks. In addition,
we must estimate what proportion of the fission tracks produced in the sample
crossed the polished surface and so became visible and therefore countable. We
must also measure [
238
U]
now
.
where
14
Plutonium was probably initially present on Earth with an abundance
∼
10% that of uranium. It
decays principally by
-particle emission, but in one in 10
4
cases it decays by spontaneous fission.
The activity on Earth of plutonium-244 is now essentially extinct since it has a half-life of only
8
10
7
2
−
60
10
−
20
.
×
yr. Thus, the proportion remaining is
∼
=
