Encyclopedia of Chemistry

ration. Narrow fission tracks from the release of high-
energy charged alpha particles burn into the material as
a result of the decay of uranium 238 to lead 206 (half-
life of 4.51 billion years) or induced by the irradiation
of uranium 235 to lead 207 (704 million years). The
number of tracks is proportional to the time passed
since the material cooled from its original molten con-
dition, i.e., fission tracks are created at a constant rate
throughout time, so it is possible to determine the
amount of time that has passed since the track accumu-
lation began from the number of tracks present. This
technique is good for dates ranging from 20 million to
1 billion years ago. U-238 fission track techniques are
from spontaneous fission, and induced fission track
from U-235 is a technique developed by controlled

irradiation of the artifact with thermal neutrons of the

U-235. Both techniques give a thermal age for the

material in question. The spontaneous fission of ura-

nium 238 was first discovered by the Russian scientists

K.A. Petrzhak and G.N. Flerov in 1940.

Potassium-Argon Dating

This method has been used to date rocks as old as 4

billion years and is a popular dating technique for

archaeological material. Potassium 40, with a half-life

of 1.3 billion years in volcanic rock, decays into argon

40 and calcium 40 at a known rate. Dates are deter-

mined by measuring the amount of argon 40 in a sam-

ple. Argon 40 and argon 39 ratios can also be used

for dating the same way. Potassium-argon dating is

radiometric dating 229

Nuclear waste storage. Steel drums containing nuclear waste at a dump at a nuclear weapons testing site. The drums contain solid
waste that is made up of transuranics, radioactive chemical elements with heavy atomic nuclei that are made in nuclear reactions.
These drums will be packed, 50 at a time, into steel cargo containers for storage aboveground. Corrosion of these containers will be
minimized because of the dry desert climate of the site. Photographed at the Area 5 Radioactive Waste Management Site at Frenchman
Flat in Nevada.(Courtesy of U.S. Department of Energy/Science Photo Library)