Page 581 - Instrumentation Reference Book 3E

Miscellaneous measurements 563

which may contain ground-based survey teams when they were investigating the radioactivity
with suitable instruments can survey the actual produced by the interaction of cosmic rays from
areas pinpointed in the aerial survey as possible outer space with air molecules. They discovered
sources of uranium. that interactions with nitrogen in the atmosphere
produced radioactive 14C which quickly trans-
23.4.1.1 Land-based radiometrical surveys formed into 14C02, forming about 1 percent of
the total C02 in the world. As cosmic rays have
Just as the aircraft can carry suitable detector been bombarding the earth at a steady rate over
systems and computing equipment, so can land- millions of years, forming some two atoms of 14C
based vehicles, which can be taken to the areas per square centimeter of the earth’s surface per
giving high-uranium indications. While similar second, then an equilibrium stage should have
electronics can usually be operated from a motor been reached, as the half-life (time for a radio-
vehicle, the detectors will have to be smaller, active substance to decay to half its original value)
especia,lly if the terrain is very rugged, when of 14C is some 5000years. As C02 enters all
manually portable survey monitors will be called living matter the distribution should be uniform.
for. These also can now incorporate a small com- However, when the human being, animal, tree, or
puter, which can perform the necessary analyses plant dies, C02 transfer ceases, and the carbon
of the signals received from potassium, uranium, already present in the now-dead object is fixed.
thorium, and background. The 14C in this carbon will therefore start to
decay with a half-life of some 5000 years, so that
23.4.1..2 Undersea surveys measurement of the 14C present in the sample
allows one to determine, by the amount of 14C
Measurement of the natural gamma radiation still present, the time elapsed since the death of
from rocks and sediments can be carried out the person, animal, tree, or plant. We expect to
using a towed seabed gamma-ray spectrometer. find two disintegrations per second for every 8 g
The spectrometer, developed by UKAEA of carbon in living beings or dissolved in sea
Harwell in collaboration with the Institute of water or in the atmosphere C02 for the total
Geological Sciences, has been used over the last carbon in these three categories adds to 8 (7.5 in
ten years and has traversed more than 10,000 km the oceans, 0.125 in the air, 0.25 in life forms, and
in surveys of the United Kingdom continental perhaps 0.125 in humus).
shelf. It consists of a NaI (Tl) crystal-photo- There are several problems associated with
multiplier detector assembly, containing a crystal radiocarbon dating which must be overcome
76mm diameter x 76nim or 127mm long, with before one can arrive at an estimated age for a
an EM[I type 9758 photomultiplier together with particular sample. First, the sample must be treated
a preamplifier and high-voltage generator which so as to release the I4C in a suitable fonn for
are potted in silicone rubber. The unit is mounted counting. Methods used are various: depending
ir? a stainless steel cylinder, which in turn is on the final form in which the sample is required
mounted in a 30-m long flexible PVC hose and whether it is to be counted in a gas counter
173 mm diameter, which is towed by a cable from (Geiger or proportional) or in a liquid-scintillation
the ship, and also contains suitable ballast in the counter.
form of steel chain to allow the probe to be One method is to transform the carbon in the
dragged over the surface of the seabed without sample by combustion into a gas suitable for count-
becoming entangled in wrecks or rock outcrops. ing in a gas-proportional counter. This can be car-
The electronics on board the ship provide four bon dioxide (COz), methane (CH4), or acetylene
channels to allow potassium, uranium, and thor- (C2H2). The original method used by Libby, in
ium, as well as the total gamma radioactivity to be which the carbon sample was deposited in a thin
measured and recorded on suitable chart record- layer inside the gas counter, has been superseded by
ers and teletypes, and provision is also made to the gas-combustion method. In this the sample is
feed the output to a computer-based multichan- consumed by heating in a tube furnace or, in an
nel analyzer. improved way, in an oxygen bomb. The gas can be
counted directly in a gas proportional counter, after
suitable purification, as COz or CH4, or it can be
23.4.2 Dating of archaeological or geological transformed into a liquid form such as benzene,
specimens when it can be mixed with a liquid scintillator and
measured in a liquid-scintillation counter.
23.4.2. I Radiocarbon dating by gas-proportional
or liquid-scintillation counting Counting systems When one considers that
The technique of radiocarbon dating was discov- there are, at most, only two I4C disintegrations
ered by W. F. Libby and his associates in 1947, per second from each 8g of carbon, producing