2. Reduced chemically to methane or ethane, or to a carbide from which
acetylene can be evolved by adding water. (See booklet cover and
description on page 59.)
The first technique is the simpler, but carbon dioxide (CO₂) contains
only one atom of carbon per molecule, whereas acetylene (C₂H₂) and
ethane (C₂H₆) each contain two. Consequently, the SPECIFIC ACTIVITY of
acetylene or ethane is twice that of carbon dioxide, other things being
equal. For that reason acetylene or ethane are the preferred gases in
some laboratories. On the other hand, they are explosive, and that
cautions other scientists into using the carbon dioxide method.
Whichever gas is used, it is first purified and then stored in a bottle
for a month or so. This storage allows for decay (disappearance) of any
radon, the gaseous radioactive product of uranium decay. Uranium
contamination is difficult to avoid at the low radioactivity levels of
¹⁴C, but the half-life of radon is only 3.82 days, so that it will decay
to an insignificant level in a month. After the storage period, the gas
is pumped into an array of instruments known as a low-background
PROPORTIONAL COUNTER, and its radioactivity is determined. This is an
involved process. First there is the matter of the BACKGROUND COUNT.
The background count of an instrument is the number of pulses (counts)
it will give per unit time when there is _no_ radioactive sample in it.
These counts are caused by cosmic rays, by radioactive contamination
always found in the vicinity of the counter, or by any contamination
inside the instrument. In ¹⁴C counting, all these sources of background
must be reduced to negligible levels. This is done in a number of ways.
For one thing, the whole assembly is constructed with surrounding walls
of lead or iron more than a foot thick. Such a shield will stop all the
GAMMA RAYS coming from radioactive contamination in the laboratory and
much of the cosmic radiation; high-energy cosmic rays and all neutrons
still will get through. Therefore, there is an ANTICOINCIDENCE RING
inside the lead shield. This is a cylindrical space completely
surrounded by GEIGER COUNTERS that are connected to each other and to
the SAMPLE COUNTER in the middle. With this arrangement, when the sample
counter and any ring counter discharge simultaneously, it is a signal
that the pulse triggering this response was caused by some energetic
particles, such as a cosmic ray, passing through the whole assembly. A
pulse recorded simultaneously on two counters is automatically rejected
from the counting mechanisms. Some instruments have been designed with a
cylinder of paraffin immediately inside the anticoincidence ring, to
slow down neutrons so that they can be captured, and with a final shield
of highly purified mercury between two cylinders of selected steel to
hold out even more unwanted radiation.
[Illustration: ]
[Illustration: _A carbon-14 counter. Sketch (above) shows arrangement of
components in photo (left)._]
Finally, the sample counters are made of specially selected metal tubing
that is extremely low in radioactive content, with a fine wire stretched
down the middle. (In some recent designs, the anticoincidence ring and
sample counter are combined in a single cylindrical housing with a thin
foil of metallized plastic between them.) A thin glass filling tube
connects the sample counter with the outside world.
[Illustration: _Detail of one type of sample-counting tube for carbon-14
work. Carbon-14 in benzene gas molecules is placed in the central cell
and mixed with a fluid that scintillates, or emits light flashes, when
exposed to radiation. Photomultiplier tubes convert the flashes to
electric signals._]
The radiocarbon-bearing gas is pumped into the sample counter through
the filling tube, and all the counts resulting from its disintegrations
are recorded electronically. The age of the sample is calculated from
the NET COUNTING RATE (the sample counting rate minus the background);
the lower the counting rate the higher the age. The upper limit of the
age that can be measured is determined by the STATISTICAL ERROR (that
is, by the measure of the instrument accuracy) in the net count. In very
old samples this error may be great enough so that the calculated age of
the sample may have little or no meaning.