Page 108 - Radiochemistry and nuclear chemistry

Radionuclides in Nature 97

useful e.g. for determining ice ages. However, due to the much larger content of
anthropogenic tritium presently, this is no longer a useful technique for such dating.
Tritium in concentrations as low as 1 TU can be measured in low background
proportional counters, and, after isotope enrichment (e.g. by electrolysis of alkaline water,
by which tritium is enriched in the remainder), down to 0.01 TU. For very low
concentrations mass spectrometry is preferred.

5.1.3. 14-Carbon
14C iS produced in the atmosphere by a variety of reactions, the most important being
between thermalized neutrons from cosmic radiation and nitrogen atoms:

n(slow) + 14N-* 14C + 1H (5.2)

This reaction occurs with a yield of approximately 22 000 atoms 14C formed per s and m 2
of the earth's surface; the global annual production rate is - 1 PBq, and global inventory
-.8 500 PBq (corresponding to --75 tons). Of this amount --140 PBq remain in the
atmosphere while the rest is incorporated in terrestrial material. All livin~ material (incl.
body tissue) has a 14C concentration of --227 Bq/kg. The half-life of t')C is 5715 y; it
decays by soft/T emission (Ema x 158 keV).
14C is also formed by reaction (5.2) in nuclear tests. From these 220 PBq is assumed to
have been injected into the atmosphere up to 1990. This 14C comes to equilibrium with
other atmospheric carbon (CO2) in 1 - 2 years. Some 14C, about 18 TBq/GW e per year,
is also released from nuclear power plants (mainly from HWR, Ch. 19 and 22). The global
atmospheric value is < 300 TBq/y.
The combustion of fossil fuel adds CO 2, which is almost free of 14C, to the atmosphere,
thus reducing the specific activity (the dilution was about 3 % for the period 1900 - 1970).
Taking all anthropogenic sources into account, a global average specific activity of modern
carbon is now 13.56 + 0.07 dpm/g C. In w we discuss dating by the "C14-method",
for which such a figure is important.

5.2. Primordial radionuclides

5.2.1. Very long-lived nuclides lighter than lead

As the detection technique for radioactivity has been refined, a number of long-lived
radionuclides have been discovered in nature. The lightest have been mentioned in w
The heavier ones, not belonging to the natural radioactive decay series of uranium and
thorium, are listed in Table 5.2. 5~ is the nuclide of lowest elemental specific activity
(---0.0001 Bq/g) while the highest are 87Rb and 187Re (each ---900 Bq/g). As our ability
to make reliable measurements of low activities increases, the number of elements between
potassium and lead with radioactive isotopes in nature can be expected to increase.