particle. Thus the Znumber of the nucleus is decreased by 1 from K (Z=19) to
Ar (Z=18), but the mass number is unchanged. This so-called gamma radiation
(g) is essentially a photon that carries a large amount of electromagnetic energy.
This transformation is very important for the atmosphere as it produces the stable
form (isotope) of argon which emanates from the potassium-containing rocks of
the earth and accumulates in the atmosphere.
Unstable heavy elements with an excess of protons in the nucleus decay to
produce radiation as an aparticle (alpha decay), which is in fact a helium (He)
nucleus, for example:
eqn. 2.13
As the aparticle loses energy, it picks up electrons and eventually becomes^4 He
in the atmosphere. As the helium nucleus contains two protons and two neutrons
the nucleus Znumber changes from that of U (Z=92) to Th (Z=90), while the
mass number decreases by 4. Another source of helium is the alpha decay of
radium (Ra):
eqn. 2.14
which also produces the inert, but radioactive, gas radon (Rn) discussed in Box
4.13.
Other heavy elements with an excess of neutrons in the nucleus decay by trans-
forming the neutron into a proton by ejecting an energized electron known as a
negative beta particle (b-). An example of beta decay is:
eqn. 2.15
As one neutron has been transformed into a proton the Znumber in the nucleus
is increased by 1 from Rb (Z=37) to Sr (Z=38), but the mass number is
unaffected.
While many radionuclides are natural, human activities have produced either
artificial radionuclides, or have greatly increased levels of otherwise natural ones.
These anthropogenic radionuclides are produced by nuclear power generation
(e.g. power stations, satellites and submarines), by reprocessing of nuclear waste
or from nuclear weapons. For example, the atmospheric testing of nuclear
weapons in the 1950s and 1960s vastly increased the concentrations of tritium
(^3 H),^14 C and^137 Cs (caesium) and dispersed them worldwide. Consequently, the
fallout of these isotopes from the atmosphere also increased, producing a char-
acteristic ‘spike’ increase in their flux to the surfaces of the oceans and the land.
This sudden arrival of radionuclide has been used to trace movements of water
masses and mixing rates in the oceans (see Box 7.1), while its burial in sediments
(e.g. saltmarshes) can be used as a time marker.
Nuclear weapons testing was deliberate; however many other releases of
radionuclides are accidental. These have included fires and spillages at nuclear
reprocessing plants resulting in releases of an assortment of nuclides to the atmos-
phere and the oceans, including super-heavy elements from the actinide group of
the Periodic Table (Fig. 2.2) such as plutonium (Pu). Similarly, accidental sinking
of nuclear submarines has released radionuclides to the bottom waters of the(^87) RbÆ (^87) Sr+b-
(^226) RaÆ (^222) Rn+a
(^238) UÆ (^234) Th+a
28 Chapter Two