Download Beryllium isotopes in geochronology Cosmogenic Be and Be

Stable
isotope
9
Be
Relative
atomic mass
9.012 1831
Mole
fraction
1
Beryllium isotopes in geochronology
Cosmogenic 10Be and 7Be isotopes are produced in the atmosphere, largely by cosmic-ray
spallation of nitrogen and oxygen. Because of its relatively short half-life (7 Be, half-life = 53 d,
compared to that of 10Be, half-life = 1.39 x 106 years), measurements of cosmogenic 7Be, and
especially the isotope-amount ratio n(7Be)/ n(10Be), can be used to study rates of atmospheric
circulation, mixing, formation of aerosols (fine solids or liquids suspended in a gas; e.g., smoke
and mist are aerosols), and particle deposition [34-37]. Cosmogenic atmospheric beryllium
isotopes (7Be and 10Be) are deposited on the Earth’s surface where they accumulate in soils,
sediments, and snow while decaying away. Thus, measurements of cosmogenic beryllium
isotopes in such deposits can be used to explore rates of soil formation, erosion, sedimentation,
and snow accumulation on time scales ranging from months (7Be) to millions of years (10 Be)
[34-37]. The minerals in rocks at the Earth’s surface interact with cosmic rays and form
substantial quantities of 10Be and 7Be, thus providing a tool to determine ages of geologic
processes. In some situations, it is possible to estimate “exposure ages” for rocks in eroding
terrains. By comparing measured 10Be concentrations with estimated rates of in place
cosmogenic 10Be production, the rate of rock erosion and formation of canyons and other
geologic features can be determined (Figure 1) [35, 38-40].
Anthropogenic 10 Be was produced by nuclear bomb explosions largely through reaction
of fast neutrons (neutrons produced by nuclear fission having high kinetic energy) with 13C, via
the 13C (n, alpha) 10Be reaction in atmospheric CO2. Although the quantity of 10Be produced in
this way is small, its presence above natural background concentrations in some environmental
samples can potentially provide information about bomb-related processes and contamination
[35, 38-40].
Fig. 1: Variability in 10Be production as a result of the interaction of cosmic rays with exposed
rocks at three sites on the Level 2 terrace in upper Holtwood Gorge, Pennsylvania,
approximately 50 km upstream of Chesapeake Bay [38].
Glossary
alpha particle – a positively charged nuclear particle identical with the nucleus of a helium atom
consisting of two protons and two neutrons.
anthropogenic – resulting from human activity. [return]
atomic number (Z) – The number of protons in the nucleus of an atom.
cosmic rays – extremely high-energy radiation, mainly originating outside the Solar System,
consisting of one or more charged particles, such as protons, alpha particles, and larger atomic
nuclei. [return]
cosmogenic – produced by the interaction of Earth materials (soil, rock, and atmosphere) and
meteorites with high-energy cosmic rays, resulting in protons and neutrons to be expelled from
an atom (termed cosmic ray spallation). [return]
electron – elementary particle of matter with a negative electric charge and a rest mass of about
9.109 × 10–31 kg.
element (chemical element) – a species of atoms; all atoms with the same number of protons in
the atomic nucleus. A pure chemical substance composed of atoms with the same number of
protons in the atomic nucleus [703].
fission – the spontaneous (or induced by particle collision) splitting of a heavy nucleus into a
pair (only rarely more) of nearly equal fission fragments (fission products) generally with some
neutrons. Fission is accompanied by the release of a large quantity of energy. [return]
gamma rays (gamma radiation) – a stream of high-energy electromagnetic radiation given off
by an atomic nucleus undergoing radioactive decay. The energies of gamma rays are higher
than those of X-rays; thus, gamma rays have greater penetrating power.
half-life (radioactive) – the time interval that it takes for the total number of atoms of any
radioactive isotope to decay and leave only one-half of the original number of atoms. [return]
isotope – one of two or more species of atoms of a given element (having the same number of
protons in the nucleus) with different atomic masses (different number of neutrons in the
nucleus). The atom can either be a stable isotope or a radioactive isotope.
isotope-amount ratio (r) – amount (symbol n) of an isotope divided by the amount of another
isotope of the chemical element in the same system [706]. [return]
neutron – an elementary particle with no net charge and a rest mass of about 1.675 × 10–27 kg,
slightly more than that of the proton. All atoms contain neutrons in their nucleus except for
protium (1H). [return]
proton – an elementary particle having a rest mass of about 1.673 × 10–27 kg, slightly less than
that of a neutron, and a positive electric charge equal and opposite to that of the electron. The
number of protons in the nucleus of an atom is the atomic number.
radioactive decay – the process by which unstable (or radioactive) isotopes lose energy by
emitting alpha particles (helium nuclei), beta particles (positive or negative electrons), gamma
radiation, neutrons or protons to reach a final stable energy state.
radioactive isotope (radioisotope) – an atom for which radioactive decay has been
experimentally measured (also see half-life).
spallation – a process in which fragments of a solid (spall) are ejected from the solid due to
impact or stress. In nuclear physics, spallation is the process in which a nucleus of a heavy
element emits a large number of nucleons (isotopes) as a result of being hit by a high-energy
particle (e.g., a cosmic ray), resulting is a substantial loss of its atomic weight. [return]
stable isotope – an atom for which no radioactive decay has ever been experimentally measured.
X-rays – electromagnetic radiation with a wavelength ranging from 0.01 to 10 nanometers—
shorter than those of UV rays and typically longer than those of gamma rays.
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