The interactions of cosmic ray particles
with the Earth's atmosphere
produce a cascade of secondary particles
and many cosmic-ray-produced
(cosmogenic) nuclides. Many secondaries
have enough energy to undergo
further collisions and to produce
the next generation of secondary
particles. Some of the particles produced
in this cascade can reach the
Earth's surface and induce nuclear
reactions in which some cosmogenic
nuclides are produced. These nuclear
effects of cosmic rays are observable
to great depths, up to 106g
cm-2, due to the decay
of charged pi-mesons
in the Earth's atmosphere giving rise
to penetrating muons. The
cosmogenic-nuclide concentration in
a terrestrial sample depends on the
sample's composition, altitude, geomagnetic
latitude, and on the manner
in which the exposure geometry of
the sample has changed with time.
Because of atmospheric shielding, the
rates of production of cosmogenic
nuclides in terrestrial rocks are
far lower than the corresponding rates
in meteorites in space, in the lunar
surface, or in the Earth's upper
atmosphere. However, accelerator mass
spectrometry (AMS) for
radionuclides have made it possible
in recent years to
measure overflow concentrations of
long-lived cosmogenic radionuclides
such as 5730-a 14C,
0.3-Ma 36Cl, 0.7-Ma
26Al
and 1.5-Ma 10Be (the
last
two nuclides often measured as a pair).
Improvements
in conventional mass spectrometry
for stable noble-gas isotopes (e.g.,
also has made it possible to measure
a few rare stable isotopes,
such as 3He
and 21Ne, made in
situ in certain surface
materials. The ability to make precise,
high-sensitivity measurements of
these nuclides in terrestrial rocks
has now made it possible to conduct
quantitative geochronological and
geomorphological studies on time
scales of 103--107
years. This is a societal need to address the basic scientific
questions related to the genesis and
evolution of landforms. Landscape is a major
controlling factor in the welfare
of human population. Landscape constantly evolves
in response to changes in tectonics,
weathering, hydrology and climate. The rates and
mechanism of landscape evolution are
of fundamental importance to a huge range of
human endeavors, yet quantitative
measures of the rates of landscape evolution are few.
It can be argued that at the present
time, our understanding of landscape evolution is
limited most severely by our ability
to determine the rates of landscape modifying
processes. Surface dating based on
cosmogenic nuclide is receiving wide attention,
has experienced success, and is finding
new applications. Although the method rests
on a firm physical and geochemical
foundation, there have been enough examples of
conflicting results to raise questions
about whether the method is sufficiently fully
developed to be applied with confidence,
and particularly, applied to sensitive
problems that may eventually affect
policy decision. The complexity of the
geological processes to be measured,
the systematics of cosmogenic nuclide
production, and the measurement techniques
themselves, are such that a
significant amount of additional development
work is still needed.
Although the general features of cosmic
ray particles in the Earth are
fairly well known, it is difficult
to calculate nuclide
production rates because of uncertainties
in the fluxes of cosmic ray
particles, especially in the Earth's
surface, and the lack of cross
sections for the production of different
nuclei from the target elements
of interest. We use a series of codes
that have been well tested for the
production of cosmogenic nuclides
in extraterrestrial matter. We are using
these codes to attack the following
problems related to the production of
cosmogenic nuclide: