In Situ-Produced Cosmogenic Isotopes

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: