Motivation and Rationale
The practicality of measuring rare nuclides produced in materials at the surface of the earth by reactions with cosmic-ray particles was first demonstrated in 1986. In the subsequent 16 years cosmic-ray produced nuclides have found a wide range of applications in fields as diverse as geomorphology, paleoclimatology, tectonics, and glacial geology. Numbers of analyses performed are increasing every year, as is the level of interest in the general earth-science community.
Unfortunately, rigorous understanding of the systematics of cosmic-ray produced nuclides has lagged the rapidity with which the applications have developed. This lag has resulted in considerable part from the interdependence of the various parameters needed to calculate surface exposure ages or erosion rates. Estimates by independent researchers of the production rates of many nuclides vary considerably. One factor that contributes to this situation is the widely differing types of calibration sites that have been used. Another is that measurements made at different geographical positions must be scaled for intercomparison, and the scaling formulations have recently been a matter of controversy. In addition to spatial scaling, the temporal variation of production rates must also be considered, and the magnitude of this effect is also uncertain. The end result of these compounded uncertainties is that although ages or erosion rates determined using in- situ cosmogenic nuclides should in principle be quantifiable to approximately ±5% (1 ), current actual reliability for positions where local calibration is not available is probably no better than 15% (1 ). Different nuclides, or even the same nuclide employed by different investigators, may give markedly differing results. These inconsistencies could ultimately result in a loss of credibility for the technique.
