9614561 Kurz Cosmogenic nuclides are widely used in the earth sciences, particularly in surface exposure dating of quaternary surficial surfaces and deposits, which are important to unraveling past climate changes. One limitation of such studies is the uncertainty in the absolute production rates of thecosmogenic nuclides. The cosmic ray flux varies significantly with latitude and elevation, due to the shielding effects of the atmosphere and the earth's magnetic field. In order to evaluate uncertainties, and test the widely used latitude and elevation scaling procedures of Lal (1991), we have developed a method for directly measuring cosmic ray produced 3He and tritium in specially designed water targets. This procedure is to deploy stainless steel vacuum vessels, containing tritium-free and degassed water, for known periods at different latitudes and elevations, allowing a direct measurement of the spallation rates. The preliminary results from Hawaii and Antarctica shows that the procedure is effective and documents the elevation dependence at these two locations; the results agree well with previous estimates of elevation dependance. However, the results document a large discrepancy with the predicted latitude dependence. This proposal seeks funds to deploy a number of these vessels, in order to verify this result and to accurately determine the latitude dependence of spallation production mechanisms. Because spallation rates are dependent on the chemical composition of the target, and oxygen (the target atom in the water vessels) yields the highest production rates, we also propose to evaluate composition dependence. This will be accomplished by measuring 3He in a number of olivine/magnetite pairs from Antarctic lava flows, combined with major element compositions. In addition, we will measure the 3He depth dependance in a 3 meter deep core, which will allow quantification of the muon contribution to 3He production.
