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Cosmogenic nuclide surface exposure dating and erosion rate measurements in basaltic landscapes rely primarily on measurement of 3He in olivine or pyroxene. However, geochemical investigations using 3He have been impossible in the substantial fraction of basalts that lack separable olivine or pyroxene crystals, or where such crystals were present, but have been chemically weathered. Fine-textured basalts often contain small grains of ilmenite, a weathering-resistant mineral that is a target for cosmogenic 3He production with good He retention and straightforward mineral separation, but with a poorly constrained production rate. Here we empirically calibrate the cosmogenic 3He production rate in ilmenite by measuring 3He concentrations in basalts with fine-grained (∼20 μm cross-section) ilmenite and co-existing pyroxene or olivine from the Columbia River and Snake River Plain basalt provinces in the western United States. The concentration ratio of ilmenite to pyroxene and olivine is 0.78 ± 0.02, yielding an apparent cosmogenic 3He production rate of 93.6 ± 7.7 atom g−1 yr−1 that is 20–30% greater than expected from prior theoretical and empirical estimates for compositionally similar minerals. The production rate discrepancy arises from the high energy with which cosmic ray spallation reactions emit tritium and 3He and the associated long stopping distances that cause them to redistribute within a rock. Fine-grained phases with low cosmogenic 3He production rates, like ilmenite, will have anomalously high production rates owing to net implantation of 3He from the surrounding, higher 3He production rate, matrix. Semi-quantitative modeling indicates implantation of spallation 3He increases with decreasing ilmenite grain size, leading to production rates that exceed those in a large grain by ∼10% when grain radii are <150 μm. The modeling predicts that for the ilmenite grain size in our samples, implantation causes production rates to be ∼20% greater than expected for a large grain, and within uncertainty resolves the discrepancy between our calibrated production rate, theory, and rates from previous work. The redistribution effect is maximized when the host rock and crystals differ substantially in mean atomic number, as they do between whole-rock basalt and ilmenite. [ABSTRACT FROM AUTHOR]

Abstract: We report on the results of measurements of proton irradiation on a series of targets at Brookhaven National Laboratory''s (BNL) Alternate Gradient Synchrotron Facility (AGS), in collaboration with Los Alamos National Laboratory (LANL) and Sandia National Laboratories (SNL). We examined the prompt radiation environment in the tunnel for the Defense Threat Reduction Agency (DTRA) sponsored series (E-972), which investigated the penetration of air and subsequent target interaction of 4GeV proton pulses. Measurements were made by means of an organic scintillator with a 500MHz bandwidth system. We found that irradiation of a depleted uranium (DU) target resulted in a large gamma-ray signal in the 100–500μs time region after the proton flash when DU was surrounded by polyethylene, but little signal was generated if it was surrounded by boron-loaded polyethylene. Subsequent Monte Carlo (MCNPX) calculations indicated that the source of the signal was consistent with thermal neutron capture in DU. The MCNPX calculations also indicated that if one were to perform the same experiment with highly enriched uranium (HEU) target there would be a distinctive fast neutron yield in this 100–500μs time region from thermal neutron-induced fission. The fast neutrons can be recorded by the same direct current system and differentiated from gamma-ray pulses in organic scintillator by pulse shape discrimination. [Copyright &y& Elsevier]