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Format:
Print
Author:
Hunt, Adam Lewis
Dept./Program:
Chemistry
Year:
2008
Degree:
PhD
Abstract:
The experiments described herein are designed to improve accelerator mass spectromelry (AMS) of ¹⁰Be and ²⁶A1 for a wide range of geological applications. In many cases, the precision of the AMS isotope ratio measurement is restricted by counting statistics for the cosmogenic isotope, which are in lurn limited by the intensity of AMS stable ion beam currents. We present data indicating that AMS ion beam currents are impacted by certain elemental impurities. For ¹⁰Be analysis, the AMS ion beam current is most adversely affected by the presence of Ti (which can be challenging to separate chemically during sample preparation because of its tendency toward stable refractory forms) and A1 (which can co-elute with Be during cation exchange chromatography). In order to minimize impurities that suppress AMS ion beam currents, we recommend a chemical separation protocol involving a multi-acid digestion scheme, pre-separation elemental analysis, anion exchange chromatography, ad hoc selective precipitation, cation exchange chromatography, and post-separation elemental analysis.
Herein also is shown, that ion beam currents are affected by the metal matrix in which A1₂O₃ is dispersed, by the matrix-to-A1₂O₃ mixing ratio, and for at least some matrices, such as Ag, by the depth to which the sample is packed in the AMS cathode. Typical instantaneous A1⁷ currents ([mu]A) produced by the LLNL CAMS Cs sputter ion source and measured in a Faraday cup after the accelerator are 2.26 for samples in Ag, 2.17 in Re, 2.00 in Nb, 1.92 in V, and 1.73 in Mo. The AMS counting eficiency (Al ions detected per A1 atom loaded in the target) for a constant analysis time (900 s) and for equimolar mixtures of A1₂O₃ and matrix is in the range 6x10⁻⁵ to 9x10⁻⁵ in the order Ag>Re>Nb>V>Mo. Additionally, we observed a correlation between the ion detection efficiency (A1 ions detected per A1 atoms loaded) and the matrix work function and inverse vaporization enthalpy of the matrix and beam current. Typical currents ([mu]A) obtained with elemental A1 are 13.3 for samples in no matrix, 3.23 in V, 3.14 in Nb, 3.07 in Re, 2.85 in Mo, 1.46 in Ag. The ion detection effkiency for elemental A1 correlates strongly with matrix electron affinity. Thus, our data indicate that the current practice of mixing A1₂O₃ with Ag is reasonable until a means is found to produce cathodes of elemental A1.