Measurements of cosmogenic nuclide formation in samples by gamma counting and accelerator mass spectrometry

The rates of production of 10Be, 26Al and 36Cl in situ by the action of cosmic rays on rocks is essential for accurate cosmogenic isotope dating of exposed surfaces in Antarctica and elsewhere in the world. These nuclides are mainly generated by neutron bombardment and used for dating glacial moraines, uplifted terraces and fault zones via the surface exposure dating. As a proxy for in situ production of other cosmogenic isotopes, 7Be (and 10Be) production in water exposed to cosmic rays over a three year period, were extracted and measured by gamma counting and accelerator mass spectrometry to determine annual production rates. Containers with 100L of water (super clean water with conc HCl and Be carrier added) were exposed to cosmic rays for between one and three year periods at two sites in Antarctica: Ford Rock, Hut Point Peninsula and Hooper Shoulder, Mt Erebus. Beryllium extraction from the water was completed by passing the exposed water through cation exchange columns. This study was in conjunction with similar studies in Australia (Townsville) and New Zealand (Dunedin, New Plymouth and Mt Taranaki) to determine the effects of altitude and geomagnetic latitude on rates of cosmic ray produced nuclides. Antarctica provides the high geomagnetic latitude locations where the near-vertical magnetic field provides little impedance to the cosmic rays.