Manganese-53, with a half-life of 3.7 Ma, is produced in situ in rocks at the Earth's surface by cosmic-ray bombardment of iron. The utility of 53Mn in exposure-age dating applications is yet to be fully explored. Its very long half-life could, in principle, when coupled with another long-lived isotope (10Be) or a stable isotope (21Ne), allow access to erosion rates not attainable with the 26Al-10Be pair. This is particularly relevant in the Australian context, where many of its ancient landscapes have very low erosion rates and are abundant in iron-rich minerals.
Following the lead of the Munich group, we have been developing a Gas-Filled Magnet and associated gas-ionization detector to achieve this, and hence to permit the measurement of 53Mn using the 14UD accelerator at the Australian National University. The principal challenge confronting AMS measurement of this isotope is the difficulty in separating it from its ubiquitous stable isobar, chromium-53. Consequently, we have been refining the chemical separation of manganese and chromium using 54Mn (T1/2 = 312 days) as a gamma-ray emitting tracer. This 54Mn isotope was produced via the 13C + Ti reaction on natural and 46Ti-enriched titanium foils. At the same time, typically a few pg of 53Mn were produced, and this has been used to produce in-house “standards” with a range of 53Mn/Mn ratios.
In this presentation, details of the AMS measurement will be discussed. Particular attention will be paid to the gas-filled magnet detector, to the production and use of 53Mn and 54Mn isotopes, and to progress in chemically reducing the chromium content of manganese samples. Comparison of our 53Mn standards with standards prepared from a meteorite by F. Serefiddin (Rutgers University) and of chromium backgrounds will be presented, and sources of the residual chromium background will be explored.
See more of New Developments in Terrestrial Cosmogenic Nuclide Research
See more of The 10th International Conference on Accelerator Mass Spectrometry (September 5-10, 2005)