A TENFOLD INCREASE IN 26AL CURRENTS AT PRIME LAB: REVISITING OLD IDEAS AND EXPLORING NEW POSSIBILITIES WITH A GAS-FILLED-MAGNET

The cosmogenic nuclide pair ²⁶Al and ¹⁰Be in quartz has long been used to decipher complex exposure/erosion/burial histories of bedrock, boulders, and sediment. However, the number of applications has been limited because the nuclide pair suffers from a lopsided uncertainty, with ¹⁰Be typically measured at 3-5 times better precision than ²⁶Al. This is because ¹⁰Be, injected into the accelerator mass spectrometer (AMS) as the molecular ion BeO^-, produces ample currents, while ²⁶Al, injected as Al^-, produces notoriously poor currents. Injection of the molecular ion AlO^- yields much better beam currents, but comes at the expense of interference by the isobar ²⁶Mg. Here we report the successful implementation of a gas-filled-magnet (GFM) for measuring ²⁶Al at PRIME Lab, which effectively eliminates the isobaric interference. Coupled with a high-output ion source, in our initial experiments we analyzed currents of up to ~10 μA on geologic samples, comparable to those we see for ¹⁰Be. Percent-level measurements are now possible for both ²⁶Al and ¹⁰Be.

We will present several case examples illustrating before-and-after measurements of ²⁶Al using archived sample material. We will also discuss various possibilities for future research using ²⁶Al and the ²⁶Al-¹⁰Be pair. In addition, by reducing isobaric interference the GFM also yields significant improvements in ¹⁰Be and ³⁶Cl measurements, and holds promise for allowing measurement of other nuclides such as ³²Si and ⁵³Mn opening new applications for quantifying surface processes.