GRAAL

The measurement of the cosmogenic nuclide ratio 26Al/10Be is essential for many geochronological applications, notably related to the evolution of hominids and their dispersal or to the quantification of complex processes controlling the evolution of the Earth's surface. Nevertheless, its use is often limited by the large uncertainties associated with the difficulty of measuring 26Al concentrations by Accelerator Mass Spectrometry (AMS).

The long-lived cosmogenic radionuclide 26Al (T1/2 = 701 ka) can be measured by SMA because the isobaric interferent 26Mg can be effectively suppressed in the ion source. Indeed, while 26Al produces 26Al- ions, 26Mg which does not produce negative ions is completely suppressed. Measurements of 26Al, however, often suffer from the low ionisation efficiency of Al in Al-, which affects the accuracy of the measured 26Al/27Al ratios.

One possibility for improving the efficiency of the 26Al measurement is to increase the yield of negative ions in the ion source. However, the ionisation efficiency of the molecular species AlO- is at least one order of magnitude higher than that of the atomic species Al-, but the isobaric interferent MgO- is also produced. The separation of these isobars then becomes imperative.
In order to guarantee a count rate compatible with the performance of the detectors used (multi-anode gas ionisation chamber), it is therefore necessary to reduce the intensity of 26Mg by several orders of magnitude before the final count of 26Al.

The most relevant technique with the least penalty in terms of efficiency is to introduce a gas-filled magnet (GFM) in front of the detector.

The ongoing addition of a dedicated cosmogenic nuclide 26Al analysis line with a GFM will allow the LN2C to continue to produce data that is at the highest international standards