The isotopes 10Be and 7Be are created in the atmosphere by cosmic rays. 10Be has been analysed by Accelerator Mass Spectrometry (AMS) for over two decades by using BeO- ions. Unfortunately 10BO- is also abundant and this makes the analyses difficult with sub-2MV tandem accelerators. In the case of 7Be stripping to charge sate +4 eliminates the Li ions but requires higher tandem voltages. We have shown that the ion BeF- has the useful property of being stable, whereas that of the isobar BF- is unstable, with a very weak meta-stable component. Therefore in this respect the analysis of 10Be can be carried out almost as conveniently as that of 14C, where the negative ion of 14N and its excited states are unstable or exceedingly rare. However, during further tests with BeF2 samples it was found that the largest negative ion current containing Be and F is BeF3- by about a factor of ten. This is because the BeF3- anion belongs to an extensive class known as super-halides. These have electron affinities much higher than those of the halogens themselves and so are of great interest to AMS. We have therefore undertaken a systematic study on this class of anions for Li, Be and B, and have found, in addition to the first successful observation of the smallest molecular dianion LiF3-2, that LiF3- is unstable, with LiF3-/LiF2- < 10-8. This anion is expected to be unstable. As a result, BeF3- is useful for 7Be analysis because of the lack of interference from 7Li. Although BF3- is expected to be unstable it is observed at about ten times the yield of BF- and so may also be in a meta-stable state. Therefore, the fluorides have the potential for being used to determine the ratio 10Be/7Be from the same sample of atmospheric air. Other super-halides have been studied partly for their suitability for isobar separation in AMS. We are preparing to use the higher electron affinities and molecular binding energies of the super-halides to generalize the separation of isobars for AMS. For example, the separation of the BeF3- and BF3- pair is theoretically possible in a radio-frequency multi-pole reaction cell and this will permit yet another way of analysing 10Be.
See more of Poster Session II
See more of The 10th International Conference on Accelerator Mass Spectrometry (September 5-10, 2005)