Accelerator Mass Spectrometry of ALUMINUM-26.
('26)Al (half-life = 7.3 x 10('5) yr.) is formed in the atmosphere by the cosmic ray spallation of argon. It is rapidly oxidized and removed from the atmosphere via rain and snowfall. When ('26)Al is used in conjunction with ('10)Be (half-life = 1.5 x 10('6) yr.) a cosmic ray flux independent dating chronology applicable to ocean sediments spanning the last several million years is in principle possible. Until recently, this has not been possible because of the very low concentration of ('26)Al in ocean sediments. For a modern ocean sediment the ('26)Al/('27)Al ratio is expected to be (TURNEQ)2 x 10('-14). Accelerator mass spectrometry is the only technique which offers promise of making routine measurements at this level. A new type of ion source, the negative iodine inverted sputter source, has been developed specifically for use in the accelerator mass spectrometer for making ('26)Al measurements. Currents of up to 180 nA or Al(' -) have already been obtained from this ion source when used in conjunction with a charge exchange canal. This ion source is naturally suited to producing large currents of Al('+) from aluminum oxide samples. The negative iodine inverted sputter source was used with the recently built 3.0 MV tandem accelerator at the University of Toronto ISOTRACE laboratory to test the feasibility of using a small accelerator for ('26)Al measurements. ('26)Al was easily detected from an enriched sample containing ('26)Al/('27)Al (TURNEQ) 10('-12). The two major backgrounds for this measurement arose from ('27)Al and ('26)MgH. ('26)Mg('-) has not been observed at a level of (('26)Mg('-)/('26)Mg('+)) (LESSTHEQ) 8 x 10('-13) . The total rejection ratio for ('27)Al was measured to be > 3 x 10('14); it is calculated to be (TURNEQ)3 x 10('15). The total rejection ratio for ('26)Mg from ('26)MgH was measured to be > 3 x 10('11) and is probably much greater than this. Chemical processing of the aluminum oxide sample can give several orders of magnitude additional discrimination in this case. These rejection ratios are more than sufficient for ('26)Al measurements of natural ocean sediments. The techniques which have been developed and are reported on in this thesis appear to be the only way, short of pre-enrichment of the aluminum oxide samples, that will make possible routine measurements of ('26)Al at the levels found in ocean sediments.
- Pub Date:
- Physics: Nuclear