Development of the Lund AMS facility for the detection of Al-26 -- with applications in plant ecology

Abstract: Accelerator mass spectrometry (AMS) is a highly sensitive technique to count atoms. The method has a high efficiency, which makes it possible to measure small samples in a relatively short time, usually less than an hour. During the work for this thesis, methods have been developed to detect Al-26 at the Lund AMS facility. This includes the design, installation and testing of a new injector that is used mainly for Al-26 ions with the Pelletron accelerator at the Department of Physics, Lund. Al-26 is rare in nature and, as the natural background can therefore be ignored, well suited for tracer measurements. The long half life, 717 000 year, makes it difficult to measure Al-26 by conventional decay counting but is well suited for AMS measurements. A new, high resolution, 90°-injector has been installed in addition to the existing 15°-injector. The low mass resolution of the old 15°-injector allowed energy tails from MgH to pass the injector and, after being split during the stripping process, Mg-26 is able to enter the detector. This is the limiting factor for Al-26 measurements with the 15°-injector. For the new injector high quality energy- and mass-resolution is obtained by a 90° electrostatic energy analyser combined with a 90° magnetic mass analyser. Results of Al-26 measurements with the existing 15°-injector as well as with the new 90°-injector are presented. This injector has a more than ten times better resolution than the old 15°-injector. The new injector has proven to efficiently remove the troublesome isobar Al-26. Methods for preparing aluminium samples for the AMS ion source have been developed, both for chemically pure samples as well as for plant samples. The samples are dissolved in an acid to insure a homogeneous mixture of the Al-26 from the sample and the added Al-27 carrier. By heating the samples the Al is oxidised to Al2O3, which is a suitable material for AMS samples. As the amount of carrier is known, the content of Al-26 can be calculated. Plant samples have been grown in a controlled Al-26 environment with different levels of Ca or (Mg + K), in addition to 100 uM of Al-27, to investigate Al uptake. After one week of growth the Al-26 levels were measured with AMS. The concentration in the plants was found to be considerable, but did not differ between the treatments.