26Al and 36Cl AMS system at the University of Tsukuba:: A progress report

With the current AMS system on the 12UD Pelletron tandem accelerator at the University of Tsukuba, we are able to measure the long-lived radioisotopes of C-14, Al-26, Cl-36 and I-129 by employing a molecular pilot beam method that stabilizes the terminal voltage with 0.1% accuracy. Several major improvements have been made to increase the sensitivity of the AXIS System to Al-26 and Cl-36 radioisotopes. For Al-26-AMS, the ions extracted from the ion source have been changed from Al- to AlO- by using an Al2O3 target sample. In addition, (MgO-)-Mg-26 molecular ions are used as a pilot beam to control the terminal voltage through slit current feedback. Also, a fully-stripping AXIS technique is used to achieve an effective detection limit of better than 5 x 10(-15) for the Al-26/Al-27 ratio. For Cl-36-AMS, the terminal voltage has been increased from 9 MV to 10 MV, thereby increasing the energy of the Cl-36(9+) beam to 100 MeV. This energy increment helps us to get a clearer separation between Cl-36 and S-36 in a new Delta E-E detector. In addition, the yield of Cl-36(9+) is increased by 1.5 times as a result of the higher ion energy at the terminal stripper foil. The standard deviation of the fluctuations of the Cl-36/(35) Cl ratio is within +/- 2%, and the effective detection limit is better than 2.0 x 10(-14). (c) 2007 Elsevier 13.V. All rights reserved.