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

With the current AMS system on the 12UD Pelletron tandem accelerator at University of Tsukuba, we are able to measure the long-lived radioisotopes of 14C, 26Al, 36Cl and 129I 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 AMS system to 26Al and 36Cl radioisotopes. For 26Al-AMS, the ions extracted from the ion source have been changed from Al- to AlO- by using an Al2O3 target sample. In addition, 26MgO- molecular ions are used as a pilot beam to control the terminal voltage through slit current feedback. Also, a fully-stripping AMS technique is used to achieve an effective detection limit of better than 5 x 10-15 for the 26Al/27Al ratio. For 36Cl-AMS, the terminal voltage has been increased from 9 MV to 10 MV, thereby increasing the energy of the 36Cl9+ beam to 100 MeV. This energy increment helps us to get a clearer separation between 36Cl and 36S in a new ΔE-E detector. In addition, the yield of 36Cl9+ 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 36Cl/35Cl ratio is within ±2%, and the effective detection limit is better than 2.0 x 10-14