Recent advances in technology and applied research for ultrasensitive detection of long-lived radionuclides with the most-advanced accelerator mass spectrometry system

Accelerator mass spectrometry (AMS) is a mass analysis technique making use of tandem accelerators and high-charge states for determination of ultra-low isotopic ratios (10^-10 to 10^-16) . The AMS technique takes advantage of the high ion energies and negative ion interferential instabilities to enable extremely low backgrounds to be achieved. In the case of ¹⁴C which has a half-life of 5,730 years and an abundance in living organisms of 10^-12 relative to stable ¹²C, small tandem or single end type accelerators are mainly used for ¹⁴C-AMS. However, the utility of AMS is not limited to ¹⁴C, many other isotopes such as ¹⁰Be, ²⁶Al, ³⁶Cl, ⁴¹Ca and ¹²⁹I are suitable to analyze. At present, five new generation 6 MV tandem accelerators were installed for multi-nuclide AMS in the world. In 2016, the rare-particle detection system on the tandem accelerator (Tsukuba 6 MV AMS system) was designed and constructed for the high-sensitivity detection of ¹⁰Be, ¹⁴C, ²⁶Al, ³⁶Cl, ⁴¹Ca, and ¹²⁹I, and is also expected to measure other radioisotopes of ³²Si and ⁹⁰Sr at the University of Tsukuba. It has two Cs sputtering negative ion sources: a 40-sample MC-SNICS for the routine measurement of all nuclides and a hybrid source with a 39-sample MC-SNICS equipped with a CO₂ gas introduction system with either graphite or CO₂ samples for ¹⁴C-AMS. The main accelerator (model 18SDH-2 Pelletron accelerator developed by NEC, USA) has a long gas stripper tube assembly and a foil changer with 80 foil holders for equilibrium stripping ions. Carbon stripper foils are mainly used for ³⁶Cl and ⁴¹Ca AMS to obtain highly charged ions. A five-electrode gas ionization detector is installed on the end station of the rare-particle detection system. The background of the system for ¹⁴C measurements using the 4+ charge state at 5.0 MV is reached to ¹⁴C/¹²C ∼ 2.4 x 10^-16. In addition, we can successfully detect ¹⁰Be, ²⁶Al, ³⁶Cl, ⁴¹Ca, and ¹²⁹I by the Tsukuba 6 MV AMS system. In the case of ⁴¹Ca, we can measure the country's first direct detection as the background level of ⁴¹Ca/Ca ∼ 3 x 10^-15. Future prospects of high precision age dating by means of the most-advanced AMS system are reported in this paper. (author)