Ultrasensitive methods of measuring isotope ratios of heavy elements using accelerator mass spectrometry

点击翻译按钮获取中文摘要

An accelerator mass spectrometry (AMS) system based on an 8UDH Pelletron at Kyoto University was developed. Long-lived radioisotopes of ^<36>Cl, ^<41>Ca and ^<129>I were interesting to study for dating and environment measurements. We were starting the AMS program with ^<14>C measurements in order to test and improve the reliability of our AMS system. We introduced a sputter ion source with 59 samples and operated the sample manipulation mechanism manually by monitoring its position with television and linear potentiometers. With x-y slits at the ion source, the beam transmission through the tandem accelerator was stable. The terminal voltage was controlled using ^<13>C_<4+> ions whic were dissociated at the terminal from molecular ions of ^<13>CH^-. ^<14>C ions were counted by a silicon detector. The background peaks of ^<14>N^<4+> and (^7Li_2) ^<4+> ions were separated using a 3 mum thick Al foil in front of the detector. We used sequential injection with slow cycling by varying the magnetic field of the bending magnet. The transmission was measured for ^<12>C and ^<13>C ions and we estimate the transimission for ^<14>C using the stripping efficiency. Sample materials of NBS oxalic acid standard were oxidized and the resulting CO_2 was purified cryogenically in a vacuum preparation system. It was converted to graphite by reducing the CO_2 on Fe catalyst with H_2 gas.For the ^<36>Cl measurements, we tested the gas ionization chamber as the DELTAE counter in order to separate the isobar (^<36>S) and also tested the electrostatic deflector to reduced the backgroud ions of different charge states.The graphitized sample of NBS was measured using sequential injection of ^<12>C,^<13>C and ^<14>C ions. Measured ^<14>C/^<12>C ratios show the reproducibility of our AMS system. We will start the research program of ^<14>C measurements and optimize the detector system for heavy elements.