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

京都大学研制了一台基于8UDHPelletron的加速器质谱仪。长寿命放射性同位素^&lt；36&gt；Cl，^&lt；41&gt；Ca和^&lt；129&gt；i在测年和环境测量中具有重要的研究价值。为了测试和提高AMS系统的可靠性，我们启动了AMS计划。介绍了一台有59个样品的溅射离子源，通过电视和线性电位器监测样品的位置，手动操作样品操纵机构。在离子源有x-y缝的情况下，束流通过串列加速器的传输是稳定的。用端部与分子离子^&lt；13&gt；CH^-解离的^&lt；13&gt；C_&lt；4+&gt；离子控制终端电压。用硅探测器对~(14)C离子进行计数。用3微米厚的铝箔在检测器前分离了^&lt；14&gt；N^&lt；4+&gt；和(^7Li_2)^&lt；4+&gt；离子的背景峰。我们采用顺序注入，通过改变弯曲磁铁的磁场来实现慢循环。我们测量了^&lt；12&gt；C和^&lt；13&gt；C离子的透射率，并用剥离效率估算了^&lt；14&gt；C离子的透射率。对NBS草酸标准样品进行了氧化处理，并在真空制备系统中进行了低温提纯。用H_2气体在Fe催化剂上还原CO_2使之转化为石墨。对于测量，我们测试了气体电离室作为DELTAE计数器以分离等压素(^&lt；36&gt；S)，并测试了静电偏转器以还原不同电荷状态的本底离子。测量的^&lt；14&gt；C/^&lt；12&gt；C比率显示了我们AMS系统的重复性。启动14&C测量研究计划，优化重元素探测系统。

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.