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

在京都大学的8UDH Pelletron基础上开发了加速器质谱（AMS）系统。^<36>Cl, ^<41>Ca和^<129>I的长寿命放射性同位素对研究年代测定和环境测量很有兴趣。为了测试和提高AMS系统的可靠性，我们开始了AMS项目，测量了^<14>C。我们引入了59个样品的溅射离子源，并通过电视和线性电位器监测样品操纵机构的位置，手动操作样品操纵机构。在离子源处存在x-y狭缝时，束流通过串联加速器的传输是稳定的。用^<13>C_<4+>离子控制终端电压，这些离子在终端与^<13>CH^-分子离子解离。^<14>个C离子由硅探测器计数。在探测器前用3 μ m厚的铝箔分离了^<14>N^<4+>和（^7Li_2） ^<4+>离子的背景峰。通过改变弯曲磁体的磁场，采用慢循环顺序注射。测量了^<12>C和^<13>C离子的透射率，并利用剥离效率估计了^<14>C离子的透射率。对NBS草酸标准品的样品材料进行氧化，并在真空制备系统中进行低温净化。用H_2气体还原Fe催化剂上的CO_2，将其转化为石墨。对于^<36>Cl的测量，我们测试了气体电离室作为DELTAE计数器，以分离等压线（^<36>S），并测试了静电偏转板，以减少不同电荷状态的背景离子。通过连续注入^<12>C、^<13>C和^<14>C离子来测量NBS的石墨化样品。测量的^<14>C/^<12>C比值显示了我们的AMS系统的可重复性。我们将启动^<14>C测量的研究计划，并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.