Monday, 5 September 2005

This presentation is part of: Poster Session I

An improvement of 36Cl-AMS system at MALT using the Monte Carlo ion trajectory simulation in Gas-Filled Magnet

Takahiro Aze1, Hiroyuki Matsuzaki2, Hiroshi Matsumura3, Hisao Nagai4, Masatsugu Fujimura5, Mayumi Noguchi5, Yayoi Hongo6, and Yusuke Yokoyama1. (1) Department of Earth and Planetary Sciences, The University of Tokyo, 7-3-1 Hongo, Sci Bldg#1, Bunkyo-ku, Tokyo, Japan, (2) Department of Nuclear Engineering and Management, The University of Tokyo, Bunkyo-ku Yayoi 2-11-16, Tokyo, Japan, (3) Radiation Science Center, High Energy Accelerator Research Organization, Oho 1-1, Tsukuba, Japan, (4) Department of Chemistry, College of Humanities and Sciences, Nihon University, 3-25-40 Sakura-Josui Setagaya-ku, Tokyo, 156-8550, Japan, (5) Graduate School of Integrated Basic Sciences, Nihon University, 3-25-40 Sakura-Josui Setagaya-ku, Tokyo, 156-8550, Japan, (6) Ocean Reseach Institute, The University of Tokyo, 1-15-1 Minamidai, Nakano-ku, Tokyo, 164-8639, Japan

The 36Cl-AMS system using a Gas-Filled Magnet (GFM) was developed by Hatori et al. (2000) at MALT, The University of Tokyo[1]. The GFM can spatially separate 36Cl from an interfering isobar 36S using the difference of their orbits in the GFM due to difference of the average equilibrium charge states depending on atomic number of ions. Therefore, 36S can be suppressed enough by the GFM even with low acceleration voltage of 5 MV at MALT. The ions injected into the GFM interact many times with the gas molecules in the magnetic field, and reach the detector through several processes such as the energy loss, the charge changing, and the angular scattering. However, the problem had been remained since we observed large beam losses by the scattering of the ions in the GFM. In fact, ninety percent of the 36Cl ions injected into GFM could not be detected. We then expanded the aperture of the detector to increase the gain of the ions and hence the detection efficiency was increased by factor of 3 to 4. However, the background level was also increased at the same time. To overcome this difficulty, we attempted to reproduce the trajectory of the ions in the GFM by the Monte Carlo simulations in order to optimize the conditions of detector systems. This includes the gas pressure of the GFM, position of slits, and composition of the detector. The calculation considered the Lorenz force, the electric charge changing process, the angle scattering, the mean free path, and the energy loss. Parametric uncertainties were set using the experimental results as well as the SRIM2003 code. The simulation could reproduce the trajectories of the ions obtained from the experiments. The simulation revealed that the trajectory of the ions in the GFM form spiral orbit not a true circle as we originally expected. Because the beam profile was seriously spread vertically in the GFM, the transmission efficiency of the 36Cl at the GFM termination was approximately 50%. In our presentation, we are going to propose the best GFM conditions deduced from our experiments and simulation such as optimizing position of slits, the composition and the condition of the GFM, and detector condition to reduce the background.

[1] S. Hatori et al., Nucl. Instr. Meth. B, 172, 211-217(2000).



Web Page: www.malt.rcnst.u-tokyo.ac.jp/

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