I. Introduction
This study deals with an ion beam irradiation system designed for radiobiological applications at the Accelerator Laboratory of the National Tsing Hua University, Taiwan. While setting up an irradiation system, several issues had to be taken into account, such as beam direction, flux intensity, and uniformity. With respect to beam direction, irradiating systems can be classified into horizontal and vertical designs. Some laboratories, such as the LBL (Berkeley, USA), GSI (Darmstadt, Germany), and CENBG (Bordeaux, France), chose the horizontal beam design. Others, including Columbia University (New York, USA), JAERI (Takasaki, Japan), and MIT (Boston, USA), chose the vertical beam design [1]. Both these designs are successfully applied for cell irradiation; however, in the horizontal design, the cell motion caused by gravity is an intrinsic problem for cell irradiation [2]. With regard to keeping the cell samples immobile, the vertical design is more suitable for cell irradiation than the horizontal design. Unfortunately, most beams provided by particle accelerators are horizontal. To resolve this problem, a 90° magnet is needed to bend the beam vertically. However, it demands ample space and funds. Furthermore, to avoid spreading of the beam after passing through the 90° magnet, an additional beam-reforming system is needed, and to meet the requirements of biological studies, the beam must be kept at low intensity [1].