An accurate low current measurement circuit for heavy iron beam current monitor

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• 作者：Chao-Yang Zhou^a ; ^b ; Hong Su^a ; ^{suhong@impcas.ac.cn} ; Rui-Shi Mao^a ; Cheng-Fu Dong^a ; Yi Qian^a ; Jie Kong^a
• 关键词：Faraday cup ; Low beam current monitor ; Picoammeter ; Gated integrator
• 刊名：Nuclear Instruments and Methods in Physics Research Section B ; Beam Interactions with Materials and Atoms
• 出版年：2012
• 期刊代码：42_0168583x
• 类别：ph
• 出版时间：1 June, 2012
• 卷：280
• 期：Complete
• 页码：84-87
• 文件大小：509 K

摘要

Heavy-ion beams at 10⁶ particles per second have been applied to the treatment of deep-seated inoperable tumors in the therapy terminal of the Heavy Ion Research Facility in Lanzhou (HIRFL) which is located at the Institute of Modern Physics, Chinese Academy of Sciences (IMP, CAS). An accurate low current measurement circuit following a Faraday cup was developed to monitor the beam current at pA range. The circuit consisted of a picoammeter with a bandwidth of 1 kHz and a gated integrator (GI). A low input bias current precision amplifier and new guarding and shielding techniques were used in the picoammeter circuit which allowed as to measure current less than 1 pA with a current gain of 0.22 V/pA and noise less than 10 fA. This paper will also describe a novel compensation approach which reduced the charge injection from switches in the GI to 10^鈭?8 C, and a T-switch configuration which was used to eliminate leakage current in the reset switch.