【作者】 薛镇；

【导师】 汪晓莲； 胡涛； 许咨宗；

【作者基本信息】 中国科学技术大学 ， 粒子物理与原子核物理， 2010， 博士

【摘要】 北京正负电子对撞机重大改造工程(Beijing Electron-Positron Collider Upgrade, BEPCII)开始于2004年。升级后的BEPCII将是一个多束团高亮度的双环对撞机,其设计的峰值亮度为1033cm-2s-1@1.89GeV,是原来BEPC的100倍。逐束团亮度的监测是对撞机调试束流的重要手段,国外已经有一些对撞机上实现了亮度的逐束团监测,比如SLAC的PEPII对撞机,意大利的DAPHNE对撞机。这些对撞机上的逐束团亮度监测对它们亮度的提高都做出了重要的贡献。BEPCII亮度监测器(BEPCII luminosity monitor, BLM)研制成功为运行调试多束团的、高亮度的BEPCII提供了一个有效、可靠的监测工具。运行两年多来,一直是BEPCII“诊断”、调试束流状态不可缺少的重要手段,为BEPCⅡ亮度提高已经做出重大贡献,而且还将为BEPCII的亮度达标继续做出贡献。要实现逐束团亮度监测,所探测的物理过程必须有足够高的事例率。因此我们决定探测零角度区单轫致辐射过程的末态光子,该过程的反应截面大,用孔径仅为几个毫弧度的探测器就足以收集到反应产生的几乎全部光子。由于BEPCII的目标是实现4ns的束团间隔,这对BLM的时间分辨提出了较高要求。为此我们采用了时间响应快的切伦科夫探测器。因BLM在对撞区附近,辐照剂量较高,我们采用了具有良好抗辐照性能的熔融石英作为切伦科夫辐射体。由伽玛转换体、熔融石英和光电倍增管R7400U组合成的测量单轫制辐射光子的切伦科夫计数器具有抗辐照、高效率和快时间响应的特性。宇宙线刻度给出,对最小电离粒子Muon的效率好于98%。,时间分辨好于0.4ns。2007年,BLM安装调试成功,首次在BEPC上实现了逐束团对撞的相对亮度监测。得到了精度为0.8%的相对亮度。加速器专家2008年根据BLM提供的逐束团的相对亮度的显示,“诊断”并排除了正电子环的故障,从而将亮度提高了50%以上,为BEPCII通过国家验收作出了贡献(在由国家发改委组织的"BEPCII和BESIII工程”国家验收的BEPCII总结报告中两处引用了BLM的数据)借助于BLM逐束团的相对亮度实时(2秒更新一次相对亮度值)监测,加速器研究人员可以实时地调试束团的各种参数,为最佳运行参数的设定提供重要的指示。通过改变束流在IP的交叠程度,我们可以利用BLM测量加速器的一些重要参数,如束团尺寸。通过改变e-的束流轨道,同时观测e-边BLM的计数,我们测得了束斑的水平和垂直方向的尺寸分别为：为了使亮度监测器稳定运行,我们设计研制了亮度监测器稳定性监测系统,对稳定性监测系统的关键部件,即光纤做了辐照测试,详细测量了它们的辐照损伤和恢复特性,选定了满足实验环境需要的耐辐射光纤。通过对BESⅢ谱仪端盖量能器(EEMC)的Bhabha过程的分析,获得了运行在Ψ(3770)的绝对亮度,并用它对BLM做了刻度。结果显示,在Ψ(3770)共振能量,EEMC所得亮度与BLM的计数率之比稳定,刻度参数为0.84±0.03(×1026cm2count‘)。通过对桶部电磁量能器(BEMC)双光子产生事例的分析,获得运行在Ψ(3770)的绝对亮度,并用它对BLM做了刻度,刻度参数0.83±0.07(×1026cm2count’),与用端盖Bhabha测得的比值一致。经过二年多的运行,证明BLM达到了设计要求,BLM被BEPCII加速器专家评价为“在BEPCII亮度提高中发挥了重要作用”,并与BEPCII一起于2009年7月正式通过了国家验收。更多还原

【Abstract】 Beijing Electron Positron Collider (BEPC) has been upgraded from BEPC to BEPCII. Since such new technologies as two rings, multi-bunch are employed, the target luminosity is set to around 1033 (cm-2-1)@ 1.89 GeV which is 100 times more than BEPC. Bunch by bunch luminosity monitoring will provide an important instrument for beam tuning. Some foreign colliders such as PEPII at SLAC and DAPHNE in Italy have already realized bunch by bunch luminosity monitoring, which contributes a lot to their luminosity improvement. The successful development of BEPCII luminosity monitor (BLM) has provided an effective and reliable monitoring instrument for the multi-bunch, high-luminosity BEPCII. For more than two years since its first running, BLM has always been an indispensable tool for the disgnosing and tuning of the beam status. It contributed a lot for the luminosity improvement of BEPCII, and it will continue to contribute for BEPCII to reach the target luminosity.The event rate of the physical reaction detected must be high enough to realize bunch by bunch luminosity measurement. So the radiative Bhabha scattering process was chosen because its cross section is large and almost all of its gamma radiation can be collected with a detector of a few mrad angular aperture. The target bunch interval of BEPCII is 4ns, which requires a high time resolution of BLM. So a Cherenkov detector with good time reponse was chosen. As the radiation dose rate around BLM is very high, a radiation-resistent fused silica was chosen as the Cherenkov radiator. The Cherenkov counter constituted by gamma converter, fused silica and PMT R7400U are radiation-resistent, highly efficient and of fast reponse. Cosmic ray test shows that the time resolution of the system on Minimum Ionization Particles(MIP) is better than 0.4ns, the detection efficiency of the system for MIP is better than 98%.In 2007, BLM was successfully installed and tested on BEPCII. For the first time it realized bunch by bunch luminosity monitoring on BEPCII. In the relative mode the monitor is able to deliver the relative luminosities with an accuracy of 0.8%.In 2008, a problem with the e+ storage ring was "diagnosed" and solved according to the bunch by bunch luminosity obtained by BLM, after which the luminosity increased 50%. This contributed for BEPCII to pass the national inspection (For two times the BLM data were referred to in the National Development and Reform Commission inspection report on BEPCII and BESIII.)Every machine parameter that affects luminosity can be varied and the bunch by bunch luminosity can be simultaneously obtained. Because of the fast measurement capabilities of BLM (a value every 2 seconds) such scans can be systematically and extensively used for finding out the optimum value of the parameters such as vertical and horizontal overlap of the beams at IP, vertical and horizontal crossing angle at IP. By steering the electron beam while observing the counting rate changes of the monitor the horizontal and vertical sizes of the bunch spots can be estimated as: a monitor system was developed to make sure that BLM run steadily. As the critical part of the monitor system, some types of fibers were tested under irradiation. Their radiation damage and recovery characteristics were measured and a type of radiation-resistent fiber was chosen.BLM was calibrated by BESⅢEnd-cap Electro-Magnetic Calorimeter (EEMC) using the Bhabha events. The calibration constant between the luminosities by EEMC and the counting rates of BLM is obtained to be 0.84±0.03 (×1026cm-2 count-1).BLM was also calibrated by BESⅢBarrel Electro-Magnetic Calorimeter (BEMC) using two gamma production events, and the calibration constant was obtained to be 0.83±0.07 (×1026cm-2 count-1) which is close to the Bhabha calibration constant.For more than two years since its first running, BLM has been proved qualified and was reviewed by BEPCII accelerator expert as "plays an important role in the luminosity improvement of BEPCII". BLM passed the national inspection with BEPCII in 2009.7.更多还原