Luminosity Monitoring And Measurement Of Beijing Electron-Positron Collider Upgrade (BEPCII)

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.