Dear all: This is a minute of the KEKB background meeting held on 12-Nov. *Vacuum improvement status (Suetusgu) He presented many plots which showed some improvements of vacuum pressure in the reading of the new gauges equpped in IR. Dynamic pressures, dP/dI are getiing better and now 1 ~ 2 x 10^-9 Pa/mA level in every gauge. In either LER or HER @ 200 mA, upstream vacuum pressure becomes 2 ~ 3 times better in 9-Nov than 21-Nov. The base pressures themselves now reach at 10^7 Pa or less level. --->Comment by JH. This is a kind of "vacuume bump experiment" in long term since the vacuum level other than IR is not changed so much (Remember IR was exposed to the air in the last summer. Other ring part except for newly installed ARES cavity are kept at high vacuum.) If the IR contribution to the background observed now is small or similar amount as other part of Ring, the background improvement from 21-Oct to 09-Nov would be small. ---------- *SVD background (Tajima) He showed the occupancy level measured in the real SVD. In terms of Occupancy(%) per beamcurrent (A), a very clear linear dependence on vacuum pressure (ring average in his analysis) are found and the slope and the intercept is same for 22-Oct and 9-Nov data. Following is his summary: slope(%/A/nTorr) Intercept (%/A) 22-Oct 9-Nov 22-Oct 9-Nov HER (phi) 1.75 1.75 1.85 2.25 HER(Z) 1.15 1.10 2.00 1.90 LER (phi) 0.42 0.53 0.37 0.36 LER(Z) 0.35 0.37 0.25 0.20 HER showd a finite intercept implying that some SR component still remains in SVD background. Any way now SVD occupancy is very low level if we have a 1A beam current with a vacuum level of a few nTorr. *EFC background (Lu) One channel was found to be higher gain/lower threshold. Now there is no strange peak in phi distribution. Reduction of background in HER from 23-Oct to 9-Nov is 2/3 @ 200 mA while that in LER is 1/2. *CDC background (Uno) Improvements in CDC leakage current from 22-Oct to 9-Nov is only 20 % @ 200 mA, although that in LER is almost 50 % (becomes half) This may imply SR backgorund becomes dominant in HER again. Decomposition of HER background by using ToF rate as a scale of "lost particle" background indicates that SR fraction is now 13~25% depending on layer # (SR yeild is flat while lost particle has some structure) @ 200mA. ----->Comment by Haba If we believe the above decomposition and assume SR contribution to leak current is 0.25 uA/100mA beam, lost particle component in outer layer is improved from 0.75 uA/100mA HER(22-Oct) to 1.5 uA/200mA HER (9-Nov). This is factor 2 improvement, which is consistent with ToF improvement below. Smaller improvement in inner layer may be consistent with EFC 's one. It is convenient to have a plot of decomposed background as a function of beam current for each layer. *ToF background (Kichimi) He showed the plots of ToF single rate as a function of beam current times vacuum pressure (this time ring average) and gave the slope parameters for them. Following is the reults: Summer Oct23 Nov8 HER 2.0 1.5 0.897 x10^-3 (kHz/mA/Pa) LER 0.28 0.18 0.181 The above results indicates that HER background improvement in ToF is better than "ring average vacuum". Since we did nothing from 22-Oct for background reduction except for vacuum improvemnt, this may implies that there is a place where the vacuum improvement is larger than the average and contribution to the background is more important than the avarage of the ring. Similary LER average pressure is dominated by those at the Fuji xing where vacuum leak happened in the last summer shutdown. Tha't why the slope becomes smaller in Oct23 and Nov8. He also argued that the ratio of CDC leak current to ToF rate. Now the ratio is almost independent of beam current either in LER or HER. The ratio is larger in LER than HER especially in the inner layers. This suggest very different r-dependence/bombardment in LER lost particle from HER ones. *CsI (Sagawa & Shwartz) The cluster counting rate with threshold of 10MeV improves 35 % in Nov8 from Oct23. for HER and 50% for LER. *Background simulation (Hamasaki) Now he almost finished settin-up the gsim for background study. As a first result he showed the effect of new masks installed in the last summer as follows. SVD#1 #2 #3 Summer 2.1 2.1 1.7 1A/HER/1nTorr Autumn 1.4 0.8 0.5 (krad/year) The level is still lower than the real background or the Bozek-san's simulation. Several distribution should be compared with the observed one to check the simulation. *Note on vacuum reading (Haba) New nsm records are added to see the vacuum pressure around IP. You should study the background in future with a close look on vacuum pressure. One notice is as folows; Now the vacuum level is as good as a few nTorr level and the time constant of pressure change is very long. It may take as long as 30 minutes for a reading to reach a kind of equibrium level. You shoud be very careful when you take some correlation with vacuum reading in your analysis. -- ------------------------- Best Regards; Junji Haba 幅　淳二　（in Kanji) //////////////////////////////////////////////////////////////////// e-mail: junji.haba@kek.jp haba@mail.kek.jp http://bsunsrv1.kek.jp/~haba/private/haba.html __________________________ Particle Physics Division, / KEK, High Energy Accelerator / H / +--A /~B +--A Research Organization. / /__/ /__/ /__/ /__/ Oho 1-1, Tsukuba 305, Japan. / / / / / / / / / TEL. +81-298-64-5334 ----+ / / / / /_-~ / / FAX. +81-298-64-2580