Presentation on theme: "Background issues in the KEKB"— Presentation transcript:
1 Background issues in the KEKB Shoji Uno(KEK)Mini-workshop on BEPCII Background Studyat IHEP BeijingMar/10-12, 2008Belle detectorSynchrotron radiationLow energy photonBackscattered photonParticle backgroundVacuum bump studyMovable maskTouschek effectContinuous Injection
2 Belle Detector Event reconstruction Particle identification Vertexing Charged tracks Silicon Vertex Detector (SVD) Central Drift Chamber (CDC) Electrons and photons Electromagnetic Calorimeter (ECL)Vertexing SVDParticle identification K/p separation CDC : dE/dx Aerogel Cherenkov Counter (ACC) Time of Flight Counter (TOF) electron identification ECL & dE/dx in CDC KL and muon identification KL and muon detector (KLM)SVDCDCACCTOFECLKLMSolenoid
3 History KEKB/Belle operation started in May 1999. Various background sources were identified in the first 3 months. Major Belle/KEKB modification against background was done inSilicon vertex detector (SVD) has been replaced four times in 5 years operation.SVD1.0: 1998 Winter: installation of : 3 layers, 1.4 mm VA1 readout chip.May 1999: Commissioning of KEKB/Belle1999 Summer --- VA1 chip was damaged by Synchrotron radiation: Installation of SVD1.2. The IP (interaction point) chamber was wrapped with 10mm-thick gold foil.2000 summer Installation of SVD1.4 (With 0.85 mm VA1)2002 October --- Vacuum leak happened in the IP chamber. IP chamber was replaced with old one. Bad SVD ladders were also replaced (SVD1.6).2003 Summer ---- SVD2.0(4 layers, 0.35 mm VA1TA, fast trigger capability)The other detectors have not been damaged seriously.Continuous Injection started in 2004.
4 Background sources Synchrotron radiation Affect on SVD and CDC.HER beam(8GeV) only.At the first three months only, Belle suffered from SR.Particle background(Electro-magnetic shower)Main background source for all detectors except for KLM.Low energy photons( a few MeV) can enter into the detector.Vacuum condition is important.Hadron productionAffect on trigger rate.( HER beam, mostly)Luminosity termPhoton and Electron (or Positron) in radiative Bhabha events can hit beam pipe around IR and produce many slow neutrons. Those neutrons can penetrate Fe shield and can produce many signals in endcap KLM.
8 Result of operationSynchrotron radiation from HER upstream magnets. This killed SVDThe low energy X ray from upstream steering magnet disappeared.Limits were set to the HER magnets.The downstream chamber is replaced with a cupper chamber
10 Backscattered SRBelle suffered from backscattered SR photons during three months just after roll in.(Jun-Aug,1999)30-60KeV photons were produced at QCS.Photons hit Aluminum chamber at 6m down stream.Backscatter photons scattered again near IP chamber.Then, entered into Belle detector.
12 SR or Particle Background Several testsHER(8GeV) or LER(3.5GeV)HERCurrent dependenceAlmost linearVacuum bump at upstream of HERTOF sensitiveCDC less sensitiveFinally, SR Background
13 Source point Measured pulse height spectra with SVD and CDC. Single cluster(SVD) or single hit(CDC) using data taken with random trigger.~30keV bump in SVD~5keV Compton shoulder and ~30keV bump in CDCHigh energy photonSource : Arc bend(forward scatter) or QCS(backscatter, Outgoing HER beam goes through off-center of QCS.)Study for orbit variationConclusion : QCS
15 Modification Changed a downstream vacuum pipe. Hit point 6m 9m (1/2)Material Al Cu (1/10)No special things : just Cu + flat surfacePut gold plates with 300mm thickness on the IP beam pipe just outside of detector acceptance.Photon can not penetrate outside of detector acceptance.
17 Results of modification Pulse height spectrum in SVD and CDC~1/10Less serious as compared with other background.Further improvement in the next year.New IP chamber200mm gold plating inside Aluminum partNew W maskFurther improvement by factor 3.
23 Particle backgroundBelle experienced huge particle (shower) background.There are holes parallel to the beam direction.The hole just out side of the vacuum chamber resulted in shower around SVD and CDC.The hole close to the end-cap detector resulted in large background at the low-angle end cap CsI.
25 Silicon vertex detector Layer-1 SVD front end chips suffered about 0.9 Mrad dose.
26 CsI calorimeter The integrated dose is shown. ForwardBackwardBarrelThe integrated dose is shown.The integrated dose is slower than beam current/luminosity increase.The occupancy for E>20MeV activity is ~1 cluster/event at 5x1033 /cm2 2/sec luminosity.
27 CsI calorimeter (cont.) Light output yield is decreasing.The barrel part can survive ~100 times dose than now.
29 Trigger/DAQ/dead time Dead time reaches ~10% at 500 Hz trigger rate.The average trigger rate is still around 400 Hz.
30 Readout Dead TimeBefore 2003 summerAfter 2003 summer
31 Beam background for a short term NEG activationAt beginning just after a long shutdown, background is worse than before.Due to opening the beam pipe for some maintenance works.Better vacuum condition after several NEG activations.Finally, the beam background becomes better after a few months operation.SVD pinCDC currentTOF rateOct Jan Feb Mar
32 Background for a long term Total CDC current(8400 wires) has kept about 1mA at the maximum beam current for each year.Vacuum condition is improving from year to year.Better masking system has been adopted for fixed masks near IP and movable masks in arc section.
39 Movable Masks D12 D9 D3 D6 Purpose To protect Belle from the beam background.During physics run and injection period.16 masks in HER and 16 masks in LER.8 horizontal + 8 vertical for each rings.Location4(H)+4(V) at D6 and 4(H)+4(V) at D3 for LER4(H)+4(V) at D9 and 4(H)+4(V) at D12 for HERD3D6D9D12
40 EffectivenessUsually, the horizontal masks are not effective. Because the horizontal tail is not so large.A few vertical masks are quite effective to reduce the beam background in the both cases for storage and injection and also for LER and HER.By factor two or more.KEKB had movable masks near IR, which were not so effective to reduce the beam background.Those masks in both of LER and HER were removed.
43 Vertical beam size changed by “size bump” Data taken 28-June 12:30~13:00LER single beamVertical beam size changed by “size bump”Beam life time supposed to followBackground could depend onBeam currentVertical beam size syshould be confirmedBeam life t1/tk might be different for different processes
44 1/t seems not scaled in i/sy sy/t*iMask?Beam size sy (mm)
45 ToFSVDCDC0EFC-fCDC1EFC-bCDC2B/i2 should be scaled for sy.
46 kvackTouschekCDC#2 leak current /iCDC#0 leak current /i1/t1/t
48 Background from vacuum and Touschek CDC#0CDC#2SVDpinToFkvac0.420.083.4840ktouschek0.110.0180.6233/ kvac0.250.2250.180.28
49 Summary of Touschek effect Beam life time and Belle background are measured as a function of LER beam size.For I~900mA, tvac= 250 min, while tTouschek =300 min (for luminosity run beam size of ~2.5mm)Belle background from Touschek is less significant (25%) than vacuum, if the contributions to the beam life are chosen at the similar level.
60 Time structure of continuous injection 200xMIP100nsec10msec2-bunch Injection in LER200msec3.5msec
61 Data size Veto time 3.5msec Originally, 2msecThere are events with larger data size after veto window.Fraction of events with larger data size is quite small.
62 Background condition(short term) Back : StorageBlue : HER injectionRead : LER injectionThe beam background is fluctuate for various accelerator condition, especially during the injection.Injection parameters and movable masks are adjusted to reduce the beam background.
63 Unstable injection background Injection background is not stable.Background at HER injection was worse before starting continuous injection mode.Background at LER injection is not stable, recently.It was less problematic before the first CIM test.The background at just injection time is higher than before and HER injection.Duration is longer than the veto time, sometimes.There are no damping ring for both electron and positron in KEKB.We have an energy compression system for positron only to reduces the energy spread.
64 SummaryVarious background sources were identified in the first 3 months. Major Belle/KEKB modification against background was done inAfter that, SR background is not so serious.We have suffered from unstable injection background time to time.Most of detectors are still working fine even for continuous injection and even for very high luminosity.No serious radiation damage. (Except for SVD1)Manageable background hits. (Except for outer most two layers in endcap KLM)