Presentation on theme: "Background issues in the KEKB Belle detector Synchrotron radiation –Low energy photon –Backscattered photon Particle background Vacuum bump study Movable."— Presentation transcript:
Background issues in the KEKB Belle detector Synchrotron radiation –Low energy photon –Backscattered photon Particle background Vacuum bump study Movable mask Touschek effect Continuous Injection Shoji Uno(KEK) Mini-workshop on BEPCII Background Study at IHEP Beijing Mar/10-12, 2008
SVD CDC ACC TOF ECL KLM Solenoid Belle Detector Event reconstruction Charged tracks Silicon Vertex Detector (SVD) Central Drift Chamber (CDC) Electrons and photons Electromagnetic Calorimeter (ECL) Vertexing SVD Particle identification K/ separation CDC : dE/dx Aerogel Cherenkov Counter (ACC) Time of Flight Counter (TOF) electron identification ECL & dE/dx in CDC K L and muon identification K L and muon detector (KLM)
History KEKB/Belle operation started in May Various background sources were identified in the first 3 months. Major Belle/KEKB modification against background was done in Silicon vertex detector (SVD) has been replaced four times in 5 years operation. –SVD1.0: 1998 Winter: installation of : 3 layers, 1.4 m VA1 readout chip. –May 1999: Commissioning of KEKB/Belle –1999 Summer --- VA1 chip was damaged by Synchrotron radiation: Installation of SVD1.2. The IP (interaction point) chamber was wrapped with 10 m-thick gold foil. –2000 summer --- Installation of SVD1.4 (With 0.85 m 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 m VA1TA, fast trigger capability) The other detectors have not been damaged seriously. Continuous Injection started in 2004.
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 production –Affect on trigger rate.( HER beam, mostly) Luminosity term –Photon 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.
Low energy photon
Beam pipe near IP
Result of operation The low energy X ray from upstream steering magnet disappeared. Limits were set to the HER magnets. Synchrotron radiation from HER upstream magnets. This killed SVD The downstream chamber is replaced with a cupper chamber
Backscattered SR problem in Belle
Backscattered SR Belle 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.
Source of backscattered photon QCS QC1
SR or Particle Background Several tests – HER(8GeV) or LER(3.5GeV) HER – Current dependence Almost linear – Vacuum bump at upstream of HER TOF sensitive CDC less sensitive Finally, SR Background
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 CDC High energy photon –Source : Arc bend(forward scatter) or QCS(backscatter, Outgoing HER beam goes through off-center of QCS.) Study for orbit variation – Conclusion : QCS
Pulse Height Spectra in CDC HER Saturation Peak Photo Absorption Compton Edge LER ~5 keV
Modification Changed a downstream vacuum pipe. – Hit point 6m 9m (1/2) – Material Al Cu (1/10) – No special things : just Cu + flat surface Put gold plates with 300 m thickness on the IP beam pipe just outside of detector acceptance. – Photon can not penetrate outside of detector acceptance.
300 m gold plates
Results of modification Pulse height spectrum in SVD and CDC – ~1/10 – Less serious as compared with other background. Further improvement in the next year. – New IP chamber 200 m gold plating inside Aluminum part – New W mask – Further improvement by factor 3.
After modification I
After modification II
After further modification
Simulation orbit for off-momentum particles
Particle background Belle 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.
Silicon vertex detector Layer-1 SVD front end chips suffered about 0.9 Mrad dose.
The 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 5x10 33 /cm 2 2/sec luminosity. CsI calorimeter Forward Backward Barrel
CsI calorimeter (cont.) Light output yield is decreasing. The barrel part can survive ~100 times dose than now.
Endcap KLM hit pattern X (cm) Y (cm)
Trigger/DAQ/dead time Dead time reaches ~10% at 500 Hz trigger rate. The average trigger rate is still around 400 Hz.
Readout Dead Time Before 2003 summer After 2003 summer
Beam background for a short term At 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. NEG activation SVD pin CDC current TOF rate Oct Jan Feb Mar
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.
Vacuum bump study
Ring Ave. D1 Straight Ave. CDC current D6D7,8D1
D3 D6 D9 D12 D1 D2 D7 D8
D1 straight D6 D7,8 I CDC /I beam (1/ )
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. Location –4(H)+4(V) at D6 and 4(H)+4(V) at D3 for LER –4(H)+4(V) at D9 and 4(H)+4(V) at D12 for HER D3 D6 D9 D12
Effectiveness Usually, 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.
Data taken 28-June 12:30~13:00 LER single beam Vertical beam size changed by “size bump” Beam life time supposed to follow Background could depend on Beam current Beam life 1/ should be confirmed k might be different for different processes Vertical beam size y
Beam size y ( m) y / i / seems not scaled in i/ y Mask?
SVD CDC0 CDC1 CDC2 ToF EFC-f EFC-b B/i 2 should be scaled for y.
CDC#0 leak current /i 1/ k vac k Touschek CDC#2 leak current /i
SVD PIN /i ToF rate /i
Background from vacuum and Touschek CDC#0CDC#2SVDpinToF k vac k touschek k touschek / k vac
Summary of Touschek effect Beam life time and Belle background are measured as a function of LER beam size. For I~900mA, vac = 250 min, while Touschek =300 min (for luminosity run beam size of ~2.5 m) Belle background from Touschek is less significant (25%) than vacuum, if the contributions to the beam life are chosen at the similar level.
After previous WS
Radiation Dose at SVD 1 st layer measurement and simulation HER 1.1A, LER 1.6A Outer-direction ~ 0 degree Inner-direction ~ 180 degree Particle-BG (LER) 22 (18) kRad/yr 14 (11) kRad/yr Particle-BG (HER) 44 (53) kRad/yr 29 (33) kRad/yr SR-BG 17 (8) kRad/yr 33 (29) kRad/yr Total 83 (79) kRad/yr 76 (73) kRad/yr (…) is 1nTorr pressure Data and simulation is consistent Touschek contribution is reduced based on measurement (Tajima)(Trabelsi)
Dose and Gain on SVD2 Dose for one year Gain variation If BG is similar, SVD2 can survive for more than 10 years.
SVD2 Hybrid Gain There is no significant degradation for two year operation.
CDC Total Current Maximum current is still below 1.2mA. Vacuum condition is improving. –Thanks for replacing a cavity-like beam pipe with a straight pipe at Tsukuba straight section in HER.
inside outside Bkg comes from outside of detector Bkg Rate (Hz/cm 2 ) Luminosity (/nb/sec) KLM Bkg Rate (Hz/cm 2 ) Much neutron background Eff. 90% Eff. 80% Eff. 70% Bkg Luminosity !! High-Bkg degrade Eff. ~9m fast neutron (mSv/2weeks) Rad-Bhabha gamma hits magnets LER HER neutron not By O.Tajima
Bkg Rate (Hz/cm 2 ) Neutron background reduction fast neutron (mSv/2weeks) LER HER polyethylene ( t 10cm) Bkg x1/3 Protect detector with Large shield or magnet covers (polyethylene etc.) Effective for ECL, too
New polyethylene shield
KLM super-layer efficiency for Layer 11 (Before) (After) Also, HV for outer two layers can be turned on. Backward shield will be installed during summer shutdown.
Time structure of continuous injection 100nsec 10 sec 3.5msec 200 sec 200xMIP 2-bunch Injection in LER
Data size Veto time –3.5msec Originally, 2msec –There are events with larger data size after veto window. –Fraction of events with larger data size is quite small.
Background condition(short term) The beam background is fluctuate for various accelerator condition, especially during the injection. Injection parameters and movable masks are adjusted to reduce the beam background. Back : Storage Blue : HER injection Read : LER injection
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.
Summary Various background sources were identified in the first 3 months. Major Belle/KEKB modification against background was done in After 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)