Background issues in the KEKB Shoji Uno(KEK) Mini-workshop on BEPCII Background Study at IHEP Beijing Mar/10-12, 2008 Belle detector Synchrotron radiation Low energy photon Backscattered photon Particle background Vacuum bump study Movable mask Touschek effect Continuous Injection

Belle Detector Event reconstruction Particle identification Vertexing  Charged tracks  Silicon Vertex Detector (SVD)  Central Drift Chamber (CDC)  Electrons and photons  Electromagnetic Calorimeter (ECL) Vertexing  SVD Particle 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) SVD CDC ACC TOF ECL KLM Solenoid

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 in 1999-2000. Silicon 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/Belle 1999 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.

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 Synchrotron radiation from HER upstream magnets. This killed SVD The 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

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 300mm thickness on the IP beam pipe just outside of detector acceptance. Photon can not penetrate outside of detector acceptance.

300mm 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 200mm gold plating inside Aluminum part New W mask Further improvement by factor 3.

After modification I

After modification II

After further modification

Particle background

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.

CsI calorimeter The integrated dose is shown. Forward Backward Barrel 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 5x1033 /cm2 2/sec luminosity.

CsI calorimeter (cont.) Light output yield is decreasing. The barrel part can survive ~100 times dose than now.

Endcap KLM hit pattern Y (cm) X (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 NEG activation 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. 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 D6 D7,8 D1 Straight Ave. CDC current

D1 D12 D2 D9 D3 D8 D6 D7

D1 straight D6 D7,8 DICDC/Ibeam D(1/t)

Movable Masks

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. 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.

Phase HER LER D6V1 0.9909 D6V2 0.9926 D6V3 0.1809 D6V4 0.1826 D3V1 0.0632 D3V2 0.0649 D3V3 0.2532 D3V4 0.2549 QCS 0.25 D9V1 0.2433 D9V2 0.0392 D9V3 0.7172 D9V4 0.5799 D12V1 0.6545 D12V2 0.5171 D12V3 0.3798 D12V4 0.3130 QC1 0.25

Touschek background

Vertical beam size changed by “size bump” 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 Vertical beam size sy should be confirmed Beam life t 1/t k might be different for different processes

1/t seems not scaled in i/sy sy/t*i Mask? Beam size sy (mm)

ToF SVD CDC0 EFC-f CDC1 EFC-b CDC2 B/i2 should be scaled for sy.

kvac kTouschek CDC#2 leak current /i CDC#0 leak current /i 1/t 1/t

SVD PIN /i ToF rate /i

Background from vacuum and Touschek CDC#0 CDC#2 SVDpin ToF kvac 0.42 0.08 3.4 840 ktouschek 0.11 0.018 0.6 233 / kvac 0.25 0.225 0.18 0.28

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.

After previous WS

Radiation Dose at SVD 1st layer measurement and simulation (Tajima) (Trabelsi) HER 1.1A, LER 1.6A (…) is simulation @ 1nTorr pressure 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 Touschek contribution is reduced based on measurement Data and simulation is consistent

Dose and Gain on SVD2 Gain variation Dose for one year 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.

Much neutron background By O.Tajima High-Bkg degrade Eff. inside outside 1.0 0.1 Bkg comes from outside of detector Bkg Rate (Hz/cm2) Eff. 70% g neutron not Eff. 80% KLM Bkg Rate (Hz/cm2) Eff. 90% Bkg a Luminosity !! Luminosity (/nb/sec) 2.2 1.8 1.5 1.6 0.8 fast neutron (mSv/2weeks) HER Rad-Bhabha gamma hits magnets LER ~9m

Neutron background reduction Bkg Rate (Hz/cm2) polyethylene (t10cm) Bkg x1/3 Protect detector with Large shield or magnet covers (polyethylene etc.) Effective for ECL, too 2.2 1.8 1.5 1.6 0.8 fast neutron (mSv/2weeks) HER LER

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.

Continuous Injection

Time structure of continuous injection 200xMIP 100nsec 10msec 2-bunch Injection in LER 200msec 3.5msec

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) Back : Storage Blue : HER injection Read : LER injection 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.

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 1999-2000. 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)