Background issues in the KEKB Belle detector Synchrotron radiation –Low energy photon...
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Transcript of Background issues in the KEKB Belle detector Synchrotron radiation –Low energy photon...
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 CDCACCTOFECLKLMSolenoid
Belle DetectorBelle DetectorEvent 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
KL and muon identification KL and muon detector (KLM)
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 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 10m-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 h
it 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 EdgeLER
~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 300m thickness on the IP beam pipe just outside of detector acceptance.– Photon can not penetrate outside of detector
acceptance.
300m 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
• 200m 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 aro
und 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 5x1033 /cm2 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
D6 D7,8D1
D3
D6
D9
D12D1
D2
D7
D8
D1 straight
D6 D7,8
IC
DC/I
beam
(1/)
Movable Masks
Movable Masks
• 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
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
LER HER
Touschek background
• 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
y
bunchi
1
...)(
Touschek
Touschek
vac
vac kkiB
i
dt
diB
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/i2 should be scaled for y.
CD
C#0
leak
cur
rent
/i
1/ 1/
kvac
kTouschek
CD
C#2
leak
cur
rent
/i
SV
D P
IN /
i
ToF
rat
e /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
ktouschek
/ 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, vac= 250 min, while Touschek =300 min (for luminosity run beam size of ~2.5m)
• 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 layermeasurement 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 simulation @ 1nTorr pressure
Data and simulation is consistentTouschek 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
1.0
0.1
Bkg comes fromoutside of detector
Bkg
Ra
te (
Hz/
cm2)
Luminosity (/nb/sec)K
LM
Bkg
Ra
te (
Hz/
cm2)
Much neutron backgroundMuch neutron background
Eff. 90%
Eff. 80%
Eff. 70%
Bkg Luminosity !!
High-Bkg degrade Eff.
~9m
2.21.81.5 1.6 0.8fast neutron
(mSv/2weeks)
Rad-Bhabha gamma hits magnets LER
HER
neutron
not
By O.Tajima
Bkg
Ra
te (
Hz/
cm2)
Neutron background reductionNeutron background reduction
2.21.81.5 1.6 0.8fast neutron
(mSv/2weeks)
LER
HER
polyethylene(t10cm)
Bkg x1/3
Protect detectorwith
Large shield ormagnet 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.
Continuous Injection
Time structure of continuous injection
100nsec10sec
3.5msec200sec
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 : StorageBlue : HER injectionRead : 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 fi
rst 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 backgroun
d time to time. • Most of detectors are still working fine even for cont
inuous injection and even for very high luminosity.– No serious radiation damage. (Except for SVD1) – Manageable background hits. (Except for outer most two l
ayers in endcap KLM)