Background issues in the KEKB Belle detector Synchrotron radiation –Low energy photon...

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 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

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

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)

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

D1 straight

D6 D7,8

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

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.

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

bunchi

Touschek

vac kkiB

Beam current

Beam life

should be confirmed

k might be different for different processes

Vertical beam size y

Beam size y (m)

/seems not scaled in i/y

B/i2 should be scaled for y.

kTouschek

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

Bkg comes fromoutside of detector

Luminosity (/nb/sec)K

Much neutron backgroundMuch neutron background

Eff. 90%

Eff. 80%

Eff. 70%

Bkg Luminosity !!

High-Bkg degrade Eff.

2.21.81.5 1.6 0.8fast neutron

(mSv/2weeks)

Rad-Bhabha gamma hits magnets LER

neutron

By O.Tajima

Neutron background reductionNeutron background reduction

2.21.81.5 1.6 0.8fast neutron

(mSv/2weeks)

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)

Background issues in the KEKB Belle detector Synchrotron radiation –Low energy photon...

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