1. 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

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
 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

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 in
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.

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 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.

5. Low energy photon

7. Beam pipe near IP

8. 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

9. Backscattered SR problem in Belle

10. 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.

11. Source of backscattered photon
QCS
QC1

12. 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

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 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

14. Pulse Height Spectra in CDC
HER
Saturation Peak
Photo Absorption
Compton Edge
LER
~5 keV

15. 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.

16. 300mm gold plates

17. 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.

18. After modification I

19. After modification II

20. After further modification

21. Particle background

22. Simulation orbit for off-momentum particles

23. 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.

25. Silicon vertex detector
Layer-1 SVD front end chips suffered about 0.9 Mrad dose.

26. 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.

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

28. Endcap KLM hit pattern
Y (cm)
X (cm)

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 Time
Before 2003 summer
After 2003 summer

31. 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

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.

33. Vacuum bump study

34. Ring Ave. D1 D6
D7,8
D1 Straight Ave.
CDC current

35. D1
D12 D2 D9 D3 D8 D6 D7

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

38. Movable Masks

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.
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

40. 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.

41. 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

42. Touschek background

43. 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

44. 1/t seems not scaled in i/sy
sy/t*i
Mask?
Beam size sy (mm)

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

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

47. SVD PIN /i
ToF rate /i

48. 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

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.

50. After previous WS

51. Radiation Dose at SVD 1st layer measurement and simulation
(Tajima)
(Trabelsi)
HER 1.1A, LER 1.6A
(…) is 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

52. Dose and Gain on SVD2 Gain variation Dose for one year
If BG is similar, SVD2 can survive for more than 10 years.

53. SVD2 Hybrid Gain
There is no significant degradation for two year operation.

54. 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.

55. 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

56. 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

57. New polyethylene shield

58. 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.

59. Continuous Injection

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

61. 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.

62. 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.

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. 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)