Transcript Operational Experience with machine backgrounds at KEKB 23 September 2003 T.Tsuboyama (KEK) History • KEKB/Belle operation started in May 1999. • Various background sources were.

Operational Experience with machine
backgrounds at KEKB
23 September 2003
T.Tsuboyama (KEK)
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.
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 lowangle end cap CsI.
Interaction region/Belle
Background to forward CsI
• Background to CsI
• Widening of energy pedestal
level was clearly observed in
the forward region.
Forward
Region
Adding particle masks
• Adding masks to vacant spaces
to stop the stray particles.
X ray from HER
• Low energy synchrotron radiation
– Synchrotron radiation from steering magnets in
the upstream of HER beam destroyed the front
end chips of SVD in one week.
– Because of the long HER strait section, generated
photons can not be stopped out side of Belle.
• The beryllium chamber was wrapped with gold foil.
(In 2000 summer, an IP chamber with 10-mm thick
gold (spattering) was installed.)
• Limits were set to current of the corresponding
magnets.
• High energy (30 keV) synchrotron radiation
– HER synchrotron light scattered back from the
down-stream chamber (crotch for LER/HER
separation) made of aluminum caused large
occupancy in CDC and SVD.
• The chamber was replaced with one made of
cupper that has smaller reflection coefficient.
• The chamber shape were changed to make crotch
farther from the IP chamber to reduce the solid
angle.
X ray energy spectrum
measured by CDC
Reflection of synchrotron radiation
• The synchrotron light from the final focus system hit the
aluminum chamber and the reflected X ray entered to Belle. The
aluminum chamber was replaced with cupper chamber and the
synchrotron dump is made further.
Aluminum chamber
Cupper
chamber
Result of operation
• The low energy X ray
from upstream
steering magnet
disappeared.
• Effect of reflected X
ray also significantly
reduced.
Synchrotron radiation from
HER upstream magnets.
This killed SVD
Limits
were set to
the HER
magnets.
The downstream
chamber is replaced
with a cupper
chamber
Current status
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KEKB Vacuum system
SVD (silicon vertex detector)
CDC (central drift chamber)
CsI (CsI calorimeter)
Trigger (Trigger rate)
Improvement of Vacuum
• The vacuum of KEKB has been improving as
shown in the figures.
– The horizontal axis shows the beam dose since
the beginning of KEKB operation.
• The KEKB vacuum group is responsible for
– Operation and maintenance of vacuum system
– Replacing broken bellows, broken masks.
– Designing and replacing vacuum components
that are heated/damaged by beam.
– Complex chamber design around the interaction
region (IR).
• HOM heat in chamber and bellows, beam monitor,
radiation shield, synchrotron light absorber,
cooling, strange-shape final focus magnets,
photoelectrons …. should be taken into
consideration.
• Owing to these efforts, the background to Belle
has been kept at an acceptable level.
Accelerator operation
• Vacuum-related backgrounds are going
down with vacuum improvement.
• A single-beam study [KEKB NIM paper,
2002] suggests the particle is not the
dominant BKG source.
• Test of Touschuk effect was done in June.
• LER beam bunch size was artificially
increased and background/beam life was
measured.
• Touschek contribution is not dominating the
background. (Thanks to large momentum
aperture of KEKB)
Summary of HER burst.
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Background burst:
The HER beam lifetime decreases significantly for short time.
Belle subsystem receive backgrounds.
Usually, hardware abort level is not exceeded. Total dose per
burst is usually smaller than 1 rad/sec.
• CDC HV system has high-speed current limitters. Therefore, HV
power supply (LeCroy) does not trip. While the current limiter is
active, the data taking is paused.
• CsI/ACC/TOF systems have not tripped by background burst.
• This phenomenon does not happen to LER.
Snapshot of the KEKB operation for best day
Silicon vertex detector
• Layer-1 SVD front end chips suffered about 0.9 Mrad
dose.
• The gain of the preamplifier decreased by 25 %
accordingly. In December 2002, bad ladders were
replaced and Gain recovered with intense tuning of the
VA1 operation parameters.
• Improvement of KEKB luminosity is faster than the
dose accumulation to SVD.
RADFET position dependence
Black – Bottom
Green – Inside the KEKB ring
Blue – Top
Red – Outside the tunnel
Central Drift Chamber
• Although the KEKB has
increased the luminosity and
beam current, wire current of
CDC has been kept in
operational range.
• This is a result of the
continuous improvement of the
vacuum.
• The cathode part is close to
limits, however, it was replaced
with a small-cell drift chamber
this summer.
Spring 2003
HER 1.1A
LER 1.5A
Cathode L=9.6x1033cm-1s-1
Inner
Main
CDC Layer #
CsI calorimeter
• The integrated dose
and PIN-diode dark
current 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.
Forward
Barrel
Backward
CsI calorimeter (cont.)
• Light output yield is decreasing.
• The barrel part can survive
~100 times dose than now.
2% loss
5% loss
10% loss
10rad
100rad
1krad
Trigger/DAQ/dead time
• Dead time reaches ~10% at
500 Hz trigger rate.
• The average trigger rate is
still around 400 Hz.
Continuous injection (CI)
• In March 2003, we did a study of continuous injection
– Tthe LER CI is and HER CI gives large noise to Belle.
• A similar test was done in end of June.
– This time LER injection is worse than HER.
• Injection rate was 5-10 Hz.
• Trigger is vetoed for 3.5 msec. at each injection.
– Dead time due to CI 3.5 %
– The autumn run will be done in CI mode.
Summary of CI (June 30)
Injection summary
Sub detectors
• TOF --- At each injection input
voltage becomes 1-15 V
(MIP=0.2V). This noise level
is within the tolerable range
(In a previous test, the
preamplifiers saturated.
Preamplifiers were replaced
with signal-cramping
capability)
• ACC(PID)
• Occupancy and N(photon)
• KLM
2003 Summer shutdown
• The CDC inner part (cathode-pad read out) is replaced
with a small-cell drift chamber.
– Z-trigger capability from cathode pads is lost.
• SVD1.6 is replaced with SVD2.0
– 4-layers. Provides fast Z-trigger from the front-end chips.
• New IP chamber made of Tantalum and Tungsten (total
weight of 40 kg). Acceptance region is made of
0.5+0.3mm thick, 15mm inner-radius Beryllium chamber,
cooled with circulation of normal-paraffin in the 0.5 mm
gap. Particle backgrounds should be effectively stopped.
(Talk by O.Tajima on Wednesday)
2003 Summer shutdown (cont.)
• Replacement of accelerator chambers next to Belle
– HOM heating in the chamber and the bellows has limited the
KEKB beam current.
– The chamber was replaced with smoother-shape chamber and
more cooling capability.