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 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.

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 a

nd CDC.• The hole close to the end-cap detector resulted in large background at the low-a

ngle 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-m 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 do

wn-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.

Cupper chamber

Aluminum chamber

Result of operation

• The low energy X ray from upstream steering magnet disappeared.

• Effect of reflected X ray also significantly reduced.

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

Current status

• 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 do

wn with vacuum improvement.• A single-beam study [KEKB NIM paper, 2

002] suggests the particle is not the dominant BKG source.

• Test of Touschuk effect was done in June.• LER beam bunch size was artificially incre

ased and background/beam life was measured.

• Touschek contribution is not dominating the background. (Thanks to large momentum aperture of KEKB)

Summary of HER burst.

• 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 % accord

ingly. 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.

CDC Layer #

Hit

rat

e/w

ire (

k Hz)

Spring 2003HER 1.1ALER 1.5AL=9.6x1033cm-1s-1

MainInner

Cathode

• 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.

CsI calorimeter

Forward

Backward

Barrel

5% loss

10rad100rad

2% loss

CsI calorimeter (cont.)

• Light output yield is decreasing.

• The barrel part can survive ~100 times dose than now.

10% loss

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.