Current BG Status at Belle

Current BG Status at Belle

Osamu Tajima

( Tohoku univ )

Assumption in this talk 100days-operation / yr 1nTorr CO pressure in simulation HER / LER = 1.1 / 1.6 A in simulation

Contents

• Design concepts for BG reduction

• BG measurements

Radiation dose

Hit rate (SVD occupancy)

Layer (radius) dependence

• Comparison data and simulation

Support Super-KEKB / Belle design

• Ideas for Less BG

SVD Upgrade in 2003 summer

rbp = 2.0 cm 3 layers Rad. hardness

rbp = 1.5 cm 　　 4 layers 　　 > 10 MRad (DSSD) 　　 > 20 MRad (reado

ut chip)

~ 1 MRad

Better vertex resolution / tracking efficiency

SVD Upgrade for Super-Belle

rbp = 1.5 cm 　　

Super-Belle• Smaller rbp (1cm)• Higher beam current• Basic design is same as SVD2 beampipe

• We must understand Current Situation• Success of beampipe design is key-point

for Super-KEKB/Belle

Beam-BG on Belle-SVD2

Synchrotron Radiation (SR)

Particle Background

Showers from scattered beam particles by Residual Gas or intra-beam scattering

Soft-SR (several keV)

Hard-SR ( keV ~ 150 keV)

Generated by upstream magnets

Backscattering from downstream

Brem. Coulomb Touschek

Reduction of Particle-BG

Particle BG ~ 70 kRad/yr

Reduction of Soft-SR

Au-coating !

crescent shapeSR-mask

some efforts


Au-coating !


Au coating absorbs low energy photon less than 8 keV


Au-coating !


Saw-tooth surface shape in Ta blind Soft-SR reflected on Ta


Au-coating !

crescent shapeSR-mask (~2.5mm)

Crescent shape SR-mask blind Be section from Soft-SR


Au-coating !


Soft-SR ~ few kRad/yr

Reduction of Hard-SR

Scattered at downstream photon-stop (OC2RE chamber)

HER e-

High energy SRis generated inOCS magnet

•Put photon-stop far place (~9m)•Chamber material: Cu

Hard-SR ~ 29 kRad/yr

Beam-BG on Belle-SVD2

Synchrotron Radiation (SR)

Particle Background

Showers from scattered beam particles by Residual Gas or intra-beam scattering

Soft-SR (several keV)

Hard-SR ( keV ~ 150 keV)

~ 70 kRad/yr @ 1st layer

few kRad/yr @ 1st layer

~ 29 kRad/yr

Generated by upstream magnets

Backscattering from downstream

Brem. Coulomb Touschek

BG measurement and Comparison with simulation

SR measurement w/ Single-Bunch HER 15 mA, with adjusting trigger timingCan measure dose w/ hit-rate (0.2 % occupancy) and energy deposition (15 keV/ch) ~20 kRad/yr dose @ 1.1 A (33 kRad/yr at max. position, =180deg) ( contribution below th. is corrected by simulation)

SVD 2.0 SVD 1.Xdatasimulation

SVD Cluster Energy Spectra in Single Beam Run

HER 0.8 A LER 1.5 A

Can extract SR from spectrum shape !?

Only Particle-BG

SR and Particle-BG

E-spectrum of HER Particle-BG

energy (keV)

#clu

ster

s/ke

V/e

vent • Diff. btw vacuum

bump on/off in HER• LER 1.5 A

HER E-spectrum of particle BG issame as LER !!

Can measure SRand particle-BG

separately

Extraction SR in HER Single Beam

50 mA 100 mA 200 mA

400 mA 600 mA 800 mA

HERParticle

SR

Hard-SR simulation

Correlation with Vacuum

Nparticle/NSR P(Pa) NSR I(A)Nparticle I(A) x P(Pa)

Average of HER whole ring

Average of HER upstream

Azimuthal Distribution of SR

33 kRad/yrat HER 1.1A


Only above threshold 10 keVSimulation complements below thereshold

simulation

29 kRad/yr

Single-Bunch 15 mA (trigger-timing is adjusted)Total 0.8 A w/ 1284 bunch (random timing)Hard-SR simulation

Azimuthal Distribution of Particle BG


43 kRad/yrat LER 1.6A

HER 0.8 A

LER 1.5 A

simulation

53 kRad/yr

simulation

21 kRad/yr

Study of Touschek EffectTouschek contribution < 20 % at collision ~ 50 % at single beam 31 % in simulation

Smaller beam-size (larger density)

larger background

If no Touschek

Touschek contributionmust be corrected

Collision run

Single beam run

Azimuthal Distribution of Particle BG


43 kRad/yrat LER 1.6A

HER 0.8 A

LER 1.5 A

simulation

53 kRad/yr

simulation

21 kRad/yr

22

18

Radiation Dose at SVD 1st layerAt Maximum Currents: 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


Outer-direction

~ 0 degree

Inner-direction

~ 180 degree






Data and simulation is consistentTouschek contribution is reduced based on measurement

• Two parameters have large uncertainty (pressure, movable mask)• It may happen that absolute values too well agree• Consistency of azimuthal distribution is important


Outer-direction

~ 0 degree

Inner-direction

~ 180 degree






Data and simulation is consistentTouschek contribution is reduced based on measurement• We can trust simulations

• Its uncertainty for abs. may be factor a few

Constraint for Occupancy (hit-rate)

At Maximum Currents (HER 1.1A, LER 1.6A)

Outer-direction

~ 0 degree

Inner-direction

~ 180 degree

Particle-BG (LER) 3 % 2 %

Particle-BG (HER) 7 % 5 %

SR-BG 2 % 4 %

Total (single beam) 12 % 11 %

Radiation Dose Occupancy (cluster size: Particle-BG 3.5 ch, SR 1.5 ch)

Collision 12 % 11 %

Energy spectra for each layers

LERsingle beam

1str ~ 2cm

2ndr ~ 4.4cm

3rdr ~ 7cm

4thr ~ 8.8cm

HERsingle beam

Layer dependence (single beam)Particle (LER) Particle (HER) SR

BG 1/(r-rbp), rbp: beampipe radius

There may be correlation BG and 1/(r-rbp)

Other sub-detectors

No large difference for BG (current diff. causes small diff. ?)

No problem

SVD 1.6

(Jun, 2003)

SVD 2.0

(Dec, 2003)

beampipe radius 2.0 cm 1.5 cm

HER/LER 1.0 / 1.5 A 1.1 / 1.6 A

CDC leak current 19 A 21 A

TOF rate 20 kHz 25 kHz

EFC rate 2.1 kHz 2.2 kHz

Ideas for Less BG

• Improvement of vacuum HER: sensitive area is upstream (0~100 m) LER: sensitive area is whole ring• How about not-straight path ? HER upstream is almost straight path• Movable mask study

Particle-BG

1/2 Particle-BG ~ 2/3 total-BG/Occ.

• Put photon-stop far place Detail will be discussed in “Belle SR” talk

SR-BG (dominated by Hard-SR)

Summary• Beampipe radius 2 1.5 cm ( 1 cm)

• Dose level is smaller (100 80 kRad/yr) Consistent with simulation

• Measure SR & Particle-BG separately using energy spectrum of SVD SR contribution ~1/3 of total

• Touschek is low < 20 % of LER-BG

• BG may decrease 1/(r-rbp)

Success of beampipe design Strong support for design in Super-B

Super-B

This method is first time in the world !?

####### backup #######

Radiation Monitors

18 kRad/yr

80 kRad/yr60 kRad/yr

before 100 kRad/yr

Dose on Si is consistent with monitor

Is monitors measuring SR ?

Be pipe

Outer-side of ring

Inner-side of ringThe 300 m Au on the manifold blinds SR-BG

Backscattered Hard-SR

BWD FWD

Most of SR photons are absorbed by Au, and converted to lower energy photons (8~14keV) via the photoelectric effect

e-

Difficulty to measure SRDose at DSSD center is same ?

Measure BGby DSSD itself

Very Rough Estimation of Dose

We can measure dose using its energy deposition - Occupancy ~ 10 % - Energy Deposition ~ 46 keV/ch - Bunch cycle 10 usec - Shaping time 2.6~3.0 usec

~100 kRad/yr - No subtraction of electrical noise, bad-ch effect - Contribution below threshold (~15keV) is not considered - Need to consider below th. for SR (low energy should be dominated by SR)

Must measure for each components

SVD Hit Occupancy (hit-rate)

• 1st layer (R=2 cm) 10~12 % (HER 1.1A, LER 1.6A)

• Before (R=2.5 cm) 7 ~ 8 % (HER 1.0A, LER 1.5A)• 2nd layer (4.3 cm) ~ 4 %, 3rd, 4th layer ~ 2 %

SVD Occupancy (hit-rate)

SVD 1.6SVD 2.0

beampipe radius

Large diff. of occ. btw 1st – 2nd layersmay come from 1/(r-rbp) relation

Single Bunch like RunHER 15 mA, with adjusting trigger timingSVD 1st layer occupancy ~ 0.2 % corresponds to ~ 4 % occupancy @ 1.1 AEnergy deposition 15.2 keV/ch corresponds to ~20 kRad/yr dose @ 1.1 A (33 kRad/yr at maximum position, =180deg) ( contribution below th. is corrected by simulation)

SVD 2.0 SVD 1.Xdatasimulation

Correlation with Vacuum

Average Pressure

(Nparticle/NSR) / P(Pa) = const

Upstream Pressure

Background at Collision

71 kRad/yr

11 kRad/yr

Total doseParticle-BGSR-BG

Run1560 (threshold ~15 keV) HER : 1.1 A, LER : 1.6 A

Consistent with expectation from single-beams

Study of Touschek (life-time)

1.4A1.1A

Touschek contribution < 20 % at collision ~ 45 % at single beam 42 % in simulation

Smaller beam-size (density)

shorter life-time and larger background

If no Touschek

Single beam run

Touschek contributionmust be corrected

1/ beam-density

Collision run

Layer dependence (collision run)Particle-BG SR-BG

BG 1/(r-rbp)

BG comes from beampipe radius ?

Large differenceof occupancy btw1st and 2nd layer

Is there reasonable reason to explain 1/(r-rbp) correlation?

Be pipe

SR is scattered / absorbed at Au coating

Backscattered Hard-SRe-

Spent particles are scattered thin-Ta region Large contribution comes from here (simulation)

Ta TaBe pipe

Current BG Status at Belle

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