Contents
• Introduction (Soft/Hard-SR, simulation tools)
• Experience at Belle
- Troubles in 1999
- Efforts for SR reduction
- Current Status
• Ideas for SR reduction in Super-B
Dose is given only for 1st layer of SVD
Synchrotron Radiation (SR)
Scattered at downstream photon-stop (OC2RE chamber)
Soft-SR
High energy SRis generated inOCS magnet
Hard-SR (backscattering)
Several keV
keV ~ 150 keV
Belle
Beam orbit in Soft-SR simulation• We can estimate real orbit from BPM• Real orbit is calculated by 2 fitting with IP constraint (<1mm)• Design orbit is adopted for Super-KEKB
Simulation Tools in Detector etc.
• Simulate down to 1 keV or less• Photoelectric-effect is important in keV range ex. Au L-edges (14keV…), Cu K-edge (8keV) etc.
Requirements
EGS4 / Geant4
Belle had miserable experience
Understanding & Reduction are Important !
Soft-SR killed readout chipCurrent limited, 100mAby high rate Hard-SR
Simulation reproduce Gain-drop
azimuthal distributionactual orbit
byT.Abe (KEK)
Solved by limiting steering magnets operation No more troubles
Reduction of Hard-SR
Put photon-stop far place : 6 9.5 m ~ 0.4 Chamber material : Al Cu ~ 1/3
Energy spectra of Hard-SR (simulation)
~1/10
Higher current 100 1100mA achieved !!
More Reduction Efforts
Au-coating !
crescent shapeSR-mask
Au coating absorbs low energy photon less than 8 keV
SVD 2.0
More Reduction Efforts
Au-coating !
crescent shapeSR-mask
Saw-tooth surface shape in Ta blind Soft-SR reflected on Ta
SVD 2.0
More Reduction Efforts
Au-coating !
crescent shapeSR-mask
Crescent shape SR-mask blind Be section from Soft-SR
SVD 2.0
Dose expectation SVD2 (r=1.5cm)
Scattered at downstream photon-stop (OC2RE chamber)
Soft-SR
High energy SRis generated inOCS magnet
Hard-SR (backscattering)
Several keV
keV ~ 150 keV
few kRad/yr
29 kRad/yr
Azimuthal Distribution of SR
33 kRad/yrat HER 1.1A
21 kRad/yrat HER 1.1A
Single-Bunch 15 mA (trigger-timing is adjusted)Total 0.8 A w/ 1284 bunch (random timing)
Only above threshold 10 keVSimulation complements below thereshold
simulation
29 kRad/yr
Hard-SR simulation
Current Status of SR at Belle
• Simulation reproduced experience• Current beampipe (r =1.5 cm) at Belle is
designed based on BG simulations• Hard-SR is dominated (~30 kRad/yr)• Data is consistent with simulation
• Success of beampipe design rbp=1.5cm
Strong support for - Design Concept in Super-B - Reliability of BG Simulation
Super-KEKB Machine Parameters
KEKB SuperKEKB
Horizontal emittance 24 nm 33 nm
x-y coupling ~ 3 % 6.4 %
x* / y* (cm) 63 / 0.7 30 / 0.3
Beam current 1.1 A 4.1 A
Crossing angle 22 mrad 30 mrad
HER-beampipe tilt 11 mrad 15 mrad
Discussion for SR-mask (rbp=1cm)
Mask height should be same as rbp=1.5 cm case ( ~2.5 mm height)
200 kRad/yr
2/3 scale 2 MRad/yr !!
Acceptable height
Discussion for SR-mask (rbp=1cm)
outside inside outside inside
If vertical direction is higher ?
200 kRad/yr
Dose distribution Sym. SR-mask
~20 kRad/yr
~50 kRad/yr
QC1L(-3m) during injection
QC2L(-7m) at stored
~10 kRad/yr
Dose distribution Sym. SR-mask
~50 kRad/yr
QC1L(-3m) at injection
QC2L(-7m) at stored
~10 kRad/yr
!! WARNING !!Soft-SR is localized
~20 kRad/yr
SR AlarmOnline Alarm system based on the SR-sim.1. Real orbit fitting with BPM (=100mm)2. SR simulation into detector Monitor Soft-SR immediately Control BG level (hopefully) Permit challenging accelerator operation
BPM Operator
simulationOrbit fit Alarm
Feed-back
Hard-SR at Super-KEKB / Belle
- No correlation with beampipe radius - Determined by beam current and solid angle HER current: 1.1 A 4.1 A beampipe tilte: 11 mrad 15 mrad 29 kRad/yr(SVD2) x (4.1/1.1) x (15/11) ~ 150 kRad/yr
In case of current configuration
For Hard-SR reduction Reduce scattering on photon-stop Put photon-stop far place
Hard-SR : materials of photon-stop
Cu:150 kRad/yr Ag:100 kRad/yr Ta:140 kRad/yr
• Heavy material reduce compton scattering• K-edge of heavier material is higher Ag looks good for Radiation Dose Wattage w/ Ag (Ta) is 36 (64) % higher than Cu
Idea for Hard-SR reduction
HER-beam
HER-beam
If we can bend beam photon-stop far place
SR
SR
If 2 times far place 1/4 BG
Summary• We had serious troubles by SR, but Solved
simulation reproduces them
• Beampipe for SVD 2.0 (rbp= 1.5 cm) was designed based on the BG simulation Measured dose agrees to expectation
• Basic design for Super-B is same as SVD 2.0, rbp= 1.0 cm scaled to 2/3 SR-mask shape is symmetric w/o 2/3 scale No serious increase for Hard-SR Soft / Hard-SR ~ 80 / 150 kRad/yr only 8 times higher than current SR-BG
No problem for Super-KEKB / Belle !
QUAD Parameterss (m) k1
SuperKEKB KEKB SuperKEKB KEKB
QCSR (+2m) + 0.72 ~ 1.68 + 1.48 ~ 2.40 - 0.46 - 0.30
QCSL (-1m) - 1.52 ~ -0.52 - 2.08 ~ 1.12 - 0.56 - 0.38
QC1L (-3m) - 3.12 ~ 2.48 - 3.61 ~ 2.99 - 0.37 - 0.32
QC2L (-7m) - 7.70 ~ 5.70 - 8.23 ~ 6.17 + 0.25 + 0.24
Based on IR_HER6.sad
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