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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

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Current BG Status at Belle

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  1. 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

  2. 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

  3. SVD Upgrade in 2003 summer rbp = 1.5 cm 4 layers > 10 MRad (DSSD) > 20 MRad (readout chip) rbp = 2.0 cm 3 layers Rad. hardness ~ 1 MRad  Better vertex resolution / tracking efficiency

  4. SVD Upgrade for Super-Belle • Super-Belle • Smaller rbp (1cm) • Higher beam current • Basic design is same • as SVD2 beampipe rbp = 1.5 cm for Super-KEKB/Belle • We must understand Current Situation • Success of beampipe design is key-point

  5. Beam-BG on Belle-SVD2 Particle Background Showers from scattered beam particles by Residual Gas or intra-beam scattering Brem. Coulomb Touschek Synchrotron Radiation (SR) Soft-SR (several keV) Generated by upstream magnets Hard-SR ( keV ~ 150 keV) Backscattering from downstream

  6. Reduction of Particle-BG Particle BG ~ 70 kRad/yr

  7. Reduction of Soft-SR  some efforts Au-coating ! crescent shape SR-mask

  8. Reduction of Soft-SR Au coating absorbs low energy photon less than 8 keV Au-coating ! crescent shape SR-mask

  9. Reduction of Soft-SR Saw-tooth surface shape in Ta blind Soft-SR reflected on Ta Au-coating ! crescent shape SR-mask

  10. Reduction of Soft-SR Crescent shape SR-mask blind Be section from Soft-SR Au-coating ! crescent shape SR-mask (~2.5mm)

  11. Reduction of Soft-SR Au-coating ! Soft-SR ~ few kRad/yr crescent shape SR-mask

  12. Reduction of Hard-SR Scattered at downstream photon-stop (OC2RE chamber) HER e- • Put photon-stop far place (~9m) • Chamber material: Cu High energy SR is generated in OCS magnet Hard-SR ~ 29 kRad/yr

  13. Beam-BG on Belle-SVD2 Particle Background ~ 70 kRad/yr@ 1st layer Showers from scattered beam particles by Residual Gas or intra-beam scattering Brem. Coulomb Touschek Synchrotron Radiation (SR) Soft-SR (several keV) few kRad/yr@ 1st layer Generated by upstream magnets Hard-SR ( keV ~ 150 keV) ~ 29 kRad/yr Backscattering from downstream

  14. BG measurement and Comparison with simulation

  15. SR measurement w/ Single-Bunch HER 15 mA, with adjusting trigger timing Can 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, f=180deg) ( contribution below th. is corrected by simulation) SVD 1.X SVD 2.0 data simulation

  16. SVD Cluster Energy Spectra in Single Beam Run Can extract SR from spectrum shape !? HER 0.8 A LER 1.5 A SR and Particle-BG Only Particle-BG

  17. E-spectrum of HER Particle-BG • Diff. btw vacuum • bump on/off in HER • LER 1.5 A HER E-spectrum of particle BG is same as LER !! #clusters/keV/event Can measure SR and particle-BG separately energy (keV)

  18. Extraction SR in HER Single Beam HER Particle SR 50 mA 200 mA 100 mA Hard-SR simulation 400 mA 600 mA 800 mA

  19. Correlation with Vacuum NSR a I(A) Nparticlea I(A) x P(Pa) Nparticle/NSRa P(Pa) Average of HER whole ring Average of HER upstream

  20. Azimuthal Distribution of SR Single-Bunch 15 mA (trigger-timing is adjusted) Total 0.8 A w/ 1284 bunch (random timing) Hard-SR simulation 33 kRad/yr at HER 1.1A 21 kRad/yr at HER 1.1A simulation 29 kRad/yr Only above threshold 10 keV Simulation complements below thereshold

  21. Azimuthal Distribution of Particle BG 44 kRad/yr at HER 1.1A simulation 53 kRad/yr HER 0.8 A 43 kRad/yr at LER 1.6A simulation 21 kRad/yr LER 1.5 A

  22. Study of Touschek Effect Smaller beam-size (larger density)  larger background Touschek contribution < 20 % at collision ~ 50 % at single beam 31 % in simulation Touschek contribution must be corrected If no Touschek Collision run Single beam run

  23. Azimuthal Distribution of Particle BG 44 kRad/yr at HER 1.1A simulation 53 kRad/yr HER 0.8 A 22 43 kRad/yr at LER 1.6A simulation 21 kRad/yr LER 1.5 A 18

  24. Radiation Dose at SVD 1st layer At Maximum Currents: HER 1.1A, LER 1.6A (…) is simulation @ 1nTorr pressure Touschek contribution is reduced based on measurement Data and simulation is consistent

  25. Radiation Dose at SVD 1st layer At Maximum Currents: HER 1.1A, LER 1.6A (…) is simulation @ 1nTorr pressure • Two parameters have large uncertainty • (pressure, movable mask) • It may happen that absolute values too well agree • Consistency of azimuthal distribution is important Touschek contribution is reduced based on measurement Data and simulation is consistent

  26. Radiation Dose at SVD 1st layer At Maximum Currents: HER 1.1A, LER 1.6A (…) is simulation @ 1nTorr pressure • We can trust simulations • Its uncertainty for abs. may be factor a few Touschek contribution is reduced based on measurement Data and simulation is consistent

  27. Constraint for Occupancy (hit-rate) Radiation Dose  Occupancy (cluster size: Particle-BG  3.5 ch, SR  1.5 ch) At Maximum Currents (HER 1.1A, LER 1.6A) Collision 12 % 11 %

  28. Energy spectra for each layers 1st r ~ 2cm 2nd r ~ 4.4cm 3rd r ~ 7cm 4th r ~ 8.8cm LER single beam HER single beam

  29. Layer dependence (single beam) Particle (LER) Particle (HER) SR BG a 1/(r-rbp), rbp: beampipe radius There may be correlation BG and 1/(r-rbp)

  30. Other sub-detectors No large difference for BG (current diff. causes small diff. ?) No problem

  31. Ideas for Less BG Particle-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 1/2 Particle-BG  ~ 2/3 total-BG/Occ. SR-BG (dominated by Hard-SR) • Put photon-stop far place •  Detail will be discussed in “Belle SR” talk

  32. Summary Super-B • 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 a 1/(r-rbp) This method is first time in the world !? Success of beampipe design Strong support for design in Super-B

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  34. Radiation Monitors 18 kRad/yr 80 kRad/yr 60 kRad/yr before 100 kRad/yr Dose on Si is consistent with monitor

  35. Is monitors measuring SR ? BWD FWD Outer-side of ring e- Be pipe Backscattered Hard-SR Inner-side of ring The 300 mm Au on the manifold blinds SR-BG Most of SR photons are absorbed by Au, and converted to lower energy photons (8~14keV) via the photoelectric effect Difficulty to measure SR Dose at DSSD center is same ? Measure BG by DSSD itself

  36. 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

  37. 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 %

  38. SVD 2.0 SVD 1.6 beampipe radius SVD Occupancy (hit-rate) Large diff. of occ. btw 1st – 2nd layers may come from 1/(r-rbp) relation

  39. Single Bunch like Run HER 15 mA, with adjusting trigger timing SVD 1st layer occupancy ~ 0.2 %  corresponds to ~ 4 % occupancy @ 1.1 A Energy deposition 15.2 keV/ch  corresponds to ~20 kRad/yr dose @ 1.1 A (33 kRad/yr at maximum position, f=180deg) ( contribution below th. is corrected by simulation) SVD 1.X SVD 2.0 data simulation

  40. Correlation with Vacuum (Nparticle/NSR) / P(Pa) = const Upstream Pressure Average Pressure

  41. Background at Collision Total dose Particle-BG SR-BG Run1560 (threshold ~15 keV) HER : 1.1 A, LER : 1.6 A 71 kRad/yr 11 kRad/yr Consistent with expectation from single-beams

  42. Study of Touschek (life-time) Smaller beam-size (density)  shorter life-time and larger background Touschek contribution < 20 % at collision ~ 45 % at single beam 42 % in simulation If no Touschek 1.4A 1.1A Touschek contribution must be corrected Collision run Single beam run 1/t abeam-density

  43. Layer dependence (collision run) Particle-BG SR-BG BG a 1/(r-rbp) Large difference of occupancy btw 1st and 2nd layer BG comes from beampipe radius ?

  44. Is there reasonable reason to explain 1/(r-rbp) correlation? Spent particles are scattered thin-Ta region Large contribution comes from here (simulation) Ta Be pipe Ta e- Be pipe Backscattered Hard-SR SR is scattered / absorbed at Au coating

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