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Calorimeter Backgrounds

Calorimeter Backgrounds. Occupancy Issues Physics impact of occupancy Types of background Radiation damage A few comments on alternatives to CsI(Tl). Calorimeter Occupancies in BaBar.

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Calorimeter Backgrounds

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  1. Calorimeter Backgrounds • Occupancy Issues • Physics impact of occupancy • Types of background • Radiation damage • A few comments on alternatives to CsI(Tl) steve playfer

  2. Calorimeter Occupancies in BaBar • Raw Digis > 0.75 MeV but can be isolated. These are important for DAQ issues. There are ~400/6580 of these from electronics noise (6%). • Default Digis > 1 MeV with a neighbour > 5 MeV are the input to clustering algorithms. • When the default digi occupancy exceeds 2000/6580 crystals (30%) the p0-finding efficiency decreases rapidly (empirical result from trickle injection data) • Clusters have a seed digi > 10 MeV and additional neighbouring digis > 1 MeV. • There are ~8 of these in multihadron physics events. • Bhabha events give clusters in endcap and backward barrel which scale with luminosity steve playfer

  3. <1ms after injection p0 signals with trickle injection >15ms after injection p0 efficiency reduced by x2 due to high multiplicity after trickle injection steve playfer

  4. Calorimeter occupancy (digis) in Physics Data A typical 45 minute run Forward Backward E n d c a p B a r r e l steve playfer

  5. BaBar calorimeter occupancies extrapolated to 2007 (random triggers) Raw Digis/3000 events Clusters/3000 events HER LER HER LER steve playfer Lumi Total Lumi Total

  6. Comments on Occupancy • BaBar extrapolations are dominated by the luminosity term • Strong forward endcap peak from (radiative) Bhabhas • Significant peak in last few rows of backward barrel (also Bhabhas) • Remainder of barrel occupancy is from low energy clusters (< 100 MeV) steve playfer

  7. Ways of dealing with occupancy • Increase digi (cluster) energy thresholds (costs efficiency and resolution) • Make tighter cuts on digi timing (limited to ~250ns by preamp shaping time) • Develop digital filtering of waveforms to resolve overlapping digis Subject of talk by Tsukamoto… steve playfer

  8. CsI(Tl) Radiation Damage • Light yield reduces with time due to formation of colour centres in crystals • Equivalent to increase in threshold and/or electronic noise • Depends quite a lot on quality of crystals • Now have plenty of data from BaBar/BELLE • Change is mostly at the front of the crystals • Affects uniformity of crystals response, but studies suggest this is not yet a problem. • No evidence of significant recovery from annealing during long down periods. steve playfer

  9. Radioactive source results (6 MeV photons) After initial steep drop, there is now a shallow logarithmic dependence on the integrated luminosity Barrel Modules 1 = forward 7 = backward Endcap rings 1 = inner 8 = outer steve playfer

  10. The steps are due to redefinitions between run periods This agrees well with the source measurements steve playfer

  11. Radiation Dose Measured by RadFETs at front of calorimeter 4.1 kRad/ab-1 in Endcap 2.5 kRad/ab-1 in Barrel Slope going down (trickle injection & lower backgrounds) Integrated luminosity fb-1 steve playfer

  12. Evidence for neutrons hitting Diodes Some annealing during down time (200 crystals) This is not a problem for calorimeter operation! (700 crystals) steve playfer

  13. Calorimeter survivability • Radiation damage reduces light output of crystals (proportional to integral luminosity) • Endcap limit of 20kRad reached after 5ab-1 • Barrel limit reached after 10-20ab-1 • Occupancy increases with luminosity and eventually limits ability to find p0s • 30% occupancy in Endcap and backward Barrel in 2007 • How much can we reduce the luminosity term? • How much can we improve occupancy with digital filtering and tighter timing cuts? • Endcap should be replaced in any upgrade • Barrel replacement is a major effort and cost! steve playfer

  14. Minimal upgrade solution • Replace Endcap crystals with pure CsI • Fast component has decay time 28ns • Light yield lower than CsI(Tl) by x20 (and shifted from 565nm to 320nm) • Readout has been demonstrated using APDs • Expected gain of x2 in effect of radiation damage • There is not much data on this! • Cheap and easy to implement! Subject of next talk … steve playfer

  15. kTeV experience with pure CsI Similar to CsI(Tl) Caveats: PMT readout Very large crystals Crystal quality? steve playfer

  16. Conclusions • Occupancy limit of 30% is reached at about: • Endcap 3x1034 (integral 2ab-1) • Barrel 3x1035 (integral 20ab-1) • Radiation damage limits are similar • Digital filtering and raised thresholds may give a bit more headroom (but not a lot) • Endcap replacement will need a fast radiation hard technology • Is pure CsI sufficiently radiation hard? • Are other alternatives such as Liquid Xenon or LSO/LYSO crystals viable and cost effective? Subject of talk by Hitlin… steve playfer

  17. BACK UP SLIDES FOR DISCUSSION steve playfer

  18. Two-Stage Model to reach 50ab-1 Now 1-3x104 LER Upgrade 1st Stage 1-2x1035 HER Upgrade 2nd Stage 5-10x1035 steve playfer

  19. Lowest Scenario = No Lumi term Highest Scenario = 2x Lumi term Radiation Dose ENDCAP Upgrade BARREL Upgrade Is this compatible with 2-stage model? Can run Barrel 2-4 years longer than Endcap. steve playfer

  20. Change from Log Linear extrapolation to one that matches existing data. Light Loss Projections ENDCAP Upgrade BARREL Upgrade Extrapolation beyond 2012 is very uncertain! (No measurements of CsI(Tl) radiation damage in large crystals beyond 50 kRad) steve playfer

  21. Usual caveat about extrapolating current backgrounds a long way and doing nothing to reduce the luminosity term Occupancy Projections steve playfer

  22. Pure CsI crystals • Fast component has decay time 28ns which is x30 faster than CsI(Tl). Solves occupancy problem. • Light yield is lower than CsI(Tl) by x20 (and shifted from 565nm to 320nm) • Readout has been demonstrated using APDs • Resolution could be comparable to CsI(Tl) • We think there is a gain of at least x2 in radiation hardness (based on one set of measurements and vague claims from manufacturers!) • No change to geometry of calorimeter • Cost is ~$4/cc which is x2 more than CsI(Tl) steve playfer

  23. Hybrid crystal solution (Bill W.) • Replace innermost few radiation lengths of Endcap with LSO crystals (rest replaced with pure CsI) • LSO(Ce) - Lutetium Oxyorthosilicate has decay time 40ns • Light yield similar to CsI(Tl) but shifted from 565nm to 420nm • Smaller radiation length 1.1cm and Moliere radius 2.3cm (CsI 1.9cm and 3.8cm) allow an increase in granularity (240 crystals per ring) or a decrease in radius of the calorimeter. • Readout has been demonstrated using APDs • Radiation hard up to 100MRad! • Very expensive - $50/cc. Only one manufacturer in the world. Major application is in PET scanners. Interesting idea. Solves radiation damage and occupancy. How much LSO can we afford? Can this solution be extended to the barrel? steve playfer

  24. LSO (or LYSO) CrystalsLutetium (+Yttrium) OxyOrthosilicate • Fast light output in 40ns. Solves occupancy problem. • Smaller radiation length 1.15cm (CsI 1.86cm) and Moliere radius 2.3 cm (CsI 3.8cm) • Believed to be radiation hard to 100MRad! • Light output is 50% (60%) of CsI(Tl), but shifted to 420nm from 550nm. • Again use APDs to read them out. • LYSO has slightly more light output than LSO, and may be easier to obtain commercially (3-4 suppliers instead of only one) • Currently the cost is ~$50/cc!! steve playfer

  25. Xenon calorimeter (David Hitlin) • Light output is within ~20ns. • Radiation length is 2.9cm • Need all of radial space between 700 and 1350mm for cryostat, Liquid Xe and readout. • Moliere radius 5.7cm. • Need sampling along shower depth for overlaps. • Light yield is similar to CsI(Tl) but at 175nm. • Use wavelength shifters and readout by APDs • Radiation hardness is not an issue • Cost of Liquid Xe is $2.5/cc • Total cost $20M + readout and mechanics? steve playfer

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