LHCb HCAL: performance and calibration

1. LHCb HCAL: performance and calibration Yu. Guz, IHEP, Protvino on behalf of the LHCb collaboration Calor2008, Pavia structure performance LED monitoring system 137Cs calibration system current status

2. LHCb HCAL: design goals Part of the experiment’s calorimetric system, intended to provide L0 high-ET hadronic trigger • Requirements: • fast (25 ns cycle) • moderate resolution is sufficient • longitudinal depth limitations • radiation tolerance: ~50 krad/year in the inner zone HCAL is a very important subdetector: it is supposed to give 70% of the L0 output

3. LHCb HCAL • The whole detector assembly • 2 independent sides, each containing 26 modules stacked on movable platform • size: 8.4 x 6.8 m2 • instrumented depth: 120 cm • cell size: • outer zone 262 x 262 mm2 • inner zone 131 x 131 mm2 • 1488 cells (608 outer + 880 inner) • Features: • built-in 137Cs calibration system for calibration in situ • LED monitoring system Installed in 2005

4. particles spacers scintillators WLS fibers light guide master plate PMT LHCb HCAL The iron-scintillator structure arranged along the beam direction was chosen: Master plates 6 mm Spacers 4 mm Scintillator 3 mm Sampling: longitudinal 20 cm lateral 2 cm 6 longitudinal sections (5.6 λI) (high energy showers not fully contained – but does not spoil the trigger operation)

5. LHCb HCAL HCAL module: self-supporting structure containing either 16 outer or 8 outer + 32 inner cells Weight : ~9.5 ton Absorber and mechanics assembly: at IHEP Protvino Optics assembly: at CERN

6. LHCb HCAL Scintillator pads: polystyrene +1.5% PTP +0.03% POPOP 256x197mm (full tile), 127x197 mm (half tile) wrapped by 100μ Tyvek • WLS fibers: KURARAY Y11(250)MS Ø1.2 mm • attenuation length ~ 3.5 m • τD ~ 7 ns • rad. hard to 500 krad • ends of fibers aluminized • compensation of light attenuation: length of contact with tile depends on depth

7. LHCb HCAL • PMT: HAMAMATSU R7899-20 • Specially designed for LHCb • bialcali photocathode, UV glass (185-650 nm) • QE 15% at 520 nm • 10 dynodes • pulse linearity: within ±2% • dark current: < 2.5 nA • max average current: 100 μA • rate effect: < 1% at I > 10 nA HV supplied by means of individual CW circuit for each PM Clipping circuit on 1.15 m coax cable is used to compensate the 7ns decay time of fibers (this cable also feeds the PMT current into the integrators of 137Cs calibration system). The parameters of the clipping circuit were optimized for the signal from hadrons

8. LHCb HCAL: performancefrom beam tests Average light yield: 105 ph.el./GeV ~3% angular dependence at higher energies: shower not fully contained in 5.6 λI

9. LHCb HCAL • Front-end electronics: • “dead timeless”: integration over 25 ns • 12 bit flash ADC • sensitivity 20 fC / ADC count • ADC samples every 25 ns • FIFO depth 256 • cell-to-cell time alignment: sampling time adjustable, step 1 ns • trigger processing: • sum of signals in 2x2 clusters • individual multiplication factor for each channel • built-in test system: charge injection

10. LHCb HCAL 25 ns HV settings for physics: correspond to Emax=15 GeV/sin(Θ) (trigger on ET) PM gains: 20k … 350kPM transit time (~1/√HV) +time of flight vary by ~5 ns Signal cable delay spread: ~2 ns Row 1 Row 2 Row 3 ▲ Row 4 Row 5 ♦Row 6 The pulse shapes from each tile row were obtained at beam test with the e- beam directed transversely into the corresponding row of a HCAL module. “Long” detector +mirrors at fiber ends: several % of signal outside 25 ns  careful time alignment is necessary for the operation @ LHC

11. LHCb HCAL: LED monitoring system • blue LEDs (WU-14-750BC) • two independent LEDs per module • adjustable LED pulse amplitude • monitoring PIN photodiode at each LED, in order to account for LED instability • light distribution with clear fibers of same length • timing of the LED flashing pulse adjustable with 1 ns step –time alignment tool

12. LHCb HCAL: LED monitoring system • The PMT gain will be continuously monitored with LEDs during the LHC run: • LEDs will be fired during the series of empty LHC bunches • significant variations of the LED amplitude recorded in run DB, for subsequent use in the offline analysis Normally, LED is more stable than PMT… 0.2% 1.5%

13. LHCb HCAL: 137Cs calibration system Six stainless steel pipes pass through the centers of each tile row (27 m per module). All modules of each half calorimeter are connected. A ~ 10 mCi 137Cs source is used.

14. LHCb HCAL: 137Cs calibration system Measurement of current: 188 8-channel integrator boards installed at the back of the HCAL nearby PMTs. Readout via the slow control bus (SPECS)(independent of the main DAQ) 4 ranges: 300 nA, 1500 nA, 9μA, 50 μA12 bit ADC Integration time 1.5 ms Currents in HCAL (MC) ETmax=15 GeV, L=2·1032 cm-2s-1 Not only for 137Cs calibration. Currents in HCAL cells will be continuously monitored during physics data taking  independent information on relative luminosity, doses etc Current, nA

15. LHCb HCAL: 137Cs calibration system The source moves at constant speed (20..30 cm/s)  the dependence of current on time I(t) can be fitted with a weighted sum of (empirically obtained) tile response functions placed at equal time intervals Δt: I, ADC counts Measured current and fitting function superimposed ci (light yield of each tile)

16. LHCb HCAL: 137Cs calibration system ±20% All the HCAL modules passed the Cs test at production: all tile responses were required to be within ±20% from average Distribution of RMS (%) of the light yield of tiles belonging to the same PMT. Average 4.7%

17. LHCb HCAL: 137Cs calibration system The precision of the 137Cs calibration was studied at beam tests: independent calibrations with Cs and 50 GeV π― coincide within 2-3% . The ratio of sensitivities to 137Cs radiation and to hadrons was measured: 41.07 (20.88) (nA/mCi)/(pC/GeV) for outer (inner) cells. The calibration precision can be affected by e.g. timing Full tile counters Half tile counters

18. LHCb HCAL: current status • Detector installed in the LHCb cavern • Photomultipliers, LED drivers, integrator boards, signal and control cables are mounted on the detector and checked • ≥99.9% of the system operational • hydraulic components and control electronics of the 137Cs system tested • Frontend electronics, components of DAQ and trigger are installed Ongoing commissioning activities: • studies with LED system: • cell-to-cell time alignment • long-term PM gain stability • trigger operation • studies with cosmic events: • coarse inter-subsystem time alignment • trigger operation • 137Cs calibration run: foreseen for mid-June HCAL July 2005

19. LHCb HCAL: current status • Cosmic trigger: coincidence of HCAL and ECAL • HCAL: all counters at G~200k; ECAL: at 300k • With CALO trigger, cosmic events seen also in PreShower/SPD and Muon system

20. Conclusions • The LHCb HCAL is a iron - scintillator sampling device with structure arranged parallel to the beam direction. The light is read out by WLS fibers to PMT • Its performance is adequate for providing L0 trigger for high-ET hadrons • It is equipped with 137Cs calibration system and LED monitoring system • The detector is installed in LHCb and operational • Currently it is under intensive tests with LEDs and cosmic events; the 137C calibration run is scheduled for mid-June • Waiting for the first LHC collisions !

21. SPARES

22. LHCb HCAL: LED monitoring system • Time alignment with LEDs. • Goal: determine optimal ADC sampling time for each cell and LED flashing delays • time alignment events: ADC sampling several (5) consecutive bunch crossings • scanning over LED flashing time, determine optimal delay for each cell account for the difference in [signal cable delay + PM transit time] within each PM group illuminated by one LED • using the PIN photodiode signal timing as a reference, we can perform time alignment between groups • knowing the time of flight, calculate optimal ADC sampling time settings BXi BXi+1 • at HV change, correct using known PM transit time dependence