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 inner)
Features:
built-in 137Cs calibration system for calibration in situ
LED monitoring system
Installed in 2005

4. LHCb HCAL
The iron-scintillator structure arranged along the beam direction was chosen:
particles
PMT
spacers
WLS
fibers
light guide
master plate
scintillators
Master plates mm
Spacers mm
Scintillator mm
Sampling:
longitudinal cm
lateral 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 ( 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: performance from beam tests
~3% angular dependence at higher energies: shower not fully contained in 5.6 λI
Average light yield: 105 ph.el./GeV

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 … 350k PM 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 LHC

11. LHCb HCAL: LED monitoring system
blue LEDs (WU BC)
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: 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 μA 12 bit ADC
Integration time 1.5 ms
Currents in HCAL (MC) ETmax=15 GeV, L=2·1032 cm-2s-1
Current, nA
Not only for 137Cs calibration.
Currents in HCAL cells will be continuously monitored during physics data taking  independent information on relative luminosity, doses etc

15. ci (light yield of each tile)
LHCb HCAL: 137Cs calibration system
The source moves at constant speed ( 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: (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
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. Waiting for the first LHC collisions !
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