lhcb hcal: performance and calibration

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LHCb HCAL: performance and calibration

Yu. Guz, IHEP, Protvino

on behalf of the LHCb collaboration

1. structure

2. performance

3. LED monitoring system

4. 137Cs calibration system

5. current status

Calor2008, Pavia

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LHCb HCAL: design goalsPart 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

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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

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LHCb HCAL

The iron-scintillator structure arranged along the beam direction was chosen:

particles

PMT

spacers

WLSfibers

light guidemaster plate

scintillatorsMaster 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)

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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

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LHCb HCAL

WLS fibers: KURARAY Y11(250)MS Ø1.2 mm attenuation length ~ 3.5 m

τD ~ 7 ns rad. hard to 500 krad

Scintillator pads:

polystyrene +1.5% PTP +0.03% POPOP

256x197mm (full tile), 127x197 mm (half tile)

wrapped by 100μ Tyvek

✜ ends of fibers aluminized

✜ compensation of light attenuation: length of contact with tile depends on depth

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LHCb HCALPMT: 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 nAClipping 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

HV supplied by means of individual CW circuit for each PM

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LHCb HCAL: performance from beam tests

)%(E

)%(Eσ 29

569 ~3% angular dependence at higher energies: shower not fully contained in 5.6 λI

Average light yield: 105 ph.el./GeV

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LHCb HCAL

P m

B uffe r In te gra tor

Vs s

Vd d

A na log C hip

B IC M O S 0 .8 um Inte gra te d c irc uit 4 c ha nne ls pe r c h ip

+

-

5 n s

2 5

6 5

2 7

4 5

1 2 M

3 3 3 7

5 1 .1

3 3

3 3 0

3 .3 k1 .5 K

2 5 n s

11 0

2 .2 n F

4 7 0 p F

2 7

2 2 n F

1 .0 7 k

A D C1 2 b its 4 0 M h z

4 p F

2 .7 k

4 .7 k

1 n F

1 n F

4 .7 k

4 .7 n F

+ 3 V

1 0 k

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

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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.

-600

-550

-500

-450

-400

-350

-300

-250

-200

-150

-100

-50

0

50

-250 -225 -200 -175 -150

25 ns

LHCb HCAL

Row 1 Row 2 Row 3 ▲ Row 4 Row 5 ♦ Row 6

PM gains: 20k … 350kPM transit time (~1/√HV) +time of flight vary by ~5 ns

Signal cable delay spread: ~2 ns

“Long” detector +mirrors at fiber ends: several % of signal outside 25 ns careful time alignment is necessary for the operation @ LHC

HV settings for physics: correspond to Emax=15 GeV/sin(Θ) (trigger on ET)

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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

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Normally, LED is more stable than PMT…

1.5%

0.2%

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

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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.

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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

Cu

rren

t,

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

Currents in HCAL (MC) ETmax=15 GeV, L=2·1032 cm-

2s-1

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LHCb HCAL: 137Cs calibration systemThe 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:

fib

10 )()(

N

ii tttfctI

ci (light yield of each

tile)

I, ADC counts

Measured current and fitting function superimposed

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All the HCAL modules passed the Cs test at production: all tile responses were required to be within ±20% from average

±20%

Distribution of RMS (%) of the light yield of tiles belonging to the same PMT. Average 4.7%

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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

Half tile countersFull tile counters

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LHCb HCAL: current status

HCAL

✔ 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 installedOngoing 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

July 2005

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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

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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 !

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SPARES

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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

at HV change, correct using known PM transit time dependence

BXi BXi+1

lhcb hcal: performance and calibration

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