1 P.Rebecchi (CERN) “Monitoring of radiation damage of PbWO 4 crystals under strong Cs 137 ...

P.Rebecchi (CERN) “Monitoring of radiation damage of PbWO4 crystals under strong Cs137 irradiation in GIF-ECAL”“Advanced Technology and Particle Physics” conference, 18 October 2001 Como

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Monitoring of radiation damage of PbWOMonitoring of radiation damage of PbWO44 crystals crystals under strong Csunder strong Cs137137 irradiation in GIF-ECAL irradiation in GIF-ECAL

- The - The PROBLEMPROBLEM- The - The IDEAIDEA- The - The SETUPSETUP- Some - Some RESULTSRESULTS


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The PROBLEMThe PROBLEM In order to discover a light In order to discover a light HiggsHiggs decaying in decaying in ggggwe need an we need an

excellent calorimeter, with a constant term of ~0.5%; so we must excellent calorimeter, with a constant term of ~0.5%; so we must have:have:

CMS-ECAL PbWOCMS-ECAL PbWO4 4 crystals are radiation hard crystals are radiation hard (no damage to (no damage to scintillation process)scintillation process) but but there are small changes in transparencythere are small changes in transparency due due to radiation exposure. The change is of the order of 2-3 % and is to radiation exposure. The change is of the order of 2-3 % and is fairly rapid (fairly rapid (hourshours).The change of transparency light will be ).The change of transparency light will be monitored in situ with a laser and a correction to particles signal monitored in situ with a laser and a correction to particles signal will be applied:will be applied:

R = R = particle particle / / monitoringmonitoring

The crystal-to-crystal variation of R must be limited to about 5% to avoid R

becoming another calibration constant.

crystal-to-crystal inter-calibration error must be limited to ~0.4%;


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The IDEAThe IDEA

The ideal solution would be to measure R for all the 71964 crystal of The ideal solution would be to measure R for all the 71964 crystal of the CMS-ECAL, but this is practically impossible;the CMS-ECAL, but this is practically impossible;

the the solution solution is to do systematic measurements of R for group of is to do systematic measurements of R for group of crystals and correlate the results with other important parameters of crystals and correlate the results with other important parameters of the crystals.the crystals.To do that, we built a setup at the General Irradiation Facility (GIF) of To do that, we built a setup at the General Irradiation Facility (GIF) of CERN. With it we can do CERN. With it we can do (at the same time)(at the same time)::

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80

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300 350 400 450 500 550 600 650 700

PWO 7365 1st irradiation at the hospital (430 Gy)

1i11irr1Irr1 + 20'Irr1 + 40'Irr1 + 24h

tran

smis

sion

(%)

Wavelength (nm)

Irradiate with a Cs137 source (E=0.662 MeV, dose rate=15 rad/h i.e. LHC conditions)Excite the crystals with SPS e beam (50 and 100 GeV)Excite the crystals with UV laser (=33710 nm, 1.55mJ/pulse, t =600 ps)Monitor the transmission of the crystals at different wave length:

LED (=45050 nm, t=500 ns) Xenon lamp (=400, 420, 440, 500, 524 nm, t=1 s)

TransmissionTransmission

UV LaserUV Laser LEDLED


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The SETUPThe SETUP

GIF is a multi-CERN-users facility, due to that three main constraints exist:GIF is a multi-CERN-users facility, due to that three main constraints exist:

very little space available;very little space available; very few days of beam per year very few days of beam per year (8-10 days)(8-10 days);; we cannot take at the same time we cannot take at the same time e data and irradiate with Caesium data and irradiate with Caesium

source;source;

UV laser can replace the UV laser can replace the e beam; beam; crystals are tested in batches of 5;crystals are tested in batches of 5; the setup can move from and back the irradiation position to the the setup can move from and back the irradiation position to the

beam, and allows to scan the 5 crystals with the electronsbeam, and allows to scan the 5 crystals with the electrons


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The SETUPThe SETUP

Cs137 Cs137 irradiationirradiation is done is done transversally to the crystaltransversally to the crystal

The The light detectorslight detectors for the crystals for the crystals are standard CMS-ECAL APDsare standard CMS-ECAL APDs

It’s very important to It’s very important to monitor monitor the the different light sources:different light sources:

a 6th crystal and a PM are installed to monitor all light sources;

a PIN diode is used to monitor the laser

The The gaingain is monitored periodically is monitored periodically with a constant charge injected to with a constant charge injected to the amplifiers (Test Pulse)the amplifiers (Test Pulse)

The 5 different The 5 different triggerstriggers(LED, (LED, Laser, Lamp, Beam, Test Pulse, Laser, Lamp, Beam, Test Pulse, Pedestals) are generated Pedestals) are generated asynchronouslyasynchronously


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Irradiation procedureIrradiation procedure

24 hours of stabilisation:24 hours of stabilisation: at the end of the stabilisation, the beam is tuned on crystal 5 (the

lowest); just before the irradiation starts, three scans per crystals with

electrons are done Irradiation starts, it lasts ~48h Irradiation starts, it lasts ~48h (~700 rad)(~700 rad) : :

during the first two hours of irradiation the scan in the beam is done every 30 minutes;

then the scan is done every hour for the next 8 hours of irradiation;

then the scan is done every 2 hours till the end of the irradiation; Sometime a recovery is done, the scan is done every two hours.Sometime a recovery is done, the scan is done every two hours. Crystals are taken out and annealed in oven.Crystals are taken out and annealed in oven.

P.S.P.S. one scan lasts ~14 minutes (~ 3 minutes per crystal). In total 7000 electrons per one scan lasts ~14 minutes (~ 3 minutes per crystal). In total 7000 electrons per crystals are kept in data, (out of 250.000 electrons hitting the crystals). This corresponds crystals are kept in data, (out of 250.000 electrons hitting the crystals). This corresponds to a dose of ~2 rad per crystal per scan.to a dose of ~2 rad per crystal per scan.


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The SETUPThe SETUP

LED boxLED box

Laser fiberLaser fiber

Lamp fiberLamp fiber

Cooling pipesCooling pipes

Crystals boxCrystals box


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The SETUPThe SETUP

PMPM

PIN diodePIN diodefor Laserfor Laser


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The SETUPThe SETUP

CoolingCooling

ScintillatorsScintillatorstrigger blocktrigger block

APDs blockAPDs block


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The SETUPThe SETUP

APDAPDTemperature probeTemperature probe

Beam Beam directiondirection

Lead radiationLead radiationscreenscreen


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The SETUPThe SETUP

Irradiation windowIrradiation window


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The SETUPThe SETUP

Beam directionBeam direction

CrystalCrystal

Temperature probeTemperature probe

Beam triggerBeam triggerscintillatorscintillator

Lamp fiberLamp fiber

Laser fiberLaser fiberLED fiberLED fiber


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The SETUPThe SETUP


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The SETUPThe SETUP

Laser boxLaser box


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Some RESULTSSome RESULTS

The results I will show are still preliminaryThe results I will show are still preliminary Results are from data taken this summer:Results are from data taken this summer:

SPS e beam of 50 and 100 GeV; 10 different production barrel crystal were irradiated, 5 of them 2

times and 5 of them 4 times.

Our Our goalsgoals of this summer were: of this summer were: confirm that UV laser can replace the confirm that UV laser can replace the e beam; beam; check the reproducibility of the results;check the reproducibility of the results; first evaluation of the crystal-to-crystal variation of Rfirst evaluation of the crystal-to-crystal variation of R.


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The BEAMThe BEAM

Drift chamber xDrift chamber x

Drift chamber yDrift chamber y

1 cm1 cm

1 cm1 cm

ADC cry1ADC cry1

ADC cry2ADC cry2

ADC cry3ADC cry3

ADC cry4ADC cry4

ADC cry5ADC cry5


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Typical irradiation curveTypical irradiation curve

330 rad330 rad


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Temperature correctionTemperature correctionCC

oo

S/S as a function of S/S as a function of T/TT/T

UV Laser: 2.5% per 0.1 CºUV Laser: 2.5% per 0.1 Cº

LED: 0.8% per CLED: 0.8% per Coo


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R calculationR calculation

LEDLED

LASERLASER

Distribution of Distribution of LASERLASER//LEDLED (i.e. R)(i.e. R)

1dim gaussian fit1dim gaussian fit

2dim linear fit2dim linear fit

Distribution of Distribution of SSLASERLASER versus versus SSLEDLED BEAMBEAM

SS LEDLE

DSS L

ASER

LASE

RSS B

EAM

BEAM


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The best:The best:

StabilisationStabilisation

IrradiationIrradiation

RecoveryRecovery

Exponential fit: Exponential fit: IrradiationIrradiation decay decay time is time is 5 times faster5 times faster than than RecoveryRecovery rime rime

(R is calculated doing the ratio of the Exponential functions. No problems from normalisation !)


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ReproducibilityReproducibility

The same crystal (9257) has been The same crystal (9257) has been irradiated several times. The irradiated several times. The variance of R is variance of R is 5%5%


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R dependence from R dependence from Irradiation N38 31/08/2000 (latest results)

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Lamp wave leght (nm)

R La

mp/

LED

6108 Russia B9

6140 Russia B9

6203 Russia B9

6207 Russia B9

6317 Russia B9

Average

(D.Sillou P.Rebecchi)

440

LED


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ConclusionsConclusions

GIF-ECAL is a setup which allows to monitor the change in GIF-ECAL is a setup which allows to monitor the change in transparency of PbWOtransparency of PbWO44 CMS-ECAL crystals due to radiation CMS-ECAL crystals due to radiation exposure (LHC dose rate);exposure (LHC dose rate);

this summer test-beam showed that:this summer test-beam showed that: UV Laser can reproduce the e beam crystal excitation (R~1.2) good reproducibility of results (~5%) the analysis is not yet fine enough in order to reduce

measurement error such that the crystal-to-crystal dispersion can be extracted

A measurement error less than 5% is needed A measurement error less than 5% is needed (actually around 15%)(actually around 15%)

improve the analysis improve the setup

SYSTEMATIC irradiations

1 P.Rebecchi (CERN) “Monitoring of radiation damage of PbWO 4 crystals under strong Cs 137 ...

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