Radiation damage in lead tungstate crystals reduces their transparency. The
calibration that relates the amount of light detected in such crystals to
incident energy of photons or electrons is of paramount importance to
maintaining the energy resolution the detection system. We report on tests of
lead tungstate crystals, read out by photomultiplier tubes, exposed to
irradiation by monoenergetic electron or pion beams. The beam electrons
themselves were used to measure the scintillation light output, and a blue
light emitting diode (LED) was used to track variations of crystals
transparency. We report on the correlation of the LED measurement with
radiation damage by the beams and also show that it can accurately monitor the
crystals recovery from such damage.Comment: 9 pages, 6 figures, LaTeX2

A.A. Derevschikov

A.E. Yakutin

A.M. Davidenko

A.N. Vasiliev

A.P. Meschanin

A.S. Konstantinov

A.V. Ryazantsev

A.V. Uzunian

Annenkov

Batarin

Brennan

D.A. Morozov

J. Butler

J. Yarba

K.E. Shestermanov

L.F. Soloviev

L.V. Nogach

N.E. Mikhalin

N.G. Minaev

P.A. Semenov

S. Stone

V.A. Batarin

V.A. Kachanov

V.I. Kravtsov

V.K. Semenov

V.N. Grishin

V.S. Lukanin

V.V. Mochalov

Y. Kubota

Y.A. Matulenko

Y.M. Goncharenko

Y.M. Melnick

English

arXiv

Batarin, V. A.

Butler, J.

Davidenko, A. M.

Derevschikov, A. A.

Goncharenko, Y. M.

Grishin, V. N.

Kachanov, V. A.

Konstantinov, A. S.

Kravtsov, V. I.

Kubota, Y.

Lukanin, V. S.

Matulenko, Y. A.

Melnick, Y. M.

Meschanin, A. P.

Mikhalin, N. E.

Minaev, N. G.

Mochalov, V. V.

Morozov, D. A.

Nogach, L. V.

Ryazantsev, A. V.

Semenov, P. A.

/Serpukhov, IHEP /Fermilab /Minnesota U. /Syracuse U.

UNT (University of North Texas) Digital Library

arXiv:physics/0504085 v1   13 Apr 2005Correlation of Beam Electron and LED SignalLosses under Irradiation and Long-termRecovery of Lead Tungstate CrystalsV.A. Batarin a, J. Butler b, A.M. Davidenko a,A.A. Derevschikov a, Y.M. Goncharenko a, V.N. Grishin a,V.A. Kachanov a, A.S. Konstantinov a, V.I. Kravtsov a,Y. Kubota c, V.S. Lukanin a, Y.A. Matulenko a, Y.M. Melnick a,A.P. Meschanin a, N.E. Mikhalin a, N.G. Minaev a,V.V. Mochalov a, D.A. Morozov a, L.V. Nogach a,A.V. Ryazantsev a,1, P.A. Semenov a, V.K. Semenov a,K.E. Shestermanov a, L.F. Soloviev a, S. Stone d,A.V. Uzunian a, A.N. Vasiliev a, A.E. Yakutin a, J. Yarba bBTeV electromagnetic calorimeter groupaInstitute for High Energy Physics, Protvino, RussiabFermilab, Batavia, IL 60510, U.S.A.cUniversity of Minnesota, Minneapolis, MN 55455, U.S.A.dSyracuse University, Syracuse, NY 13244-1130, U.S.A.AbstractRadiation damage in lead tungstate crystals reduces their transparency. The cali-bration that relates the amount of light detected in such crystals to incident energyof photons or electrons is of paramount importance to maintaining the energyresolution the detection system. We report on tests of lead tungstate crystals, readout by photomultiplier tubes, exposed to irradiation by monoenergetic electron orpion beams. The beam electrons themselves were used to measure the scintillationlight output, and a blue light emitting diode (LED) was used to track variations ofcrystals transparency. We report on the correlation of the LED measurement withradiation damage by the beams and also show that it can accurately monitor thecrystals recovery from such damage.Key words: Scintillating crystal, lead tungstate, energy calibrationPACS: 61.80.-x, 29.40.VjPreprint submitted to Elsevier Science 14 December 20051 IntroductionElectromagnetic calorimeters built of the lead tungstate (PbWO4, PWO) scintillatingcrystals will be used in several high energy physics experiments, such as ALICE andCMS at the CERN LHC [1,2,3]. This work was done for studies of the electromagneticcalorimeter for BTeV at the FNAL Tevatron Collider [4]. Unfortunately, BTeV wasterminated by the U.S. Dept. of Energy.Over the last several years, the PWO crystals have been extensively studied at theInstitute for High Energy Physics (IHEP) in Protvino, Russia [5,6,7,8,9,10]. In particular,the studies confirmed that the PWO light output degrades under irradiation by high-energy electron or pion beam of high intensity. The light loss has a tendency to exhibitsaturation when the dose rate is kept at a constant level. On the other hand, the lightoutput changes whenever the radiation rate changes. Thus, they have to be monitoredcontinuously during operation to maintain excellent energy and space resolutions of thecalorimeter.Monte-Carlo study indicates that there would be enough electrons and positrons fromphoton conversion near the interaction region and semileptonic B decays to calibrate thedetector in-situ. The rates of collecting electrons/positrons for the calibration vary indifferent areas of the calorimeter, ranging from less than 1 hour to about several hours ofdata taking to achieve the calibration accuracy of (0.2−0.3)%. Any changes in the crystallight output over this time scale must be monitored using the transparency measurements.On the other hand, the transparency monitoring does not have to be stable over muchlonger time scale like a month since the electron calibration takes care of that part.We presume that the PWO light output degradation is caused by the changes in thetransparency since no evidence of the scintillating mechanism damage has been found sofar [11]. Thus, a highly stable reference light pulser sending light through the crystal canbe used to measure the transparency changes and predict the scintillation light outputchanges from the PWO crystals.When the coefficient of light absorption changes by ∆α, the light transmission changesby a factor of e−∆αλ, where λ is the path length that light must travel from the source tothe light detector. It is expected that ∆αλ < 0.1 for several hours of data taking, and itis relevant to use a linear approximation. Therefore the fractional loss is given by ∆αλ.Considering that the path lengths for scintillation and monitoring light are different, thefractional losses for the two processes will be different, but are expected to be proportional.The dose rate profiles induced by electron and pion beams are significantly differentlongitudinaly [8]. This could potentially lead to different proportionality constants forelectron or pion irradiation. Our studies addressed those issues for different crystals andunder different conditions. A dedicated test beam run took place at the IHEP-Protvinotest beam facility [6] in November-December 2002.1 corresponding author, email: ryazantsev@mx.ihep.su2Irradiation of PWO crystals has been done using high-intensity 34 GeV pion beam and23 GeV electron beam. Following the run, the crystals were left for a long-term naturalrecovery for over a 3-months period. Their light transmittance changes were measuredwith the LED monitoring system [9]. Results are presented in this paper.2 Test beam apparatus and irradiation procedureThe IHEP-Protvino test beam facility is described in details in [7]. The major componentsare momentum tagging system and a prototype of the PWO crystal calorimeter. Theprototype is a 5×5 matrix of crystals from two vendors, Bogoroditsk and Shanghai, andis installed in a thermo-stabilized light-tight box on a moving platform. All the crystalsare rectangular in shape, with a 27×27 mm2 cross-section and a 220 mm length. Theywere instrumented with 6-stage R5380Q Hamamatsu photomultiplier tubes (PMT).Before irradiation, scintillation light output of the crystals was measured with the use ofa low intensity electron beam. All results presented in this paper are normalized to theresults of this very first calibration.All crystals received from 500 rad to 1.5 krad of integral dose over the entire studies.During the electron irradiation runs, position of the electron peak itself was used tomonitor the light output continuously. During the irradiation by pions, data taking runsalternated with calibration runs by low intensity electron beam, to monitor changes in thecrystals light output. For the pulse height analysis, only electrons that hit the central part(2×2 mm2) of the crystal’s front face were selected using the data from drift chambers.Light transmittance change in the crystals was measured continuously with the use of ablue (470 nm) LED. Optical fibers guided light from the LED to the front side (oppositefrom the end where the PMT was attached) of the crystals. The typical path length ofLED light in the crystal approximately equals the length of the crystal. The light comesout of the optical fiber with a characteristic full angle spread of 25◦; this angle is reducedto 11◦ as the light enters the crystal from air. Thus, the path length of the light in acrystal should be increased by 1/ cos 11◦, i.e. order of 2%. As for the scintillation lightfrom incident particles, taking into account that this light is emitted isotropically andthe crystal is wrapped with Tyvek (diffuse reflecting material), its average path length tothe photocathode is longer due to the multiple reflections. In addition, the LED systemmonitors the transparency of the crystal at a specific wavelength (in our case, 470 nmwas chosen partially due to the availability of blue LEDs) and thus does not sample theentire spectrum of scintillation light. The radiation damage effect is less severe at 470 nmthan at 430 nm, which is the center of the PWO scintillation emission peak. From theseconsiderations, we expect that the ratio, K, of the light loss factors for the LED signaland the particle signal should be less than 1.30.860.880.90.920.940.960.9810.75 0.8 0.85 0.9 0.95 1Relative electron signalRelative LED signal 123456irradiationrecoveryK = 0.59 ± 0.03Fig. 1. Blue LED vs. scintillation signal relative changes under pion irradiation for Shanghai T9crystal. Points representing the data of the calibration runs are fitted to a straight line. Theslope, K = 0.59, was obtained with an accuracy of ±5%.3 Experimental results3.1 Data from calibration runsThe mean pulse heights of the scintillation signal and of the LED signal were obtainedusing the data from the calibration runs and normalized to the results of the very firstcalibration. Fig. 1 shows an example of the correlation between the relative changesof the LED signal vs. relative changes of the scintillation signal. Points 1–4 representmeasurements taken during the pion irradiation period; they fit very well to a linearfunction. Points 5–6 were taken when the crystal started to recover. By this we mean thatthe high intensity pion beam moved away from this crystal and onto other crystals, thusthe dose rate on the spot decreased significantly and the light output started to restore,as was measured in the subsequent calibration runs. It has to be noted that points 5–6agree very well with the same linear fit applied to the data taken during the irradiationperiod. The fit function is shown below:1 − y = K(1 − x) , (1)where x and y are relative electron and LED signals, respectively. For this particularcrystal, the proportionality coefficient was obtained to be K = 0.59 with an accuracyof ±5%.The same calculations were applied for all the crystals that have been irradiated eitherby electrons or pions. Distributions of the coefficients obtained from a linear fit of the4EntriesMeanRMS             19 0.6263 0.6077E-01PionLED/electronNumber of crystals EntriesMeanRMS             19 0.5842 0.9136E-01LED/electronNumber of crystalsElectron02468100 0.2 0.4 0.6 0.8 10123450 0.2 0.4 0.6 0.8 1Fig. 2. Linear fit coefficients between the blue LED and electron signals as a result of (a) pionirradiation; (b) electron irradiation.LED vs. electron dependencies for each crystal are presented in Fig. 2 for (a) pion and (b)electron irradiation. The mean values of the two distributions are the same within errors.3.2 Continuous electron calibrationAll electrons incident within 3×3 array of the crystals and satisfying the condition, thatthe energy deposit over 9 crystals in this array was within ±10% of the beam energy, wereselected for this analysis. The electron irradiation data were subdivided into smaller datasets, each set corresponded to 15 min of data taking. The mean signals for the 9 crystalsin the array were calculated for each of the subsets. A standard inverse matrix iterationprocedure of crystal calibration required not more than 6 iterations.Fig. 3(a) shows the electron signal and the blue LED signal vs. time for one of thecrystals during the electron irradiation period when the average dose rate was 20 rad/h;both signals have been normalized to the light output measured at the beginning ofthe irradiation period. Fig. 3(b) shows the correlation between relative electron signaland relative blue LED signal. The linear fit coefficient K = 0.596 is computed withmuch better accuracy of ±0.3% than in the case of discrete calibration runs in the pionirradiation data.Fig. 4 demonstrates how accurate the energy correction can be in a single crystal withthe use of a stable light calibration source if the coefficient K in formula 1 is known. Theresults presented here were obtained over 35 hours of data taking. While the responsechanged over each 15-minutes period, the effect was corrected according to the changein the LED signal and with the knowledge of the linear fit coefficient that is shown inFig. 3(a). The corrected energy distribution fitted by Gaussian has σ equal to 0.2%.Fig. 4(b) shows the distribution of Gaussian σ computed for 19 crystals with mean at0.25% and r.m.s. about 0.07%.50.90.9510 20 40-  LED-  ElectronTime, hoursRelative signal0.960.9810.95 1K = 0.596 ± 0.002Relative electron signalRelative LED signalFig. 3. (a)The dependence of electron and blue LED normalized signals on time and (b)theircorrelation for Shanghai T9 crystal during the electron irradiation period.  10.38    /    10Constant   19.69Mean   1.000Sigma  0.1879E-02Corrected electron signalNumber of events EntriesMeanRMS             19 0.2500E-02 0.6689E-03SigmaNumber of crystals010200.98 0.99 1 1.01 1.0201234560 0.002 0.004Fig. 4. (a) Distribution of the electron signals corrected for the LED signals over 35 hours ofcontinuous crystal irradiation fitted by Gaussian (σ=0.2%). (b) Distribution of Gaussian sigmavalues for 19 crystals.4 Long time crystals recoveryWe observed that the transparency of the crystals recovered upwards to its level beforeirradiation. Light output from the crystals was constantly monitored with the blue LED60.850.90.9510 500 1000 1500 2000Fit functionf(t) = a + b×(1 - e-t/τ1) + c×(1 - e-t/τ2)τ1 =   131 ±   21τ2 = 2097 ± 112Time, hoursLED signalFig. 5. Dependence of the blue LED signal on time over 2200 hours of natural crystal’s recovery.The parameters a,b and c are as follows : a=0.855±0.005, b=0.097±0.06, c=0.048±0.016.during for more than 3 months. Fig. 5 shows typical recovery process for one of thecrystals. This crystal was irradiated with the dose rates which varied from 15 to 30 rad/hand accumulated 2.2 krad absorbed dose.The experimental results were fitted with function:f(t) = a + b × (1 − e−t/τ1) + c × (1 − e−t/τ2). (2)Besides continuous monitoring of the changes in the crystals light transmittance with theLED, at the end of the recovery period of more than 2200 hours, a calibration run with alow intensity electron beam was taken. The results of the calibration run were comparedto those from the crystals calibration at the end of the irradiation run. Fig. 6 shows thaton average the light output from the crystals degraded to 86% of its initial level at theend of the irradiation run but naturally recovered up to 98%.5 ConclusionsThe goal of this study was to confirm that the electromagnetic calorimeter made of leadtungstate crystals read out by photomultiplier tubes can be continuously calibrated tothe required accuracy with the use of LED-based monitoring system within a period of 1day or shorter.We studied crystals behaviour under electron or hadron irradiation and whether thechanges in their responses to electrons would scale well with their response to the blueLED.We found that the relative changes of the LED and electron signals can be approximated70123456780.6 0.8 1 1.2 1.4 1.6EntriesMeanRMS             18 0.8557 0.4967E-01Relative electron signalNumber of crystals01234560.6 0.8 1 1.2 1.4 1.6EntriesMeanRMS             18 0.9799 0.4185E-01Relative electron signalNumber of crystalsFig. 6. (a) after irradiation; (b) after the 2200 hours recovery.by a linear function in both the electron and pion irradiation studies. The obtained linearfit coefficients are consistent with each other. This strongly suggests that in the realexperimental environment, where the crystals will be irradiated by a mixture of hadrons,gammas and electrons, linear fit will be sufficient for the calorimeter’s calibration.When the electron data were corrected for the transmission loss, which is due to irradi-ation, using the LED data, the corrected energy measurements are constant to within±0.25%. This satisfies one of the most important technical requirements of modernexperiments.Over the 3-months long recovery period that followed the irradiation run we found thatthe light output of the crystals restored from an average of 86% to 98%. It was also foundthat, for a given crystal, correlations between electron signal and blue LED signal arelinear and are the same if measured during irradiation or during the recovery period.6 AcknowledgementsThis work was supported by the U.S. National Science Foundation and the Departmentof Energy as well as the Russian Foundation for Basic Research grant 02-02-39008.References[1] A.A. Annenkov, et al., Nucl. Instr. and Meth. A490(2002) 30.8[2] ALICE Collaboration, Technical Proposal, CERN/LHCC/95-71, Geneva, 1995.[3] CMS Collaboration, The Electromagnetic Calorimeter Project Technical Design Report,CERN/LHCC 97-33, CMS TDR 4 (1997).[4] A. Kulyavtsev, et al., Proposal for an Experiment to Measure Mixing, CP Violation andRare Decays in Charm and Beauty Particle Decays at the Fermilab Collider - BTeV, May2000;G.Y. Drobychev, et al., Update to Proposal for an Experiment to Measure Mixing, CPViolation and Rare Decays in Charm and Beauty Particle Decays at the Fermilab Collider- BTeV, March 2002.[5] T. Brennan, et al., Nucl. Instr. and Meth. A494 (2002) 313.[6] V.A. Batarin, et al., Nucl. Instr. and Meth. A510 (2003) 211 (e-Print ArXivhep-ex/0208012).[7] V.A. Batarin, et al., Nucl. Instr. and Meth. A510 (2003) 248 (e-Print ArXivhep-ex/0209055).[8] V.A. Batarin, et al., Nucl. Instr. and Meth. A512 (2003) 484 (e-Print ArXivhep-ex/0210011).[9] V.A. Batarin, et al., Nucl. Instr. and Meth. A534 (2004) 486 (e-Print ArXivphysics/0311119).[10] V.A. Batarin, et al., Nucl. Instr. and Meth. A530 (2004) 286 (e-Print ArXivphysics/0312063).[11] V.A. Batarin, et al., Nucl. Instr. and Meth. A540 (2005) 131 (e-Print ArXivphysics/0410133).9

Correlation of beam electron and LED signal losses under irradiation and long-term recovery of lead tungstate crystals

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Correlation of Beam Electron and LED Signal Losses under Irradiation and
  Long-term Recovery of Lead Tungstate Crystals

Correlation of Beam Electron and LED Signal Losses under Irradiation and Long-term Recovery of Lead Tungstate Crystals

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