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R.Novotny a* , S. F. Burachas d , W. Döring a , V. Dormenev b ,

Radiation hardness and recovery processes of PWO crystals at –25 o C. R.Novotny a* , S. F. Burachas d , W. Döring a , V. Dormenev b , Y. M. Goncharenko c , M. S. Ippolitov d , A. Hofstaetter e , M. Korzhik b , V. Manko d , Y. M. Melnick d , O. Missevitch b ,V. V. Mochalov c ,

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R.Novotny a* , S. F. Burachas d , W. Döring a , V. Dormenev b ,

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  1. Radiation hardness and recovery processes of PWO crystals at –25oC R.Novotnya*, S. F. Burachasd, W. Döringa, V. Dormenevb, Y. M. Goncharenkoc, M. S. Ippolitovd, A. Hofstaettere, M. Korzhikb, V. Mankod, Y. M. Melnickd, O. Missevitchb,V. V. Mochalovc, V. Ryazantsevc, P. A. Semenovc, G. Tamulaitisf, A. V. Uzunianc, A. Vasilievd, A. N. Vasilievc and for the PANDA collaboration aII. Physics Institute, University Giessen, Heinrich-Buff-Ring 16, 35392 Giessen, Germany bINP, Belarus State University, 11 Bobruiskaya, 220030 Minsk, Belarus cInstitute for High Energy Physics, Protvino, Russia dR.RC Kurchatov Institute, Moscow, Russia eI. Physics Institute, University Giessen, Heinrich-Buff-Ring 16, 35392 Giessen, Germany fVilnius University, Vilnius, Lithuania SCINT 07 R.Novotny 1 SCINT 07 R.Novotny 1

  2. high resolution calorimetry with PWO • CMS – HYCAL – DVCS – ALICE - PANDA • the experimental facility at IHEP • experimental results • interpretation • decay kinetics – defects - impurities • consequences, further experiments and outlook SCINT 07 R.Novotny 2

  3. high resolution calorimetry with PWO + fast and very dense scintillator: t<10ns Xo=0.9cm + radiation hard @RT: CMS-ECAL - low light yield: medium-energy application improvement of the photon statistics by: • increase of light output PWO-II • operation well below room temperature s / E % excellent energy resolution achievable at low temperatures photon energy / MeV SCINT 07 R.Novotny 3

  4. EMC @PANDA - FAIR PHOS @ALICE LHC PHOS (PHOton Spectrometer) 17920 PWO channels (22x22x180mm3) study of initial phase of HI collision: via direct photons, high pTg, p0 20000 PWO channels (20x20x200mm3) hadron physics with anti-protons envisaged operation at T = -25oC SCINT 07 R.Novotny 4

  5. transmission / % wavelength / nm both experiments can expect a lower radiation dose all tests of radiation hardness have been so far performed at RT dose: 1013 protons Ep = 90 MeV @ KVI, Groningen • no permanent damage due to defect formation • activation due to proton induced reactions • reduction of optical transmission • can be handled by monitoring SCINT 07 R.Novotny 5

  6. the experimental facility at IHEP Protvino schematic layout SCINT 07 R.Novotny 6

  7. cooling machine -source crystal container SCINT 07 R.Novotny 7

  8. dose rate profile /a.u. 137Cs -source PWO air cooling machine crystal container SCINT 07 R.Novotny 8

  9. dose rate 20rad/h relat. PM-current irradiation time / h the samples to be tested SCINT 07 R.Novotny 9

  10. @RT • small tranmission loss • fast saturation • @ T=-23oC • increase of light yield • no saturation due to • slow recovery similar behavior at lower dose rate of 2rad/h (upper limit @PANDA) the typical behavior SCINT 07 R.Novotny 10

  11. recovery after irradiation transmittance @455nm / a.u. time / h at low temperatures no improvement of the transmittance with time SCINT 07 R.Novotny 11

  12. 2 rad/h 20 rad/h -25oC light yield / pe/MeV 0oC PMT output / a.u. +25oC PWO-II - 28 integration gate / ns +20 C -25 C PWO-II - 30 time / h irradiation of PWO-II crystals SCINT 07 R.Novotny 12

  13. recovery process at +20C recovery process at -25C irradiation of PWO-II crystals PMT output / a.u time / h SCINT 07 R.Novotny 13

  14. irradiation of PWO-II crystals * LY – light yield measured by RINC, Minsk SCINT 07 R.Novotny 14

  15. irradiation of PWO crystals of CMS-type: different concentration of dopants scintillation signal crystal #94 (large) crystal #136 (small) relative PM response blue LED signal dose rate 2rad/h @ -25oC time / h SCINT 07 R.Novotny 15

  16. interpretations and (?) solutions R.-Y. Zhu et al, IEEE Trans. on Nucl. Scí. (2004) @RT Dk / Dk0 transmittance / % induced absorption / m-1 wavelength / nm M.Korzhik et al. time / s irradiation time / min SCINT 07 R.Novotny 16

  17. long release times: V.Dormenev, M.Korzhik et al. • analysis of TSL data up to 230K delivers no slow time constants • at –10oC or –25oC • center @580meV (observed in pure PWO) • should be suppressed by La/Y doping • deep trap @700meV (Frenkel defect) • induced absorption @400nm • release time t ~ 2.8h @RT • release time t = 125h @ -10oC • t = 808h @ -25oC • operation of the calorimeter at –10oC not sufficient SCINT 07 R.Novotny 17

  18. results are related to radiation induced structural changes in inclusions of variable valency tungstate oxide complexes WO3-x due to re-arrangement of oxygen ions. S.Burachas et al., J. Crystal Growth 293(2006)62 the changes proceed slower at reduced temperatures thermal energy insufficient for recovery 1,2: b/a red.annealing 1´,2´: b/a n-irradiation recovery after fast neutron irradiation origin of slow recovery processes S. Burachas et al. SCINT 07 R.Novotny 18

  19. induced absorption of PWO after irradiation: 20krad (60Co) @ RT exc.@325nm T=250K • Mo as a contributor to the optical absorption • induced in PWO (A.Hofstaetter et al.) SCINT 07 R.Novotny 19

  20. temperature of maximum thermal decay depends strongly on the Mo-content. crystals were irradiated with 50keV x-rays thermoluminescence of PbWO4/PbMoO4 mixed crystals Hofstaetter, R. Oeder, A. Scharmann, et al. phys. stat. sol. (b) 89, 375 (1978) SCINT 07 R.Novotny 20

  21. responsible trap: MoO4 tetrahedron identification via EPR SCINT 07 R.Novotny 21

  22. ESR measurements identify the complex in PWO-II crystals SCINT 07 R.Novotny 22

  23. M. Böhm, R. Grasser et al., J. de Physique C6, 508 (1980) optical absorption due to (MoO4)3- ? • difference in optical absorption • of CaWO4:Pb: • irradiation with x-rays @T = 77 K • subsequent annealing SCINT 07 R.Novotny 23

  24. optical absorption Emax=2.35eV FWHM=0.8eV wavelength / nm energy / eV is it relevant for PWO-II crystals? SCINT 07 R.Novotny 24

  25. in contrast: evaluations of the TSL-parameters in the temperature range between 180K and 300K by Korzhik et al. lead to trap-lifetimes of 4 < t < 250s@ 248K SCINT 07 R.Novotny 25

  26. consequences, further experiments and outlook 1/3 ESR intensity / a.u. bleaching wavelength / nm • optimum experimental resolution and stabilization (constant term) • will rely on very sophisticated monitoring • online bleaching probably not! SCINT 07 R.Novotny 26

  27. consequences, further experiments and outlook 2/3 irradiation of PWO crystal @ -25oC with 1.2MeV photons (60Co) @GI and measurement of optical transmission of the cooled crystal @ -25oC • more conclusive investigations • modification of doping or further reduction of impurities (Mo?) SCINT 07 R.Novotny 27

  28. consequences, further experiments and outlook 3/3 • operation at higher temperature: test performed at T=  0oC ! 3x3 matrix of PWO-II crystals 20x20x200mm3 readout with LAAPDs SCINT 07 R.Novotny 28

  29. irradiation of PWO-II crystals 2 rad/h 20 rad/h PMT anode signals under different modes of crystal irradiation by 137Cs gamma-source +20○C -25○C

  30. irradiation of PWO-II crystals Recoveryprocessat+20C Recoveryprocess at-25C

  31. CB 0 -0.25 -0.5 -0.75 -1 eV -4.1 (WO4)3- (WO4)3- - SE3+ Pb+ - VO (MoO4)3-  325 nm VB

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