1. 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, V. Ryazantsev c, P. A. Semenov c, G. Tamulaitis f, A. V. Uzunian c, A. Vasiliev d, A. N. Vasiliev c and for the PANDA collaboration a II. Physics Institute, University Giessen, Heinrich-Buff-Ring 16, 35392 Giessen, Germany b INP, Belarus State University, 11 Bobruiskaya, 220030 Minsk, Belarus c Institute for High Energy Physics, Protvino, Russia d R.RC Kurchatov Institute, Moscow, Russia e I. Physics Institute, University Giessen, Heinrich-Buff-Ring 16, 35392 Giessen, Germany f Vilnius University, Vilnius, Lithuania Radiation hardness and recovery processes of PWO crystals at –25 o C 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. SCINT 07 R.Novotny 3 high resolution calorimetry with PWO +fast and very dense scintillator:   <10ns X o =0.9cm +radiation hard @RT: CMS-ECAL -low light yield: medium-energy application improvement of the photon statistics by: increase of light outputPWO-II operation well below room temperature photon energy / MeV  / E % excellent energy resolution achievable at low temperatures

4. envisaged operation at T = -25 o C PHOS (PHOton Spectrometer) 17920 PWO channels (22x22x180mm 3 ) study of initial phase of HI collision: via direct photons, high p T ,   PHOS @ALICE LHC EMC @PANDA - FAIR 20000 PWO channels (20x20x200mm 3 ) hadron physics with anti-protons SCINT 07 R.Novotny 4

5. no permanent damage due to defect formation activation due to proton induced reactions reduction of optical transmission can be handled by monitoring dose: 10 13 protons E p = 90 MeV @ KVI, Groningen wavelength / nm transmission / % both experiments can expect a lower radiation dose SCINT 07 R.Novotny 5 all tests of radiation hardness have been so far performed at RT

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

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

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

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

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

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

12. irradiation of PWO-II crystals light yield / pe/MeV integration gate / ns -25 o C 0oC0oC +25 o C time / h PMT output / a.u. 2 rad/h20 rad/h +20 C-25 C PWO-II - 28 PWO-II - 30 SCINT 07 R.Novotny 12

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

14. Crystal ID LY * @ RT [p.e./MeV] ratio, DC@-25C/ DC@+20C signal loss (%) after 310 h of irradiation @ 2 rad/h &-25C DC Blue LED Red LED 3 19.13.450199 27 19.53.42071 28 19.63.41951 29 17.83.436136 30 19.83.532103 37 21.13.32983 * LY – light yield measured by RINC, Minsk irradiation of PWO-II crystals SCINT 07 R.Novotny 14

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

16. interpretations and (?) solutions wavelength / nm transmittance / % @RT R.-Y. Zhu et al, IEEE Trans. on Nucl. Scí. (2004) induced absorption / m -1 irradiation time / min M.Korzhik et al. SCINT 07 R.Novotny 16 time / s  k /  k 0

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

18. SCINT 07 R.Novotny 18 origin of slow recovery processes S. Burachas et al. results are related to radiation induced structural changes in inclusions of variable valency tungstate oxide complexes WO 3-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 recovery after fast neutron irradiation 1,2: b/a red.annealing 1´,2´: b/a n-irradiation

19. induced absorption of PWO after irradiation: 20krad ( 60 Co) @ 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. Hofstaetter, R. Oeder, A. Scharmann, et al. phys. stat. sol. (b) 89, 375 (1978) thermoluminescence of PbWO 4 /PbMoO 4 mixed crystals SCINT 07 R.Novotny 20 crystals were irradiated with 50keV x-rays temperature of maximum thermal decay depends strongly on the Mo-content.

21. responsible trap: MoO 4 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. optical absorption due to (MoO 4 ) 3- ? difference in optical absorption of CaWO 4 :Pb: irradiation with x-rays @T = 77 K subsequent annealing M. Böhm, R. Grasser et al., J. de Physique C6, 508 (1980) SCINT 07 R.Novotny 23

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

25. SCINT 07 R.Novotny 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 <  < 250s @ 248K

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

27. consequences, further experiments and outlook 2/3 more conclusive investigations irradiation of PWO crystal @ -25 o C with 1.2MeV photons ( 60 Co) @GI and measurement of optical transmission of the cooled crystal @ -25 o C 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=  0 o C ! 3x3 matrix of PWO-II crystals 20x20x200mm 3 readout with LAAPDs SCINT 07 R.Novotny 28

31. +20 ○ C -25 ○ C 2 rad/h20 rad/h PMT anode signals under different modes of crystal irradiation by 137 Cs gamma-source irradiation of PWO-II crystals

32. Recovery process at +20C Recovery process at -25C irradiation of PWO-II crystals

33. VB CB  0 -0.25 -0.5 -0.75 eV -4.1 (WO 4 ) 3- (WO 4 ) 3- - SE 3+ (MoO 4 ) 3- Pb + - V O 325 nm