1. Luminescent detectors of ionising radiation. L. Grigorjeva, P. Kulis, D. Millers, S. Chernov, M. Springis, I. Tale IWORDI-2002 7-12 Sept. Amsterdamm Institute of Solid State Physics University of Latvia

2. Scope IWORDI-2002 7-12 Sept. Amsterdamm Storage materials Luminescent imaging systems Imaging plates for detection of slow meutron fields Radiation energy storage materials for detecting of slow neutrons LiBaF 3 Storage processes, nature of radiation defects Photostimulated luminescence Thermostimulated decay of radiation defects (feeding) Tungstate scintillators Two types of tungstates. Excited state absorption. Optical absorption of self-trapped carriers. Formation of luminescence centers. Conclusions.

3. Luminescent radiation transformers IWORDI-2002 7-12 Sept. Amsterdamm ScintillatorsStorage materials RadiometersLuminescent imaging plates DosemetersStorage imaging plates

4. Sample of slow neutron imaging IWORDI-2002 7-12 Sept. Amsterdamm Ignitron

5. Radiation energy storage materials for detecting of slow neutrons field IWORDI-2002 7-12 Sept. Amsterdamm Existing photoluminescent imaging plates Composite materials Neutron converter + storage phosphor (GdO / BaFBr-Eu) New materials Storage media using Li – containing compounds Gd- containing compounds ( ternary fluorides & oxides)

6. LiBaF 3 Storage processes IWORDI-2002 7-12 Sept. Amsterdamm Absorption spectrum of color centers, created by x-irradiation at RT Accummulation kinetics during X-irradiation at RT

7. IWORDI-2002 7-12 Sept. Amsterdamm LiKY 2 F 8 Storage processes Optical absorption of LiKYF 8 undoped crystals, induced by X- irradiation (W-tube operating at 45 kV, 10 mA) at RT for various time, min: 1- 68; 2- 130; 3- 210; 4-350; 5- 620.

8. LiBaF 3 Photostimulated read-out IWORDI-2002 7-12 Sept. Amsterdamm

9. LiBaF 3 Nature of the absorption bands (a) EPR spectrum of LiBaF 3 :Fe crystal, x-irradiated and measured at RT for a magnetic field orientation B ll [111]. (b) calculated EPR spectrum for a magnetic field orientation B ll [111] with parameters of the table 1. Crystal structure of LiBaF3 with F- centre. Fluorine vacancy has 2 Li neighbours (I) in the first shell and 8 fluorine neighbours (II) in the second shell. ShellNuclei data LiBaF 3 IsotopeSpin(%)Nucla (mT)b (mT) I Li 7 3/292.520.910.07 Li 6 17.50.340.03 II F 19 1/210083.200.45

10. LiBaF 3 Photostimulated luminescence IWORDI-2002 7-12 Sept. Amsterdamm Photostimulated luminescence with 420 nm light at 85 K Preliminary X-irradiation at:    O : 85 K

11. IWORDI-2002 7-12 Sept. Amsterdamm LiBaF 3 Photostimulated luminescence

12. LiBaF 3 Thermostimulated read- out IWORDI-2002 7-12 Sept. Amsterdamm Decay kinetics of X- irradiation created absorption bands peaked at 270 nm; 317 nm and 420 nm Curves R – pure LiBaF 3 samples Curves O – sampkes dopod by oxygen. Activation energy of the main decay stage estimated by the Glow Rate Technique: R- sample 0,42 eV O- sample 0,78 – 0,83 eV I pure LiBaF 3 (R- samples) decay of the F-type centers are governed by mobile fluorine atoms trapped in the course of irradiation by antistructure defects Li Ba. In heterovalent oxygen doped LiBaF 3 (O- samples) F-centre migration and recombination with fluorine atoms trapped by complexes O Li V F is governed by mobile anion vacancies.

13. Tungstate scintillators Led tungstate: Large radiation hardness Good stopping power for ionizing radiation Low scintillation output at RT Led tungstate - main scintillator in the large electromagnetic calorimeter at CERN. Problem: is it possible an efficient use of this material at low temperature ? Cadmium tungstate: The luminescence matches well with the spectral sensitivity curve of semiconductor photodetectors. High stopping power of X-ray is high The scintillation output is somewhat bellow to the estimated level. Cadmium tungstate - known scintillator used for computed X-ray tomography. Problem: can the properties of material to be improved? IWORDI-2002 7-12 Sept. Amsterdamm

14. Tungstate scintillators Structure IWORDI-2002 7-12 Sept. Amsterdamm Crystallogphically, depending on the size of metal ion, tungstate phosphors normally exist in two structure modifications, : scheelite-type (C 6 4h ) = stolzite wolframite-type (C 4 2h ) =raspite Lead tungstate: both forms. Cadmium tungstate: only wolframite type.

15. Tungstate crystals Luminescence spectra The luminescence center: tungstate-oxygen complex. Scheelites: WO 4 2- (~ 400 nm) Wolframites: WO 6 6- (~500 nm) IWORDI-2002 7-12 Sept. Amsterdamm Room temperatures: The luminescence mechanism : decay of self-trapped exciton. The luminescence spectra peaks for CdWO and ZWO are close and corresponds to the sensitivity of semiconductor photodetector, whereas for PWO and CaWO peaks are shifted to the blue region.

16. IWORDI-2002 7-12 Sept. Amsterdamm Transient absorption of PWO bellow 1.4 eV : the self-trapped electron ( black curve – the high energy wing of band is shown). Transient absorption of CdWO & CaWO peaks at 2.5 eV and it overlaps with the luminescence band.

17. Kinetics Luminescence & Transient absorption The decay kinetics of luminescence and transient absorption matches well. Consequences: the transient absorption is due to luminescence center excited state. IWORDI-2002 7-12 Sept. Amsterdamm

18. Tungstates The formation of luminescence center The rise time of luminescence follows the decay time of transient absorption bellow 1.4 eV. Consequences: The release rate of self-trapped electron governs the luminescence center formation time. The luminescence center is an self trapped exciton! The scintillations are limited by both - luminescence center formation and decay time. IWORDI-2002 7-12 Sept. Amsterdamm

19. Kinetics Luminescence & Transient absorption The decay kinetics of luminescence and transient absorption matches well. Consequences: the transient absorption is due the transition to the next excired state of luminescence center (self trapped exciton). IWORDI-2002 7-12 Sept. Amsterdamm

20. Tungstates Self trapping of electrons / holes IWORDI-2002 7-12 Sept. Amsterdamm Self-trapped carriers (electrons and/or hole) are precursors of self-trapped exciton.

21. Conclusions Tungstates The scintillations from PWO at low temperature became significant longer, because of limitation by both - excited state formation and decay time. Excited state absorption from luminescence center is observed in all tunstates (CdWO, PWO, CaWO, ZnWO) studied. The scintillation efficiency in CdWO is lower than estimated due to overlaping of emission and transient absorption. The self-trapped charge states are involved in evciton formation in tungstates. IWORDI-2002 7-12 Sept. Amsterdamm Radiation energy storage in fluoroperovskites LiBaF 3 represents a perspective material for development of storage imaging plates for imaging of slow neutron fields The radiation defects responsible for the main absorption bands in LiBaF 3 are due to creation of F-type centers Photostimulation in the main absorption bands results in decay of F-type centers followed by recombination luminescence The theroactivated decay of radiation created defects is governed by ionic mobility in fluorine sublattice; the decay mechanism depecds on deviation from stoichiometry

22. Institute of Solid State Physics University of Latvia

23. Scope IWORDI-2002 7-12 Sept. Amsterdamm