Luminescent detectors of ionising radiation .

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

2. Scope • Storage materials • Luminescent imaging systems • Imaging plates for detection of slow meutron fields • Radiation energy storage materials for detecting of slow neutrons • LiBaF3 • 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. IWORDI-2002 7-12 Sept. Amsterdamm

3. Luminescent radiation transformers Scintillators Storage materials Radiometers Luminescent imaging plates Dosemeters Storage imaging plates IWORDI-2002 7-12 Sept. Amsterdamm

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

5. Radiation energy storage materials for detecting of slow neutrons field 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) IWORDI-2002 7-12 Sept. Amsterdamm

6. LiBaF3 Storage processes Accummulation kinetics during X-irradiation at RT Absorption spectrum of color centers, created by x-irradiation at RT IWORDI-2002 7-12 Sept. Amsterdamm

7. LiKY2F8 Storage processes Optical absorption of LiKYF8 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. IWORDI-2002 7-12 Sept. Amsterdamm

8. LiBaF3 Photostimulated read-out IWORDI-2002 7-12 Sept. Amsterdamm

9. Shell Nuclei data LiBaF3 Isotope Spin (%) Nucl a (mT) b (mT) I Li7 3/2 92.5 2 0.91 0.07 Li6 1 7.5 0.34 0.03 II F19 1/2 100 8 3.20 0.45 LiBaF3 Nature of the absorption bands 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. (a) EPR spectrum of LiBaF3: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. IWORDI-2002 7-12 Sept. Amsterdamm

10. LiBaF3 Photostimulated luminescence • Photostimulated luminescence with 420 nm light at 85 K • Preliminary X-irradiation at: • · : 205 K • -- : 190K • : 160 K O : 85 K IWORDI-2002 7-12 Sept. Amsterdamm

11. LiBaF3 Photostimulated luminescence IWORDI-2002 7-12 Sept. Amsterdamm

12. LiBaF3 Thermostimulated read- out Decay kinetics of X- irradiation created absorption bands peaked at 270 nm; 317 nm and 420 nm Curves R – pure LiBaF3 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 LiBaF3 (R- samples) decay of the F-type centers are governed by mobile fluorine atoms trapped in the course of irradiation by antistructure defects LiBa. In heterovalent oxygen doped LiBaF3 (O- samples) F-centre migration and recombination with fluorine atoms trapped by complexes OLiVF is governed by mobile anion vacancies. IWORDI-2002 7-12 Sept. Amsterdamm

13. 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? Tungstate scintillators IWORDI-2002 7-12 Sept. Amsterdamm

14. Tungstate scintillators Structure Crystallogphically, depending on the size of metal ion, tungstate phosphors normally exist in two structure modifications, : scheelite-type (C64h) = stolzite wolframite-type (C42h)=raspite Lead tungstate:both forms. Cadmium tungstate: only wolframite type. IWORDI-2002 7-12 Sept. Amsterdamm

15. Tungstate crystalsLuminescence spectra Room temperatures: • The luminescence mechanism: • decay ofself-trapped exciton. • The luminescence center: tungstate-oxygen complex. Scheelites:WO42- (~ 400 nm) Wolframites: WO66- (~500 nm) 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. IWORDI-2002 7-12 Sept. Amsterdamm

16. 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. IWORDI-2002 7-12 Sept. Amsterdamm

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 bellow1.4 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 Self-trapped carriers (electrons and/or hole) are precursors of self-trapped exciton. IWORDI-2002 7-12 Sept. Amsterdamm

21. Conclusions • Radiation energy storage in fluoroperovskites • LiBaF3 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 LiBaF3 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 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

22. Institute of Solid State Physics University of Latvia

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