Luminescent detectors of ionising radiation.

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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
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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.

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Luminescent radiation transformers
Scintillators
Storage materials
Radiometers
Luminescent imaging plates
Dosemeters
Storage imaging plates
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Sample of slow neutron imaging
Ignitron
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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)
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LiBaF3 Storage processes
Accummulation kinetics during X-irradiation at RT
Absorption spectrum of color centers, created by
x-irradiation at RT
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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.
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LiBaF3 Photostimulated read-out
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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 LiBaF3Fe 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.
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LiBaF3 Photostimulated luminescence

• Photostimulated luminescence with 420 nm light at
  85 K
• Preliminary X-irradiation at
• 205 K
• -- 190K
• 160 K
• O 85 K

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LiBaF3 Photostimulated luminescence
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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.
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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?

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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.
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Tungstate crystals Luminescence spectra
Room temperatures

• The luminescence mechanism
• decay of self-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.
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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.
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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.
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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.

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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).
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Tungstates Self trapping of electrons /
holes
Self-trapped carriers (electrons and/or hole) are
precursors of self-trapped exciton.
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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.

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Institute of Solid State Physics University of
Latvia
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Scope
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