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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. IWORDI-2002 7-12 Sept. Amsterdamm. Scope. Storage materials Luminescent imaging systems - PowerPoint PPT Presentation
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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
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
• 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.
Luminescent radiation transformers
IWORDI-2002 7-12 Sept. Amsterdamm
Scintillators Storage materials
Radiometers Luminescent imaging plates
Dosemeters Storage imaging plates
Sample of slow neutron imaging
IWORDI-2002 7-12 Sept. Amsterdamm
Ignitron
Radiation energy storage materials for detecting of slow neutrons field
IWORDI-2002 7-12 Sept. Amsterdamm
Existing photoluminescent imaging plates
Composite materialsNeutron converter + storage phosphor
(GdO / BaFBr-Eu)
New materials
Storage media using Li – containing compounds
Gd- containing compounds
( ternary fluorides & oxides)
LiBaF3 Storage processes
IWORDI-2002 7-12 Sept. Amsterdamm
200 300 400 500 600 700 800 9000.0
0.5
1.0
1.5
2.0
2.5
LiBaF3
Opt
ical
den
sity
, nm
Absorption spectrum of color centers, created by x-irradiation at RT
0 50 100 150 200 250 300 3500,0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
1,0
1,1
1,2
B 270 nm C 320 nm D 430 nm E 630 nm
AB
S
Time, min
Accummulation kinetics during X-irradiation at RT
IWORDI-2002 7-12 Sept. Amsterdamm
LiKY2F8 Storage processes
200 300 400 500 600 700 8000.0
0.1
0.2
0.3
0.4
0.5
0.6
5
4
3
2
1
, nm
Opt
ical
den
sity
LiKYF8
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.
LiBaF3 Photostimulated read-out
IWORDI-2002 7-12 Sept. Amsterdamm
200 300 400 500 600 700 800-1.0
-0.5
0.0
LiBaF3
Bleaching wavelength: 430 nm 270 nm 320 nm
Optica
l den
sity di
fferen
ce
Wavelength (nm)
6 5 4 3 2Photon energy (eV)
IWORDI-2002 7-12 Sept. Amsterdamm
LiBaF3 Nature of the absorption bands
(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.
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.
LiLi
LiLi
Li
4.017 A°
[100][110]
1
1111
1111 1
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
a)
b)
LiBaF3 Photostimulated luminescence
IWORDI-2002 7-12 Sept. Amsterdamm
Photostimulated luminescence with 420 nm light at 85 K
Preliminary X-irradiation at:
O : 85 K
IWORDI-2002 7-12 Sept. Amsterdamm
LiBaF3 Photostimulated luminescence
LiBaF3 Thermostimulated read- out
IWORDI-2002 7-12 Sept. Amsterdamm
250 300 350 400 450 500 550 600
0,0
0,2
0,4
0,6
0,8
1,0
300 400 500 600
0,6
0,8
1,0
1,2
270 nm band 317 nm band 420 nm band
LiBaF33O
2O
3R1R
2R
1O
Opticaldensity
Nor
mal
ized
opt
ical
den
sity
Temperature (oK)
Wavelength (nm)
420317270
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.
Tungstate scintillatorsLed 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
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 (C64h) = stolzite
wolframite-type (C42h) =raspite
Lead tungstate: both forms.
Cadmium tungstate: only wolframite type.
Tungstate crystals Luminescence spectra
• The luminescence center: tungstate-oxygen complex .
Scheelites: WO42- (~ 400 nm)
Wolframites: WO66- (~500 nm)
300 350 400 450 500 550 600 650 7000.0
0.2
0.4
0.6
0.8
1.0
Inte
nsity
, (a.
u.)
waveleght, (nm)
PbWO4
CaWO4
CdWO4
ZnWO4
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.
Transient absorption spectra of tungstate crystals
1.0 1.5 2.0 2.5 3.0 3.5 4.00.1
0.2
0.3
0.4
0.5
0.6
0.7
Op
tica
l den
sity
Energy, (eV)
PbW O4
ZnW O4
CdW O4
CaW O4
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.
Kinetics Luminescence & Transient absorption
0 10000 20000 30000 40000-6
-4
-2
0
2
4
CaWO4
10 sln (I
lum &
OD
)
X Axis Title
lum. 2.9 eV abs. 2.3 ev
0 5000 10000 15000 20000
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
14 s
ZnWO4
ln (I
lum &
OD
)
time, (ns)
lum. 2.1 eV abs. 2.8 eV
0 2 4 6 8 101.5
2.0
2.5
3.0
3.5
CdWO4
0.6 s 11 s
ln (I
lum &
OD
)
time, ()s
lum. at 2.6 eV abs. at 2.5 eV
0 2000 4000 6000 8000 10000-1
0
1
2
3
4
4 s
LNT
PbWO4
ln (I
lum &
OD
)
time, (ns)
abs. 1.3 eV lum. 2.8 eV
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
Tungstates The formation of luminescence center
1.0 1.5 2.0 2.5 3.0 3.5 4.00.25
0.30
0.35
0.40
0.45
0.50
0.55
0.60
LNT
PbWO4
OD
E, (eV)
0 ns 200 ns delay
0 20 40 60 80 100 120 140 1600
5
10
15
20
25
rise
=90-95 ns
LNT
I (a.
u.)
time, (ns)
PbWO4
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
Kinetics Luminescence & Transient absorption
0 2000 4000 6000 8000 10000-1
0
1
2
3
4
4 s
LNT
PbWO4
ln (I
lum &
OD
)
time, (ns)
abs. 1.3 eV lum. 2.8 eV
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
Tungstates Self trapping of electrons / holes
PbWO4 CaWO4 ZnWO4 CdWO4
electron + + - + ?
hole - + + -
Tdeloc 50 K 160 K 75 K -
ESR + + + -
Eabs ~1.0 eV 1.7 eV~1.2 eV
- ~1.2 eV
IWORDI-2002 7-12 Sept. Amsterdamm
Self-trapped carriers (electrons and/or hole) are precursors of self-trapped exciton.
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• 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
Institute of Solid State Physics University of Latvia
Scope
IWORDI-2002 7-12 Sept. Amsterdamm