Transcript Luminescent detectors of ionising radiation.

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
•
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
Radiometers
Luminescent
imaging plates
Storage materials
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
New materials
Storage media using
Composite materials
Li – containing compounds
Neutron converter + storage phosphor
Gd- containing compounds
(GdO / BaFBr-Eu)
( ternary fluorides & oxides)
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LiBaF3
Storage processes
2.5
1,2
1,1
LiBaF3
2.0
1,0
0,9
0,8
ABS
Optical density
0,7
1.5
0,6
B 270 nm
C 320 nm
D 430 nm
E 630 nm
0,5
0,4
0,3
1.0
0,2
0,1
0,0
0
0.5
0.0
200
300
400
500
600
700
800
900
50
100
150
200
250
300
350
Time, min
Accummulation kinetics during
X-irradiation at RT
l, nm
Absorption spectrum of color centers,
created by x-irradiation at RT
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LiKY2F8
0.6
Storage processes
5
LiKYF8
4
Optical density
0.5
0.4
0.3
3
2
0.2
1
0.1
0.0
200
300
400
500
600
700
800
l, nm
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
LiBaF3
Photostimulated read-out
Photon energy (eV)
6
5
4
3
2
Optical density difference
0.0
LiBaF3
-0.5
Bleaching wavelength:
430 nm
270 nm
320 nm
-1.0
200
300
400
500
600
700
800
Wavelength (nm)
IWORDI-2002 7-12 Sept. Amsterdamm
LiBaF3
Nature of the absorption bands
Li
Li
Li
a)
°A
11 4.017
11
Li
Li
11
11
1
b)
1
[100]
[110]
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.
Shell
I
II
Nuclei data
LiBaF3
Isotope
Spin
(%)
Nucl
a (mT)
b (mT)
Li7
3/2
92.5
2
0.91
0.07
Li6
1
7.5
0.34
0.03
F19
1/2
100
3.20
0.45
8
(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.
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LiBaF3
Photostimulated luminescence
Photostimulated luminescence
with 420 nm light at 85 K
Preliminary X-irradiation at:

--
19

16
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
Optical 1,2
density
1,0
Normalized optical density
1,0
0,8
0,8
420
317
0,6
0,6
3R
1R
Curves R – pure LiBaF3 samples
270
1O
300
400
500
Curves O – sampkes dopod by oxygen.
600
Wavelength (nm)
0,4
0,2
2R
2O
LiBaF3
3O
270 nm band
317 nm band
420 nm band
Activation energy of the main decay stage
estimated by the Glow Rate Technique:
0,0
R- sample 0,42 eV
250
300
350
400
450
o
Temperature ( K)
500
550
600
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
1.0
Room temperatures:
• The luminescence mechanism:
decay of self-trapped exciton.
0.8
Intensity, (a.u.)
• The luminescence center:
tungstate-oxygen complex .
Scheelites: WO42- (~ 400 nm)
Wolframites: WO66- (~500 nm)
PbWO4
CaWO4
CdWO4
ZnWO4
0.6
0.4
0.2
0.0
300
350
400
450
500
550
600
650
700
waveleght, (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 spectra of tungstate crystals
0.7
PbW O4
ZnW O4
CdW O4
CaW O4
Op ti ca l den si ty
0.6
0.5
0.4
0.3
0.2
0.1
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Energy, (eV)
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
3.5
4
3
PbWO4
CdWO4
2.5
0.6 ms
ln (Ilum & OD)
ln (Ilum & OD)
3.0
2.0
abs. 1.3 eV
lum. 2.8 eV
lum. at 2.6 eV
abs. at 2.5 eV
11 ms
LNT
2
4 ms
1
0
1.5
0
2
4
6
8
-1
10
time, (m)s
0
2000
lum. 2.1 eV
abs. 2.8 eV
ln (Ilum & OD)
ln (Ilum & OD)
10000
CaWO4
ZnWO4
-1.5
14 ms
-2.0
8000
lum. 2.9 eV
abs. 2.3 ev
2
-0.5
-1.0
6000
time, (ns)
4
0.0
4000
0
-2
10 ms
-4
-2.5
0
5000
10000
time, (ns)
15000
20000
-6
0
10000
20000
30000
40000
X Axis Title
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
25
0.60
LNT
trise=90-95 ns
0.50
0.45
OD
I (a.u.)
15
0 ns
200 ns delay
LNT
0.55
PbWO4
20
PbWO4
10
0.40
0.35
5
0.30
0
0
20
40
60
80
100
time, (ns)
120
140
160
0.25
1.0
1.5
2.0
2.5
3.0
3.5
4.0
E, (eV)
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
4
abs. 1.3 eV
lum. 2.8 eV
3
ln (Ilum & OD)
PbWO4
LNT
2
4 ms
1
0
-1
0
2000
4000
6000
8000
10000
time, (ns)
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
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.
IWORDI-2002 7-12 Sept. Amsterdamm
Institute of Solid State Physics University of Latvia
Scope
IWORDI-2002 7-12 Sept. Amsterdamm