luminescent detectors of ionising radiation

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


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


Scintillators Storage materials

Radiometers Luminescent imaging plates

Dosemeters Storage imaging plates

Sample of slow neutron imaging


Ignitron

Radiation energy storage materials for detecting of slow neutrons field


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


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


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


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)


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


Photostimulated luminescence with 420 nm light at 85 K

Preliminary X-irradiation at:

O : 85 K


LiBaF3 Photostimulated luminescence

LiBaF3 Thermostimulated read- out


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?


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.

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


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


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).


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.

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.


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


luminescent detectors of ionising radiation

Documents

kpreliminary xirradiation

course of irradiation

x irradiation wtube

crystal structure of

fluorine neighbours

absorption bandsiwordi

mobile fluorine atoms

amsterdammdecay kinetics