Scintillation efficiency improvement by mixed crystal use

A.Gektina),   A. Belskyb),   A.Vasil’evc)

a)Institute for Scintillation Materials, 60 Lenin Ave., 61001 Kharkiv, Ukraineb)Institut Lumière Matière, UMR5306 Université Lyon 1‐CNRS, Université de Lyon, 

Villeurbanne, 69622, Francec)Skobeltsyn Institute of Nuclear Physics, Lomonosov Moscow State University, 

Leninskie Gory 1(2), 119991, Moscow, Russia

Motivation and outlines

• Can we use advantages of the last years theoretical studies for some practical use?  We mean the model of thermalisation stage responsibility for the later survival of electron excitation

• What is following from the general model? How to apply  this knowledge to doped crystals?

• Can we manage the thermalization distance by :– doping (rare solutions)– move to the mixed crystals (heavy solutions)

• Experimental data… old data, last results,  perspectives

• Alternative mechanisms

2 4 6 8 10 12 140

40

80

120

Lu2S3:Ce

SrI2:Eu

LuI3:Ce

Gd2O2S:Tb

LiI:Eu

NaI (80K)

NaI:Tl LaCl3La C l3

CsILaBr3

ZnS:Ag

bromides oxides fluorides sulfides chlorides iodides

β=2.5

band gap Eg (eV)

Phot

on Y

ield

(pho

tons

/keV

)

LSOCaF2:Eu

P.Dorenbos, SCINT, 2009

Scintillator efficiency:

Nph = β S Q

β =heE

E

–

γ

Eγ quantum energyEe-h = ~ 2.4 EgS energy transfer efficiencyQ luminescence center efficiency

Maximal Maximal scintillatorscintillator light yieldlight yield

Q is ~ 1 for many typical activators, Ce, Eu etc

S is also ~1 for many hosts.

1-5% of uniform distributed activator minimizes the transfer length to 2-5 a (lattice parameters)

β – e-h creation efficiency is a key to the new material search and investigation

Primary stages of scintilltion(track formation and energy relaxation)

Main contributors to the track theory developments :Main contributors to the track theory developments :

• R.T.Williams, …. see ‐ SCINT – O2.9, O2.19, O6.6• A.N.Vasil’ev… see – SCINT – O5.6, O5.9• S.Kerisit, Z.Wang,  F.Gao…• A.Canning … see –SCINT – O7.5• V.Nagirnyi, M.Kirm… See – SCINT ‐ O2.2• W.Setyawan… et al

Q

S

β

Capture by defects

Capture by defectswhd

Density-dependent STH+e→STEreactionweh-ex

Ionization by fast electrons, e-einelastic scattering, Auger cascade

elec

tron

s

hole

s exito

ns

STH

s STE

s

γ

Thermalization, formation of track structure

Density-dependent STE-STE quenching;Thermal STE quenching; STE quenching on

defects wexq

~90%~10%~90%

STE emission wexr

10–14 s

10–11 s

10–8 s

t

Scintillation process; from the absorption to photon emission

Nph = β S Q A.Vasil’ev, A.Gektin - O5.6 (yesterday)

6

R

R0

3D3DRecombination probabilityP

R0 R

1R/R0

⎩⎨⎧

><

=00

0

,,1

RrrRRr

Peheh

ehBlack sphere

( )ehOns rRP −−= exp1

TkRe

BOns

=ε

2

Coulomb

ε=5.7 T=300K ROns=10 nmT=77K ROns=38 nmT=10K ROns=300 nm ???

For thermalized excitations ROns/reh

Spatial distribution of thermalized electrons (binary crystals)

E0

r, nm

CsI

ROns, 300K Yield, 300K Yield, 77K

CsI 9.87 nm 0.24 0.44NaI 9.05 nm 0.34 0.58

0 1 2 3 4 5 60.0

0.2

0.4

0.6

0.8

1.0

CsI NaI

Rec

ombi

natio

n yi

eld

Electron kinetic energy, eV

Yield vs Ekin, 300K

Analytical estimation

ROns, 77K

ROns, 300K

ROns, 10K

0 100 200 300 400 500 600 700 80010-4

10-3

10-2 NaI CsI

Spa

tial d

istri

butio

n, n

m-1

r, nm

See Vasil’ev, O5.6

Electron‐hole separation and recombination

Thermalization

Diffusion of thermalized carriers

Geminate recombination Bimolecular recombination and escape

Electron‐hole separation and recombination

Thermalization

Diffusion of thermalizedcarriers

Geminate recombination Bimolecular recombination and escape

1 2 3 4 5

0.2

0.4

0.6

0.8

1.0

stochastic e‐hrecombination & escape

geminate e‐hrecombination

Thermalization length / Onsager radius

How we can manage thermalization length?

Two ways for e-h separation management

• Doped/activated crystals (rare solutions)

• Mixed crystals (hard solutions)

What we have to do to improve the yield?

The goal is to concentrate e-h pairs at the distance less then Onsager radius, to minimize the volume of stochastic recombination and escape losses.

Z. Wang, Y. Xie, B. D. Cannon… 2011

See also S.Gridin…O5.9

N

ROns RROns

( ) ( )

,4ln3Ei2

tanh~

241

4ln1

2tanh

~

38

0

2

0*

2

22

0*

20

2,

0

⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛Ω⎟

⎟⎠

⎞⎜⎜⎝

⎛ Ω⎟⎟⎠

⎞⎜⎜⎝

⎛=

Ω′

Ω′⎟⎟⎠

⎞⎜⎜⎝

⎛Ω

′⎟⎟⎠

⎞⎜⎜⎝

⎛ Ω⎟⎟⎠

⎞⎜⎜⎝

⎛= ∫

Ω

LO

e

B

LO

eB

E

LOLOLOB

LO

eBeLOe

ETkmm

a

EdE

ETkmm

aEle

LO

h

h

hhh

h

h

ε

ε

Coupled processes of thermalization and spatial diffusion

Mean square of the thermalization distance

Spatial distribution function

where thermalization length is

( )( ) EdESEDr

e

kine

kinee

E

E

R

EE′

′′

=>< ∫→0

062

( )( )( ) ( )⎟

⎟⎠

⎞⎜⎜⎝

⎛−=

02

2

03

2

0 23exp63,

eeeeee El

rElrElrf

π

( ) TkEee BerEl →>

Possible mechanisms of reduction of electron‐hole separation in solid solutions

Modification of hot stage of relaxation o Modification of phonon spectrum (additional phonon 

branches)o Modification of electron spectrum – increasing of elastic 

scattering (Bragg scattering in case of regular crystal)

Modification of diffusion of thermalizedcarriers

o Non‐uniformity of solution (in particular, clusterization) and scattering

o Anderson localization of carriers in disordered systems

Modification of phonon spectrum (additional phonon branches)

0 10 20 30 40 50 60 700.00.20.40.60.81.00.00.20.40.60.81.00.00.20.40.60.81.00.00.20.40.60.81.00.00.20.40.60.81.0

0 10 20 30 40 50 60 70

x=1

ELO, meV

x=0.75

x=0.5

Den

sity

of L

O s

tate

s

x=0

x=0.25

Density of LO states

Mixed – AxB1-xC crystal model

0.0 0.2 0.4 0.6 0.8 1.00

5

10

15

20

25

Solid solution of binary crystalsAxB1-xCELO(AC)=50 meVELO(BC)=10 meV

Mea

n th

erm

aliz

atio

n le

ngth

, nm

x

Electron kinetic energy 0.5 eV 1 eV 3 eV 5 eV

0.0 0.2 0.4 0.6 0.8 1.00.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Pro

babi

lity

of g

emin

ate

reco

mbi

natio

n

x

Electron kinetic energy 0.5 eV 1 eV 3 eV 5 eV

Solid solution of binary crystalsAxB1-xCELO(AC)=50 meVELO(BC)=10 meV

Mean thermalization length vs X concentration

Probability of geminate recombination

Thermalizationlength decrease

Recombination probability increase

Modification of electron spectrum – increasing of elastic scattering (Bragg scattering in case of regular crystal)

0.0 0.2 0.4 0.6 0.8 1.00

5

10

15

20

25

Solid solution of binary crystalsAxB1-xCELO(AC)=50 meVELO(BC)=10 meV

Mea

n th

erm

aliz

atio

n le

ngth

, nm

x

Electron kinetic energy 0.5 eV 1 eV 3 eV 5 eV

0.1

1

0 5 10 15 20 25

Solid solution of binary crystals AxB1-xC ELO(AC)=50 meV ELO(BC)=10 meV

0.05

Mean distance 1/2 (nm)

Ele

ctro

n en

ergy

(eV)

x=0 x=0.5 x=0.95 x=1

kBT

15

Scintillator kinetics

ln(I)

t

Luminescence quenching due to interaction in the regions with high concentration of

excitations

Long component (migration)

Rising part(migration)

Modification of kinetics in scintillators

M. Moszyński, et al. Study of Pure NaI at RT and LNT, IEEE TNS 2003

( )B

ttAI +

+ 201~

Essentially non-exponential decay kinetics for pure NaI

From the theory to experiment

• The database… Old and new experimental data

• The yield and decay kinetics analysis…• Selection of proper experimental conditions (the same growth and components, activator content structure etc)

• No “masked phenomena”

Mixed halides.   CsI‐CsBr

A. Gektin, N. Shiran, V. Shlyahturov and A. Belsky, Proceedings of SCINT’95, Delft, 1995

UV-luminescence decay kinetics in doped CsI crystals. 1 – CsI; 2 –CsI-RbI; 3 – CsI-CsCl; 4 – CsI-CsBr.

A.N. Belsky, A.V.Gektin, V.V.Mikhailin et al., Preprint ISC-91-3, Kharkov, 1991

First note – 1987

(Kubota… Gektin, Shiran)

Fast CsI scintillator…

Mixed halides.   CsI‐CsBr

3.0 3.5 4.0 4.5 5.0 5.5

1 - CsI-10%CsBr2 - CsI- 1%CsBr3 - CsI-99%CsBr

321

Люминесценция

(отн

.ед.

)Энергия фотона (эВ)

Inte

nsity

, a.u

.Photon energy (eV)

A.N. Belsky, A.V. Gektin et al., Proceedings of SCINT’95, Delft, 1995

X-ray emission spectra and decay kinetics of CsI1-xBrx solid solutionsC

OU

NT

S

TIME (ns)

1988

19

( )( ) ( ) ( )( ) ( )rdd

r

rlum

tRn

tnntIτπ

ττ erf

321

exp,3

0

20

0

−+

−=

r

rr

0 1 2 3 4 5

0.001

0.01

0.1

1

10 Ilum(t)

t/τr

n0Rd-d3=1010.10.01

Dipole-dipole transfer:

M. Kirm et al, PRB 79, 233103 (2009)

Rd‐d=2.1 nm for CdWO4

R.T.Williams et al, PSS(b) 248, 426 (2011)

Rd‐d=2.9 nm for CsI

Interaction of excitations in the regions with high excitation concentration

M. Kirm, V.Nagirnyi O2.2

0 20 40 60 80 1000

1x104

2x104

3x104

4x104

0 20 40 60 80 100

101

102

103

104CsI–1% BrCsI–10% Br

Time, ns

N

Time, ns

lgN

(2005)CsI-CsBr (data reconstruction)

* Pulse intensity increase withsimultaneous decay time shortening

• Similar behavior for CsI:CsCl

• Problem is the limited solubility

Mixed fluorides:  CexLa1‐xF3

10 20 30 40 500

1x105

2x105

3x105

4x105

0 20 40 60 80 1000

1x105

2x105

3x105

4x105CexLa1-xF3

Inte

nsity

, a.u

.

Time, ns

Ce 0.05% Ce 1% Ce 10% Ce 50% CeF3

CeF3Ce concentration, %

LaF3

Pulse shape and decay kinetics of CexLa1-xF3

X-ray excitation (10 keV).Left – original linear scale data; right –intensity vs cation mixture rate.

A.N. Belsky, A.V. Gektin et al., Proceedings of SCINT’95, Delft, 1995

Mixed halides 20 years late

Emission spectra under UV excitation0,0 0,2 0,4 0,6 0,8 1,0

0

20

40

60

80

100

120

140

LaBr3

activator CeCl3 activator CeBr3 R

elat

ive

light

out

put,

%

molar ratio of LaCl3/LaBr3LaCl3

GE Research

Sulphides:   Ca1‐xSrxS

Се3+ luminescence spectra for the solid solution (mixed crystals) of Ca1-xSrxS at X-ray excitation (30 kW, 10 mA)

0,0 0,2 0,4 0,6 0,8 1,06,0x102

8,0x102

1,0x103

1,2x103

1,4x103

1,6x103

Inte

nsity

, a.u

.

Sr concentrarion, %

Ca1-xSrxS: Ce

X-ray

Intensity (curve 1) of X-ray luminescence of ZnxCd1-xS-Ag and phosphorescence kinetics (2) from CdS concentration

[ A.Gurvich – in Russian -А.М.ГурвичРентгенолюминофоры и рентгеновскиеэкраны. М., Атомиздат, 1976]

1976 !

[A. Belsky et al., J. Phys.: Condensed Matter 5 (1993) 9417-9422]

Oxides: LuYAP

(2000-2001)

Lu0.5Y0.5AlO3‐Се

Excitation of Lu0,5Y0,5AlO3-Се

Excitation and luminescence spectra of (Lu,Y)AlO3-Ce

5 10

LuAlO3

YAlO3

Энергия фотона (эВ)

Интенсивность

(отн

.ед.

)In

tens

ity, a

.u.

Energy, eV

4 5 6 7 8 9 10 11

III

ЗП

Экситон

Ce 5d(e)

Ce 5d(t2)

Ce 4f

ВЗ

III

Энергия фотона (эВ)

Интенсивность

(отн

.ед.

)

Energy, eV

Inte

nsity

, a.u

.

CB

Exc

VB

[A.N. Belsky, W. Blanc, C. Dujardin et al., Proceedings of SCINT’99, Moscow, 1999]

0,0 0,2 0,4 0,6 0,8 1,0

Y3+ concentration, mole fractionYAlO3-Ce

Inte

nsity

, a.u

.

LuAlO3-Ce

excitonic

consecutive capture of e– and h+

Luminescence ofLu0,5Y0,5AlO3-Се

Scintillation in (Lu,Y)AlO3‐Ce

Gamma excitation amplitude spectra of (Lu,Y)AlO3-Ce.(Single photon counting)

Light output of solid solution of (Lu,Y)AlO3-Cedepending on Y cocentration. Bridgemen growth from the same raw material (A.Petrosian) (set one -▼, set 2 - ▲, set 3 - ●)

0,0 0,2 0,4 0,6 0,8 1,00

50

100

150

200

250 Bridgman Czochralski approximation

Ligh

t Yie

ld (B

GO

-100

%)

Y concentration, %

(Lu, Y)AlO3: Ce43% Y

50% Y

Загрузка канала

анализатора

B G O

0 500 1000 1500 2000

100% Y0% Y

К а н а лы а м п л и т уд н о го а н а л и за т о р а

Channels

Inte

nsity

, a.u

.

[A.N. Belsky, W. Blanc, C. Dujardin et al., Proceedings of SCINT’99, Moscow, 1999]

Mixed Borates – (Lu‐Sc)BO3:Ce

Mixed oxides  ‐ borates ‐ Lu0.75Y0.25BO3:Eu

[D. Spassky, ISMART 2012, Dubna, 2012]

Luminescence spectra of

Lu0.75Y0.25BO3:Eu3+

Eex = 5.4 eV (1) and Eex = 5.9 eV (2).

Mixed oxides  ‐ borates ‐ Lu0.75Y0.25BO3:Eu

[D. Spassky, ISMART 2012, Dubna, 2012]

Excitation spectra of

LuxY1-xBO3:Eu3+

withx = 0 (curve 1), x = 0.25 (2), x = 0.5 (3), x = 0.75 (4)x = 1 (5),

λem = 590 nm, T = 300 K.

Oxides (intrinsic luminescence) ‐ ZnxMg1‐xWO4

50

100

150

Zn0.91 0

10.90.80.70.60.1 0.2 0.40.3 0.5

MgW

O4

Zn0.

6Mg 0

.4W

O4

Zn0.

5Mg 0

.5W

O4

Zn0.

7Mg 0

.3W

O4

Zn0.

9Mg 0

.1W

O4

ZnW

O4

Ligh

t out

put,

%

0

0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1

Mg

I.Tupitsina, D.Spassky, 2013Private communication

2013D.Spassky – O2.18

Excitation and luminescence spectra

Mixed oxides, (Ce,La)PO4

Excitation spectra of solid solution (Ce,La)PO4 at Ce absorption area and its intensity vs Ce concentration

0 20 40 60 80

0,1

0,2

0,3

0,4

0,5

0,6

0,7

Inte

nsity

, a.u

.Ce concentration, %

(Ce, La)PO4

4 6 8 10 12

0,1

1

Ce4++e-CBVB

5d4f

Ce05%

Ce01%

Ce40%&80%

Ce1%

Ce30%Ce50%Ce70%

Интенсивность

люмин

есценц

ии (о

тн.ед.

)

Энергия фотона (эВ)

Energy, eV

Inte

nsity

, a.u

.

[A.N. Belsky, 2000]

0

1

2

3

4

5

6

7

5 10 15 200

1

2

I, a.

u.LSO

75% Lu

50%Lu

40%Lu

10% Lu

GSO

Eg2Eg

Ce3+

A

Ce3+

I, a.

u.

E, eV

Eg2Eg

B

20 40 60 80

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0

Ce1 Ce2

I, ar

b. u

n.

Lu conc, at.%

Excitation spectra of 400 nm luminescence at Т=300К.

Important! The best efficiency of carrier multiplication and transfer to luminescence centers corresponds to 50:50 Gd:Lu rate

Ratio of intensities I(30 eV)/I(7 eV) of Се1 (400 nm) and Се2 (530 nm) excitation.

Mixed oxides, silicates  ‐ LGSO:Ce

See details - O.Sidletskiy – O2.5

Mixed oxides, silicates  ‐ LGSO:Ce

Light yield in LGSO:Ce crystals with monoclinic C2/c structure vs. host composition

1. O. Sidletskiy, V. Bondar, B. Grinyov, et al. J. Cryst Growth, 312 (2010) 6012. O.Sidletskiy, A. Belsky, A. Gektin, et al. Crys Growth & Des, (2012), 12, 441

Oxides, garnets – GAGG and YAGG

Mixed oxides, garnets  ‐ YAGG:Ce

[1] O.Sidletskiy, V. Kononets, K. Lebbou, S.Neicheva, O.Voloshina, V.Bondar, V.Baumer, K.Belikov, A. Gektin, M– F.Joubert., Materials Research Bulletin, Vol. 47, No. 11. (2012),

pp.3249-3252

Ga concentration, %A

rea

unde

r em

issi

on b

and,

a.u

.Integral radioluminescence yield vs Ca concentration

Scintillation yield and radioluminescence intensity dependence for mixed YAGG

Mixed oxides  ‐ YAGG:CeEnergy storage in mixed YAGG:Ce scintillators

TSL (Thermo luminescence) for YAGG:Ce

TSL efficiency for YAG:Ce and YAGG:Ce with different Ga concentration

8

Ga doping allows to decrease the energy storage in YAGG comparing to YAG:Ce crystals

Inte

nsity

, a.u

.

а b

Tmax, oC Ea, eV Tmax, 

oC Ea, eV

YAG: Ce 102 1.17 250 1.75

YAGG (40%Ga):Ce 80 1.02 217 1.56

YAGG (60%Ga):Ce ‐ ‐ 119 0.9

Traps parameters

e‐h separation and/or conduction band modification?

CB

VB

CB

VB

CB

VB

YAG YAGG (40%Ga)

YAGG (60%Ga)

E, эВ

Eg=6.5эВ

1 эВ

1.75эВ

1.17эВ 1.56э

В 0.9эВ

Band structure change with Ga doping.

9

Ga doping (shift to mixed crystals)

* Decrease the CB bottom level

• Decrease of shallow traps influence

• ….

M.Nikl …

* There are some alternativemechanisms that influence to light yield with similar or even higher rate

** Crystal performance, initial purity and activator concentration are crucial for theexperimental study of phenomena

*** Decay time measurement could be moreefficient for the model verification than yield test

**** We need in more detailed theoretical estimations for dopedand mixed crystals

CONCLUSIONS

CsI-CsBr

0 20 40 60 80 1000

1x105

2x105

3x105

4x105

Inte

nsity

, a.u

.

CeF3

Ce concentration, %

LaF3

CexLa1-xF3

Gd2(AlxGa1-x)5O12:Ce

LGSO:Ce

YAGG:Ce

0,0 0,2 0,4 0,6 0,8 1,00,6

0,8

1,0

1,2

1,4

1,6

Inte

nsity

, a.u

.

Sr concentrarion, %

X-ray

Ca1-xSrxS

0,0 0,2 0,4 0,6 0,8 1,00

50

100

150

200

250

Ligh

t Yie

ld (B

GO

-100

%)

Y concentration, %

(Lu, Y)AlO3: Ce

50

100

150

Zn0.91 0

10.90.80.70.60.1 0.2 0.40.3 0.5

MgW

O4

Zn0.

6Mg 0

.4W

O4

Zn0.

5Mg 0

.5W

O4

Zn0.

7Mg 0

.3W

O4

Zn0.

9Mg 0

.1W

O4

ZnW

O4

Ligh

t out

put,

%

0

0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1

Mg

ZnxMg1-xWO4

0,0 0,2 0,4 0,6 0,8 1,00

20

40

60

80

100

120

140

LaBr3

activator CeCl3 activator CeBr3

Rel

ativ

e lig

ht o

utpu

t, %

molar ratio of LaCl3/LaBr3LaCl3

LaCl3-LaBr3

Thank you for your attentions!