A.Gektina) A. Belskyb) A.Vasil’evc) · Scintillation efficiency improvement by mixed crystal use...
Transcript of A.Gektina) A. Belskyb) A.Vasil’evc) · Scintillation efficiency improvement by mixed crystal use...
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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
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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
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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
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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
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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)
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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
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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
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Electron‐hole separation and recombination
Thermalization
Diffusion of thermalized carriers
Geminate recombination Bimolecular recombination and escape
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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
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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
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( ) ( )
,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 →>
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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
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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
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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
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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
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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”
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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…
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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
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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
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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
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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
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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]
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Oxides: LuYAP
(2000-2001)
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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
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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]
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Mixed Borates – (Lu‐Sc)BO3:Ce
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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).
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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.
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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
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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]
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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
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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
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Oxides, garnets – GAGG and YAGG
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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
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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
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Thank you for your attentions!