ナノ材料を用いた高速シンチレータ開発
東北大学大学院工学研究科 応用化学専攻
越水 正典

Scintillators and scintillation detectors
Scintillation

Ionizing radiation

Scintillator

Photon detector(e.g. PMT, PD)

TimeDetection signalVoltage

High-energy X-ray detection applications> 20 keV: APD is not suitable because fast response
and high detection efficiency cannot
simultaneously be achieved.

・High counting rate measurements(e.g. diffraction measurements using intense beam)

Scattering
intensity

・Nuclear resonant scattering measurements
Prompt component

3~5 orders of magnitude!!
Component to be analyzedTimeCharacterization @ PF

Today’s contents
1. Fast scintillation mechanism.
2. Fast scintillators using Auger-free luminescenceor self-trapped exciton luminescence.
3. Fast scintillators having nanostructures.

Possible fast scintillation mechanism
・Auger-free luminescence (e.g. BaF2)
・Self-trapped exciton luminescence (e.g. CsI)（・Charge transfer luminescence in Yb-doped crystals(e.g. Lu2O3:Yb)）
・Luminescence from organic fluorophores(e.g. plastic scintillators)
・Wannier excitons luminescenceFast scintillators developedby us(without nanostructures)

Auger-free luminescence(CsCaCl3)
420 photons/MeV
t: 2.3 ns (58%)

Auger-free luminescence(Cs2ZnCl4) - 1

t: 1.8 ns (>95%)

Auger-free luminescence(Cs2ZnCl4) - 2

Time resolution: ~0.7 ns

630 photons/MeV @67.4 keV~1000 photons/MeV @662 keV

Self-trapped exciton luminescence(BaCl2)950 photons/MeV

t: 1.5 ns (34%)

Development of fast scintillators
having nanostructures

Advantages of Nanostructured for fast
scintillation materials
1. Use of quantum confinement effects
Fabrication of low-dimensional quantum confinement
structure in self-organized manner.
Wannier exciton luminescence can be utilized at RT.
Fast and efficient scintillation is expected.

2. Incorporation of inorganic nanoparticles
into plastic scintillators
Novel approach to synthesize loaded organic scintillators.
Detection efficiency can be effectively enhanced.

1. Use of quantum confinement effects

Use of Wannier exciton luminescence
Wannier exciton:
Hydrogen-atom-like bound stateof an electron-hole pair

Conduction band

Binding energy

Several meV～several hundreds meVEmission lifetime < 10 ns
in direct-gap semiconductors

Wannier exciton
energy level

Valence band

Binding energy > kT-Thermally stable exciton

2D quantum confinement- quantum well structure
EConduction bandBarrier

Quantum Barrier
well

Wannier excitons in 2D structure

Large binding energy- Exciton is stable at RT
Enhanced oscillator strength- Fast radiative decay

Valence bandRequirements and difficulties inscintillator applications
Typical synthesis methods of quantum well structure- Thin film deposition techniques

Thickness: ~ several microns- too thin to detect X-ray or g-ray.
Thin film deposition technique is not suitable.

Organic-inorganic perovskite-type compounds
Organic amine layer(Barrier layer)
Inorganic PbX42- layer(Well layer)
Wannier excitons are confined.

Quantum well structure
is formed in self-organizedmanner.

Composition and structure
Chemical formula:(RNH3)2(CH3NH3)m-1PbmX3m+1

Organic : RNH3+
Inorganic : PbX42c

m: Thickness of the inorganic layer- Influence on exciton confinement.
X: Halogen- Influence on band-gap energy
in inorganic layer.
RNH2: alkylamine- Influence on crystal structure.
energy transfer process?

baGeneral trend of scintillation
properties
Thickness of the inorganic layer

Light yield was the highest for m=1.
(Well layer was monomolecular thickness.)
HalogenBromide was best. Iodide was second best.
RNH2: alkylamine

Much different for different amine molecules.
(Amine molecules: Phe, Ben, C4)

Single crystal growth
Diffusion of poor solvent Solution in good solvent
Poor solvent

CaCl2 pellets

Crystal growth bygradual decreasein solubility.

Max. size: 20×20 mm2 ×4mmt(Phe was used in organic layers.)

Luminescence spectra
Intensity [arb.units]

6000

Phe

5000

Free exciton line
+ defect band

4000
3000
2000
1000

Phe: Highest intensity

BenC40350

400

450

Wavelength [nm]500

Pulse height spectra for 67.4 keV X-ray
Significantly different light yield forcrystals with different amine.
13,000 photons/MeV for Phe crystal.

10000

C4Ben

Counts

1000

YAP

Phe
100

C4: butylamine,
Ben: benzylamine,
Phe: Phenetylamine

10

10200

400

600

Channel

800

1000

Scintillation decay (Phe)100000Decay time constant：7.5 nscounts

10000(Main component)

1000
100
101-50050100150200time (ns)Experimental data

Fitted curve

250

Scintillation decay

Intensity [arb.units]

C4PheBenPhebut < 10 ns

0102030

Time [ns]405060Gamma-ray detection
100000

57Co

GSO

10000

10000

137Cs

Phe

Phe

1000

countcount

1000

100

GSO

100

10101

1
0

100

200

channel

300

200

400

600

channelLight yield of Phe was 1.5 times of GSO.
→ 15,000 photons/MeV

800

Scintillation properties of Phe
Light yield: 12,000 photons/MeV (67.4 keV)
15,000 photons/MeV (>100 keV)
Comparable to rare-earth-doped crystals

Scintillation decay time constant: 10 ns
Much faster than rare-earth-doped crystals

Crystal structure
BrC4 IBrBrPbPb

BrBrBrBen IBrBrBrBrPb

PbPb

BrBrBr

BrBr

Phe Ⅰ

Ben Ⅱ Br

C4 : 155°
Ben : 150°
Phe : 151, 152°150.0°
90. 1° 90. 0°

150.0°
90. 1° 93. 7°

Pb

Pb–Br–Pb angle
Pb

Br

Br

Br

154.7°
89. 4° 90. 7°

154.6°
89. 1° 89. 4°

Pb

C4 Ⅱ

BrBr

Br

Br

152.0°
88. 6° 96. 2°

Phe ⅡBr

151.0°
83. 7° 88. 3°

Pb

PbPbPbBrBrBrBr

Larger distortion between
Octahedra in Phe and Ben.

Crystal structure
C4
180.0°

3.00

Ben

3.01

179.8°

Phe180.0°

2.98

2.98

Almost no distortionin octahedra.

180.0°

171.1° 3.20

2.88

172.7°

Pb ion is located off
the octahedra center.

Significant distortion inside octahedra in Phe.

Use of luminescence fromsemiconductor nanocrystals

CsPbCl3 nanocrystals in CsCl:Pb
single crystals

Luminescence spectraScintillation properties in CsCl:Pb
single crystals
CsCl:10%Pb(Pb aggregated)

CsCl:10%Pb(Pb isolated)

0.42 ns (27%)
2.0 ns (62%)

1.7 ns (86%)

18.3 ns (11%)

17.0 ns (14%)

Scintillation yield: < 500 photons/MeV

2. Incorporation of inorganicnanoparticles
into plastic scintillators

Expected scintillation mechanism
High-energy photons

Interaction withhigh-energy Photons;

Inorganic nanoparticles

secondary electrons
are emitted.
Plastic scintillator domain

Scintillation fromplastic scintillator
domainHigh-Z inorganic nanoparticles are incorporated
in order to obtain high interaction efficiency withhigh-energy photons.

Why nanoparticles?
Mie scattering- Scattering of light at the scatterer larger than
the wavelength.

The wavelength of scintillation: several hundreds nm

The nanoparticle size should be less thanseveral hundreds nm in order to achievetransparency for scintillation light.

Preparation method 1- two step synthesis
Preparation of nanoparticles

・Preparation by solution methods.
・Commercially available nanoparticles can also be used.
Preparation of organic-inorganic hybrid materials
Evaporation of solvents

Mixture oforganic solvents and waterSample plate
polymer
nanoparticle
Light-emitting molecule(DPO+POPOP or B-PBD)

Plastic scintillators loaded withcommercial ZrO2 nanoparticles

Photographs

1 wt%

10 wt%

20 wt%

30 wt%

Scintillation spectraPulse height spectra of 67.4 keV X-ray
10000

ZrO2 1 wt%
ZrO2 10 wt%
ZrO2 20 wt%
ZrO2 30 wt%
ZrO2(3 nm) 10 wt%

counts1000

100

10Detection efficiency was more than twice higher
than NE142 for 10~30 wt% materials.
120120

220

320

420

520

620

720

channelClear peak was observed for plastic scintillator
incorporated with nanoparticles of 3 nm size.

Time resolution curvePlastic scintillators loaded with
HfSiO2 nanoparticles
by one-pot method

Preparation method 2- one step synthesis
Nanoparticles formation Evaporation of solvents

Sample plate

Mixture oforganic solvents and waterpolymer
Precursors of nanoparticles
Light-emitting molecule

Hf-Si oxide nanoparticles were synthesizedin polymers by one step synthesis.

Photograph

TEM image

Nanoparticle size: less than 300 nm
250 nmDecay kinetics
Decay time constant: 2.5 ns

Pulse height spectra of 67.4 keV X-ray
Detection efficiency was
twice higher than NE142.
NE142

Plastic scintillators incorporated withHf oxide nanoparticles

Absorption probability

Plastic scintillators incorporated withnanoparticles – Future prospect

Element B

Element C

Element A

X-ray energy

Incorporated element can be chosen according
to the X-ray energy to be detected.

Further prospects for organic
scintillators loaded with nanoparticles
モリブデン酸化物ナノ粒子添加
液体シンチレータ

100Mo原子核

中性子

中性子

b線

ニュートリノレス
二重ベータ崩壊b線
b線
モリブデン酸化物ナノ粒子

二次電子

b線

液体シンチレータ
シンチレーションの生じる領域

Summary
1. Use of quantum confinement effects

Efficient scintillation due toquantum confined Wannier exciton was achieved.
Scintillation process should be revealed.
2. Incorporation of inorganic nanoparticles
into plastic scintillators
Enhancement in detection efficiency
has successfully been demonstrated.
Nanoparticle size and distribution is the key
to improve scintillation properties.

Future prospects
1. Use of quantum confinement effects

・Control over crystal structure → Higher light yield
・Growth of larger crystals.
・Search for crystals having semiconductor precipitates.
2. Incorporation of inorganic nanoparticles
into plastic scintillators
・Synthesis of smaller nanocrystals withgood dispersion in plastic scintillators.
・Synthesis of nanocrystals exhibiting fast scintillation.

Thank you for yourkind attention.