NSS-MIC 2014, Seattle, USA

Barium-based bright scintillators as transparent ceramic

IEEE - NSS 2014, SEATTLE

M. Gascón1, R. Gaume2, T. Shoulders2, L. Boatner3, E. Bourret1, G. Bizarri1

1Lawrence Berkeley National Laboratory2University of Central Florida3Oak Ridge National Laboratory

NSS-MIC 2014, Seattle, USA

MotivationMotivation

Develop and optimized a ceramic production process for halide compounds while maintaining single crystal performance

Why Ceramics?

● Flexible processing (large size)● Low cost & reproducibility

● Ba-based halides are abundant, ● inexpensive suitable for mass production● Barium has a single site for Eu doping● BaBrI, BaBrCl are less hygroscopic than SrI2(Eu) & LaBr3(Ce)

Project goals

Why Halides?

G. Gundiah et al. Journal of Luminescence 138 (2013) 143–149

Performance Oxides Halides

Single Crystals Fair to poor Good to excellent

Ceramics remains degrades

Why Barium–based halides?

NSS-MIC 2014, Seattle, USA

Halide-ceramics (challenges and possible solutions)Halide-ceramics (challenges and possible solutions)

Issues Consequences Impact on scintillation Possible solutions

A Highly reactive compoundsPoor starting materials

Impurities Light quenching PurificationIn-house powder synthesis

B Hygroscopicity H2O contamination Performance degradation Ultra dry processing

C Oxygen contamination O2 related defects Light absorption Defect engineering

D Inhomogeneous dopant distribution

Non-uniformity in L.O.

ER degradation Ceramics

Ceramics specifics

E Grain boundaries Traps/Defects Light scattering and absorption

Defect engineering

F Non-cubic lattice Anisotropic light scattering

Lower transparencyLight absorption

Structure stabilization

→ Using cubic or non-cubic materials with almost isotropic optical properties→ high purity precursors→ high density (sintering)→ reduce number of pores → pores smaller than light wavelength→ grain size below visible light wavelength → more uniform grain size→ avoid contamination→ annealing

How to achieve a high optical transparent ceramic?

NSS-MIC 2014, Seattle, USA

Ceramic Fabrication: toolsCeramic Fabrication: tools

Hot Isostatic Press (HIP)

Uniaxial Hot press

Casting Spark plasma sintering

Vacuum furnace Spray-drier

Filtering

NSS-MIC 2014, Seattle, USA

A) Starting Materials: Impurities identificationA) Starting Materials: Impurities identification

Raman Microscopy

Carbon contamination was found on commercial

powder

in-house powder synthesis ● BaCO3 + HCl = BaCl2 + CO2 + H2O● Remove all organic contaminants

BaCl2

Solution

NSS-MIC 2014, Seattle, USA

A) Starting Materials: PurificationA) Starting Materials: Purification

Steps toward transparency

Sintering: impact of starting material

Spray-drier

• Nearly 100% density for Sigma anhydrous powder, lower density for Alfa and dy-hydrate powders

o Large particle size for Alfa (platelets shapes) → low sintering ability

o Full density for 2h de-hydration (300C)+ 10h (750C, 40 MPa)

Fra

ctio

n al d

e ns i

ty

NSS-MIC 2014, Seattle, USA

B) Hygroscopicity: Ultra-dry processing B) Hygroscopicity: Ultra-dry processing

7”

Power to heating

coil Thermocouple

Vacuum/Gas

Inside view of the pressing chamber

● Pressure up to 100MPa (uniaxial)● Temperature up to 600 C● Vacuum level down to 10-3 torr● Stainless steel die● Glovebox-compatible air-tight canister

Low T, High P Hot-Pressing System

9”

Overall view of the system

NSS-MIC 2014, Seattle, USA

B) Hygroscopicity: Ultra-dry processing B) Hygroscopicity: Ultra-dry processing

450C, 87 MPa, 2h0.4 mm thick

280C, 123 MPa, 30 minNaCl buffer

1.7 mm thick

Undoped BaCl2 - spray dried powder

300C, 123 MPa, 30 minNaCl buffer

1.7 mm thick Eu:BaCl2 - spray dried

powder

NSS-MIC 2014, Seattle, USA

C) Oxygen contaminationC) Oxygen contamination

Thermally activated

non-radiative pathway

Fair performance at low temperature, poor performance at room temperature

Optical emission X-ray luminescence

Optical signature of oxygen contamination

NSS-MIC 2014, Seattle, USA

Light re-absorption mechanism

C) Oxygen contaminationC) Oxygen contamination

• Burn off organic contaminantso No carbon contamination

o Less colored samples

o Good optical response, no Eu oxidation

O2 treatment

Oxygen center energy

trapping

Poor quantum efficiency

LatticeEuropium

Oxygen center

• Two type of emitting centers

o Divalent Eu

o oxygen related center

Interaction between both leads to performance degradation

radiative quenching non-radiative quenching

Tube Furnace

NSS-MIC 2014, Seattle, USA

D) Dopant DistributionD) Dopant Distribution

Time resolved Microscopy

LaBaCl2

Energy resolved Microscopy

Setup: UV confocal Microscope320 ns

355 ns

BaCl2(5%Eu) ceramics Decay time mapping

Non uniformity in L.O.

Emission intensity mappingBaCl2(5%Eu) ceramics

→ Decay time and Emission shows significant variation in the light output

NSS-MIC 2014, Seattle, USA

E) Grain BoundariesE) Grain Boundaries

Unknown signature BaCl2 signature

BaCl2 signature

Raw materials

Raman is a very useful tool to

investigate the ceramic formation

dynamics

Are defects/impurities segregated at grain boundaries?

Grain BoundaryLaBaCl

2

BaCl2 signature

NSS-MIC 2014, Seattle, USA

Thermoluminescence of BaCl2 vs BaCl2(5%Eu)

Optically stimulated Luminescence

E) Traps/Defects + trapping mechanismsE) Traps/Defects + trapping mechanisms

Europium incorporation

does not change the

properties of shallow traps

NSS-MIC 2014, Seattle, USA

F) Non-cubic vs cubic structureF) Non-cubic vs cubic structure

Str

uct

ure

XR

D

[1] Edgar et al. Journal of Applied Physics 107. Vol 8. Apr 2010. pp 083516-083516-7

Transparency vs structure

Transmission improves ...

→ when the cubic phase is achievedOrthorhombic

Cubic

BaCl2

LaBaCl2

→ BaCl2 presents a cubic phase between 962 and 925C.

→ Trivalent lanthanum substitution on a divalent barium site

leads to cubic form [1].

FILMETRICS FT-10

NSS-MIC 2014, Seattle, USA

F) Non-cubic vs cubic - AnnealingF) Non-cubic vs cubic - Annealing

Transparency vs annealing

Transmission improves ...

Is the transparency the limiting

Factor?

→ after certain annealing steps

Ann

ealin

g

Ceramics made by casting

NSS-MIC 2014, Seattle, USA

In summaryIn summaryLight Output & Energy Resolution

Conclusions

Ceramic Size (mm3)

E.R. (%)

LO (Ph/MeV)

LaBaCl2(Eu) 450 13.6 11,000

CeBaCl2(Eu) 450 13.8 5,000

CeBaCl2

450 15.3 5,000

Single Crystals

Crystal structure

E.R (%)

LO (Ph/MeV)

NaI(Tl) cubic 6.7 44,000

BaCl2(Eu) orthorh. 3.6 50,000

BaBrCl(Eu) orthorh. 3.6 50,000

Single Crystals as REFERENCES

THIS WORK

→ A high degree of transparency has been achieved→ LO and ER are still far from single crystals but... → Cleaner starting materials will certainly make more transparent ceramics and better energy resolution is expected

NSS-MIC 2014, Seattle, USA

Final slideFinal slide

Future work

● Transparent and cubic halide ceramics out of Ba-base compounds have been fabricated.● Transparency has been improve as the cubic phase has been achieve and also after

certain annealing temperatures have been applied. ● Microscopy measurements has shown the segregation of contaminants to the grain

boundaries limiting the scintillation performance.

● Explore other Ba-base compounds● Optimize temperature and pressure conditions to achieve more transparent ceramics● Purification of the raw materials to minimize contaminants

● Achieve uniformity of the melt to minimize the dopant segregation to the grain boundaries.

Conclusions