the quest for new radiation detector materials · pdf filethe quest for new radiation detector...

1SLAC AdvancedInstrumentation SeminarMarch 19, 2008

The Quest for New RadiationDetector Materials

The Quest for New RadiationDetector Materials

• The ideal semiconductor radiation detector

• The ideal scintillator radiation detector

• Limitations of available radiation detector materials

• Strategies for finding better detector materials

• Theoretical efforts at LBNL to predict new scintillators

• Experimental efforts at LBNL to find new scintillators

Stephen Derenzo

Lawrence Berkeley National Laboratory


The Ideal Semiconductor Radiation DetectorThe Ideal Semiconductor Radiation Detector

• Large crystals at low cost

• Good stopping power for gammas and neutrons

• Band gap 1.5 to 2 eV (maximum electron-holeproduction with low leakage current)

• Both electron and hole carriers have high mobility inan electric field (v = E!)

• Minimum concentration of impurities and nativedefects that trap carriers (long lifetime !)

• 662 keV => 130,000 e-h pairs

fwhm = 2.35 f / 400,000 < 0.1% f = Fano factor " 0.1


Limitations of Available SemiconductorDetector Materials

Limitations of Available SemiconductorDetector Materials

Si Ge† CZT* HgI2 PbI2 AlSb

Density (g/cc) 2.33 5.35 5.76 6.36 6.16 4.22

Atten. Length** (mm) 44.6 23.7 20.1 13.9 14.1 27.2

Photofraction** 0.0016 0.043 0.18 0.38 0.40 0.16

Band gap (eV) 1.12(I) 0.67(I) 1.7 2.1(D) 2.4(D) 1.6(I)

E(pair) (eV) 3.6 3.0 5.0 4.2 4.9 5.1

!(e–) 1400 40,000 1350 100 8 "400

!(h+) 480 40,000 120 4 2 "500

Fano factor "0.1 0.08 >0.2 ??

E(fwhm) 662 keV 0.2% "1% ??

†Ge must be cooled (LN) *Cd0.9Zn0.1Te **511 keV


The Ideal Scintillator Radiation DetectorThe Ideal Scintillator Radiation Detector

• Large crystals at low cost

• Good stopping power for gammas and neutrons

• Band gap 2.5-5 eV (maximum photon production at a useablewavelength)

• Efficient transport of carriers to luminescent centers (minimumtrapping on very slow or non-radiative centers)

• Photon production proportional to energy deposited

• Quenching temperature above room temperature

• Excellent optical transparency

• Fast response (ns to !s, depending on application)

• 662 keV => 60,000 e-h pairs => 30,000 photons => 20,000 electrons in photodetector

fwhm = 2.35 / 20,000 < 1.7%


Good Energy Resolution Requires High Luminosity,Proportional Response, and Optical Clarity

Good Energy Resolution Requires High Luminosity,Proportional Response, and Optical Clarity

From P. Dorenbos, “Light output and energy resolution of Ce3+ doped

scintillators,” Nucl Instr Meth, vol. A486, pp. 208-213, 2002.

0%

2%

4%

6%

8%

10%

12%

0 2,000 4,000 6,000 8,000 10,000 12,000 14,000

En

erg

y R

eso

luti

on

@ 6

62 k

eV

(fw

hm

)

Luminosity (photoelectrons / MeV)

BGOGSO

Lu3Al

5O

12:ScLSOBaF

2

YAlO3:Ce

CsI:Tl

K2LaCl

5:Ce

NaI:TlCaI

2:Eu

RbGd2Br

7:Ce

LaBr3:Ce

LaCl3:CeTheoretical Limit

(Counting Statistics)


Limitations of Available ScintillatorsLimitations of Available Scintillators

Desired properties

"photo/("photo+ " Compton) (.5 MeV)

Density

Photons per MeV

Energy resolution

Decay time (ns)

Photoelectrons/MeV/ns*

Cost per CC

BGO

0.43

7.1

8,200

13%

300

2.6

$10

NaI(Tl)

0.18

3.7

40,000

7%

230

18

low

BaF2

0.19

4.9

1800

10%

< 1

200

low

LSO

0.34

7.4

!20,000

11%

40

50

$100

BGO = Bi4Ge3O12 LSO = Lu2SiO5:Ce

Ideal

>0.43

>7

>100,000

<2%

< 1

>10,000

low

* Photoelectrons per ns = 0.1 (Photons) / (Decay time)

(Primarily for gamma ray detection)


BGO Compared to Recently DiscoveredCerium-Activated Scintillators

BGO Compared to Recently DiscoveredCerium-Activated Scintillators

BGO LSO LPS LuYAP LaBr3 LuI3

Luminosity (ph/MeV) 8,200 25,000 26,000 12,500 60,000 90,000

E(fwhm) (662 keV) 12% 10% 10% 8% 2.5% <8%

Decay Time (ns) 300 40 38 25, 200 25 30

Density (g/cc) 7.1 7.4 6.2 7.4 5.3 5.6

Atten. Length* (mm) 11 12 15 13 22 18

Photofraction* 43% 34% 31% 27% 14% 29%

Wavelength (nm) 480 420 385 390 370 470

Natural Radioactivity? No Yes Yes Yes No Yes

Hygroscopic? No No No No Yes Yes

* 511 keV


Scintillation MechanismsScintillation Mechanisms

Core band (full)

6p 5d

4f

Conduction band (empty)

Valence band (full)

Core band (full)

Core-valence

<1 ns weak

[BaF2 fast]

se l f-trapped

e x c i t o n

slow or weak

[CsI, BaF2 slow]

Tl+

Pb2+

slow

Bi3+

[BGO, NaI:Tl]

6s

5d

4f

Ce3+

!30 ns

[LSO, LaBr3]

Energy


105 Years ofInorganic

ScintillatorDiscovery

105 Years ofInorganic

ScintillatorDiscovery

ZnS(Ag)

CaWO4

NaI(Tl)

CdWO4

CsI(Tl)

CsF

CsI

LiI:Eu

Silicate glass:Ce

CaF2:Eu

CsI(Na)

BaF2 (slow)

BaF2 (fast)

Bi4Ge3O12

YAlO3:Ce

Gd2SiO5:Ce

(Y,Gd)2O3:Eu,Pr

CeF3

PbWO4

LuAlO3:Ce LuBO3:Ce

LuPO4:Ce

RbGd2Br7:Ce

Lu2SiO5:Ce

LaCl3:Ce, LaBr3:Ce

CeBr3

CdS:In, ZnO:Ga

1900 1920 1940 1960 1980 2000 2020

1900 1920 1940 1960 1980 2000 2020

LuI3, Lu2Si2O7:Ce

For a more complete list, seehttp:/scintillator.lbl.gov


Scintillation Mechanism in CodopedSemiconductors

Scintillation Mechanism in CodopedSemiconductors

• Direct-gap semiconductor host with Eg = >2.5 eV

• Prompt (<50 ps), efficient trapping of hot holes by dopant ions

• Fast (~1 ns) recombination with donor band electrons

Dopant hole trap

h!

Non-radiative hole trap

Valence band

Conduction band

Energy

Donor band


Direct-Gap Semiconductor ScintillatorDirect-Gap Semiconductor Scintillator

• Direct band gap 2.2 to 3.5 eV

• 5 to 7 eV per electron-hole pair

• Fundamental limit 200,000 photons/MeV

• Fundamental limit 1.4 % fwhm at 662 keV

• Shallow acceptor and donor dopants (near band-edge emission)

• Allowed radiative transition (" 1 ns decay time)

• Cerium decay time >20 ns decay time (< 5% in first ns)

Advantages: (1) ultra-fast decay time(2) maximum potential luminosity








OutlineOutline


Groups Exploring New Detector MaterialsGroups Exploring New Detector Materials

• Delft University, Netherlands

• LBNL/UCB

• LLNL

• Radiation Monitoring Devices, Inc.

• ORNL

• PNNL

After decades of neglect by DOE Office of Science, significantsupport has been recently provided by DOE Nonproliferation(NA22) and DHS Domestic Nuclear Detection Office


Paths to New Radiation DetectorsPaths to New Radiation Detectors

First principles theory;Informatics based on known materials

Candidate synthesisCrystalline powders; thin films

Screening measurements (band gap)

Thermal analysis for crystal growth

Semiconductors Scintillators

Produce crystals;Characterize as detectors

Industrial-scale production


Eu63

Zr40

Sr38

Y39

Na11

Li3

Stable Elements for Ce3+ Doped CandidatesStable Elements for Ce3+ Doped Candidates

Mn25

V23

Cr24

Fe26

Co27

Ni28

Cu29

Zn30

Ga31

Ge32

As33

Se34

Kr36

Nb41

Mo42

Ru44

Rh45

Pd46

Ag47

Cd48

In49

Sn50

Sb51

Te52

Xe54

Re75

Ta73

W74

Os76

Ir77

Pt78

Au79

Hg80

Tl81

Pb82

Bi83

Ar18

C6

N7

Ne10

He2

Be4

H1

Nd60

Pr59

Sm62

Tb65

Dy66

Ho67

Er68

Tm69

Yb70

Lu71

Toxic (Be, Se, Tl) and Radioactive elements (K, Rb, Lu) excluded

Ce58

Ba56

Hf72

Cs55

La57

Gd64

Al13

Si14

P15

B5

Br35

I53

S16

Cl17

F9

O8

Mg12

Sc21

Ti22

Ca20

K19

Rb37


Natural Radioactive IsotopesNatural Radioactive Isotopes

• K-40 0.012% 1.3 x109 yr (gamma 10%) 31 d/s/gram

• Rb-87 27.85% 5 x 1010yr (beta– 100%) 860 d/s/gram

• La-138 0.09% 1.05 x1011 yr (gamma 70%) 0.81 d/s/gram

• Lu-176 2.6% 4 x1010 yr (gamma 180%) 49 d/s/gram

• Hf-174 0.18% 2 x 1015 yr (alpha 100%) 0.000067 d/s/gram


511 keV Photofraction vs. Z511 keV Photofraction vs. Z

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

30 40 50 60 70 80 90 100

! =

"p

"p +"c

(511 keV)

La

Atomic number (Z)

Bi

Th

Gd

WLu

Ge#

#

#

##

#

#

!

Te#

Br#

Cd#


Traditional ExperimentalDetermination of Crystal Structure

Traditional ExperimentalDetermination of Crystal Structure

• Melting or thermal diffusion of integer ratios to attempt to make small

crystals (e.g. GeO2 and Bi2O3 in 3:2 ratio produces Bi4Ge3O12)

• X-ray beam ==> Laue diffraction pattern

• Solve for the periodic atomic coordinates

• Publish synthesis and structure in Acta Crystallographica

• Discard crystals

• 96,000 entries in the 2005 Inorganic Crystal Structure database (ICSD)

Thousands of heavy crystals are known but have not

been explored as nuclear detector materials


PhaseDiagram forLaCl3 and

KCl

PhaseDiagram forLaCl3 and

KCl








OutlineOutline


Factors for Good Cerium-Activated LuminosityFactors for Good Cerium-Activated Luminosity

• Energy gap (lower for higher photon yield)

• Electron-hole production (vs. phonons) per keV

• Ce 4f to valence band energy difference (lower forefficient hole trapping

• Ce 5d to must be below conduction band or excitedCe3+ is not stable(i.e. Ce 4f and 5d levels must be in the forbiddenband of the host crystal)


Scintillation MechanismsScintillation Mechanisms

Core band (full)

6p 5d

4f

Conduction band (empty)

Valence band (full)

Core band (full)

Core-valence

<1 ns weak

[BaF2 fast]

se l f-trapped

e x c i t o n

slow or weak

[CsI, BaF2 slow]

Tl+

Pb2+

slow

Bi3+

[BGO, NaI:Tl]

6s

5d

4f

Ce3+

!30 ns

[LSO, LaBr3]

Energy


Projected DOS plots for LaBr3:Ce (LDA, DFT) Projected DOS plots for LaBr3:Ce (LDA, DFT)

Ce3+

La3+

Br-

Ef

4f 5d

5d

4p

• Ground State

Calculation

• Valence band

maximum 4p Br

• Conduction band

minimum 5d La

• Ce 5d and La 5d

hybridize

• need better

characterization of

(Ce3+)* state

4s


Constrained LDA for (Ce3+)* state Constrained LDA for (Ce3+)* state

1. explicitly set the occupancy of Ce 4f states to zero

2. set the occupancy to one above the f states

3. plot the spatial distribution of the excited electron in gray

LaBr3:Ce LaI3:Ce

Ce - blue

La - yellow

Br, I - red

Gray:

Excited (Ce3+)* electroncharge density


Cs2NaYBr6:CeCs2NaYBr6:Ce

Gray:

Excited (Ce3+)*electron chargedensity


LaAlO3:CeExample of

a Non-Scintillator

LaAlO3:CeExample of

a Non-Scintillator

Ce - blue

La - green

Al - orange

O - red


Ba2YCl7:Ce - New ScintillatorDiscovered by these Calculations

Ba2YCl7:Ce - New ScintillatorDiscovered by these Calculations








OutlineOutline


LBNL Scintillator Discovery ProcessLBNL Scintillator Discovery Process

Synthesis design

Robotic dispenser

X-ray diffractometry

Bar code samples

X-ray luminescence spectra Pulsed x-ray time response

Luminosity, decay

times and fractions

Candidate selection

Furnace array

Spectral

components

Crystalline phase

identification

Database captures all synthesis and characterization data and flags “winners”


Robotic Dispensing Station(ChemSpeed Technologies)

Robotic Dispensing Station(ChemSpeed Technologies)


Array of 30 furnaces (1200˚C)Array of 30 furnaces (1200˚C)

5 racks of 6 furnaces each

Features• PID controllers• 4 different atmosphere(N2, Ar, H2/Ar, O2/Ar)• Differential temperaturemeasurements


Furnace with Differential Thermal AnalysisFurnace with Differential Thermal Analysis


Hand Bar Coder and SamplesHand Bar Coder and Samples


X-Ray Diffractometer and Sample ChangerX-Ray Diffractometer and Sample Changer

Mar, Inc.2D X-RayImager

X-Raygenerator

Bruker Nonius 591 rotating water-cooled anode, Max 50 kV, 100 mA

16 sample changer


X-RayDiffractionPattern for

YAlO3

X-RayDiffractionPattern for

YAlO3

2#

0

50

100

150

200

20 25 30 35 40 45 50 55 60

(a)

Intensity

2!

0

20

40

60

80

100

120

20 25 30 35 40 45 50 55 60

(b)

2!

Intensity

Powderdiffractiondatabase

Our data


X-Ray Luminescence Spectrometerand Sample Changer

X-Ray Luminescence Spectrometerand Sample Changer

Computer-controlled samplechanger with barcode reader

Spectrometer withorder-sortingfilters, twogratings, and CCDreadout

Automaticuploading of datato database


X-Ray-Excited Wavelength SpectrumX-Ray-Excited Wavelength Spectrum

0

5

10

15

20

200 300 400 500 600 700 800 900 1000

Rela

tive inte

nsi

ty

Wavelength (nm)

Y2SO

2:Tb


Pulsed X-Ray Time Response MeasurementsPulsed X-Ray Time Response Measurements

Sample

Fluorescent Emissions

X-ray Tube

Time to Analog Converter

Stop

Start

Data Acquisition Computer

Microchannel photomultiplier Tube

+ 30 kV

Ti-sapphire laser

Nd:YAG Pump laser

Doubler crystal

Photodiode

Pulse Height Anlyzer

Discriminator

Discriminator

Sample changer identical to that of the X-ray luminescence spectrometer


Two Ultra-Fast Semiconductor ScintillatorsTwo Ultra-Fast Semiconductor Scintillators

0

1000

2000

3000

4000

5000

6000

0 200 400 600 800 1000

Inte

nsity

Time (ps)

ZnO:Ga

CdS:In


Sample Changer on Pulsed X-Ray SystemSample Changer on Pulsed X-Ray System


ProgressProgress

• Almost all of the facility shown was constructedfrom June 2006 to June 2007

• Since then we have synthesized over 2000samples of over 500 compounds

• Of these, 7 are potentially bright new heavyscintillators and 6 others are worthy of further study

• Results were presented at SCINT2007 at WakeForest and 2007 IEEE NSS in Hawaii

• Possibly only 1 of 2 of these can be easily grown

• See http://scintillator.lbl.gov for a table of manypublished scintillation properties


The LBNL DNDO ProjectResearch Team

The LBNL DNDO ProjectResearch Team

Investigators

Stephen Derenzo, PIMarvin WeberMartin JanecekWilliam MosesAndrew Canning

Anurag ChaudhryRoss BuchkoLin-Wang WangNiels JensenEdith Bourret-Courchesne, Co-PIYetta Porter-ChapmanRamesh BoradeThomas Budinger

Supporting Staff

Martin BoswellQi PengJames PowellSteve HanrahanKatie Brennan

David WilsonChris Ramsey

Physics

Computation

Chemistry/Materials Sciences

Mechanical


How Does Non-ProportionalityAffect Resolution?

How Does Non-ProportionalityAffect Resolution?

Scintillator Crystal

IncidentGamma

Ray

Knock-OnElectron

Delta Ray

FluorescentX-Ray

AugerElectron


Light Output per keV canDepend on Electron Energy

Light Output per keV canDepend on Electron Energy

0.9

1.0

1.1

1.2

1.3

1.4

1 1 0 100 1000

NaI:Tl

CsI:Tl

CsI:Na

Re

lati

ve

Lig

ht

Ou

tpu

t

Electron Energy (keV)

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1 1 0 100 1000

CaF2:EuLSOYAPBGOGSOBaF2

Re

lati

ve

Lig

ht

Ou

tpu

tElectron Energy (keV)

Figure 8.8 From W. Mengesha, T. Taulbee, B. Rooney and J. Valentine,

“Light yield nonproportionality of CsI(Tl), CsI(Na), and YAP,” IEEE

Trans Nucl Sci, vol. 45, pp. 456-461, 1998.


Collimated 1 mCi Source

Scintillator onHybrid Photodiode

30% HPGeDetectors,10 cm away fromScintillator

SLYNCI — Scintillator Light YieldNon-proportionality Characterization Instrument

SLYNCI — Scintillator Light YieldNon-proportionality Characterization Instrument

Compton Coincidence Apparatus (LBNL/LLNL)(Measures Non-Proportionality in <1 Day)

Compton Coincidence Apparatus (LBNL/LLNL)(Measures Non-Proportionality in <1 Day)


ConclusionsConclusions

• There are thousands of known compounds that have never beenexplored as radiation detector materials

• There are many more compounds yet to be discovered• Some of these will provide substantial advances in detector

properties that are currently well below fundamental limits

• The prizes– New heavy-atom semiconductors that are easy to grow as large

crystals and have good electron transport– New heavy-atom scintillators that are easy to grow as large crystals

and have desirable combinations of elemental composition, responsetime, and energy resolution


What Crystals Can Exist?What Crystals Can Exist?

“One of the continuing scandals in the physical sciences isthat it remains in general impossible to predict thestructure of even the simplest crystalline solids from aknowledge of their chemical composition”

John Maddox, editor of Nature, 1988

This states a grand challenge whose solution couldreveal a vast number of new candidate materials


Why Are Crystals So Important?Why Are Crystals So Important?

• Semiconductor Charge Collection Detectors

– High carrier mobility ! - requires long scattering times

– Long carrier lifetime ! $ requires no trapping on defects

– Trapping length = !!E (want > 1 m)

• Scintillator Detectors

– Want trapping on luminescent center >> trapping on defectsExample:Ce-activated glass is a poor scintillator because carriers trapon defects before exciting the Ce.But if Ce are excited directly with UV, the fluorescence isefficient

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