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
2SLAC AdvancedInstrumentation SeminarMarch 19, 2008
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
3SLAC AdvancedInstrumentation SeminarMarch 19, 2008
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
4SLAC AdvancedInstrumentation SeminarMarch 19, 2008
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%
5SLAC AdvancedInstrumentation SeminarMarch 19, 2008
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)
6SLAC AdvancedInstrumentation SeminarMarch 19, 2008
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)
7SLAC AdvancedInstrumentation SeminarMarch 19, 2008
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
8SLAC AdvancedInstrumentation SeminarMarch 19, 2008
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
9SLAC AdvancedInstrumentation SeminarMarch 19, 2008
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
10SLAC AdvancedInstrumentation SeminarMarch 19, 2008
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
11SLAC AdvancedInstrumentation SeminarMarch 19, 2008
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
12SLAC AdvancedInstrumentation SeminarMarch 19, 2008
• 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
OutlineOutline
13SLAC AdvancedInstrumentation SeminarMarch 19, 2008
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
14SLAC AdvancedInstrumentation SeminarMarch 19, 2008
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
15SLAC AdvancedInstrumentation SeminarMarch 19, 2008
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
16SLAC AdvancedInstrumentation SeminarMarch 19, 2008
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
17SLAC AdvancedInstrumentation SeminarMarch 19, 2008
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#
18SLAC AdvancedInstrumentation SeminarMarch 19, 2008
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
19SLAC AdvancedInstrumentation SeminarMarch 19, 2008
PhaseDiagram forLaCl3 and
KCl
PhaseDiagram forLaCl3 and
KCl
20SLAC AdvancedInstrumentation SeminarMarch 19, 2008
• 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
OutlineOutline
21SLAC AdvancedInstrumentation SeminarMarch 19, 2008
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)
22SLAC AdvancedInstrumentation SeminarMarch 19, 2008
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
23SLAC AdvancedInstrumentation SeminarMarch 19, 2008
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
24SLAC AdvancedInstrumentation SeminarMarch 19, 2008
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
25SLAC AdvancedInstrumentation SeminarMarch 19, 2008
Cs2NaYBr6:CeCs2NaYBr6:Ce
Gray:
Excited (Ce3+)*electron chargedensity
26SLAC AdvancedInstrumentation SeminarMarch 19, 2008
LaAlO3:CeExample of
a Non-Scintillator
LaAlO3:CeExample of
a Non-Scintillator
Ce - blue
La - green
Al - orange
O - red
27SLAC AdvancedInstrumentation SeminarMarch 19, 2008
Ba2YCl7:Ce - New ScintillatorDiscovered by these Calculations
Ba2YCl7:Ce - New ScintillatorDiscovered by these Calculations
28SLAC AdvancedInstrumentation SeminarMarch 19, 2008
• 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
OutlineOutline
29SLAC AdvancedInstrumentation SeminarMarch 19, 2008
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”
30SLAC AdvancedInstrumentation SeminarMarch 19, 2008
Robotic Dispensing Station(ChemSpeed Technologies)
Robotic Dispensing Station(ChemSpeed Technologies)
31SLAC AdvancedInstrumentation SeminarMarch 19, 2008
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
32SLAC AdvancedInstrumentation SeminarMarch 19, 2008
Furnace with Differential Thermal AnalysisFurnace with Differential Thermal Analysis
33SLAC AdvancedInstrumentation SeminarMarch 19, 2008
Hand Bar Coder and SamplesHand Bar Coder and Samples
34SLAC AdvancedInstrumentation SeminarMarch 19, 2008
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
35SLAC AdvancedInstrumentation SeminarMarch 19, 2008
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
36SLAC AdvancedInstrumentation SeminarMarch 19, 2008
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
37SLAC AdvancedInstrumentation SeminarMarch 19, 2008
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
38SLAC AdvancedInstrumentation SeminarMarch 19, 2008
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
39SLAC AdvancedInstrumentation SeminarMarch 19, 2008
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
40SLAC AdvancedInstrumentation SeminarMarch 19, 2008
Sample Changer on Pulsed X-Ray SystemSample Changer on Pulsed X-Ray System
41SLAC AdvancedInstrumentation SeminarMarch 19, 2008
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
42SLAC AdvancedInstrumentation SeminarMarch 19, 2008
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
43SLAC AdvancedInstrumentation SeminarMarch 19, 2008
How Does Non-ProportionalityAffect Resolution?
How Does Non-ProportionalityAffect Resolution?
Scintillator Crystal
IncidentGamma
Ray
Knock-OnElectron
Delta Ray
FluorescentX-Ray
AugerElectron
44SLAC AdvancedInstrumentation SeminarMarch 19, 2008
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.
45SLAC AdvancedInstrumentation SeminarMarch 19, 2008
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)
46SLAC AdvancedInstrumentation SeminarMarch 19, 2008
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
48SLAC AdvancedInstrumentation SeminarMarch 19, 2008
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
49SLAC AdvancedInstrumentation SeminarMarch 19, 2008
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