Performance optimization of large diameter SrI(Eu) detector assemblies

Performance optimization of large diameter
SrI2(Eu) detector assemblies
(manufacturing notes)
Ivan Khodyuk, Stacy Swider, Amlan Datta, Maria Hackett, Stephanie Lam, and
Shariar Motakef
CapeSym, Inc., MA, USA
Presented at SPIE Optics + Photonics: Hard X-Ray, Gamma-Ray, and Neutron Detector Physics XX
August 20, 2018 at the San Diego Convention Center

MEETING YOUR NEEDS
CapeSym partners with its customers to match the specifications
and form factors required for each sensor module.
CapeSym company overview
CapeSym, Inc. SPIE 2018: Optics + Photonics 2
TEAM
CapeSym’s R&D team includes physicists,
chemists, electrical engineers, mechanical
engineers, and software engineers.
BACKGROUND
Founded in 1992 as Cape Simulations, CapeSym, Inc. is now a multi-faceted company offering novel technical crystals for
nuclear detection, including ScintiClear™ and elpasolite scintillators, and TlBr and CdZnTe semiconductors.
FACILITY
Our 12,000 sq. ft. facility in Natick, MA, USA

Outline and definitions
1- Overview of SrI2(Eu) properties
2- Scintillation detectors design and optimization
a) General considerations
b) Read-out selection
c) Crystal optimization
d) Encapsulation
e) Signal acquisition
Performance – energy resolution
Large diameter – 38.1 mm and larger
SrI2(Eu) – high performance scintillator material
ScintiClear™ – SrI2(Eu)-based scintillator manufactured by CapeSym using proprietary crystal
growth and purification methods

Light Yield 80,000 ph/MeV
Energy Resolution
@ 662keV
~3%
Decay Time 1-3 µs
Emission Range 400-480 nm
Max Emission 430 nm
Density 4.59 g/cm3
Zeffective 50
Intrinsic activity <0.05 Bq/cm3
Moisture
Sensitivity
Hygroscopic
(similar to NaI(Tl))
Refractive Index 1.85
Thermal shock up to 10 ˚C/min
SrI2(Eu) was originally proposed as a radiation detector by
R. Hofstadter in 1968.
In 2008 scientists from Lawrence Livermore National Laboratory (USA)
spearheaded the development of SrI2(Eu) into a leading edge radiation detector.
SrI2(Eu) high performance scintillator
Linear attenuation coefficient, 1/cm

• Sr has naturally stable isotopic
composition.
• Intrinsic activity of SrI2(Eu) is up to forty
times smaller compare to LaBr3(Ce).
• Sr is the 15th most abundant element on
Earth.
• Although Strontium Iodide salt becomes
corrosive when exposed to air, Strontium
is fundamentally non toxic.
SrI2(Eu) has low to no internal activity
Internal activity spectrum of a standard 1.5” SrI2(Eu) in comparison with LaBr3(Ce), as measured inside a Pb castle with 2 inch thick walls.

High energy resolution in the entire energy range
Energy resolution can be as good as 2.8% at 662keV and about 2.2% at 1332keV.

Unambiguous Identification
Clear detection of both WGPu (414 keV) and HEU (186 keV) and
separation between 214Bi and 137Cs (609 and 662 keV).
High energy resolution in the MeV range, and no internal activity.

How to achieve high scintillation performance?
High performance crystal

Matching readout

Matching readout
Appropriate encapsulation

Matching readout
Optimized electronics

Matching readout
High performance radiation spectrometer
+
+
+
=

Matching readout
High performance radiation spectrometer
+
+
+
=
Start design
here

Readout selection - PMTs
Spectral sensitivity and Quantum efficiency Size and form-factor
Ø2” Ø3” Ø5”
Other shapes are available:

Readout selection – PMTs (sensitivity and QE)
0
10000
20000
30000
40000
SrI2_Eu(0,1mol%)_Exc=290nm_T=300K
SrI2(Eu)emission
BA
SBA
UBA
EGBA
BA – bialkali
SBA – super-bialkali
UBA – ultra-bialkali
EGBA – extended green bialkali

Readout selection –PMTs (sensitivity and QE)
Photochathode
Material
Curve code Peak QE,
nm
Typical ER,
% at 662keV
BA 400K 420 4%
SBA 440K 380 3%
UBA 441K 400 3%
EGBA 444K 420 3%
SrI2(Eu) emission max 430nm
High quantum efficiency is more
important than spectral match! 0
10000
20000
30000
40000
SrI2(Eu)emission
BA
SBA
UBA
EGBA

Readout selection – PMTs (size and form)
d46mm d70mm d111mm
Ø PMT ≠ Ø Photocathode

Readout selection – PMTs (photocathode)
6
255.54
0.95
KEY Channel No. (proportional to LY)
Relative Light Yield (normalized to response at position 3)Location #
0 1 2 3 4 5
0
1
2
3
4
5
Y(cm)
X (cm)
244.4
249.3
254.2
259.1
264.0
268.9
273.8
278.7
283.6
Channel No.

Readout selection – PMTs (photocathode)
6
255.54
0.95
KEY Channel No. (proportional to LY)
Relative Light Yield (normalized to response at position 3)Location #
0 1 2 3 4 5
0
1
2
3
4
5
Y(cm)
X (cm)
244.4
249.3
254.2
259.1
264.0
268.9
273.8
278.7
283.6
Channel No.
16%

Readout selection – SiPMs
J-series C-series
300 350 400 450 500 550 600
0
10000
20000
30000
40000
300 350 400 450 500 550 600
0
10000
20000
30000
40000
50000
emissionspectra
50%
40%
Important factors:
1) Photon detection efficiency; 2) Active area vs total area ratio

Standard readout limitations
• PMTs – SBA
• SiPMs
ArrayJ-60035-64P-PCB
Very promising, but:
1) Expensive (economy of scale applies)
2) Pad to pad variations
3) Performance deterioration above 40 degC
4) Square + dead space in between
Very reliable, but:
1) Economy of scale doesn’t apply
2) Performance variation
3) Bulky and fragile
4) Limited choice
5) Sensitive to magnetic field

Crystal size vs scintillation performance
Let’s compare performance of Ø38.1mm vs Ø46mm, and Ø50.8mm ScintiClear
crystals on 2” (R6231-100) and 3” (R6233-100) SBA PMTs.
ScintiClear diameter,
mm
R6231-100 – 2” PMT
ER at 662keV, %
R6233-100 – 3” PMT
ER at 662keV, %
38.1 2.9% 3.0%
46.0 2.9% 3.1%
50.8 4.3% 3.1%
Insensitive area on the photocathode perimeter is detrimental to ScintiClear scintillation performance.
Photocathode sensitive area must be ≥ than crystal front face.

Surface treatment
Optically polished surface is important for optimal performance
Rough
After shaping
Additional
polishing

SrI2(Eu) crystals anisotropy
0 500 1000
0.0
0.5
1.0
1.5
Normcounts
MCA channel
Æ38.1 x 38.1 mm3
SrI2(Eu)
Side A
Side B
Canberra 2005 preamplifier, and 2025 AFT research amplifier (coarse gain 20, shaping mode 12us) were used to shape
the signal and send it to Pocket MCA 8000A digitizer from AmpTek. Data analyzed in OriginPro2018
“Side A” “Side B”
A
B A
B
Side B has high
energy shoulder!
662keV
137Cs
137Cs

Canberra 2005 preamplifier, and 2025 AFT research amplifier (coarse gain 20, shaping mode 12us) were used to shape
the signal and send it to Pocket MCA 8000A digitizer from AmpTek. Data analyzed in OriginPro2018
A
B A
B
“Side A”
850 900 950 1000 1050
0.0
5.0x10-1
1.0x100
SubtractedData
Baseline X
Peak Analysis
Fitting Results
BaseLine:Exponential
Adj. R-Square=9.87820E-001 # of Data Points=163
Degrees of Freedom=160SS=1.75327E-001
Chi^2=1.09580E-003
Date:8/14/2018Data Set:[Book1]Sheet1!C
Peak Index Peak Type Area Intg FWHM Max Height Center Grvty Area IntgP
1 Gaussian 31.40339 31.15829 0.94683 940.80504 100
600 620 640 660 680 700 720
0.0
5.0x10-1
1.0x100
SubtractedData
Baseline X
Peak Analysis
Fitting Results
Chi^2=1.57355E-004
Date:8/14/2018Data Set:[Book1]Sheet1!B
1 Gaussian 20.11298 19.38761 0.97459 661.96755 100
“Side B”
Gaussian fit
2.9%
Same 2.9%
but with
shoulder
“Side A” “Side B”
137Cs
137Cs

“Side A”
850 900 950 1000 1050
0.0
5.0x10-1
1.0x100
SubtractedData
Baseline X
Peak Analysis
Fitting Results
Chi^2=1.09580E-003
Date:8/14/2018Data Set:[Book1]Sheet1!C
1 Gaussian 31.40339 31.15829 0.94683 940.80504 100
600 620 640 660 680 700 720
0.0
5.0x10-1
1.0x100
SubtractedData
Baseline X
Peak Analysis
Fitting Results
Chi^2=1.57355E-004
Date:8/14/2018Data Set:[Book1]Sheet1!B
1 Gaussian 20.11298 19.38761 0.97459 661.96755 100
“Side B”
High energy shoulder is caused by impurities or crystal
inhomogeneities but not necessarily self absorption.
Gaussian fit
2.9%
Same 2.9%
but with
shoulder
0 20 40 60 80 100 120 140
0
20
40
60
80
100
Absorption(%)
Thickness (mm)
at 662keV
at 1332keV
Gamma attenuation is anisotropic by definition
ScintiClear™ is a new high-performance SrI2(Eu)-based scintillator manufactured using proprietary purification and
crystal growth process that improves its inherently excellent performance.

Self-absorption?
137Cs
101.6mm
50.8mm
Length of the crystal versus performance

Self-absorption?
137Cs
101.6mm
50.8mm
Step 1 – Measurement 101.6 mm long crystal
0 500 1000 1500 2000
-200
0
200
400
600
800
1000
1200
1400
1600
Counts
MCA channel
ScintiClear
ER = 3.2%
Peak max = 1034
Canberra 2005 preamplifier, and 2025 AFT research amplifier (coarse gain 20, shaping mode 12us) were used to
shape the signal and send it to Pocket MCA 8000A digitizer from AmpTek. Data analyzed in OriginPro2018

Self-absorption?
137Cs
76.2mm
50.8mm
Step 2 – Cut the crystal
Line of cut
25mm

Self-absorption?
137Cs
76.2mm
50.8mm
Step 3 – Measurement 76.2 mm long crystal
0 500 1000 1500 2000
-200
0
200
400
600
800
1000
1200
1400
1600
Counts
MCA channel
ScintiClear
ER = 3.2%
Peak max = 1245
Canberra 2005 preamplifier, and 2025 AFT research amplifier (coarse gain 20, shaping mode 12us) were used to
shape the signal and send it to Pocket MCA 8000A digitizer from AmpTek. Data analyzed in OriginPro2018
0 500 1000 1500 2000
0
2000
4000
6000
8000
Counts
MCA channel
ScintiClear
and so on…

Self-absorption?
137Cs Length of the crystal versus performance
100 80 60 40 20
3.00
3.05
3.10
3.15
3.20
Energyresolution,%
Crystal length, mm
ScintiClear
100 80 60 40 20
1000
1050
1100
1150
1200
1250
1300
Peakposition
Crystal length, mm
ScintiClear
Performance is governed by the volume where gamma radiation is absorbed.
Self-absorption doesn’t influence energy resolution.

Effects of encapsulation – Ø25.4mm+
Larger size crystals Ø25.4mm and up – minor deterioration
-14% Light Output
Comparable resolution
0 500 1000
0.0
0.5
1.0
Normalizedcounts
MCA channel
ScintiClear Æ51x102mm3
Bare
Packed
Peak = 1002
ER = 3.2%
662keV
Peak = 860
ER = 3.2%

Effects of encapsulation – smaller crystals
0 500 1000 1500
0.00
0.05
0.10
0.15
0.20
0.25
0.30
Peak = 1294
ER = 3.1%
Normalizedcounts
MCA channel
ScintiClear 12.7mm cube
Bare
Packed
Peak = 1507
ER = 2.9%
Smaller size crystals are more challenging to pack.
We always observe performance deterioration for
packed crystals.
Solution : direct coupling to a readout

Detector encapsulation
ScintiClear™ RIID detector
cores comply with ANSI
N42.34 environmental
standards for temperature
and thermal shock.

ScintiClear benefits from long shaping/integration time
Standard Spectroscopic Scintillation electronics:
1) Canberra 2005 preamplifier
2) Ortec 672 - 10μs /Canberra 2025 - 12μs
3) MCA
Shaping and integration time - analog
No special signal processing is required
to achieve high energy resolution

Insufficient integration time = poor ER
Decay time of ScintiClear crystals is size and
shape dependent, and ranges from 1 to 3μs on
average.
Optimal ER achieved with ~15-25μs
digital integration time.
ScintiClear benefits from long shaping/integration time
Time, us
Signal,mV
Shaping and integration time - digital
Other digital DAQ systems:

Developer’s kit – performance guaranteed
Key features:
• Integrated PMT and DAQ
• USB-powered and controlled
• Web-browser base GUI
• Open source API in Python and C++
• Energy resolution <3.3% at 662keV
• -30˚C to +55˚C operation temperature
• Digital pulse-shape acquisition capabilities
ScintiClear kit – plug and measure
Benefits:
• Ready to use in 30 minutes
• Parameters optimized for ScintiClear
• OEM ready

Thank you for your attention.
SPIE 2018: Optics + Photonics

Performance optimization of large diameter SrI(Eu) detector assemblies

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