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Overview of Strontium Iodide Scintillator Materials
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Lawrence Livermore National Laboratory
PI: Nerine Cherepy (LLNL)
Co-Investigators: L Boatner (ORNL), A Burger (Fisk), K Shah (RMD)
PM: Steve Payne (LLNL)
DNDO PMs: Alan Janos, Austin KuhnLawrence Livermore National Laboratory, P. O. Box 808, Livermore, CA 94551
This work performed under the auspices of the U.S. Department of Energy by
Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344
Overview of Strontium Iodide Scintillator MaterialsApril 1, 2010
Funded by
DHS/DNDO
LLNL-PRES- 426327
2
Lawrence Livermore National Laboratory
Official Use Only
Official Use Only Official Use Only CFP06-TA01-LL01 Cherepy
Title: High Resolution Scintillator Materials and Detectors
Disclaimer: The GFM is offered to the chosen vendors as an option.
The data and analyses presented in this document represents a best
effort of the contractors (LLNL, RMD, Fisk and ORNL), the accuracy
of which is not expressly or implicitly guaranteed by the
Department of Homeland Security. Any suggested procedures are
suggestions only and are not guaranteed by the government.
3
Lawrence Livermore National Laboratory
Official Use Only
Official Use Only Official Use Only CFP06-TA01-LL01 Cherepy
Title: High Resolution Scintillator Materials and Detectors
We explored the Alkaline Earth Halide scintillators and identified
SrI2(Eu) as the best candidate
b-excited emission spectra
LY
(Photons/MeV)
Resolution
(662 keV)
SrI2 undoped <60,000 6.7%
SrI2(Eu) 90,000 2.6%
SrBr2(Eu) 25,000 7%
BaI2(Eu) 40,000 8%
CaI2(Eu) 110,000 ---
LaBr3(Ce) 60,000 2.6%
SrI2(Eu) offers excellent light yield
and proportionality
SrI2(Eu) is more proportional than LaBr3(Ce)
1.15
1.10
1.05
1.00
0.95
0.90
Re
lative
Lig
ht Y
ield
5 6 7
102 3 4 5 6 7
1002 3 4 5 6 7
1000Electron Energy (keV)
RMD SrI2(0.5%Eu)
ORNL SrI2(4%Eu)
ORNL SrI2(6%Eu)
NaI(Tl)
LaBr3(Ce)
4
Lawrence Livermore National Laboratory
Official Use Only
Official Use Only Official Use Only CFP06-TA01-LL01 Cherepy
Title: High Resolution Scintillator Materials and Detectors
SrI2(Eu) should match LaBr3(Ce) performance with PMT readout
Property LaBr3(Ce) SrI2(Eu) Comparison
Melting Point 783 ºC 538 ºC Less thermal stress
Handling Easily cleaves Resists cracking Better processing
Light Yield 60,000 Ph/MeV 90,000 Ph/MeV Higher
Proportionality contribution ~2.0% ~2.0% Favorable
Inhomogeniety 0% >1% (current) Impurities and surfaces being
addressed
Decay time 30 nsec 0.5-1.5 msec Fast enough to avoid
deleterious signal pile-up
Radioactivity La ~ intrinsic bckgd None Less noise
Hygroscopic / air sensitive? Very Very Similar
absorption (2x3”, 662 keV) 22% 24% Similar
Quantity LaBr3(Ce) SrI2(Eu)
PMT Efficiency 35% 35%
Inhomogeneity 0% 0%
Resolution (total) 2.5% 2.3%
reflector loss = 0.5%/bounce; material loss = 0.2%/cm; noiseless PMT; 2x3” ; 662 keV
Predicted resolution with optimized readout and crystal quality
5
Lawrence Livermore National Laboratory
Official Use Only
Official Use Only Official Use Only CFP06-TA01-LL01 Cherepy
Title: High Resolution Scintillator Materials and Detectors
For gamma ray spectroscopy, SrI2(Eu) can meet or exceed LaBr3(Ce)
300
250
200
150
100
50
0
Co
un
ts
16015014013012011010090
Energy (keV)
5.24%
6.81%
LaBr3(Ce)
SrI2(Eu)Co-57
10
2
4
100
2
4
1000
2
4
Co
un
ts
40035030025020015010050
Energy (keV)
NaI(Tl) LaBr3(Ce)
SrI2(Eu)
Ba-133
400
300
200
100
0
Co
un
ts
800700600500400300200
Energy (keV)
NaI(Tl) LaBr3(Ce)
SrI2(Eu)
Cs-137Am-241
-spectrum
Co-57
-spectrum
Ba-133 -spectrum
Cs-137
-spectrum
16
12
8
4E
ne
rgy R
esolu
tion
(%
)
100 1000
Gamma Energy (KeV)
LaBr3(Ce)
SrI2(Eu)
NaI(Tl)
Scintillator Photopeak Efficiency
(662keV, 5x5x7.5 cm3)
LY
(Ph/MeV)
Resolution
(662 keV)
NaI(Tl) 18% 40,000 ~6.5%
LaBr3(Ce) 20% 60,000 ≤3%
SrI2(Eu) 22% 90,000 ≤3%
6
Lawrence Livermore National Laboratory
Official Use Only
Official Use Only Official Use Only CFP06-TA01-LL01 Cherepy
Title: High Resolution Scintillator Materials and Detectors
We have been acquiring thermal data for feedstock and crystal
growth optimization
Expansion coefficients indicate cracking
due to anisotropy not a problem
Dehydration of feedstock complete by 350ºC
-50
-40
-30
-20
-10
0
He
at flo
w (
uW
)
600500400300200100
Temperature (°C)
SrI2 EuI2
Hydrate desorption
Melting
The crystal structures of SrI2 and EuI2are both orthorhombic and exhibit
very similar lattice parameters
7
Lawrence Livermore National Laboratory
Official Use Only
Official Use Only Official Use Only CFP06-TA01-LL01 Cherepy
Title: High Resolution Scintillator Materials and Detectors
Distribution coefficient of Eu in SrI2 is approximately 1.0
• Due to well-matched lattice constant and thermal
properties between SrI2 and EuI2 there is no
observable segregation effect
• Strontium iodide crystals are growable with high
Eu doping and uniformity
Ionic Radii:
Sr = 1.40 Å
Eu = 1.41 Å
Melting Points:
SrI2 = 538ºC
EuI2 = 580ºC
Density of SrI2 = 4.55 g/cm3
8
Lawrence Livermore National Laboratory
Official Use Only
Official Use Only Official Use Only CFP06-TA01-LL01 Cherepy
Title: High Resolution Scintillator Materials and Detectors
Crystals can be handled in a variety of ways
1.5 in
(1) Boule in ampoule (2) Boule vacuum packed in plastic
(3) Best domain harvested, cut
and polished then vacuum
packed in plastic
(4) Cut and
polished crystal
in “openable”
hermetic
enclosure
Top view Side view
1.75 in
9
Lawrence Livermore National Laboratory
Official Use Only
Official Use Only Official Use Only CFP06-TA01-LL01 Cherepy
Title: High Resolution Scintillator Materials and Detectors
…Difficult to avoid some level of
light-trapping in SrI2(Eu)
“Light-trapping” occurs in Eu2+ doped scintillators
Successive emissions, followed by re-absorption then re-emission (etc.), causes
effective lengthening of decay- no problem unless accompanied by a loss mechanism
Eu2+
CB
VB
freabsorbed = 80%
10
Lawrence Livermore National Laboratory
Official Use Only
Official Use Only Official Use Only CFP06-TA01-LL01 Cherepy
Title: High Resolution Scintillator Materials and Detectors
Inch-scale crystals directly coupled to PMT exhibit inhomogeneous
lineshape due to light-trapping
• Collimation experiment reveals potential of each crystal to achieve
high resolution
• Light trapping alone readily correctable via digital readout
• Light trapping in combination with surface absorption will result in
poor performance ― surface finish is crucial
Analog pulse height spectrum using Cs-137 of
unencapsulated crystal reveals some tailing to
high energy of the photopeak at 662 keV
Analog pulse height spectrum acquired with
collimated Cs-137 source reveals
inhomogeneity due to light-trapping
11
Lawrence Livermore National Laboratory
Official Use Only
Official Use Only Official Use Only CFP06-TA01-LL01 Cherepy
Title: High Resolution Scintillator Materials and Detectors
Digital readout may be employed to improve energy resolution
• Inverse correlation between decay time and pulse height, Cs-137 source
• Events may be corrected based on pulse shape, and energy histogram
made more accurate
12
Lawrence Livermore National Laboratory
Official Use Only
Official Use Only Official Use Only CFP06-TA01-LL01 Cherepy
Title: High Resolution Scintillator Materials and Detectors
Optics of encapsulated crystals impact light trapping
• Collimation study with Cs-137 source indicates encapsulated crystal has
more uniform light-trapping than crystal directly on PMT window
• Likely due to presence of intervening window, resulting in more
homogeneity in average ray pathlength and angle
13
Lawrence Livermore National Laboratory
Official Use Only
Official Use Only Official Use Only CFP06-TA01-LL01 Cherepy
Title: High Resolution Scintillator Materials and Detectors
Second-generation hermetically-sealed scintillator package has been developed
Assembling Strontium Iodide Detector Canister
Step 1: Saw cut the sample to length using a .008” diameter diamond wire
and mineral oil
Step 2: Grind the sample into a cone on a Strasbaugh hand grinding
spindle and a 15 micron diamond plate and mineral oil
Step 3: Polish the sample using the same Strasbaugh machine and a
Buehler Texmet lap, 3 micron polycrystalline diamond and mineral oil
Assembling the package
Step 1: Epoxy window into recess of top flange. Epoxy tube to bottom
flange
Step 2: “Tack” sample to inside of window using Norland UV cured optical
adhesive and Norland Opticure light gun
Step 3: Wrap sample with Teflon tape
Step 4: Insert “O” ring into top flange
Step 5: Bolt top and bottom flange together using supplied anodized bolts
Step 6: Place white reflective disc on top of Teflon wrapped sample
Step7: Fill tube with Avian Technologies processed barium sulfate powder
(predried in oven). Include a teaspoon of desiccant powder
Step 8: Epoxy lid to end of tube
Step 9: Set canister under UV lamp to cure Norland UV cement
New package screws together, minimizes
metal and window thicknesses
OLD
NEW
14
Lawrence Livermore National Laboratory
Official Use Only
Official Use Only Official Use Only CFP06-TA01-LL01 Cherepy
Title: High Resolution Scintillator Materials and Detectors
Crystal encapsulation design being optimized for light coupling and seal
against environment
• We have developed encapsulation methods that provide stable
performance
• Conventional approaches for Sodium Iodide encapsulation appear to be
directly adaptable to Strontium Iodide
15
Lawrence Livermore National Laboratory
Official Use Only
Official Use Only Official Use Only CFP06-TA01-LL01 Cherepy
Title: High Resolution Scintillator Materials and Detectors
We consistently obtain <4% resolution at 662 keV with
encapsulated crystals
• Pulse height spectrum acquired with Cs-137 source using PMT and standard
analog readout electronics exhibits ~3.2% resolution at 662 keV
• Direct replacement of NaI(Tl) by SrI2(Eu) into existing detectors should require
only a shaping time modification
Volume = 11.7 cm3
#52
#33a
Crystal #52
Crystal #33a
16
Lawrence Livermore National Laboratory
Official Use Only
Official Use Only Official Use Only CFP06-TA01-LL01 Cherepy
Title: High Resolution Scintillator Materials and Detectors
References
1. N.J. Cherepy, G. Hull, A. Drobshoff, S.A. Payne, E. van Loef, C. Wilson, K. Shah, U.N. Roy, A. Burger, L.A.
Boatner, W-S Choong, W.W. Moses “Strontium and Barium Iodide High Light Yield Scintillators,” Appl. Phys.
Lett. 92, 083508, (2008).
2. R. Hawrami, M. Groza, Y.Cui, A. Burger, M.D Aggarwal, N. Cherepy and S.A. Payne, “SrI2, a Novel Scintillator
Crystal for Nuclear Isotope Identifiers,” Proc. SPIE, 7079, 70790 (2008).
3. C.M. Wilson, E.V. Van Loef, J. Glodo, N. Cherepy, G. Hull, S.A. Payne, W.S. Choong, W.W. Moses, K.S. Shah,
“Strontium iodide scintillators for high energy resolution gamma ray spectroscopy,” Proc. SPIE, 7079,
707917, (2008).
4. N.J. Cherepy, S.A. Payne, S.J. Asztalos, G. Hull, J.D. Kuntz, T. Niedermayr, S. Pimputkar, J.J. Roberts, R.D.
Sanner, T.M. Tillotson, E. van Loef, C.M. Wilson, K.S. Shah, U.N. Roy, R. Hawrami, A. Burger, L.A. Boatner,
W.-S. Choong, W.W. Moses, “Scintillators with Potential to Supersede Lanthanum Bromide,” IEEE Trans.
Nucl. Sci. 56, 873-80 (2009).
5. E.V.D. van Loef, C.M. Wilson N.J. Cherepy, G. Hull, S.A. Payne, W- S. Choong, W.W. Moses, K.S. Shah,
“Crystal Growth and Scintillation Properties of Strontium Iodide Scintillators”, IEEE Trans. Nucl. Sci., 56,
869-72 (2009).
6. N. J. Cherepy, B. W. Sturm, O. B. Drury; T. A. Hurst. S. A. Sheets, L. E. Ahle, C. K. Saw, M. A. Pearson, S. A.
Payne, A. Burger, L. A. Boatner, J. O. Ramey, E. V. van Loef, J. Glodo, R. Hawrami, W. M. Higgins, K. S. Shah,
W. W. Moses, “SrI2 scintillator for gamma ray spectroscopy ,” Proc. SPIE, 7449, 7449-0 (2009).
7. J. Glodo, E.V. van Loef, N.J. Cherepy, S.A. Payne, and K.S. Shah “Concentration effects in Eu-doped SrI2,”
IEEE Trans. Nucl. Sci., in press (2010).
8. S A Payne, N J Cherepy, G Hull, J D Valentine, W W Moses, W-S Choong, “Nonproportionality of Scintillator
Detectors: Theory and Experiment”, IEEE Trans. Nucl. Sci., 56, 2506-2512 (2009).