This document summarizes research on strontium iodide scintillator materials conducted by Lawrence Livermore National Laboratory. It finds that SrI2 doped with europium is a promising scintillator that offers high light yield and energy resolution comparable to lanthanum bromide. The document outlines thermal and optical properties of SrI2 and describes crystal growth techniques. It presents results demonstrating less than 4% energy resolution at 662 keV can be achieved with encapsulated SrI2 crystals and analog readout.

1. Lawrence Livermore National Laboratory
Overview of Strontium Iodide Scintillator Materials
April 1, 2010
Funded by
DHS/DNDO
PI: Nerine Cherepy (LLNL)
Co-Investigators: L Boatner (ORNL), A Burger (Fisk), K Shah (RMD)
PM: Steve Payne (LLNL)
DNDO PMs: Alan Janos, Austin Kuhn
Lawrence Livermore National Laboratory, P. O. Box 808, Livermore, CA 94551
This work performed under the auspices of the U.S. Department of Energy by LLNL-PRES- 426327
Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344

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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.
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We explored the Alkaline Earth Halide scintillators and identified
SrI2(Eu) as the best candidate
b-excited emission spectra SrI2(Eu) is more proportional than LaBr3(Ce)
1.15
RMD SrI2(0.5%Eu)
ORNL SrI2(4%Eu)
ORNL SrI2(6%Eu)
1.10
Relative Light Yield
NaI(Tl)
LaBr3(Ce)
1.05
1.00
0.95
0.90
5 6 7 2 3 4 5 6 7 2 3 4 5 6 7
10 100 1000
Electron Energy (keV)
LY Resolution
(Photons/MeV) (662 keV)
SrI2 undoped <60,000 6.7%
SrI2(Eu) 90,000 2.6% SrI2(Eu) offers excellent light yield
SrBr2(Eu) 25,000 7% and proportionality
BaI2(Eu) 40,000 8%
CaI2(Eu) 110,000 ---
LaBr3(Ce) 60,000 2.6%
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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
Predicted resolution with optimized readout and crystal quality
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
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For gamma ray spectroscopy, SrI2(Eu) can meet or exceed LaBr3(Ce)
Am-241 400 Cs-137
-spectrum Cs-137 NaI(Tl)
-spectrum
300 LaBr3(Ce)
Counts
SrI2(Eu)
200
100
0
200 300 400 500 600 700 800
Energy (keV)
6.81% 16
300
Energy Resolution (%)
LaBr3(Ce)
250 LaBr3(Ce) SrI2(Eu)
12
Counts
200 Co-57 SrI2(Eu)
Co-57
-spectrum
NaI(Tl)
150 8
100
50 5.24%
4
0
90 100 110 120 130 140 150 160
Energy (keV) 100 1000
Gamma Energy (KeV)
4 NaI(Tl)
LaBr3(Ce)
2 Scintillator Photopeak Efficiency LY Resolution
1000 SrI2(Eu)
(662keV, 5x5x7.5 cm3) (Ph/MeV) (662 keV)
Counts
4
2
NaI(Tl) 18% 40,000 ~6.5%
100
4 Ba-133 -spectrum
Ba-133 LaBr3(Ce) 20% 60,000 ≤3%
2
10 SrI2(Eu) 22% 90,000 ≤3%
50 100 150 200 250 300 350 400
Energy (keV)
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We have been acquiring thermal data for feedstock and crystal
growth optimization
Dehydration of feedstock complete by 350ºC
0
-10 Hydrate desorption
Heat flow (uW)
-20
SrI2
-30 EuI2
-40 Melting
-50
100 200 300 400 500 600
Temperature (°C)
Expansion coefficients indicate cracking
due to anisotropy not a problem The crystal structures of SrI2 and EuI2
are both orthorhombic and exhibit
very similar lattice parameters
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Distribution coefficient of Eu in SrI2 is approximately 1.0
Ionic Radii:
Sr = 1.40 Å
Eu = 1.41 Å
Melting Points:
SrI2 = 538ºC
EuI2 = 580ºC
Density of SrI2 = 4.55 g/cm3
• 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
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Crystals can be handled in a variety of ways
(1) Boule in ampoule (2) Boule vacuum packed in plastic
(3) Best domain harvested, cut (4) Cut and
and polished then vacuum polished crystal
packed in plastic in “openable”
hermetic
enclosure
1.75 in
Top view Side view
1.5 in
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“Light-trapping” occurs in Eu2+ doped scintillators
CB
Difficult to avoid some level of Eu2+
…
light-trapping in SrI2(Eu)
VB
freabsorbed = 80%
Successive emissions, followed by re-absorption then re-emission (etc.), causes
effective lengthening of decay- no problem unless accompanied by a loss mechanism
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Inch-scale crystals directly coupled to PMT exhibit inhomogeneous
lineshape due to light-trapping
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
• 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
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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
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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
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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
NEW 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
OLD Step 9: Set canister under UV lamp to cure Norland UV cement
New package screws together, minimizes
metal and window thicknesses
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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
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We consistently obtain <4% resolution at 662 keV with
encapsulated crystals
Crystal #52 Volume = 11.7 cm3
#52
Crystal #33a
#33a
• 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
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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).
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