History and Current Status of Strontium Iodide Scintillators

Lawrence Livermore National Laboratory
History and current status of
strontium iodide scintillators
August 7, 2017
This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and has been supported
by the Department of Homeland Security, Domestic Nuclear Detection Office, under competitively awarded IAA HSHQDC-09-x-00208 / P00002, the U.S. Department of Energy
National Nuclear Security Administration, Defense Nuclear Nonproliferation Research and Development Office of the under Contract DE-AC03-76SF00098 and the Enhanced
Surveillance Program. This support does not constitute an express or implied endorsement on the part of the Government.
Nerine J. Cherepy, Patrick R. Beck, Stephen A. Payne, Erik L. Swanberg, Brian Wihl, Scott E. Fisher,
Steven L. Hunter, Peter A. Thelin, Cordell J. Delzer, Shayan Shahbazi, Lawrence Livermore National Laboratory;
Arnold Burger and team, Fisk University; Kanai S. Shah, Rastgo Hawrami and team, Radiation Monitoring Devices, Inc.;
Lynn A. Boatner and team, Oak Ridge National Laboratory; Michael Momayezi and team, Bridgeport Instruments, LLC ;
Kevin T. Stevens, Mark H. Randles, Denys Solodovnikov and team, Northrop Grumman SYNOPTICS

2
Union
Materials
The original R&D team
Crystals - US
Crystals - Japan
PMT-mounted assemblies
6 in3
RMD
4 in3
Northrop
Grumman

3
Where did SrI2(Eu) come from?
• SrI2(Eu) – perfect match between Sr & Eu ionic radii = high doping, with high uniformity
X
X
AX2
Strontium Iodide: Uniform Eu-doping, No cleavage plane, No intrinsic radioactivity
Equivalent to LaBr3(Ce) - Stopping and Energy resolution
• CaI2(Eu) reported by Hofstadter in 1960’s  100,000 Ph/MeV, but laminar growth habit
• In 2006, our team started growing the nice, orthorhombic crystal BaI2(Eu)  60,000 Ph/MeV,
BUT  Eu not a good match to Ba ionic radius, poor doping uniformity and poor energy resolution
Ionic Radii:
Sr = 1.40 Å
Eu = 1.41 Å
Ba = 1.56 Å
Ca = 1.26 Å
• Later, we found out that Hofstadter had tried SrI2(Eu) in 1960’s, but had used Eu3+ dopant at very low
concentrations  LY lower than NaI(Tl), so he abandoned it and no papers in literature on SrI2(Eu)
Alkaline Earth Iodides

4
Strontium Iodide matches the resolution, cost, and volume available for
Lanthanum Bromide, without the undesirable intrinsic radioactivity
STANDARD
Gamma
Scintillation
Detectors
SrI2(Eu)
• 3% resolution
• Standard size 40 cm3
• $120-200/cm3
• No self-activity
NaI(Tl)
• >6% resolution
• 2000 cm3
• $5/cm3
LaBr3(Ce)
• 3% resolution
• Standard size 40 cm3
• $200/cm3
• Self-activity
1.5” diameter X 4” long
4 in3
7 in3 crack-free boules
1.5” DIAMETER X 4” LONG
• Encapsulated crystals – up to 6 in3
• PMT-mounted crystals – 1.5” x 1.5” std
2” diam 1.5” diam
1.5” x 4”
2” x 2”
Union Materials
(Japan)

5
4
3
2
1
I(a.u.)
700600500400300
wavelength (nm)
2500
2000
1500
1000
500
0
STE
Eu
2+
Impurity-mediated
recombination
Eg = 5eV
Exciton
Absorption, 100 um SrI2(3%Eu)
updoped SrI2
4
3
2
1
I(a.u.)
700600500400300
wavelength (nm)
2500
2000
1500
1000
500
0
Eu
2+
Absorption, 100 um SrI2(3%Eu)
000
800
600
400
200
0
1.2MeV1.00.80.60.40.20.0
Energy
undoped SrI2
SrI2(3%Eu)
FWHM=
5.28%
FWHM=
2.85%
Strontium Iodide scintillator characteristics
Undoped:
t = 0.42 ms
3% Eu-doped 1 in3
encapsulated crystal:
t = 3.3 ms average decay time
Digitized scintillation
pulses, 662 keV
Radioluminescence
Eu-doped Undoped
Undoped SrI2(3%Eu)
Cs-137 Pulse Height Spectra
R =
5.3%
R =
2.8%
1.0
0.9
0.8
0.7
0.6
0.5
Rel.LY
1
2 3 4 5 6
10
2 3 4 5 6
100
2 3 4 5 6
1000
Electron Energy (keV)
Undoped SrI2
SrI2(3%Eu)
Light Yield Proportionality
Energy(MeV)
RNP =
3.8%
2.2%
• Sturm, B.W.; Cherepy, N.J.; Drury, O.B.; Thelin, P.A., Fisher, S.E., Payne, S.A. Burger, A, Boatner, L.A. Ramey, J.O., Shah, K.S. Hawrami,
R. “Effects of packaging SrI2(Eu) scintillator crystals,” Nucl. Instr. Meth. A, 652, 1 , 242-246 (2011).
• B.W. Sturm, N.J. Cherepy, O.B. Drury, P.A. Thelin, S.E. Fisher, A.F. Magyar, S.A. Payne, A. Burger, L. A. Boatner, J.O. Ramey, K.S. Shah,
R. Hawrami, “Evaluation of Large Volume SrI2(Eu) Scintillator Detectors,” IEEE Nuc. Sci. Symp. Conf. Record, p. 1607-1611 (2010).

6
SrI2(Eu) matches LaBr3(Ce) performance with PMT readout
Property LaBr3(Ce) SrI2(Eu) Comparison
Melting Point 783 ºC 538 ºC  Less thermal stress
Handling Cleaves Resists cracking  Better processing
Light Yield 60,000 Ph/MeV 85,000 Ph/MeV  Higher
Proportionality contribution to
resolution at 662 keV
~2.0% ~2.0%  Similar
Decay time 30 nsec 2 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
Resolution at 662 keV (total,
calculated best achievable)
~2.6% ~2.6%  Similar
• 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).
• P.R. Beck, et al, “Strontium iodide instrument development for gamma spectroscopy and radioisotope identification,” SPIE Optical Engineering+
Applications, 92130N-92130N-9 (2014).

7
Gamma spectroscopy with SrI2(Eu) - significant resolution
improvement over NaI(Tl) and no intrinsic radioactivity like LaBr3(Ce)
• Better resolution of SrI2(Eu) allows more rapid, high confidence determination of
radioisotope ID’s with smaller detectors than for NaI(Tl)
• Noise from intrinsic radioactivity in LaBr3(Ce) is problematic for detecting weak sources
High Energy Resolution No Intrinsic Radioactivity

8
“Light-trapping” in Eu2+ doped scintillators is a type of optical
scatter that increases the effective decay time
Successive emissions, followed by re-absorption and re-emission (etc.), causes effective
lengthening of decay - no problem unless accompanied by a loss mechanism
…
Eu2+
CB
VB
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., 57, 1228 (2010).
Self-
absorption

9
• Electron nonproportionality is fit to a model of carrier capture
by activators
• From this data, we calculate the gamma nonproportionality,
which has a different shape
• To calculate resolution, we start with the predicted single-
electron resolution, and then include fluctuations in the
gamma-generated electrons and finally the photon statistics
SrI2(Eu) is highly proportional – electron and gamma
nonproportionality measurements fit models well
2% at 662 keV –
nonproportionality limit
Patrick R. Beck, Stephen A. Payne, Steven Hunter, Larry Ahle, Nerine J. Cherepy, and Erik L. Swanberg, “Nonproportionality
of Scintillator Detectors. V. Comparing the Gamma and Electron Response,” IEEE Trans. Nucl. Sci., 62, 1429 (2015).

10
4 in3 Strontium Iodide crystal from Northrop Grumman exhibits light yield of
84,000 Ph/MeV and resolution of 2.9% at 662 keV with digital readout
600
400
200
0
counts
6004002000
Energy (keV)
3.6%
800
600
400
200
0
counts
6004002000
Energy (keV)
2.9%
Gamma spectroscopy tests of encapsulated crystal
Analog readout with 12 us
shaping time:
R (662 keV) = 3.5%, with
additional non-Gaussian
tail
Digital readout with 30 us
fixed integration time:
R (662 keV) = 3.6%, peak
is Gaussian
Digital readout with 30 us
fixed integration time plus
on-the-fly correction
factor :
R (662 keV) = 2.9%
Large (4 in3) crystal provides 2.9% resolution,
comparable to results with small crystals Encapsulated crystal
4 in3

11
Second Taper 58% 23.1 13% 3.3% 3.0%
Third Taper 100% 20.5 23% 3.2% 2.9%
Taper stage
% of Length
Tapered
Vol
(cm3)
Vol
Lost
R(662 keV)
analog
R(662 keV)
digital
Right Cylinder 0% 26.5 0% 5.3% 3.5%
First Taper 18% 25.2 5% 3.3% 3.1%
Tapering study to determine best geometry using a 1” x 2” long crystal
indicates that only a small taper needed

12
1.0
0.8
0.6
0.4
0.2
0.0
Rel.LY
3002001000-100
Temperature (°C)
Decrease from RT to
150 °C of 8%
SrI2(5%Eu)
• T-dependence of LY is comparable for SrI2(Eu) and LaBr3(Ce)
• Minimal temperature stabilization required in instrument
Eu-doped Strontium Iodide offers a light yield that is stable around room
temperature, and has potential to be useful at high temperatures
M.S. Alekhin, J.T.M. de Haas, K.W. Krämer, I.V. Khodyuk,
L. de Vries, P. Dorenbos, “Scintillation properties and self
absorption in SrI2:Eu2+” IEEE TNS , 58, 2519-2527 (2010).
LaBr3(5%Ce) Decrease from RT
to 150 °C of 5%
M.D. Birowosuto,P. Dorenbos, K.W. Krämer, H.U. Gudel
“Ce3+ activated LaBr3-xIx: High-light-yield and fast-
response mixed halide scintillators” J. Appl. Phys,
103 , 103517 (2008).

13
Development over 6 years has improved functionality, capability,
and performance of the SrI2(Eu) handheld spectrometer
2011:
• 1” encapsulated SrI2(Eu)
• Separate Digitizer + MCA
• 4.9” x 3.5” OQO computer
2012
2013

14
RIID capabilities often are overstated by claiming the ability to
identify a large number of sources simultaneously
• Easy spectra can be resolved with poor resolution scintillators, such as NaI(Tl)
• Even six sources are easy to resolve without overlapping features:
• Am-241, Co-57, Sn-113, Na-22, Cs-137, Co-60
• With fewer counts from more distant or minimally shielded sources this spectrum
becomes extremely challenging for peak finding algorithms
Co-60
Co-60
Cs-137
Na-22
Sn-113
Co-57
Am-241
Na-22
Measured Data

15
Radionuclide Analysis Kit (RNAK) provides multiple-
regression full spectral analysis
RadioNuclide Analysis Kit (RNAK):
 A collection of automated
algorithms and tools to analyze
radiation measurements and
identify radionuclides
 Provides more accurate and
complete analysis of
radionuclide mixtures, shielding,
and activity than other
algorithms
 Uses large-scale optimization techniques to find all of the best nuclide
identification solutions that are consistent with the measurement:
• Estimates shielding material, volume, and mass
• Estimates maximum SNM mass that could also be present (helps find items
of concern hidden by benign radioactive materials)
• Automatic calibration adjustment, background modeling, and pile-up compensation
Nelson, K., “Bayesian Optimization Algorithm for Radionuclide Identification: RNAK – RadioNuclide Analysis Kit,”
Lawrence Livermore National Laboratory Report, LLNL-TR-484722, (2011).

16
Am-241
Co-57
Ba-133
Cs-137 Ho-166m
Na-22
Co-60
Eu-152 Ra-226
Th-232
NaI(Tl):
•1.5” x 1.5”
•R(662keV) = 6.9%
Two detectors are being extensively tested to evaluate RNAK
radioisotope identification performance
Detectors tested
SrI2(Eu) measured data
SrI2(Eu):
•1.5” x 1.5”
•R(662keV) = 3.2%
22Na 54Mn 57Co
60Co 65Zn 113Sn
133Ba 137Cs 152Eu
166mHo 177mLu 226Ra
232Th U isotopes
239Pu
Sources tested

17
Mr. ID – Mobile radioisotope Identification Detector
Detector:
• Gamma: 1.5” x 1.5” SrI2(Eu)
Physical:
• Dimensions (W x L x H): 4” x 11” x 6”
• Weight: 6 lbs
• Housing: Delrin, anodized aluminum
Battery:
• Li-ion 18650 rechargeable batteries
• Internal battery for hot swapping
• Micro-USB charging capability
Operation time:
• 8 hours with tablet
• 12 hours with on-board screen only
Environmental:
• Operating Temp: -10 to 40°C
Input/Output:
• USB: 2.0, micro-AB socket, on tablet
• WLAN: WiFi 802.11 b/g/n
SrI2(Eu)
Android Tablet Interface
WATERFALL PLOT GAMMA SPECTRUM
Th-232
Contacts: Patrick Beck, beck30@llnl.gov
Nerine Cherepy, cherepy1@llnl.gov
Funded by
DHS/DNDO
Performance:
• Energy Range: 30 keV – 3 MeV
• Energy Resolution: 3% @ 662 keV
• Identification: LLNL Radionuclide Analysis Kit (RNAK)
with ANSI N42.34 (>60 radionuclides)
• Categories: Industrial, Medical, SNM, NORM
Software:
• Counting: Count Rate, Cumulative Counts
• Gamma Spectroscopy:
o Manual or Automatic Search
o Radionuclide Identification
o Temperature stabilization
o Energy calibration source - Cs-137
• Tablet Display:
o Gamma Spectrum
o Waterfall Spectral Plot
o Radioisotope Search
Reachback: Via cellular network
File Format: ANSI N42.42

18
RNAK working well in Mr. ID
ID complete
with only
180 counts
in spectrum
ID at 1000
counts in
spectrum
SrI2(Eu) ID’s
Am-241 spectrum 5X
faster than NaI(Tl)
SrI2(Eu) ID’s
radiological “soup”
of Cs-137, Ra-226,
Na-22, Th-232
in <18 seconds
18 sec3 sec

19
Patrick Beck (lead engineer for Mr. ID) describing its use

20
2” x 2” SensL SiPM
Strontium Iodide on large area SiPM - good gamma spectroscopy
SrI2(Eu) on SiPM array
Pathway to compact detector  SiPM
Microsecond decay
of SrI2(Eu) does not
saturate SiPM
25
20
15
10
5
0
kCounts
8006004002000
Energy (keV)
PMT
SiPM
1" x 1" SrI2(Eu) encapsulated crystal
on SenSL SiPM array
Gaussian Fit - R(662 keV) = 3.0%

21
Strontium and Europium Iodide feedstock available from many vendors
Different as-received batches of EuI2 –
zone refining effective if needed
Less iodide decomposition – higher purity
A. With oxide as starting material:
1. Eu2O3 (s) + 4HI (aq) + 2NH4I (aq)  2EuI3 (aq) +3H2O (l) +2NH3 (g)
2. (with heating) EuI3 * 6H2O  EuI2 (s) +I2 (g) + H2O (g)
B. With hydroxide as starting material:
1. Sr(OH)2 (s) + 2HI (aq)  SrI2 (aq) + 2H2O (l)  SrI2*6H2O
C. With carbonate as starting material:
1. 2SrCO3 (s) + 2I2(s) + N2H4  2SrI2*2H2O + 2CO2 (g) + N2 (g)
or
1. SrCO3 (s) + 2HI (g)  SrI2 (s) + H2O (g) + CO2(g)
~2008

22
Thermal analytical data reveals that feedstock can be dried from
hydrates under vacuum
Expansion coefficients indicate cracking
due to anisotropy not a problem
Dehydration of feedstock complete by 350ºC
-50
-40
-30
-20
-10
0
Heatflow(uW)
600500400300200100
Temperature (°C)
SrI2
EuI2
Hydrate desorption
Melting
Crystal structure of SrI2 and EuI2 -
• Orthorhombic
• Similar lattice parameters
Feedstock melted under vacuum to
remove any residual adsorbed
water prior to crystal growth
Thanks to Cheng Saw & Mark Pearson (LLNL)
30oC
210oC
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 ,” SPIE Optical Engineering+ App, 7449, 7449-0 (2009).

23
On stoichiometry HI depleted HI saturated
• One small crack
• No evidence of I2 vapor
• Few cracks
• Black deposits above
melt
• Crystal completely
fractured
• Pink offgas (I2)
Best Workable with filtration Unworkable – crystals
brown-colored
Arnold Burger (Fisk University) found that stoichiometry of melt is key
to avoiding secondary phases and cracking

24
Frit filtration developed by Lynn Boatner at ORNL is expedient and
effective means of feedstock purification prior to growth
Excess I2 gas being
desorbed during vacuum
melting through quartz frit
• Upper chamber after
vacuum melt filtration of EuI2
• Black and brown filtrate -
significant impurities
removed (oxides,
hydroxides, hydrates)
L. A. Boatner, J. O. Ramey, J. A. Kolopus, R. Hawrami, W. M. Higgins, E. van Loef, J. Glodo, K. S. Shah, P. Bhattacharya, E. Tupitsyn, M. Groza, A. Burger N. J. Cherepy, S. A Payne,
“Bridgman Growth of Large SrI2:Eu2+ Single Crystals: A High-performance Scintillator for Radiation Detection Applications,” J. Crystal Growth, 379, 63-68 (2013).

25
APL Engineered Materials (Urbana, IL) feedstock generally requires
little or no pre-purification prior to crystal growth

26
LLNL partners with other national labs, academia, small and large
businesses to deploy our new materials into detectors
Funded by DHS
Funding/Investment
Funding/Investment Gap
Manufacturing – Innovation Process
Union
Materials
Crystals - US
Crystals - Japan
PMT-mounted assemblies
Feedstock - US
NEXT:
• Detectors
developed and
commercialized
• SrI2(Eu) detectors
deployed for
improved
Radioisotope ID in
the field

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