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Crystal growth of large diameter strontium iodide scintillators using in Situ stoichiometry monitoring

  • CapeSym, Inc.
  • CapeSym, Inc.
  • CapeSym, Inc.
Crystal Growth of Large Diameter Strontium
Iodide Scintillators Using In Situ Stoichiometry
Amlan Datta, Member, IEEE, Stephanie Lam, Stacy Swider, Shariar Motakef
Abstract- The scintillation radiation detection community is on
the cusp of a major breakthrough with the potential deployment
of Europium-activated Strontium Iodide (SrI2:Eu) detectors for
medical imaging and homeland security applications. Compared
to the traditional scintillators (such as NaI), SrI2 provides much
better energy resolution and light output. The crystal growth of
SrI2:Eu has been impaired for a long time due to cracking
problems, which makes it highly unreliable and nonreproducible.
This significantly increases the cost of the material which in turn
impedes wide-scale deployment and limits its advantages over
other scintillators. In this paper, we demonstrate a technique of
growing crack-free SrI2:Eu crystals by monitoring the
stoichiometry of the melt atmosphere during processing and
crystal growth. Using the feedback information from the in situ
monitoring technique, the stoichiometry of the melt was corrected
and multiple crack-free SrI2:Eu crystals of diameters 1.5 inches
were repeatedly grown using Bridgman configuration with no
visible inclusions, bubbles or defects whatsoever.
n recent years, europium-doped strontium iodide
(SrI2:Eu2+) has emerged as a promising scintillation
material for gamma-ray spectroscopy with extremely high light
yield and proportional response, exceeding that of the widely
used high performance commercial scintillator LaBr3:Ce3+.
Energy resolution of this material is close to 3% with 662keV
gamma rays, about 2 times better than the commonly used
members of the iodide family such as NaI and CsI. The main
challenge impeding wide-spread application of this material is
the unavailability of crack-free low-cost large crystals of SrI2.
The effort demonstrated in this study will enrich the knowledge
to grow larger SrI2 scintillator crystals.
Over the past few years, extensive research has been
performed on the development of crystal growth processes for
this scintillator [1,2]. Due to very limited interaction of this
iodide salt with the quartz surface, generally quartz is used as a
growth container. Usually, vertical Bridgman technique is
Manuscript received December 30, 2016.
Amlan Datta is with CapeSym Inc., Natick, MA 01760 USA (telephone:
508-653-7100 ext. 216, e-mail:
Stephanie Lam is with CapeSym Inc., Natick, MA 01760 USA (telephone:
508-653-7100, e-mail:
Stacy Swider is with CapeSym Inc., Natick, MA 01760 USA (telephone:
508-653-7100, e-mail:
Shariar Motakef is with CapeSym Inc., Natick, MA 01760 USA (telephone:
508-653-7100, e-mail:
employed for the growth of this low melting point scintillator.
SrI2 has orthorhombic crystal structure (oP24) with lattice
constants of a = 15.22, b = 8.22, and c = 7.90 [3]. It has been
shown that the anisotropy in thermal expansion coefficients
along the crystallographic axes is not the primary cause of
cracking in SrI2 crystals [4].
The main issues that hinder the progress of these materials
include hygroscopicity, oxidation and stoichiometry variations.
During the processing of these highly hygroscopic materials,
they absorb moisture and forms stable hydrates. The high
surface area of the starting materials in powder or bead forms
significantly increases the probability of such reactions (for
example: 1,2). Hence, it is imperative that before starting the
crystal growth, the starting material needs to be dehydrated
thoroughly. Due to high vapor pressure of halides, it is
extremely difficult to maintain the stoichiometric conditions in
halide crystals during its processing and growth. This in turn
increases the chances of cracking, and degradation of the
crystalline and charge transport properties of the detectors.
Recently, several techniques such as “melt-aging” [5] and melt
pumping [1] has been shown to improve the crystalline quality
of the halide scintillators. Also, different growth methods and
crucible materials have shown some promise [6,7]. However,
these techniques are highly empirical and lacks reproducibility.
There are no published results which demonstrates multiple
Bridgman-grown SrI2:Eu crystals with no cracking and can be
used by the entire scientific community as a reliable technique.
Most certainly the reason behind this is the variation in the
quality of the starting materials and differences in handling
SrI2+2H2O→Sr(OH)2+2HI (1)
SrI2+H2O→SrO+2HI (2)
There are two main factors which will help eliminate the
cracking problem in the crystal growth of SrI2 crystals:
eradicating nucleation centers of microcracks and stopping the
propagation of already initiated cracks. In our approach, we
report the reproducibility of the technique along with relevant
statistics. The versatility of our process ensures its applicability
to almost any halide scintillator or semiconductor.
In this study, we implemented an in-situ monitoring
technique for processing and crystal growth of halide materials.
The experimental set up is shown in Fig. 1. The charge material
was comprised of anhydrous 4N+ SrI2 and 5N EuI2 beads from
EMD Performance Materials. The ampoule with a quartz frit
for melt filtering was thoroughly cleaned and etched using aqua
regia and HF. It was vacuum baked for 24 hours. The beads
were then loaded in it inside an Ar glovebox with a moisture
content below 0.5ppm. The ampoule was connected to the gas
monitoring system using a gas manifold. The gas monitoring
system consists mainly of a Residual Gas Analyzer (RGA) and
a pressure reduction system. The pressure reduction system
basically consists of a long capillary tube and a needle valve
which reduces the pressure from approximately 102 Torr to 10-
5 Torr range. As we heat up the SrI2:Eu2+ charge material, we
alternated between vacuum and flowing a dry gas while
measuring the RGA response. After passing the molten charge
material through a quartz frit, it was collected in a vessel.
During the processing of this melt, the RGA data was collected
at various stages. This RGA data was used as a feedback to
regulate the melt processing process.
Fig. 1. Experimental set up for the melt processing set up for SrI2:Eu2+
Fig. 2 and Fig. 3 show typical RGA responses from the
SrI2:Eu2+ melt processing set-up while flowing a dry gas over it.
The ambient vacuum baseline shows the presence of trace
amounts of Nitrogen and Argon. When the molten SrI2:Eu2+ is
processed properly, oxide and hydroxide species leave the melt
over time.
We grew several 1 in. and 1.5 in. diameter SrI2:Eu2+ crystals
after processing the melt in a similar system (Fig. 4 and Fig. 5).
The crystals had no bubbles, cracks or any other visible
crystalline defects. The crystals were taken out of the quartz
growth ampoules in an Ar gas purged glovebox. The crystals
were cut using a 0.3mm diameter diamond wire saw in dry
mineral oil. Crystals were then ground using Buehler CarbiMet
2 SiC grinding papers: 600 grit (P600) and 1200 grit (P1200).
The entire processing of the crystals was performed while
covered with dry mineral oil. Large crack free 1.5 in. diameter
crystals with lengths up to 1.75 in. were prepared. The polished
crystals were then wrapped with ESR reflector and the pulse
height spectra for 137Cs source was measured on a Hamamatsu
R6231-100 photomultiplier. The energy resolution of the
662keV peak for a 1.5 in. x 1.5 in. SrI2:Eu2+ scintillator crystal
was estimated to be 3.3% (Fig. 6). The light yield was estimated
to be 75000±2000 photons/MeV. With tapering of another 1.5
in. x 1.5 in. SrI2:Eu2+ crystal, an energy resolution of 3% was
obtained (Fig. 7). Fig. 8 shows the hermetically packaged
tapered SrI2:Eu2+ crystal as a final processing step in the
scintillator detector fabrication process.
Fig. 2. RGA data during SrI2:Eu2+ melt processing (10 to 25 amu).
Fig. 3. RGA data during SrI2:Eu2+ melt processing (34 to 45 amu).
Fig. 4. 1 in. diameter SrI2:Eu2+ crystals grown using the process described
above. This technique is highly reproducible and repeatable. These crystals
shown here are in the growth ampoules, and the dark marks are on the inner
diameter of the quartz ampoule and not a crystalline defect.
Fig. 5. 1.5 in. diameter SrI2:Eu2+ crystals grown using the process described
above. This technique is highly reproducible and repeatable. These crystals
shown here are in the growth ampoules, and the dark marks are on the inner
diameter of the quartz ampoule and not a crystalline defect.
Fig. 6. Pulse height spectra from a 1.5 in. x 1.5 in. SrI2:Eu2+ crystal with
662keV resolution of 3.3% and light yield of 75000 photons/MeV.
Fig. 7. Pulse height spectra from a tapered 1.5 in. x 1.5 in. SrI2:Eu2+ crystal
with 662keV resolution of 3%.
Fig. 8. Hermetically packaged tapered 1.5 in. x 1.5 in. SrI2:Eu2+ crystal
Multiple SrI2:Eu2+ scintillator crystals were grown using an
innovative melt processing technique which involves an in situ
monitoring and feedback process. This technique demonstrated
high reliability and reproducibility, and hence provides higher
yield over the other reported techniques. All the large diameter
SrI2:Eu2+ crystals grown using this technique were free from
cracks and defects. The large 1.5in. x 1.5in. SrI2:Eu2+
scintillator detectors demonstrated an energy resolution as high
as 3% and a light yield of approximately 75000 photons/MeV.
With these developments, now it is possible to deploy high
resolution and high light yield SrI2:Eu2+ scintillators for lower
cost field applications.
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E Van Loef, J Glodo, K. S. Shah, E. Rowe, P. Bhattacharya, E. Tupitsyn,
M. Groza, A. Burger, N. J. Cherepy, and S.A. Payne, “Bridgman growth
of large SrI2:Eu
2+ single crystals: A high-performance scintillator for
radiation detection applications,” J. Crys, Growth, vol. 379, pp. 63-68,
[2] E. V. van Loef, C. M.Wilson, N.J. Cherepy, G. Hull, S.A. Payne, W.S.
Choong, W. W. Moses, and K.S. Shah, Crystal Growth and Scintillation
Properties of Strontium Iodide Scintillators,” IEEE Trans. Nucl. Sci., vol.
56, pp. 869-872, 2009.
[3] H. Barnighausen, H. Beck, H. Grueninger, E. T. Rietschel, N. Z. Schultz,
Kristallogr., vol. 128, pp. 430, 1969.
[4] N. Cherepy, L. Boatner, A. Burger, K. Shah, S. Payne, A. Janos, A. Kuhn,
“Overview of Strontium Iodide Scintillator Materials,” LLNL
presentation, April 1, 2010. (LLNL-PRES-426327).
[5] Y. T. Wu, M. Zhuravleva, J. A. Johnson, H. Wei, M Koshchan and C. L.
Melcher, Effects of melt aging and off-stoichiometric melts on
CsSrI3:Eu2+ single crystal scintillators,” Phys. Chem. Chem. Phys., vol.
18, pp.8453, 2016.
[6] R. Král, V. Jarý, J. Pejchal, S. Kurosawa, K. Nitsch, Y. Yokota, M. Nikl,
and A. Yoshikawa, “Growth and Luminescence Properties of Single
Crystals Prepared by Modified Micro-Pulling-Down Method”, IEEE
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[7] Y. Yokota, T. Ito, S. Yasuhiro, S. Kurosawa, Y. Ohashi, K. Kamada, and
A. Yoshikawa, “Growth of 1.5-in Eu : SrI2 Single Crystal and
Scintillation Properties,” IEEE Trans. Nucl. Sci., vol. 63, no. 2, pp. 467,
April 2016.
Thallium Bromide (TlBr) is a promising room-temperature radiation detector candidate with excellent charge transport properties. Currently, Travelling Molten Zone (TMZ) technique is widely used for growth of semiconductor-grade TlBr crystals. However, there are several challenges associated with this type of crystal growth process including lower yield, high thermal stress, and low crystal uniformity. To overcome these shortcomings of the current technique, several different crystal growth techniques have been implemented in this study. These include: Vertical Bridgman (VB), Physical Vapor Transport (PVT), Edge-defined Film-fed Growth (EFG), and Czochralski Growth (Cz). Techniques based on melt pulling (EFG and Cz) were demonstrated for the first time for semiconductor grade TlBr material. The viability of each process along with the associated challenges for TlBr growth has been discussed. The purity of the TlBr crystals along with its crystalline and electronic properties were analyzed and correlated with the growth techniques. Uncorrected 662 keV energy resolutions around 2% were obtained from 5 mm x 5 mm x 10 mm TlBr devices with virtual Frisch-grid configuration.
Full-text available
Single crystals of SrI<sub>2</sub>:Eu and SrI<sub>2</sub>:Ce/Na were grown from anhydrous iodides by the vertical Bridgman technique in evacuated silica ampoules. Growth rates were of the order of 5-30 mm/day. Radioluminescence spectra of SrI<sub>2</sub>:Eu and SrI<sub>2</sub>:Ce/Na exhibit a broad band due to Eu<sup>2+</sup> and Ce<sup>3+</sup> emission, respectively. The maximum in the luminescence spectrum of SrI<sub>2</sub>:Eu is found at 435 nm. The spectrum of SrI<sub>2</sub>:Ce/Na exhibits a doublet peaking at 404 and 435 nm attributed to Ce<sup>3+</sup> emission, while additional impurity-or defect-related emission is present at approximately 525 nm. The strontium iodide scintillators show very high light yields of up to 120 000 photons/MeV, have energy resolutions down to 3% at 662 keV (Full Width Half Maximum) and exhibit excellent light yield proportionality with a standard deviation of less than 5% between 6 and 460 keV.
We grew 1.5-in Eu doped SrI <sub xmlns:mml="" xmlns:xlink="">2</sub> (Eu : SrI <sub xmlns:mml="" xmlns:xlink="">2</sub> ) bulk single crystal by a modified vertical Bridgman (VB) method using a removable chamber and high-frequency induction heating. Asgrown 1.5-in Eu : SrI <sub xmlns:mml="" xmlns:xlink="">2</sub> bulk single crystal had no visible crack and inclusion in the crystal. In the transmittance and α-ray radioluminescence spectra, large absorption below 433 nm and emission peak at 433 nm were observed, respectively. Each polished Eu : SrI <sub xmlns:mml="" xmlns:xlink="">2</sub> specimen indicated 56 000 ~ 62 000 ph/MeV light yield and 3.3 ~ 3.9% energy resolution. The decay times of the specimens were 0.61 ~ 0.67 μs.
This paper reports on the preparation and growth of Eu-doped strontium iodide (SrI2) single crystals by modified micro-pulling-down method. Influence of the Eu2+ and Eu3+ (introduced in form of EuI2 and EuCl3) on the luminescence and scintillation properties is analyzed. Sufficient optical quality of prepared single crystals was obtained allowing further characterization of their absorption, radio- and photoluminescence spectra. Photoluminescence and scintillation decay time of grown SrI2 crystals irradiated under γ-ray source were evaluated as well.
Ternary halide scintillators are commonly prepared from a mixture of commercially available binary halides. The initial binary halides may contain excess halogen ions or have different volatilities, which could lead to loss of stoichiometry of the resulting ternary halide crystals and potentially negatively affect optical and scintillation properties. In this work, the effects of vacuum aging of the melt (melt aging) and use of off-stoichiometric melts via introduction of excess CsI on the crystal quality and scintillation properties of CsSrI3:Eu2+, a promising scintillator for gamma-ray detection applications, are investigated. The phase purity of the grown samples was confirmed by powder X-ray diffraction and differential scanning calorimeter measurements, and the existence of matrix composition variations is revealed by energy-dispersive X-ray spectroscopy analyses. An abnormal relationship between the full energy peak and the shaping time, i.e. full energy peak broadening or existence of two full energy peaks, in the melt-aged and off-stoichiometric samples is observed. It is ascribed to a slow scintillation decay event in a time scale between 15 and 50 ms. For the CsSrI3:Eu2+ single crystal grown from a stoichiometric melt without melt aging treatment, an energy resolution of 5.0% at 662 keV and a light yield of 48000� �2000 photons per MeV can be achieved at a size of 1.4 cm3.
Single-crystal strontium iodide (SrI2) doped with relatively high levels (e.g., 3-6%) of Eu2+ exhibits characteristics that make this material superior, in a number of respects, to other scintillators that are currently used for radiation detection. Specifically, SrI2:Eu2+ has a light yield that is significantly higher than LaBr3:Ce3+—a currently employed commercial high-performance scintillator. Additionally, SrI2:Eu2+ is characterized by an energy resolution as high as 2.6% at the 137Cs gamma-ray energy of 662 keV, and there is no radioactive component in SrI2:Eu2+—unlike LaBr3:Ce3+ that contains 138La. The Ce3+-doped LaBr3 decay time is, however, faster (30 ns) than the 1.2 μs decay time of SrI2:Eu2+. Due to the relatively low melting point of strontium iodide (˜515 °C), crystal growth can be carried out in quartz crucibles by the vertical Bridgman technique. Materials-processing and crystal-growth techniques that are specific to the Bridgman growth of europium-doped strontium iodide scintillators are described here. These techniques include the use of a porous quartz frit to physically filter the molten salt from a quartz antechamber into the Bridgman growth crucible and the use of a "bent" or "bulb" grain selector design to suppress multiple grain growth. Single crystals of SrI2:Eu2+ scintillators with good optical quality and scintillation characteristics have been grown in sizes up to 5.0 cm in diameter by applying these techniques. Other aspects of the SrI2:Eu2+ crystal-growth methods and of the still unresolved crystal-growth issues are described here.
  • H Barnighausen
  • H Beck
  • H Grueninger
  • E T Rietschel
H. Barnighausen, H. Beck, H. Grueninger, E. T. Rietschel, N. Z. Schultz, Kristallogr., vol. 128, pp. 430, 1969.
Overview of Strontium Iodide Scintillator Materials
  • N Cherepy
  • L Boatner
  • A Burger
  • K Shah
  • S Payne
  • A Janos
  • A Kuhn
N. Cherepy, L. Boatner, A. Burger, K. Shah, S. Payne, A. Janos, A. Kuhn, "Overview of Strontium Iodide Scintillator Materials," LLNL presentation, April 1, 2010. (LLNL-PRES-426327).
Effects of melt aging and off-stoichiometric melts on CsSrI
  • Y T Wu
  • M Zhuravleva
  • J A Johnson
  • H Wei
  • C L Koshchan
  • Melcher