SrI2, A novel scintillator crystal for nuclear isotope identifiers

SrI2, a Novel Scintillator Crystal for Nuclear Isotope Identifiers

R. Hawrami1,2, , M. Groza 1,Y.Cui1, A. Burger1, M.D Aggarwal2

Nerine Cherepy3 and Stephen A. Payne3

1) Department of Physics, Fisk University, Nashville, TN 37208, USA

2) Department of Physics, Alabama A&M University, Normal, Al 35762, USA

3) Lawrence Livermore National Lab, Livermore, CA 94550, USA

The growth and scintillating properties of undoped and Eu2+

doped Strontium Iodide

indicate excellent potential for gamma ray spectroscopy. Energy resolution at 662 keV was

found to be as good as 2.7% at 662 keV. The effect of purification by zone refining was also

studied and crystal growth of SrI2 by the Bridgman technique was found to be less subject to

cracking compared to the growth of lanthanum halide scintillators.

Introduction

The search for high light yield single crystal scintillators having the potential for low

cost, large volume, and improved energy resolution for gamma ray detection has increased in

recent years due to a larger demand for radioisotope identifiers. (1-2). In the past few years, a

number of other materials that have been investigated have extremely promising properties,

including LaBr3:Ce and LaCl3:Ce (3-5 ). The inorganic scintillator currently providing the

highest energy resolution is LaBr3(Ce), ~2.8% at 662 keV, but it is highly hygroscopic and its

growth is still challenging. Barium Iodide is a material offering higher Z (ρ=5.1 g/cm3 and

Zeff=54.1) and equivalent or higher light yield than LaBr3 (Ce) ( 6-8). Since the alkaline earth

cations are divalent, divalent activators, such as Eu2+

, are most suitable. (9). High light yields are

well-known for many iodide scintillators, such as CsI(Tl) (65,000 Ph/MeV) and LuI3(Ce) (about

100,000 Ph/MeV). A few reports of scintillation from the family of alkaline earth halides have

been published, Calcium Iodide which exhibits light yields in the vicinity of 100,000 Hard X-Ray, Gamma-Ray, and Neutron Detector Physics X, edited by Arnold Burger, Larry A. Franks, Ralph B. James,

Proc. of SPIE Vol. 7079, 70790Y, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.798335

Proc. of SPIE Vol. 7079 70790Y-12008 SPIE Digital Library -- Subscriber Archive Copy

Photons/MeV and has been activated with many dopants, including Tl+

and Eu2+

(9). Thallium

activated Calcium iodide, CaI2(TI), was shown to have a light output essentially equal to that of

NaI(Tl) (11). CaI2 activated with europium and also pure Cal2, yield pulse heights approaching

200% relative to typical NaI(Tl) samples, and energy resolution of 5.2% (FWHM) at 662 keV

has been reported for CaI2(Eu). This is significantly better than any resolution figures obtained

for NaI(Tl). It should be pointed out that calcium iodide has several characteristics which limit

its usefulness. It is a highly deliquescent material, being considerably more sensitive to moisture

than sodium iodide. It has a hexagonal crystal structure with pronounced basal cleavage which

may be initiated by thermal stress or shock (10-11), though it is nearly impossible to grow

substantial CaI2 crystals due to its platelet growth habit. Efforts to demonstrate scintillation from

Barium halides have indicated promising results, particularly for BaI2(Eu), exhibiting gamma

light yields of 35,000 Ph/MeV (8) and BaBr2(Eu) measured at 32,000 Ph/MeV (12).

Our earlier report on SrI2(Eu) (8) characterized the scintillation performance of a

SrI2(0.5%Eu) crystal. It exhibited energy resolution of 3.7% at 662 keV. Subsequently, we have

approached optimization of SrI2(Eu) in two directions. The first has been to carefully purify the

SrI2 feedstock by zone refining. The second has been to increase the Eu doping level. In this

report, we describe the zone refining methodology and scintillation properties of undoped and

Eu2+

doped SrI2, including the accurate determination of melting points. The effect of

purification by zone refining was studied and crystal growth was performed using the Bridgman

technique. Finally, scintillation performance of both undoped and doped SrI2(Eu) is described.

Proc. of SPIE Vol. 7079 70790Y-2

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Experimental:

A single crystal of SrI2 was grown and zone refined. Ultra dry SrI2 powder with purity of

99.9% from Alfa Aesar was used. The 200 g of SrI2 powder was placed in a freshly cleaned and

pre-baked quartz ampoule of (14 mm outside diameter and 2mm thickness of 4 feet long). The

ampoule was sealed under high dynamic vacuum of 4x10-6

torr. The synthesis was carried out in

a resistance-heated furnace. The synthesized material was then sealed in a conically tipped quartz

ampoule. Then the ampoule was transferred into a horizontal zone refining system, as shown in

Figure 1, insert pictures are the photos of the SrI2 crystal sections cut from the zone-refined

boule. The zone refining system heating profile was 8-10 oC/hour up to 400

oC, then 5-6

oC/hour

up to 580 oC. Then, the furnace was moved at a rate of 10 cm/day for 8 days for the first cycle,

and this process was then repeated at the faster rate of 100 cm/day for 15 cycles.

Figure 1. Photograph of the zone refining system. The insert shows sections taken from the

first-to-freeze and last-to-freeze sections of the SrI2 ingot after the refining process. Proc. of SPIE Vol. 7079 70790Y-3

Zone-refined material was then used to grow undoped and Eu2+

Strontium Iodide SrI2

using the vertical Bridgman technique at a rate of ~10 mm/day. Crystals were grown by melting

the entire charge, taken from the pure section of the zone refined boule, with added EuI2 powder

at 5% doping level, prior to the start of the run and subsequently lowering of the ampoule

through the heating zone. Solidification starts at the tip, which acts as a seed for random crystal

growth along the length of the tip and the remainder of the crystal. After complete solidification,

the ampoules were allowed to cool to room temperature at a rate of 40oC/day. Simultaneous

differential scanning calorimetry (DSC)/thermogravimetric analysis (SDT) were performed to study the

phase transition, melting point and the thermal stability of the material using TA instruments model SDT

2960. For scintillation measurements of the undoped SrI2 crystals, a Hamamatsu R6427

photomultiplier tube (PMT) was used for recording pulse height spectra with 662 keV gamma

rays from a 137

Cs source, while a Hamamatsu R6231U (Ultrabialkali photocathode) PMT was

used to measure the pulse height spectrum from the SrI2(Eu) crystal.

Results and Discussion:

Zone Refining and Growth of SrI2: The SrI2 material was successfully purified by zone

refining as shown in Figure 1, and the pure refined material was harvested from the middle of the

ingot. The Bridgman-Stockbarger technique was employed to grow SrI2 and SrI2:5%Eu2+

crystals which were found to be free of cracks; after sectioning the ingot with a wire saw; the

samples were lapped and polished with mineral oil as a lubricant and avoiding moisture. Figure

2, shows a photographic of zone refined pure and impure Bridgman technique SrI2 Scintillator

crystals and Figure 3, shows a photograph zone refined SrI2:5%Eu2+

Bridgman technique

scintillator crystals.


Sr112

i!JJlP.FJ1!ILILIijILI

/II/III//iiiuuh//un pu19 71

Srlz'1" )HLIIIIULI(IWIUU

.Sr12A'

4 — a 1? 2+711 .' /0 tU

Figure 2. Photograph of Bridgman grown, zone-refined SrI2 for the first-to-freeze and last-to-freeze sections of the SrI2 ingot after the refining process. .

Figure 3. A photograph of SrI2: 5%Eu doped grown using Bridgman technique. The crystals are

immersed in mineral oil to protect them from moisture.

Fist –to- freezeLast –to- freeze


0.0—

628°C

-0.3-i I 536°CI 'I'/ i--.-

i::•

Srl1ImpureZR.

SrI1Pu

p•

p•

P

•I

•P

100 200 300 400 600

0Temperature ( C)

Thermal Analysis: The phase transition, melting point, and the thermal stability of the

material during the heating and cooling cycle were studied using differential scanning

calorimetry (DSC) under a dry nitrogen flow. Figure 4 shows the thermogram spectra of the pure

and impure sections from SrI2 zone-refining growth. The samples were heated from room

temperature to 800 °C followed by cooling to 200 °C, at a constant rate of 20 °C/min. the

melting points of the material was determined to be ~528°C and~536°C respectively, no phase

transition was observed up to the melting point of the material. However, these different melting

points indicate the effect of purification of material after the zone refining process.

Figure 4. DSC and TGA spectrum of a single crystal SrI2 showing the melting point and heat flow of the

pure and impure zone refined samples.


Scintillation Performance: The polished crystals were characterized for their

scintillation performance. Figure 5 shows the pulse height spectrum of a Bridgman grown

zonerefined pure section of SrI2 under 137

Cs γ-rays excitation at 662keV. The energy resolution

obtained was about 9% and the estimated light yield was approximately 22,000 photons/MeV.

Figure 6 shows the spectrum of Bridgman grown zone refined impure area of SrI2 under 137

Cs γ-

rays excitation at 662keV. The energy resolution obtained was about 12% and light yield was

estimated to be approximately 22,000 photons/MeV. Bridgman grown zone refined SrI2:5%Eu

was tested as a scintillator at LLNL under 137

Cs γ-rays excitation at 662 keV (Figure 7). Energy

resolution obtained was 2.7% and light yield was estimated to be about 110,000 photons/MeV

(referenced to LaBr3(Ce) acquired under the same conditions). The high light yield and excellent

light yield proportionality of SrI2(Eu) indicate that even better energy resolution is possible with

this material, which now matches the scintillation performance of LaBr3(Ce), and appears to be

considerably easier to grow..


2500

21100

1500

1000

500

0100 200 4110 1110 000

Enagy (key)

Sri2 ZR Undoped Unpure Area

R— 12%, Smaple sizeL.Y — 22,000 Ph/MeV

12x10x4 mm3

3000

2000

1000

0

100 200 300 400 500 600 700 800

Energy Key)

7000

6000

5000

4000

Figure 5. Pulse height spectrum of 662 keV γ-rays from a 137

Cs source measured at room

temperature using a pure zone refined, undoped SrI2 section.



temperature with a section taken from the impure section of the zone refined SrI2 ingot.




temperature of a SrI2(5%Eu) crystal. Inset shows the characteristic blue emission of the crystal

under UV excitation.

Conclusions

We have successfully zone refined and grown the new scintillator crystal SrI2 using the

Bridgman technique. The melting point of pure SrI2 was found to be 536 0C, while an impure section

from the zone refined boule exhibited a melting point of 528 0C. This melting point differential indicates

the success of our zone refinement for purification. Undoped crystals were found to produce an

estimated gamma light yield of 22000 photons/MeV, and energy resolution at 662 keV of 9% (pure

section) and 12% (impure section). We have also grown SrI2(5%Eu), measuring its gamma light yield at

11000 photons/MeV) and energy resolution at 662 keV of 2.7%.


Acknowledgements

This work was supported by the Domestic Nuclear Detection Office in the Department of

Homeland Security (Alan Janos) and by the National Nuclear Security Administration, Office of

Nonproliferation Research and Development (NA-22, Robert Mayo) of the U.S.DOE under Contract No.

DE-AC03-76SF00098, and this work was performed under the auspices of the Center for Physics and

Chemistry of Materials under Cooperative Agreement HRD-0420516 (CREST program) with the

National Science Foundation. This work was also supported under NSF HBCU-RISE program HRD-

0531183.

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SrI2, A novel scintillator crystal for nuclear isotope identifiers

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