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

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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

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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

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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

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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).