Novel Scintillators Based on Metal Organic Frameworks 

F. P. Doty

Sandia National LaboratoriesLivermore, CA

Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s

National Nuclear Security Administration under contract DE-AC04-94AL85000.

Sandia MOF Team• Synthesis and characterization

– Christina Bauer (UCLA)– Ragu Bhakta– Noel Chang (UCB Chemistry)– Ronald J. T. Houk

• Modeling– Ida M. B. Nielsen

• Ion Microprobe– Gyorgy Vizkelethy

• Principal investigators– Mark D. Allendorf– F. P. Doty

Available PSD Scintillators Poor Match to Fission Spectrum

• Fission neutrons:– Sources: 235U, 241Pu– Spectra peak < 1 MeV

• Applications:– Detecting nuclear proliferation– Long term monitoring– Active interrogation– Neutron radiography

Liquid organic scintillators – Low detection efficiency – Combustible– Toxic solvents– Delayed light quenched by oxygen

Efficiency of liquid scintillator NE213 (EJ 301 equivalent) compared to the normalized Watt spectrum. The evaluated efficiency was obtained for a cylinder (diameter 53 mm, height 101 mm) using a 252Cf source (86 kBq) incident perpendicular to the cylinder axis, threshold 0.15 MeV, and acquisition time 210 hours.

Data from J. Cub, et al., Nuclear Instruments and Methods A274 (1989) 217-221

0.01

0.1

1

0.1 1 10

En / MeV

Pro

bab

ilit

y

0.01

0.1

1

Eff

icie

ncy

Normalized Spectrum n(E) Neutrons Detected P(E)

NE213 Evaluated Efficiency Integrated Neutrons Detected

Liquid scintillators using PSD detect

<20% of 235U fission neutrons

Brighter, more proportional scintillator materials are needed to increase efficiency

Volume dependence of PSD FOM

• PSD Relies on fast timing• Multiple interactions and

increased optical paths rapidly degrade timing and figure of merit

• Alternate signatures would enable larger volume, higher sensitivity

The figure of merit, M, at the gate of 0.3 MeV recoil electron energy versus the initial delay of 250 ns wide gate. The same dependences for the M factor normalized to the photoelectron number.

Moszynski, M.; Costa, G. J.; Guillaume, G.et al."Study of n discrimination with NE213 and BC501A liquid scintillators of different size," Nucl. Inst. Meth. Phys. Res. A 1994, 350, 226.

Limited Practical Volume

New scintillator materials are needed to increase volume and sensitivity

Stilbene MOFs: multiple structures result from the same starting materials under different synthetic conditions

“3D” cubic structure• DEF, 105 °C/ 16 hours• Interpenetrated, IRMOF-type structure• Open porosity• Surface area = 580 m2/g

“2D” hexagonal structure• DMF, 70 °C/16 hrs; 85 °C/ 4 hrs• Dense material

Formula unit: ZnL3(DMF)2

L = 4,4’-stilbenedicarboxylic acid

Formula unit: Zn4OL3

C. A. Bauer, T. V. Timofeeva, T. B. Settersten, B. D. Patterson, V. H. Liu, B. A.Simmons, M. D. Allendorf, J. Am. Chem. Soc. 2007, 129, 7136.

Fluorescence Spectra Correlate with Structure

• Increased fluorescence lifetimes

– Stilbene in solution: 1 < 100 ps

– LH2: 1 = 0.73 ns, 2 = 2.49 ns

– 2D MOF: 1 = 0.2 ns, 2 = 0.95 ns

– 3D MOF: = 0.50 ns

Single crystal fluorescence and excitation spectra

Dec

reas

ing

linke

r in

tera

ctio

ns

C. A. Bauer, et al., J. Am. Chem. Soc. 2007, 129, 7136.

Ion Beam Induced Luminescence (IBIL)

Fluorescence/excitation curves for the 3D stilbene MOF, compared with the IBIL emission. Inset shows optical micrograph under UV illumination.

Fluorescence/excitation curves for the 2D stilbene MOF, compared with the IBIL emission. Inset shows optical micrograph under UV illumination.

F. P. Doty, C. A. Bauer, P. G. Grant, B. A. Simmons, A. J. Skulan, and M. D. Allendorf, Radioluminescence and radiation effects in metal organic framework materials, Proc. of SPIE Vol. 6707 67070F-2 (2007)

IBIL Spectra Less Structure Dependent

Electronic spectra reflect molecular structure, intermolecular interactions, and excitation mode

Chromophore interaction: MOF-S1 < MOF-S2 < Linker IBIL: Broadened, varying degrees of vibronic structure More diffuse spectra suggest distortions Shift to longer wavelength ---

Dimer formation? Multiple luminescent states?

1.2

1.0

0.8

0.6

0.4

0.2

0.0

No

rma

lize

d In

ten

sity

700600500400300Wavelength (nm)

3D MOF IBIL

Fluorescence: Excitation Emission

1.2

1.0

0.8

0.6

0.4

0.2

0.0

Nor

mal

ized

Inte

nsity

2D MOF IBIL

Fluorescence: Excitation Emission

1.2

1.0

0.8

0.6

0.4

0.2

0.0

Nor

mal

ized

inte

nsity

SDCAH2

IBILFluorescence:

Emission Excitation

E(Stokes)

E(Stokes) ~ large

E(Stokes): intermediate

E(Stokes):~ small

Linker

MOF-S2

MOF-S1

F. P. Doty, C. A. Bauer, A. J. Skulan, P. G. Grant, and M. D. Allendorf , Adv. Mater. 2009, 21, 95–101

Analysis of IBIL Vibronic Progressions

Linker crystal SDCH2 MOF-S1 MOF-S2

F. P. Doty, C. A. Bauer, A. J. Skulan, P. G. Grant, and M. D. Allendorf , Adv. Mater. 2009, 21, 95–101

Excited state determines the fluorescence and IBIL spectra: • Fitted Franck-Condon structure with a Gaussian line shape, • Assumed constant vibrational frequency and variable line widths, • Obtain vibrational frequency of 1994 cm-1 for the MOF progressions and 1202 cm-1 for SDCH2

Both frequencies are similar to an intense band in the Raman spectrum of trans-stilbene that is assigned to the totally symmetric C(ethenyl)–C(phenyl) stretch.

Dose Dependence of IBIL

Decay of IBIL signal as a function of dose, comparing the stilbene MOFs with anthracene.

Lines are fits of the IBIL intensity to a stretched exponential function.

Inset: Proton ion-beam-induced luminescence spectra for MOF-S2, obtained using a continuous 3 MeV proton beam depositing a 5 Mrad/s dose rate for thin samples.

Spectra were taken at 2 s intervals.

F. P. Doty, C. A. Bauer, A. J. Skulan, P. G. Grant, and M. D. Allendorf , Adv. Mater. 2009, 21, 95–101

source

PM tube

PIN diode

h

Ortec 142A

Scintillating Powder Decay Time

F. P. Doty, Ronald J. T. Houk, R. K. Bhakta, Ida M. B. Nielsen, Gyorgy Vizkelethy, Mark D. Allendorf, SPIE conference on CBRNE Sensing X, Orlando, FL United States 16 April 2009

MOF S2 Scintillation Response

MOF S2 alpha luminescence

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

10 100 1000

delay / ns

Co

un

ts

t-1

F. P. Doty, Ronald J. T. Houk, R. K. Bhakta, Ida M. B. Nielsen, Gyorgy Vizkelethy, Mark D. Allendorf, SPIE conference on CBRNE Sensing X, Orlando, FL United States 16 April 2009

Least squares fit components

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

10 100 1000

delay / ns

Co

un

ts

MOF S2 data

exp1: 54%

exp2: 11%

1/t: 35%

total: 100%

Timing Characteristics of MOF S2

• Exponential components: 65%

– 1 2.6 ns, 2 23 ns• Slow component: 35%

– 1/t time dependence

Timing Spectral Dependence

• No Filter: 5124 cph• SWF: 2125 cph• Relative count rate with SWF: 41%

All Components due to Emission from the Same Excited State

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

350 400 450 500 550 600 650

Wavelength / nm

QE

or

Re

lati

ve

Inte

ns

ity

.

2D IBIL

QE Bialkali PMT

IBIL*QE

SWF 450

42% 58%

Long and Short Wavelengths Show Exactly the Same Decay

MOF S2 alpha luminescence

10 100 1000delay / ns

No

rma

lize

d C

ou

nts No Filter

SWF 450

t-1

source

PM tube

PIN diode

h

Ortec 142A

Short wavelength Pass filter

• Fluorescence – Excitation Produces Only

Neutral Excited States– Main (fast) Fluorescence from

Singlet Monomers– Minor (delayed) Component

from Neutral Dimers

• Radioluminescence– Excitation produces ions– Ionized Monomers form Stable

Dimer Cations– Dimer Cations Capture e- to form

Neutral Excited Dimers– Radiative Relaxation to Dissociated

Ground State

MOF S2 Luminescence: Working Hypothesis

*DeD Hot electrons

hMMD *MDMD *Thermalized electrons0* ADAD Trapped electrons

Three Timing Components due to Three Paths to Neutral Dimer:

Tunneling from traps should depend on particle LET

LET dependence of MOF S2 scintillations

• 241Am excitation• PIN diode detector• System resolution 200ps

• 137Cs excitation• PIPS detector• System resolution 10ns

0.001

0.01

0.1

1

10

0 100 200 300 400

Delay / ns

No

rma

lize

d C

ou

nts MOF S2 beta

1/(time) fit

0.001

0.01

0.1

1

10

0 100 200 300 400

Delay / ns

No

rma

lize

d C

ou

nts MOF S2 alpha

1/(time) fit

12% of counts delayed

35% of counts delayed

3-fold increase in delayed light for alphas

Summary and conclusions

Acknowledgements This work was funded by: DOD, Defense Threat Reduction AgencySandia, Laboratory Directed R&D program

Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company,for the United States Department of Energy’s National Nuclear Security Administration

under contract DE-AC04-94AL85000.

•Properties of the first MOF-based scintillators have been evaluated.

•Light-yield, timing, radiation tolerance, and particle discrimination characteristics of the first compositions are comparable to commercial scintillators.

•These materials present a unique opportunity for both understanding and engineering processes determining detection limits, sensitivity, and particle discrimination performance of neutron sensitive materials.

•Our approach utilizes the synthetic control made possible by MOFs to produce framework structures designed to probe the mechanisms of excitation, transport and relaxation through various pathways, both radiative and nonradiative.

•Combined with detailed modeling this will quantify the dominant mechanisms determining spectral and temporal components of the luminosity..

3c. Excitons in Scintillator Radiation Detector Materials. Excitons are a fundamental means by which energy is transported in scintillators, including organic and inorganic compounds. For example, when organic materials are irradiated with neutrons, the triplet excitons formed are known to migrate, and upon encountering other excitons, they self-annihilate giving rise to singlet excitons with a characteristic delayed emission that is a signature for neutrons (known as pulse shape discrimination, PSD).

We are soliciting both theoretical and experimental optical projects that elucidate the dynamics of excitons in terms of their migration and interactions, as they pertain to these crucial mechanisms of scintillators: pulse shape discrimination and nonproportionality. Aspects of excitons that are of interest include, but are not limited to, diffusion rate and distance, radiative and nonradiative lifetimes, exciton-exciton Auger upconversion, and trapping by activators and defects.

Applicants in this area are strongly encouraged to team with a DOE laboratory that has experience developing practical radiation detectors.

An application should not be submitted in response to this Notice of Interest, but may be submitted after release of the FOA. The FOA is anticipated to be released on or about June 22, 2009.

Notice of Interest concerning the upcoming release of a Funding Opportunity Announcement (FOA) to solicit research and development (R D) applications regarding proliferation detection.