Novel Scintillators Based on Metal Organic Frameworks F. P. Doty Sandia National Laboratories Livermore, 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.
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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
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
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
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
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.