Transcript Advances in Radiation Detector Materials and Technologies

Advances in Radiation Detector Materials and
Technologies
Lynn A. Boatner
Center for Radiation Detection Materials and Systems (CRDMS)
Oak Ridge National Laboratory
Presented at
Rutgers University
January 8, 2011
Research at the CRDMS is supported in part by the DOE Office of Nonproliferation Research and Development, NA-22, in the Nuclear
Security Administration and in part by the Domestic Nuclear Detection Office of the Department of Homeland Security.
Radiation Detection by Scintillators or
Electronic Materials
Scintillator
Detector
Photomultiplier
Incident Radiation
Si Photodiode
Eye
Incident
Radiation
Light Output
(usually visible to
near visible)
SCINTILLATION DETECTION
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ELECTRONIC DETECTION
Gamma-Ray Scintillators
3 Major Processess:
ABSORPTION OF GAMMA- OR X-RAY PHOTONS AND CONVERSION INTO
CHARGED PARTICLES (ELECTRON-HOLE PAIRS).
Direct process: for E> 1.02 MeV the gamma ray directly produces an
electron-positron pair with the same total energy (pair production).
Compton scattering: gamma-ray energy is divided between a scattered
photon and a recoil electron
Photoelectric effect: the absorbed photon generates a fast electron and
a hole in a deep core level of an ion with the two carrying all of the energy
of the original photon.
ENERGY TRANSFER FROM THE ELECTRONIC EXCITATIONS TO THE
LUMINESCENCE CENTERS.
A complex and not well-understood process.
EMISSION OF THE SCINTILLATION PHOTONS:
Occurs with a quantum efficiency Q that represents the fraction of
excited centers that actually emit a scintillation photon.
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-Ray Detection Using a Scintillation Crystal
γ
Requirements for a Good Scintillator
β
S
L
Q
hυ
1. High Light Output (Photons/Mev)
NaI: Thallium  38,000
BGO  8,200
2. Short Decay Time (Nano Sec)
3. Wavelength Match to Detector
4. High Density (>6 g/cm3)
5. Chemical Stability
6. Radiation Hardness
7. Cost
8. Crystal Growth
NaI: Thallium  1948, Hufstader
BGO (Bi4Ge3O12)  1973, Weber & Monchamp
LSO (Lu2SiO5)  1992, Melcher & Schweitzer
β -ray to electronic excitation
S fraction transferred to luminescence centers
Q quantum efficiency of the emission step
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http://scintillator.lbl.gov/
State of the Art Scintillators
Material
Light Yield
(photons/MeV)
38,000
65,000
8,200
70,000
Resolution @ 662keV
(%)
5.5
6.2
12.7
2.8
CeCl3
46,000
3.4
LSO(Ce)
39,000
7.9
SrI2 : Eu (6%)
BC-408 Plastic
GS-20 Li Glass ($2930
120,000
10,600
4,100
2.7
17
NaI(Tl)
CsI (Tl)
BGO
LaBr3(Ce)
for 1-inch round, 2mm
thick/ $4,739 for 6.2-inch
square, 2mm thick plate)
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Among the alkaline earth halides, Strontium Iodide
(Eu) possesses the most promising characteristics
Advantages:
•Low melting point → reduced temperature gradients
•Size and structure match between SrI2 and EuI2 → unity distribution coefficient
•High light yield, proportional → superior resolution
•Congruent, since binary compound → no compositional gradients
•Near-UV emission → ideal match to PMT response
•Microsecond decay → enlarges dynamic range for pulse height spectrum
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A New Grain Selector
A new grain-selector geometry has
been incorporated into the 2.5” OD
Bridgman growth ampoules since it
was found that two grains would
sometimes propagate into the largediameter growth chamber during the
growth process – even the case of
long straight grain selectors that
incorporated a bulb configuration.
Similar grain-selector geometries are
uses in the growth of the metal and
alloy single crystals – including single
crystal high-performance-alloy turbine
blades – for nucleation suppression.
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Strontium Iodide Eu2+ Crystal Growth
Enlarged view of the quartz frit
that is used to filter the molten
SrI2:Eu2+ salt that then flows into
the Bridgman ampoule prior to
sealing under vacuum.
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Two-inch diameter single crystal of SrI2(Eu)
grown at Oak Ridge National Laboratory.
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Crystal 68f (Fisk University)
was
encapsulated
in
a
standard aluminum can, and
its performance is equivalent
to the best RMD crystal.
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Elpasolites
Cs2LiYCl6
CLYC
• High light yield
•
Cs2LiLaCl6
CLLC
•
Cs2LiLaBr6
CLLB
•
•
•
www.rmdinc.com
a dynasil member company
70,000 to 180,000 ph/neutron
High gamma equivalent
>= 3 MeV
High energy resolution
2-3%
Pulse height discrimination
Pulse shape discrimination
Cubic structure
Summary of Properties
Cs2LiLaCl6:Ce Cs2LiLaBr6:Ce
CLLC
CLLB
Density, g/cm3
3.5
Emission, nm
290CVL,
400Ce3+
Cs2LiYCl6:Ce
CLYC
4.2
3.3
410Ce3+
290CVL,
390Ce3+
1 CVL,
60, 400, …
55, 270, …
1 CVL,
40, 1800, …
Max. light yield,
ph/MeV
~ 35,000
~ 60,000
~ 20,000
Light yield
ph/n
~ 110,000
~ 180,000
~ 70,000
GEE, MeV
~3.1
3.2
~3.1
Best ER @662 keV
3.4
2.9
3.9
Excellent
Possible
Excellent
Decay time, ns
PSD
www.rmdinc.com
a dynasil member company
Bridgman Grown Crystals
Solidification zone
Melt zone
1 in CLYC
1 in
CLLC
www.rmdinc.com
a dynasil member company
CLLB
CLYC: 6Li vs.
natLi
intensity, counts/sec/gram
2.0
CLYC:Ce
3.4 MeV
Am/Be Spectra
6
>95% Li
1.5
Enrichment significantly
improves detection of
thermal neutrons.
1.0
0.5
6
7% Li (natural)
477 keV
0.0
0.0
0.5
1.0
3.3 MeV
1.5
2.0
2.5
energy, MeV
www.rmdinc.com
a dynasil member company
3.0
3.5
New Metal-Organic Scintillators
Investigations of alternate methods for growing large single crystals of rareearth halide scintillators from organic solutions have led to the discovery of a
new metal-organic scintillator crystal. This new scintillator material is a
methanol adduct of cerium trichloride with the formula: CeCl3(CH3OH)4.
Large transparent single crystals of this material were grown from a seeded
anhydrous methanol solution in a controlled-temperature bath, and the
molecular structure was subsequently determined by single-crystal x-ray
structure analysis.
The CeCl3(CH3OH)4 metal-organic scintillator is applicable to x-ray, gamma-ray,
alpha-particle, and neutron detection, and this new finding offers the promise
of identifying other similar metal-organic molecular systems that offer the
potential for serving as efficient radiation detector materials that can
potentially be grown in large sizes using solution-growth methods.
Most recently the scintillator La(4%Ce)Br3(CH3OH)4 has been discovered.
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Crystal Structure
Perspective view of the CeCl3(CH3OH)4 adduct showing the bridging role of the chlorine atoms. The
basic CeCl3(CH3OH)4 crystal data resulting from the single crystal x-ray structural refinement are:
M = 374.64, monoclinic structure, space group P21/c (no. 14), a = 8.7092(5), b = 18.5100(9), c =
8.2392(4) Å, β = 108.946(1)°, V = 1256.2(1) Å3, Z = 4, and Dcalc = 1.981 g/cm3.
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Large faceted single crystal of
CeCl3(CH3OH)4 grown from an
anhydrous methanol solution shown on a cm scale. The
platinum wires used to hold the
seed crystal on the growth
platform are visible through the
crystal. Crystal growth was
allowed to continue for a total
growth time of 24 hrs - at which
time the crystals were removed
from the vessel, rinsed clean of
the solution in fresh anhydrous
methanol, dried, and sealed under
dry inert gas.
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Energy Spectra
Energy spectrum of the CeCl3(CH3OH)4 metal-organic scintillator single crystal obtained using
662 keV gamma rays from a 137 Cs 1  Curie source. The light yield is ~ 230% of that of a BGO
reference crystal - yielding a light yield of ~16,600 photons/MeV without corrections for the
photomultiplier tube efficiency. The energy resolution was determined to be 11.4% for this
specimen.
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X-ray Luminescence
X-ray-excited luminescence spectrum for a single crystal of CeCl3(CH3OH)4 measured
in both transmission and reflection geometries using an x-ray tube operated at 35 kV
as an excitation source. The peak of the luminescence occurs at ~ 365 nm.
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Transparent Polycrystalline Ceramic
Scintillators
Glass Scintillators
Why would we want these?
Single crystal growth is a time-consuming, expensive, and rate-limiting
process.
Transparent polycrystalline ceramic scintillators and glass scintillators offer
an alternative approach to scintillator synthesis that eliminates single
crystal growth.
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Lu2O3:Eu
Synthesis and Post Synthesis Treatment
•Lu2O3 and Eu2O3 (5 wt. %) powders combined physically
•Powder heated in vacuum to dry
•Hot pressed at 1530°C with 262 kg/cm2 of pressure
•Annealed with flowing oxygen for 72 hours at 1050°C
Photograph of a Lu2O3:Eu ceramic before
(right) and after (left) annealing in an oxygen
atmosphere. Hot pressing technique tends
to draw oxygen out of the host lattice,
creating a dark color in the densified body.
This coloration can be removed by
annealing in an O2.
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Photograph of a Lu2O3:Eu
ceramic excited by a 30kV
continuous X-ray source.
Photographs of a transparent
Lu2O3:Eu ceramic (~1mm thick)
LSO:Ce
Synthesis and Post-synthesis
Treatment
•High quality LSO:Ce powder produced by
Nichia Corporation (Japan) used
•Powder heated in vacuum to dry
•Hot pressed at 1400°C with 337 kg/cm2 of
pressure for 2 hours
•Annealed in vacuum at 1050°C/108h
Photograph of a LSO:Ce
ceramic before (left) and after
(right) annealing in vacuum
Photograph of an LSO:Ce ceramic
(0.6 mm thick). Note that no backlight is used in this photograph.
•Annealed in water vapor at 1050°C/32h
•Annealed in air at 1150°C/32h
Transmission electron
microscopy (TEM) image of
LSO:Ce powder from Nichia
Corporation
Particle size distribution of the Nichia LSO:Ce
powder used to make the LSO ceramic.
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Scanning electron microscopy
(SEM) image of LSO:Ce powder
from Nichia Corporation.
Scintillating pulse shape of a LSO:Ce polycrystalline ceramic
excited by 662 KeV gamma photons. The solid line represents
single- and three-exponential (+ noise) fits to the experimental
data . The decay time constants and contribution of faster
components in comparison to the decay time of about 42 ns
generally accepted for single crystal LSO.
Energy spectra (for 662 keV excitation photons) of the LSO:Ce refernce crystal (the light yield for this crystal was
~30,000 photons/MeV) and the LSO:Ce ceramic at various post-sintering annealing stages. Symbol “A” denotes a
ceramic with a 2 mm thickness after annealing in vacuum, “A1” denotes a 0.7 mm thick piece of the former ceramic after
additional annealing in water vapor, and “A1a” the same after additional annealing in air.
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New scintillators for gamma ray
spectroscopy developed for DHS
and DOE. (upper left) SrI2(Eu)
single crystal under UV excitation,
(upper right) GYGAG(Ce) ceramic,
(bottom) two Bi-loaded polymers
under UV excitation.
Comparative Gamma Spectroscopy with SrI2(Eu), GYGAG(Ce) and Bi-loaded Plastic Scintillators
N.J. Cherepy, Member, IEEE, S.A. Payne, Member, IEEE, B.W. Sturm, Member, IEEE, J.D. Kuntz,
Z.M. Seeley, B.L. Rupert, R.D. Sanner, O.B. Drury, T.A. Hurst, S.E. Fisher, M. Groza, L. Matei, A. Burger,
Member, IEEE, R. Hawrami, Member, IEEE, K.S. Shah, Member, IEEE, and L.A. Boatner
IEEE Transactions on Nuclear Science, IEEE/NSS Proceedings 2010 (Submitted for publication)
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GLASS SCINTILLATORS
HOW CAN WE IMPROVE THEIR
PERFORMANCE?
Glass Scintillator Parameter Space
Composition
(Glass-forming space)
Cladding Phosphate
Lead Phosphate
Silicate
Germanate
Arsenate
…
Activation
Ce,Pr,Nd,Eu,Tb,Yb
Co-doping
Structure *
Phosphate glass only
Phosphate chain length
Post-synthesis Treatment
Time
Temperature
Atmosphere
* B. C. Sales, J. O. Ramey, L. A. Boatner, and J. C McCallum, “Structural in equivalence of the IonDamaged-Produced Amorphous State and the Glass State in Lead Pyrophosphate,” Phys. Rev. Lett. 62,
(10) 1138-1141 (1989).
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Energy Spectra of Ce Doped Ca-Na
Phosphate Glasses
137Cs
1μCi γ source
662 keV γ photons
0.5 μs shaping time
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Current Uses of 3He
Cryogenics below 1 K, laser research, guided missiles
No known alternative
Medical imaging of lungs
Unique capability
Security applications
Looking for alternatives
Oil well logging
Need alternatives
Nonproliferation
Low probability of finding alternatives
Neutron polarization
No known alternatives
Neutron scattering detectors
Need alternatives for large area coverage
Ron Cooper
29 September, 2009
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Gas Detectors
n He H  H 0.76MeV
3
3
1
 5333barns
Gas Proportional Counter
~25,000 ions and electrons
(~4x10-15 coulomb) produced per neutron
Anode
Fill gas
Cathode
R
-
+
HV
E-field
radius
1. 1 part in 104 of natural helium
2. Obtained from Tritium decay
1. 3T → 3He + e- + antineutrino
2. Half-life is 12.3 years
3. Tritium is produced in reactors
mainly for nuclear weapons
4. The Watts-Bar reactor, near Oak
Ridge - scheduled for tritium
production - delayed
5. Accelerator option, 40 3MeV
accelerators with 1A beam current
each, -20k liters/year 120MW of beam
power!
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Alternative Thermal Neutron Converters
6Li(n,α)
reaction:
n+
→ 3H + 4He Q-value=4.78 MeV
E3H=2.73 MeV E4He=2.05 MeV
Cross Section → 940 b
6LiF coatings - Chemical Stability
Ranges: 3H-32.1 microns; 4He-6.11 microns nabs-174 microns
10B(n,α) reaction:
n + 10B → 4He + 7Li [ground state] Q-value=2.792 MeV
n + 10B → 4He + 7Li [excited state] Q-value=2.310 MeV
ELi =0.84 MeV E4He =1.47 MeV
Cross Section → 3836 b
10B coatings
Ranges: 7Li-1.6 microns; 4He-3.6 microns nabs -19.9 microns
Boron straws
10BF (gas)
3
6Li
Other reactions:
157Gd(n,γ) (Natural Gd, cross section 49,000 b)
113Cd(n,γ)
Less useful gamma-rays and conversion electrons
Range calculations: McGregor, D. S., et al., NIM A 500 (2003)
272-308
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The challenge is to
balance thermal
neutron conversion
efficiency and charged
particle transport
(while minimizing
and/or rejecting
gamma-ray response)
Prototype 6Li-lined gas detector that incorporates a Li-coated Mo cathode,
a 0.001” stainless steel anode wire, and a Xe fill gas at one atmosphere.
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Typical preamplifier output pulse from a neutron interaction. Note the
large amplitude of the pulse, which after subsequent amplification in a
spectroscopy amplifier, leads to a count in the neutron “peak”
(>channel number 1000) shown in the pulse height spectra.
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DETAILS OF THE NEUTRON RESPONSE PORTION OF THE PULSE HEIGHT
SPECTRA ARE SHOWN FOR THE 6Li-LINED PROPORTIONAL COUNTER USING
AN AmLi MODERATED NEUTRON SOURCE -- UNSHIELDED SOURCE
(BLACK), A 2” THICK Pb-SHIELDED SOURCE (BLUE), AND FOR A CdSHIELDED SOURCE (RED).
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