184
K. A. S. Fessler et al.
Keywords Uranium hexafluoride · Quantum cascade laser · Infrared
spectroscopy · Isotope analysis · Uranium isotopes · High performance infrared
Introduction
Parties to the Treaty on the Non-Proliferation of Nuclear Weapons (the NPT) which
enrich uranium to various grades for energy, medicinal, and defense applications are
required to adhere to treaty terms to ensure nonproliferation of nuclear weapons.
The International Atomic Energy Agency (IAEA) is responsible for verification of
these safeguard measures contained in the treaty. Traditionally, mass spectrometry
or radiometric analysis has been used to determine the ratio of uranium isotopes
in a sample. Mass spectrometry offers unparalleled accuracy and sensitivity but
has been limited to expensive laboratory-based measurements due to the size and
complexity of the instrumentation and often requires extensive sample preparation,
greatly increasing sample analysis time [1]. In contrast, radiometric techniques can be
field deployable but require large sample sizes and have not demonstrated adequate
sensitivity or repeatability for treat monitoring [2].
Infrared (IR) absorption spectroscopy is a nondestructive optical technique sensitive to isotopic mass that detects molecular vibrations and rotations upon the absorption of infrared light. As different chemical bonds absorb IR light at different quantized frequencies, IR spectroscopy can be used for chemical structure analysis, chemical fingerprinting, and chemical imaging [3–5]. The unique resonance frequency of
a given bond depends on the strength of the bond, the mass of the bound atoms, and
the symmetry of the molecule being excited. Due to its broad detection capabilities,
IR absorption spectroscopy is an excellent spectroscopic tool for measuring analytes
of interest in process facilities.
Recent improvements in infrared laser and detector technologies have dramatically increased the analytical capability of spectroscopic-based quantification techniques, while simultaneously reducing equipment costs, experimental complexity,
footprint, and analysis time [6–9]. These advances have made high performance
infrared spectroscopy a potential approach for real-time isotopic analysis of the
isotopic distribution of uranium in uranium hexafluoride (UF 6 ) gas. Grigor demonstrated the determination of uranium isotopics to about 5% accuracy using an infrared
spectrometer and a tunable diode laser centered at 7.75 μm (1290 cm
−1 wave
number). The isotopic shift between
238 U and
235 U is about 0.65 cm
−1 at the vibrational combination band near 1291 cm
−1 . Nabiev and co-workers published a thorough study of the ν 1 + ν 3 infrared absorption band of the UF 6 molecule using a
quantum cascade laser-based spectrometer, similar to the one used in this work, and
demonstrated the ability to use quantum cascade laser technology to determine the
isotopic ratio of
235 U/
238 U of various isotopic mixtures of UF 6 gas at room temperature and moderate pressures (10-70 Torr). The focus of this work is to improve the
accuracy and sensitivity of UF 6 isotopic analysis and advance the technology toward
a field deployable instrument, as previously outlined by Fessler and co-workers [10].
K. A. S. Fessler et al.
Keywords Uranium hexafluoride · Quantum cascade laser · Infrared
spectroscopy · Isotope analysis · Uranium isotopes · High performance infrared
Introduction
Parties to the Treaty on the Non-Proliferation of Nuclear Weapons (the NPT) which
enrich uranium to various grades for energy, medicinal, and defense applications are
required to adhere to treaty terms to ensure nonproliferation of nuclear weapons.
The International Atomic Energy Agency (IAEA) is responsible for verification of
these safeguard measures contained in the treaty. Traditionally, mass spectrometry
or radiometric analysis has been used to determine the ratio of uranium isotopes
in a sample. Mass spectrometry offers unparalleled accuracy and sensitivity but
has been limited to expensive laboratory-based measurements due to the size and
complexity of the instrumentation and often requires extensive sample preparation,
greatly increasing sample analysis time [1]. In contrast, radiometric techniques can be
field deployable but require large sample sizes and have not demonstrated adequate
sensitivity or repeatability for treat monitoring [2].
Infrared (IR) absorption spectroscopy is a nondestructive optical technique sensitive to isotopic mass that detects molecular vibrations and rotations upon the absorption of infrared light. As different chemical bonds absorb IR light at different quantized frequencies, IR spectroscopy can be used for chemical structure analysis, chemical fingerprinting, and chemical imaging [3–5]. The unique resonance frequency of
a given bond depends on the strength of the bond, the mass of the bound atoms, and
the symmetry of the molecule being excited. Due to its broad detection capabilities,
IR absorption spectroscopy is an excellent spectroscopic tool for measuring analytes
of interest in process facilities.
Recent improvements in infrared laser and detector technologies have dramatically increased the analytical capability of spectroscopic-based quantification techniques, while simultaneously reducing equipment costs, experimental complexity,
footprint, and analysis time [6–9]. These advances have made high performance
infrared spectroscopy a potential approach for real-time isotopic analysis of the
isotopic distribution of uranium in uranium hexafluoride (UF 6 ) gas. Grigor demonstrated the determination of uranium isotopics to about 5% accuracy using an infrared
spectrometer and a tunable diode laser centered at 7.75 μm (1290 cm
−1 wave
number). The isotopic shift between
238 U and
235 U is about 0.65 cm
−1 at the vibrational combination band near 1291 cm
−1 . Nabiev and co-workers published a thorough study of the ν 1 + ν 3 infrared absorption band of the UF 6 molecule using a
quantum cascade laser-based spectrometer, similar to the one used in this work, and
demonstrated the ability to use quantum cascade laser technology to determine the
isotopic ratio of
235 U/
238 U of various isotopic mixtures of UF 6 gas at room temperature and moderate pressures (10-70 Torr). The focus of this work is to improve the
accuracy and sensitivity of UF 6 isotopic analysis and advance the technology toward
a field deployable instrument, as previously outlined by Fessler and co-workers [10].