How Lasers Can Help with Nuclear Nonproliferation Monitoring

Seeing the Light in LPP
Although light from plasmas is easy to collect, the difference in the wavelengths of light that specific molecules emit is more difficult to decipher. And uranium is so reactive with oxygen in the explosion fireball that it creates many different uranium oxide combinations. These molecular combinations can be anywhere from one uranium atom paired with a single oxygen atom, to multiple uranium atoms bonded to as many as eight oxygen atoms.

Multiple uranium species immediately complicate how spectroscopy deciphers simple light collection. These uranium species emit light in a such a tight color spectrum with such small differences in wavelengths that each wavelength is only beginning to be matched with its respective uranium oxide transition.

The effect of oxygen on uranium laser produced plasmas. A more intense flash of light associated with uranium monoxide is seen when more oxygen is present. However, with more oxygen the plasmas do not persist as long.

The researchers zoomed in on the tight spectrum of wavelengths using narrow-band filters the team had previously developed. These narrow-band filters work by isolating the light emitted at specific wavelengths so that only the wavelengths associated specific species are collected and analyzed.

One filter measured only atomic uranium, and another measured uranium oxide in the plasma during the laser pulses. The team then measured the light emitted from the plasma as they increased oxygen in the environment, watching to see how the chemistry changed in the presence of more oxygen.

Using precisely timed snapshots of the plasma (called fast-gated imaging), Harilal and his team directly observed how uranium monoxide and uranium atoms moved through the LPP over time and by location. This let them see how and where the species were formed and how they persisted as the plasma plume expanded and dissipated.

The team found that uranium oxides are formed further from the target, where lower temperatures favor molecular recombination. Uranium oxides also form at later times in the lifetime of the plasma. When more oxygen is present, the plasmas don’t last as long.

Understanding the evolution of uranium atoms to uranium monoxide to higher oxides is critical for predictive modeling of explosion events. Precise, experimentally validated models mean more effective nuclear nonproliferation monitoring and better overall understanding of uranium chemistry.

In addition to helping researchers better understand uranium plasma chemistry, the laser-based techniques used in this work are also under development for in-field, remote nonproliferation monitoring as well. Since laser ablation coupled with optical emission spectroscopy measures light emitted from a plasma, data collection can be done from a safe, standoff distance that requires no sample handling. This technique has implications for nuclear forensic and safeguards monitoring.