Uranium is actually a rather common element on Earth. You can find it in small quantities within most rocks, but 99.3% of that uranium is the relatively harmless uranium-238. It’s the much more rare uranium-235 that sustains nuclear reactions and is vital in the production of nuclear weapons. Naturally, being able to detect this isotope of uranium is important, and scientists from the University of Michigan think they’ve found a way to do that at a distance using lasers.
The physical difference between uranium-238 and 235 is miniscule — uranium-238 has three more neutrons than 235 does. That has a significant impact on its properties, making it useful in nuclear power plants and in nuclear weapons. In order to do that you need to enrich uranium it by concentrating the 235 isotope. Facilities that do this are highly regulated, and a “rogue” enrichment operation is bad news for everyone. That’s something authorities want to track down, but how?
A concentration of uranium-235 higher than the trace background levels indicates an enrichment operation. Scientists can differentiate these two isotopes from each other using a variety of methods. Although, most of them require close proximity to the sample. The laser-based system developed by U of M nuclear engineer Igor Jovanovic and his colleagues does not suffer that limitation.
The team used very short, high energy laser pulses to identify uranium isotopes. Plasma is momentarily formed when a laser of this sort (known as a femtosecond filament laser) strikes an object, which in this case leads the uranium atoms to form a bond with oxygen in the atmosphere around them. By analyzing the optical and acoustic spectrum of the resulting uranium oxide molecules, the researchers confirmed there’s a measurable difference between natural and enriched uranium.
This experiment was conducted in a laboratory setting at Penn State with real weapons-grade enriched uranium from the school’s reactor. Thus, the team believes the results are replicable in real life. They also note that air does not interfere with the measurements. The testing was done a a distance of several meters, but it could work at distances up to a mile.
The system could potentially detect enriched uranium in dust around an illegal enrichment facility, or simply verify enrichment more easily at an authorized reactor. The team envisions detection lasers being mounted on trucks or drone aircraft. The use of femtosecond filament lasers is not limited to uranium detection. It could be useful in identifying a great number of elements at a distance.