Separation by Scattering: Scientists take on an isotope challenge

You can split sunlight into a vibrant array of colors by sending it through a prism, as fans of physics (or Pink Floyd) know well, or by bouncing it off a mirror through a refractive medium like water. In an exciting but less colorful way, a team of researchers from the University of Chicago recently demonstrated in the American Physical Society’s journal Physical Review Letters that you can split neon gas into the specific varieties, or isotopes, of neon that compose the gas in an analogous way. This could be a more cost- and energy-efficient method for enriching isotopes, a key component in many medical technologies, energy systems, and other applications.

In a new method to separate isotopes, atoms directed at a specially crafted crystal surface bounce off at different angles based on their different quantum wavelengths. (The red track represents a lighter isotope, the blue track a heavier one.)
Image Credit: Arin R. Greenwood, Federico Giberti, and Kevin J. Nihill.

Hydrogen is hydrogen and neon is neon, right? Well, only kind of. If you dive a little deeper into what it means to be a chemical element, it quickly becomes clear that things aren’t quite so simple. Here’s a brief review of physical chemistry 101:

• Elements are defined by the number of protons in an atom
• Protons and neutrons are bound together in the nucleus of an atom
• Atoms with the same number of protons but different numbers of neutrons are called isotopes
• The number of protons + the number of neutrons in an atom = the atomic mass number

For example, neon, Ne, is tenth in the periodic table because it has 10 protons. However, neon comes in several isotopes. Three of them are stable and naturally occurring: 20Ne, 21Ne, and 22Ne. The superscript number to the left of the chemical symbol tells you the atomic mass number, so 20Ne has 10 neutrons, 21Ne has 11, and 22Ne has 12. The rest of neon’s isotopes are radioactive and decay quickly, most in under a second.

As long as an atom is neutral, it has the same number of electrons as protons. This means that all neutral atoms of the same element have basically the same chemical and physical properties, regardless of their atomic mass number. Most differences between isotopes of the same element boil down to a difference in mass (neutrons are heavy) or a difference in the structure of the nucleus.

By exploiting these differences, scientists can do things like label biological samples undergoing a chemical reaction using different isotopes. In this way, they can trace samples through a reaction without influencing the results. Additionally, some isotopes are better suited than other isotopes of the same element for specific applications in medical imaging, cancer treatments, energy production, microelectronics, forensic analysis, future applications in quantum computing, and more.

Taking advantage of these applications requires the ability to separate an element into its isotopes. If not completely separated, then one needs to at least “enrich” the amount of the desired isotope in a sample by removing as much as you can of the unwanted isotope(s). Since isotopes of the same element have basically the same physical and chemical properties, this can be a challenge. In many cases, it requires a repetitive multi-step process or a more expensive and energy intensive method, such as exciting the isotope you’re after with a laser.

This new work by Kevin Nihill, Jacob Graham, and Steven Sibener presents a totally different enrichment method that is more akin to separating white light into its colors. It relies on a unique instrument in Sibener’s lab that he and his team have built up over the years for in-depth studies of gas-surface collisions. “This is a very sophisticated experiment that involves a rotating detector that weighs over a ton,” says Sibener. This instrument wasn’t designed for enriching isotopes, but the team’s expertise and experimental achievements set them up for success in this new challenge.

The team started with a gas of neon atoms that reflected the relative proportions of 20Ne and 22Ne in nature. From this, they formed a well-aligned supersonic beam just a few millimeters across. This beam was unusual in that all of the atoms moved at the same speed. The only real difference between the isotopes was their mass, everything else was the same.

The combination of mass differences and uniform speed enabled the researchers to exploit a quantum mechanical effect. Isotopes of different masses moving at the same velocity have different wavelengths due to quantum effects at the atomic level (remember that like photons, atoms have both particle and wave properties).

When you take such a beam of atoms and collide it with a precisely patterned crystal surface, as the team did in this experiment, the beam is scattered off of the surface. The cool thing is that the atoms are scattered at slightly different angles based on their wavelength. In their experiment, the team saw different isotopes emerge from the collision at different angles, like different wavelengths of light emerging from a prism. Furthermore, the collision led to slight differences in the average velocities of the scattered 20Ne and 22Ne isotopes. The researchers suggest that in areas where both isotopes overlap in space, they can be separated based on their arrival times.

Achieving the maximum separation between isotopes requires “scrupulous consideration of the experimental setup,” say the researchers, as even in their experiment the two isotopes weren’t completely separated. However, their research demonstrates that this technique works and offers suggestions for improving the results even more.

“This accomplishment builds on the insights from many prior graduate students and postdoctoral fellows, and emphasizes the ongoing importance of long-term investment in fundamental science,” says Sibener. “It was not by accident that we were well-positioned to take on this challenge. It was the realization that all of [our previous work] could be used for an entirely new project that has made this such an exciting topic to explore!”

Kendra Redmond

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