A New Way to Stop & Store X-rays

X-rays reveal broken bones and objects hidden in airport luggage. They detect abnormalities in breast tissue, examine blood vessels while arteries are being repaired, and kill cancer cells. X-rays illuminate structures in crystals and stars. Although x-rays are an extremely useful tool already, the future looks bright for new applications. Among other projects, scientists are working on ways to control the movement of x-rays more precisely in order to use them for next generation methods of storing and transmitting information.

X-rays are a type of electromagnetic radiation with higher energy than visible light. The pictures created by sending x-rays through objects are also called x-rays, but here we’re talking about the rays themselves.
Image Credit: NASA’s Imagine the Universe

X-rays are a high energy form of electromagnetic radiation, and they can be focused just like other types of light. However, x-rays can be focused onto even smaller areas than visible light, thanks to a property of light called the diffraction limit. This means that information processing systems based on x-rays could be extremely compact.

Captivated by this idea, PhD student Xiangjin Kong at the Max-Planck-Institut für Kernphysik and his advisor Adriana Pálffy recently demonstrated a new way that an x-ray pulse can be mapped and stored. Storing an x-ray pulse is a first step toward being able to control the velocity and other important properties of x-rays, which are necessary if you want to use x-rays to manipulate information.

Their theoretical work, published last week in Physical Review Letters, describes a physical system in which you send an x-ray pulse into a very thin material containing a layer of iron nuclei. You do this while an external magnetic field is turned on. Turn the magnetic field off while the x-ray pulse is inside of the material, and the pulse is stopped and stored in the iron nuclei. Turn the magnetic field back on, and the pulse is restored and continues traveling as if nothing happened.

This image shows the simulated movement of the pulse inside of the material. The magnetic field is turned off at approximately 185 nanoseconds (ns) and back on again at approximately 310 ns.
Image Credit: Xiangjin Kong and Adriana Pálffy.

Of course, it’s not quite that simple. The properties of the magnetic field, material, and x-rays all have to be precisely chosen. To better understand this, we can look at a technique that is used to slow down and store visible light. The method is physically different from this new x-ray storage method, but the researchers found that the two systems are described by similar mathematical equations.

First, recall that an atom is made of a nucleus containing protons and neutrons, surrounded by orbiting electrons. These electrons can be in different states—the lowest energy ground state, or higher energy excited states. You can bump an electron up to an excited state by giving it some energy and it will stay there for a short time, until it decays by emitting a photon and returning to the ground state.

In certain cases, you can use lasers to stimulate the bumps and drive atomic transitions. Lasers can not only move electrons from one state to another, but can also create something called atomic coherences between two states.

The x-ray system that Kong and Pálffy describe is similar to one that involves shining two lasers with different, carefully chosen energies onto a material. The lasers interact with the atoms in the material in such a way that the first laser slows down the photons from the second laser, called the probe laser (for more on this, see “Slow Light” here). When the first laser is turned off, the photons from the probe laser that are inside the material are transformed into atomic coherences and trapped. When the first laser is turned back on, the photons are set free.

Like atoms, the nuclei of atoms have ground states and excited states. The new x-ray storage system acts like the one described above, but uses a magnetic field (instead of a laser) to transform x-ray pulses into coherences of the iron nuclei (instead of atomic coherences). The coherences can be restored to the original x-ray pulse as long as the magnetic field is turned back on within about 100 nanoseconds.

In the last few years scientists have proposed and designed alternative methods for storing x-rays, but Kong and Pálffy predict that this method will be more reliable, easier to set-up experimentally, and more flexible in terms of the storage time and other parameters. The next step will be to try it out and see what happens experimentally!

—Kendra Redmond

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