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Improving X-Ray Imaging with a Streak Camera

Photonics Spectra
Dec 2007
Device provides high resolution simultaneously in time and space.

Hank Hogan

When using x-rays to study what is very small and fleeting, researchers have had two choices: develop a short-pulse x-ray source, an approach with a potential price tag in the hundreds of millions, or develop a fast enough detector — cheaper, but still a challenging task.

Now a group from Lawrence Berkeley National Laboratory in Berkeley, Calif., has taken a step in both directions. It demonstrated an x-ray streak camera that has a temporal resolution of 233 fs and a spatial resolution of 10 μm. It is the first device with such a high resolution in time and space simultaneously.

PRX-ray_figure_small.jpg

A schematic shows the setup of a high-resolution x-ray streak camera. Incoming x-rays strike a photocathode, and the generated electrons travel through a magnetic lens to strike a detector. Courtesy of Jun Feng, Lawrence Berkeley National Laboratory.


In some sense, though, that resolution is really just in space. “The streak camera uses a very neat idea which converts the fast time information into space information to be recorded by a CCD detector,” explained staff scientist Jun Feng.

Streak cameras pull off this feat of turning time into space by dispersing incoming photons into two perpendicular directions. One axis produces spatial information, while the other yields temporal data. Intensity also is recorded. The cameras have a photocathode that incoming photons strike after passing through a slit. The generated electrons are swept away by an accelerating mesh and deflected by sweep electrodes. When the electrons strike a recording device, their location yields time and space information, while their number yields intensity.

What makes the investigators’ streak camera different from previous versions is the use of a carefully designed extraction-mesh and sweep plate for acceleration and streaking. Another key improvement is the use of a large-aperture magnetic solenoid lens design that sits downstream of the deflection plates. The resulting magnetic lens operates outside the vacuum chamber holding the guts of the streak camera. According to the researchers, this arrangement significantly reduces vacuum and thermal issues.

Feng said that the device also includes a photoconductive switch design that reduces jitter and that generates ultrafast high voltage. He summed up the situation by noting that the camera was the result of the integration of many new technologies.

The researchers tested the device using 266-nm light from a Ti:sapphire femtosecond laser made by Coherent Inc. of Santa Clara, Calif., because there are no suitable x-ray sources. Using a gold photocathode, they showed a 10-μm resolution in space with a static image. With the same setup, they demonstrated that the system could resolve 233-fs pulses with 10-μm spatial resolution.

Along with appropriate x-ray sources, the streak camera could be used to study magnetic materials fashioned into thin films, multilayer constructs and nanostructures. Exploration of the origin of the magnetic phenomena in these objects could produce smaller magnetic bits and faster magnetic switching for data storage and other applications.

According to Feng, the new streak camera technology is promising but must be improved still further if research targets are to be met. “The goal by the year 2010 is to conduct x-ray probe experiments in reaction dynamics at subfemtosecond resolution.”

Applied Physics Letters, Sept. 4, 2007, Vol. 91, 134102.


GLOSSARY
detector
1. A device designed to convert the energy of incident radiation into another form for the determination of the presence of the radiation. The device may function by electrical, photographic or visual means. 2. A device that provides an electric output that is a useful measure of the radiation that is incident on the device.
photocathode
An electrode used to release photoelectric emission when irradiated, making it then the irradiated negative electrode of a phototube.
x-ray streak camera
A diagnostic instrument that uses a photocathode design to see a broad range of x-ray radiation by streaking the photographic image continuously in time. It produces time- and space-resolved measurements of x-ray emission from fusion targets.
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