LBL Researchers build tabletop x-ray device

March 4, 1994

By Jeffery Kahn, [email protected]

Since 1988, Life Sciences Division researchers Steve Derenzo and Bill Moses have combed through the world of materials, looking for light-emitting compounds called scintillators. As they searched for better scintillators for detectors used in medicine, science, and industry, they labored under a handicap.

To test candidate materials, Derenzo and Moses traveled to the National Synchrotron Light Source at Brookhaven National Laboratory in New York. The light source allowed them to identify which materials glow when hit with ionizing radiation, and characterize then. But to use the New York facility, Derenzo and Moses had to move and assemble more than a ton of equipment, and make sure it operated during the short window of beamtime allotted them.

Frustrated, the scientists decided to end their reliance on remote facilities. Recently, with electronics engineer Matt Ho and graduate student Stephen Blankespoor, they designed and built a novel pulsed x-ray system which now sits on a tabletop in their Bldg. 55 laboratory. Built in collaboration with Hamamatsu Photonics of Japan, the tabletop device allows immediate evaluation of samples.

"We are looking at inorganic scintillators for the detection of gamma rays," Derenzo says. "Right now, we have no reliable way of predicting what substances are good scintillators. So, we have to actually test materials.

"With better scintillators, we can improve the spatial and time resolution in a variety of detectors and sensors. This would produce not only more detailed images but also images of other phenomena that are not observable with current scintillators."

Positron emission tomography, which records chemical activity in the body, is limited by the performance of available scintillators. PET medical images usually rely on bismuth germanate (BGO) scintillators. LBL's PET scanner, which has the highest resolution in the world, can record structures as small as 2.6 millimeters.

Derenzo and Moses believe they will be able to find new scintillators five times brighter and ten times faster that can double the spatial resolution of the LBL instrument. Likewise, they believe comparable performance improvements are feasible for other scientific detectors that rely on scintillators.

Their new tabletop system will play a key role in the search process. It combines a laser diode and a light-excited x-ray tube. Essentially, the tube is a single-stage photomultiplier with a photocathode and tungsten anode. Light from the laser liberates photoelectrons at the photocathode which then strike the anode, producing x-rays.

Moses says the technique used for testing samples with the tabletop device is fairly simple. "We use the tabletop x-ray source to expose a sample to a short, intense burst of x-rays. Then, we see if the sample glows. This is done with a photomultiplier tube which counts photons."

Thus far during their search for new scintillators, Derenzo and Moses have examined roughly 600 materials. They are winnowing through available compounds looking for materials that are bright, in terms of their light output, but that also are fast.

"Light doesn't just come pouring out of a scintillator instantly," Moses says. "But the shorter the elapsed time between excitation of the scintillator and its emission of photons, the better.

Scintillation is rare in nature as most materials bombarded with ionizing energy dissipate it as heat rather than as light. Previous searches have required crystal samples but because relatively few compounds are readily available as crystals, Derenzo and Moses have devised techniques that allow them to use powder compounds to identify promising materials.

Before a compound can be used in a detector, it must be grown into crystals. Derenzo and Moses discovered that lead sulfate has scintillator properties that could extend the capabilities of PET scanners, but up until now, nobody has figured out how to grow crystals economically on a commercial scale.

Several private companies are working on this project, as is LBL researcher John Apps. The group has measured the properties of some small synthetic crystals of lead sulfate grown by Radiation Monitoring Devices of Watertown, Mass. These were presented at the 1993 IEEE Nuclear Science Symposium in San Francisco.