Tiny details crucial for building particle detector prototype (Pittsburgh Post-Gazette)

Gary Wilkin's attempt to clamp a wire that's thinner than a human hair is complicated by the fact that he can't see it against his white gloves.
Pam Panchak, Post-Gazette

Tiny details crucial for building particle detector prototype

Gary Wilkin is flat on his back below a shiny, 6-foot-high contraption made of long plastic tubes and broad metal plates. He's trying to find and clamp a dangling wire that's five times thinner than a human hair.

It's not going well.

"These wires are giving me fits," said Wilkin, a physics department technician at Carnegie Mellon University.

Six feet above, graduate student Zeb Krahn is feeding the wispy, gold-plated tungsten wire down the center of one tube, but Wilkin is having a hard time even seeing the wire at the bottom, much less clamping a 5-gram weight to it.

When he does clamp it, the clamp slips. When the clamp doesn't slip, any kink in the wire causes it to break.

"We make two steps forward," observed physics professor Curtis Meyer, "and one back."

This, believe it or not, is a big-time particle physics experiment. Or, at least, this is how a big-time particle physics experiment gets off the ground.

The CMU team is building a prototype for a particle detector that is expected to be installed at the Jefferson National Accelerator Facility in Newport News, Va., by the end of the decade. It would be part of an experiment called GlueX that will look for subatomic particles so exotic that even jaded physicists call them exotic.

These particles, called exotic mesons, could include species that contain none of the elementary particles called quarks that make up protons, electrons and garden-variety mesons, but that are comprised entirely of gluons, the force-carrying particles that normally hold quarks together. Failure to find such particles would undermine a theory known as quantum chromodynamics, which describes the interaction of gluons and quarks.

U.S. Department of Energy officials in April took an initial step toward a $250 million upgrade of Jefferson's Continuous Electron Beam Accelerator Facility (CEBAF). The upgrade would increase the energy of the 7/8-mile, racetrack-shaped accelerator to 12 billion electron volts. About $40 million of that total would be spent on the GlueX experiment.

Meyer is one of the leading investigators for GlueX and, along with Wilkin, an experienced hand when it comes to building particle detectors.

The 6-foot-long, 4-foot-diameter detector Meyer, Wilkin and Krahn are building would be only the first of three stages of the GlueX detector; a former student of Meyer, Daniel Carman of Ohio University, is working with Jefferson lab scientists to build the other stages.

Graduate student Zeb Krahn checks one of the prototype's Mylar plastic tubes, which have not proven to be as rigid as hoped.
Pam Panchak, Post-Gazette

The first-stage detector will consist of a bundle of more than 3,300 gas-filled, aluminized plastic tubes, each with a thin wire running its length. In operation, the CEBAF's electron beam will be converted into a photon beam, which in turn will be aimed at a liquid helium target at the center of the detector.

When the high-powered light beam hits the helium, it should generate a variety of subatomic particles that will fly out through the detector. When a particle flies through one of the detector's tubes, it will ionize the gas inside, completing an electrical circuit between the electrically charged tube and the grounded wire at its center. This will generate a signal telling experimenters that a particle passed through the tube.

Rather than running parallel to each other, about a third of the tubes will be angled, creating thousands of intersections between tubes. By identifying which tubes generate signals and by identifying the intersections between those tubes, the scientists will be able to determine the trajectory of each particle to within a tenth of a millimeter, Meyer said. The trajectory will help them identify the mass and other characteristics of each particle.

The first stage of the detector is designed to find slower particles with sharply curving trajectories; later stages will be oriented to detector faster moving particle with straighter trajectories.

Not if you build it, but how.

At this point, however, the CMU team is simply trying to figure out how to build it. To minimize deflection and loss of energy from the particles, the detector needs to be made of light materials, Meyer said. And maximizing the electrical field within each half-inch-diameter tube requires that the wires be as thin as possible.

The result is a device of remarkable delicacy. Building the prototype, which will include only about 200 tubes, will tell the researchers not only if the design will work, but will help them determine the best way to build it.

Meyer said it will take another year to finish building and testing the prototype; construction of the actual detector probably will take three years.

Already, the researchers are looking for alternatives to the Mylar plastic used in the tubes, which aren't as rigid as they had hoped.

Last week, Wilkin and Krahn began grappling with how to wire each tube, a task that seems better suited to a seamstress than a machinist. Wilkin, using a jeweler's loupe to help him clamp the thin wire, said he also could use some black-colored surgical gloves, noting that the thin wire is all but invisible against his white surgical gloves.

Up above, Krahn has his own problems, noting he can't push the wire into the tube. If the wire gets stuck, "you have to stop, pull it out, snip it off. It can be very frustrating when you're tired."

"I'll happily trade places with you, pal," Wilkin said.

For Krahn, a graduate student in physics, gaining experience in constructing an experiment is priceless. Most particle physics experiments are so large and run so long that few students get a chance to see them come together; many earn doctoral degrees without getting their hands on the hardware, he noted.

"I love this stuff," he added.