Probing the Quirks of Quarks - A String of Pearls; Hall of Mirrors (Rensselaer Magazine)

Jefferson Lab Senior Scientist Charles Sinclair '60: "I can't but believe for a moment that it will lead to discoveries and inventions that will make life better for humanity."
Enlarged version of this photo.


Probing the Quirks of Quarks

A String of Pearls

Rensselaer's most visible technical contribution to the Jefferson Lab has been the design, construction, and installation of six so-called "Cerenkov" radiation detectors for the spectrometer in Hall B. If the rest of the Jefferson Lab can be thought of as a huge electron microscope, the spectrometers in Halls A, B, and C are its eyepieces. Each uses different detection technologies so that a variety of experiments may be performed.

The spectrometer in Hall B is made up of four different types of detectors; the Cerenkov detector that Rensselaer built at a project cost of $1.8 million plays a unique role. A Cerenkov detector works on the principle that while neither an electron nor anything else can ever go faster than the speed at which light travels in a vacuum, electrons and other charged particles can indeed whiz through some materials, like heavy C4F10 gas, faster than light itself can. When that happens an electromagnetic shock wave is produced and a photon, or a bit of light, is generated. That's Cerenkov radiation, and that's what Rensselaer's equipment was built to detect.

These electrons leave a string of photons in their wake, like strings of pearls. Looking at those paths is a roundabout but very precise way of knowing the presence, speed, and behavior of the electrons and to differentiate them from other high-speed particles.

But while that might be simple in concept, it was extraordinarily difficult to put into practice. The Hall B detector, the size of a large house with six large rooms filled with particle-detection devices, is staggering in its complexity.

In the Cerenkov detector, about 100 photons of visible light (an incredibly small luminosity) are produced by an electron as it travels through the gas, and as many as possible need to be guided through the detector's complex shape to reach light-sensitive photomultiplier tubes, which convert the light to electronic signals. To accomplish this, Stoler and his colleagues had to design and construct a very sophisticated system with about 600 mirrors having 40 different shapes. Even with the complex mirror system, only a few photons actually reach the photomultiplier tubes.

The curved paths traversed by the electrons and other particles are tracked by a variety of detectors built by scientists from collaborating universities and national laboratories in the United States, France, Italy, Russia, Armenia, and Korea. At the heart of the Hall B spectrometer there are about 100,000 position sense wires, each of which produces an electrical signal when a particle passes near.

Hall of Mirrors

The project began in 1991 and seemed straightforward in the beginning, even though the completed Cerenkov detector would end up being one of the largest and most complex ever built. "We're quite proud of our achievement because we ran into many unforeseen difficulties in design and installation," Stoler says.

Most of the Rensselaer participants have come from the nuclear and particle physics groups. Besides Stoler these have included Physics Professors Gary Adams, James Napolitano '77, and Paul Yergin. Many undergraduate and graduate students from around the world have also had the opportunity to work on various aspects of the world-class facility over the years.

John Price, a Rensselaer postdoctoral research associate, has been working on-site in Arlington for the last three years, with responsibility for making sure the detector was assembled and installed correctly.

Mechanical Engineering Professors Kevin Craig and Warren DeVries and two of their students played key roles, as did Electric Power Engineering Professor Mietek Glinkowski and his student. Rensselaer has awarded three master's degrees in engineering for work related to this project.

The first major challenge was to design the 40 different shapes of the mirrors and then find a suitable material from which to make the 600 high-quality mirrors for the six Cerenkov detectors. Glass was no good because it absorbs particles and ultraviolet light, including Cerenkov radiation. Material after material was ruled out because of these issues and also because of stringent requirements for weight, strength, and moldability. Finally Lexan® polycarbonate was settled on, with a reflective surface of vacuum-deposited aluminum coated by magnesium fluoride.

"With Professor Craig and his students we set up a veritable mini-factory at Rensselaer to produce the 600 mirrors needed. It took two years to build all of them and another three years to assemble the detector and align the mirrors on-site," says Stoler.

Because the equipment in Hall B is so sensitive and space is so limited, the placement of even routine items like cables and controls was problematic. That led to the project's next major challenge: shielding the photomultiplier tubes from the Hall B spectrometer's extremely powerful torroidal magnetic field. Although the Rensselaer team knew that photomultiplier tubes don't work in a magnetic field, they expected shielding them to be routine. What they didn't realize was that the tubes would have to be tucked into tight spaces right next to the superconducting coils that produce the torroidal magnetic field. Shielding became a major issue.

"This was a difficult problem," Stoler says, "but Professors Adams and Glinkowski and their student solved it by inserting each tube in a specially shaped chamber consisting of multiple layers of metal with different magnetic shielding properties."

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