Since the mid-20th century, the standard photodetector in high-energy physics has been the photomultiplier tube. While PMTs tend to be accurate to within 100 picoseconds—that's 100 trillionths of a second—a collaboration between Argonne National Laboratory, the University of Chicago, Fermilab and several other institutions has created a photodetector that is even more accurate.
A typical PMT is a roughly foot-long tube that amplifies a captured photon by converting its signal to many electrons. The photon strikes a photocathode, which releases an electron, and that electron then strikes a metal plate that causes several more electrons to be released. The process repeats over several more plates, and a shower of electrons grows in size down the length of the tube. The electrons are detected as a signal at the end, heavily amplified from the initial photon's energy.
|An 8-inch-square microchannel plate, installed at Fermilab's nickel-chromium coating facility, reflects the image of scientist Eileen Hahn, PPD. Hahn develops the "electroding" process for MCPs. Pasha Murat, PPD, Fermilab|
Starting in 2003, University of Chicago physicist Henry Frisch posited that the future of high-energy physics lay in the development of a detector with new capabilities, including picosecond resolution, at least 100 times faster than typical PMTs. Many were skeptical.
"Everybody knew you couldn't do it, but no one could say why," Frisch said. He and a few others put together a collaboration to find out if it indeed was possible.
After several years of R&D, the collaboration has created what was once thought impossible: an 8-inch-by-8-inch microchannel plate, or MCP, with picosecond resolution. The device contains approximately 80 million pores, each 20 microns wide and functioning like a PMT in miniature. However, instead of using metal plates, the interior of the pore is coated with an emissive layer approximately 20 nanometers thick, as well as a slightly thicker resistive layer that can transmit signals.
"If an electron hits the wall of one of the pores, it generates electrons in a very similar fashion to these metal structures," said Fermilab physicist Erik Ramberg, who is a member of the collaboration. A voltage across the plate draws the electrons to a set of closely spaced readouts.
While the plates were in development, Fermilab established the first test beam run at MTEST. Technician Greg Sellberg – whom Frisch calls a "magician" – figured out how to make connections to 1,024 sensors in a 2-inch-by-2-inch prototype MCP, establishing proof of concept. Fermilab physicist Anatoly Ronzhin, an expert in PMTs, helped design the test run and consulted on photocathode design.
Fermilab also plays a major role in production. Physicist Pasha Murat manages a supply chain from Argonne to Fermilab, with initial production paced at two MCPs per week. Eileen Hahn, an engineering physicist and leader of the Thin Film Group, designed a new apparatus for applying a coating of nickel-chromium alloy to the MCPs through evaporation. The process, termed electroding, creates a lead for the voltage that is applied across the plate.
Argonne physicist and project manager Bob Wagner said the plates are showing a time resolution close to a picosecond. That would allow devices such as Cherenkov water detectors not only to see particles coming from a collision, but also to pick out their point of origin.
"These detectors are a game-changer," Frisch said. "Now we have to focus on making them and getting them into the field."