Watching the Detectors

Hidden behind a series of old doors and makeshift walls in the subbasement of Osmond Laboratory on the Penn State University Park campus lies a 47-meter- long tube constructed of large, ancient-looking pipe. Originally built in the 1950s, its outward appearance obscures just how impressive and rare the long-cell X-ray beam facility is and its important role in Penn State’s leadership in X-ray astronomy.

“It was originally built—I’m told by graduate students and postdocs—to study the hyperfine structure of hydrogen,” said David Burrows, professor of astronomy and astrophysics. “In the early 1980s, when Gordon Garmire came to Penn State to start up an X-ray astronomy group, it had been sitting idle. He discovered that it would hold a vacuum, so set it up with an X-ray source at one end and experiments to test instruments at the other.”

Garmire, Evan Pugh Professor Emeritus of Astronomy and Astrophysics, used the long-cell X-ray beam in developing X-ray instruments that were tested on sounding rocket missions. He went on to develop the ACIS X-ray camera that is at the heart of NASA’s Chandra X-ray Observatory. Chandra is one of two NASA X-ray satellites—Swift being the other—with major Penn State contributions. Penn State astronomers are now using the long-cell to test X-ray detectors and optics that are being developed for some possible large and small missions of the future.

“These include the Lynx X-ray Surveyor, which could be the next large NASA mission for X-ray astronomy and would enable studies of the birth and evolution of the rst black holes in the universe,” said Abe Falcone, research professor of astronomy and astrophysics. “As well as BlackCAT, which is a proposed CubeSat—a low-cost miniature satellite—to monitor X-ray transients from events such as gamma-ray bursts and gravity- wave triggers.”

Burrows and Falcone lead a team of postdocs and students that designs and builds cutting-edge X-ray detectors. These instruments are put through a series of tests—first in the lab, then in the long-cell, and then on sounding rockets—to demonstrate their effectiveness before they can be incorporated into full-scale X-ray satellites.

“The long-cell is important because it allows us to test our detectors in an environment that mimics the conditions they will experience in space prior to actually being launched,” said Falcone. “X-rays traveling from a source in deep space arrive at our detectors traveling in essentially parallel lines. Unlike optical light, which you can easily focus using lenses and mirrors, X-rays are incredibly difficult to focus. Without the aid of mirrors, the best way to approximate parallel X-rays is to make an X-ray beam as long as possible. In the long cell, the divergent X-rays travel far enough to appear close to parallel when viewed by an instrument that has small dimensions relative to the length of the beam.”

Having the long-cell X-ray beam has bene ted Penn State astronomers since Garmire first started his work here. It also serves as a resource for researchers from other institutions.

“There are only a handful of X-ray beams as long, or longer, in the world,” said Burrows. “Having this world- class piece of equipment here makes our work easier and has helped keep Penn State at the leading edge of X-ray astronomy.”