|Berkeley Lab Far-Infrared Detectors in Orbit|
|Contact: Lynn Yarris, [email protected]|
On August 25, 2003, the Space Infrared Telescope Facility (SIRTF) was launched into orbit from Cape Canaveral aboard a Delta-2 rocket. The last of NASA's four Great Observatories, SIRTF is the largest infrared telescope ever sent into space, designed to capture images of previously hidden celestial objects.
SIRTF's launch was anxiously followed by two researchers with Berkeley Lab's Materials Sciences Division (MSD). They've been involved with the SIRTF project for nearly 20 years and soon, with the successful launch, they expect to see the unprecedented cosmic views they helped obtain.
Imaging in the far-infrared
SIRTF's images will be captured via a 0.85 meter telescope and three sophisticated infrared spectroscopy tools. One of these tools, the Multiband Imaging Photometer for SIRTF (MIPS) will provide highly sensitive deep-space probing and mapping at far-infrared wavelengths ranging from 24 to 160 microns, or about 50 to 300 times longer than the wavelengths of visible light.
Detectors for two of the three arrays at the heart of MIPS were developed and partly fabricated under the leadership of Eugene Haller and Jeffrey Beeman, MSD scientists experienced at developing doped and ultrapure germanium crystals for gamma-ray detectors.
"NASA approached us in 1984 asking for a detector that would be a good absorber of far-infrared radiation, but which would generate very small dark currents in the absence of radiation," says Haller, who also holds a faculty position at UC Berkeley. (Dark current is generated by the thermal motion of a detector's atoms, which can produce spurious signals.)
Says Beeman, "What they were asking for had never been developed, and we had no means to measure the extremely small signals and dark current that they were describing. About midway through the project, budget cuts forced NASA to rethink the size of SIRTF, and the ensuing delay gave us time to improve our device technology and testing so we could meet NASA's requirements."
Because interstellar gas clouds and space dust effectively absorb visible and ultraviolet photons, observations in the infrared portion of the electromagnetic spectrum -- in particular the far infrared -- can reveal objects and phenomenon that would otherwise be invisible to astronomers. Far-infrared radiation is especially valuable for studying the history of early star formation and the evolution of galaxies and planetary systems. The far infrared is also good for the study of small stars, extrasolar planets, and molecular clouds that give off heat but are too dim to be viewed by visible light.
Despite its advantages, far-infrared astronomy has scarcely been tapped until now. Since Earth's atmosphere blocks most far-infrared radiation, this type of astronomy cannot be done from ground observatories. For years scientists had to rely on balloon-borne telescopes or telescopes mounted aboard high-flying jets, with imaging systems that lacked sensitivity and could only collect a few pixels at a time.
The situation improved somewhat with earlier infrared space telescopes such as IRAS (Infrared Astronomical Satellite) and, more recently, ISO (Infrared Space Observatory). SIRTF, however, takes infrared astronomy to a whole new level, and MIPS is the first true detector system designed specifically for far-infrared wavelengths.
Building the detectors, stressed and unstressed
For the two detector arrays they helped develop, Haller and Beeman mounted a highly specialized and extensive search of semiconducting materials. The best choice proved to be a germanium crystal doped with gallium that had first been grown by Berkeley Lab researcher Bill Hansen in 1970.
To boost the wavelength range of some of their gallium-doped germanium detectors, Haller and Beeman subjected crystals to mechanical stress. This implemented an effect Haller first studied in collaboration with MSD colleague Paul Richards.
Explains Haller, "Under mechanical stress, a semiconductor like germanium doped with p-type impurities such as gallium can detect lower-energy photons than it can when unstressed, because of unique changes in its valence band structure. The energy required to break a charge carrier from the gallium acceptor at close to zero Kelvin can be cut in half by applying large mechanical stress."
The unstressed gallium-doped germanium detectors that Haller and Beeman fabricated are deployed in a 32 by 32 configuration that can image photons at wavelengths of 70 microns, or collect spectra from 50- to 100-micron photons. This array is 100 times larger than the array aboard ISO that operated at this wavelength, and each detector is 30 times more sensitive than those in ISO.
The mechanically stressed crystals are deployed in 40 detectors, arranged in a 2 by 20 configuration that can image 160-micron photons. This is ten times the number of 160-micron detectors in ISO. Each of these new detectors is also ten times more sensitive than their ISO counterparts.
The gallium-doped germanium detectors, in combination with an array of 16,384 detectors (128 by 128) made from arsenic-doped silicon, enable MIPS to "see" heat sources radiating at around 20 Kelvin (-253 degrees Celsius), which is a glow 100 times more faint than any previous infrared telescope could see.
Because the MIPS detectors are so highly infrared-sensitive, they and the rest of SIRTF's science instruments have to be cooled down with liquid helium to a temperature of about to 1.5 Kelvin. Otherwise, the detectors would be "blinded" by their own heat radiation. Building detectors that could operate at temperatures of near absolute zero was just one of the many challenges that Haller and Beeman had to overcome.
"These detectors also had to withstand the violent shaking that occurs during launch as well as the many Gs of thrust from the Delta-2 rocket," says Haller.
Another challenge, Haller says, was creating electrical contacts that could operate in such ultracold temperatures. For this, he and Beeman used ion implantation to create heavily doped regions near the surface of the semiconductors.
"Together with evaporated thin metallic films, these heavily doped regions act as perfect electrical contacts," Haller says.
In addition to the detectors, Haller and Beeman also had to invent low-power calibrators, sources of far-infrared light that can be used to test the detectors before and during the mission. The calibrators allow astronomers to perform measurements that can determine whether a detector's performance has been impaired by thermal instabilities or radiation damage. They're also required to serve as "IR flood sources" that can help reset detectors after a radiation problem.
"When testing showed that the calibrators would be critical to the success of MIPS, we decided there needed to be redundancy at each usage point in case of a failure during the launch or other problems," Beeman says. "The devices are designed to be mounted as a pair at each calibration point on MIPS. There are a total of 10 calibration devices on board."
It will probably be late in the fall of 2003 before Haller and Beeman's detectors have sufficiently cooled to begin recording images. Already, however, images of distant stars have been taken from SIRTF as part of an "aliveness test" during the instrumentation power-up sequences.
"Our detectors and calibrators have been so thoroughly tested, we're confident they'll perform as they were designed to do," Beeman says.