George H. Rieke
Erick T. Young
TUCSON, Ariz. -- A team at Steward Observatory of the University of Arizona
has built the first true detector arrays for the far infrared.
These arrays will be mounted into an instrument for the Space Infrared
Telescope Facility (SIRTF), a major NASA
observatory to be launched in late 2001. UA astronomer George Rieke, who
heads the Multiband Imaging Photometer experiment for SIRTF, will deliver
the arrays to Ball Aerospace in Boulder, Colo., where MIPS is being
assembled, on Monday.
One of the arrays operates at 70 micrometers, 130 times longer wavelength
than visible light. It has 1,024 detectors, about 100 times larger than a
previous array operating in space at this wavelength. Each detector has
about 30 times the sensitivity of those in the earlier array.
The other UA-built array works at 160 micrometers, about 300 times the
wavelength of visible light, with 10 times as many detectors each 10 times
as sensitive as for the best previous array.
All objects above the coldest possible temperature, "absolute zero" or _459
F, emit infrared, or heat, radiation. The new arrays will reveal celestial
objects at temperatures between about -430 F and -300 F. This range includes
the outer zones of possible planetary systems around nearby stars, where the
solar system shows a population of small, icy objects known as the Kuiper
Belt (named for the late UA planetary scientist Gerard Kuiper).
The arrays will also help find very young galaxies at the edge of the known
Universe. When massive stars first form, they manufacture dust that can blot
them out at visible wavelengths and shift their energy to the far infrared.
As a result, many of the first galaxies in the Universe may be hidden from
optical pictures such as the Hubble Deep Field, but will glow brightly in
the spectral region where the new arrays operate.
Exploring the Universe has been hampered in the far infrared because
previous instruments had to rely on only a few detectors. Astronomers faced
a problem analogous to taking a picture using the light meter, reading one
point at a time, rather than using a camera with an array of detectors that
records all the points in the picture at once. Because the new arrays
provide such a large advance in far infrared picture-taking capability, it
is likely that astronomers using them will happen onto new discoveries not
currently anticipated or imagined.
To avoid blinding the new arrays with its own heat radiation, the SIRTF
telescope will be cooled with liquid helium to just 8 degrees F above
absolute zero (or to _ 451 F). The detectors themselves need to be cooled to
_457 F, just 2.5 degrees F above absolute zero.
One of the challenges in building the arrays was to develop a new type of
integrated circuit amplifier to read out the detectors while operating at
these ultra-cold temperatures. Each detector had also to be isolated from
the miniscule power (1 millionth of a Watt) from its amplifier so it would
not get heated above its best operating temperature. The detector material,
germanium with a trace of gallium impurity, requires special processing not
used with conventional silicon-based electronics. Because the germanium is
only modestly efficient in absorbing the far infrared photons, the arrays
had to be built with the detectors mounted "sideways" so the light travels a
total path of more than an eighth of an inch in the material. (In visible
light detectors, the light is typically absorbed in 1/10,000 inch).
For the array operating at the longer wavelength, the germanium must be kept
squeezed nearly to its breaking point in a very strong steel clamp. At the
same time these technical issues were resolved, the arrays had to be built
to NASA_s stringent standards for reliability so they could be used in a
major space mission.
UA astronomer Erick Young led the development team of Michael Bradley
(technician), James Cadien (research specialist), James Davis (instrument
designer), Lacinda Davidson (quality assurance engineer), Karl Gordon
(senior research associate), Todd Horne (research specialist), Douglas Kelly
(staff scientist), Dak Knight (technician), Frank Low (professor), Gil
Rivlis (staff scientist), Rick Schnurr (engineer), and Greg Winters
(electronics engineer). The arrays are being integrated into the Multiband
Imaging Photometer for SIRTF (MIPS).The Principal Investigator for MIPS is
UA professor George Rieke and the project manager is Debra Wilson.
Development of these arrays has continued through the 15 years since the UA
to build MIPS. See http://mips.as.arizona.edu
The material for the detectors was grown and processed by Eugene Haller and
Jeffrey Beeman of Lawrence Berkeley National Laboratory.
SIRTF is managed through the Jet Propulsion Laboratory for NASA.