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New optical detector could revolutionize astronomy
Physicists at Stanford have developed a new optical detector so sensitive that
it can clock the arrival of a single particle of light and measure its energy
with exceptional precision.
When applied to light coming from celestial objects, the device's ability to
directly measure the location, arrival time, and energy of individual photons
could have a revolutionary impact on optical astronomy, say its inventors,
Stanford physics Professor Blas Cabrera and his research team.
Not only can this detector measure all of an individual photon's important
attributes, but it can do so throughout the infrared, optical and ultraviolet
portions of the spectrum, the physicists report in the Aug. 10 issue of the
journal Applied Physics Letters.
The basic sensor, called a superconducting transition edge sensor (TES), was
invented with Department of Energy support as part of a physics experiment
called the Cryogenic Dark Matter Search and patented by Stanford in 1997. The
experiment is being operated on campus and involves more than 40 scientists
from eight institutions, Stanford, University of California-Berkeley,
University of California-Santa Barbara, Case Western Reserve University,
University of Santa Clara, San Francisco State University, Lawrence Berkeley
National Laboratory and Fermilab.
The sensor is a critical element in a new detector designed to detect
elementary particles called WIMPs. These Weakly Interacting Massive Particles
have been proposed as one possible explanation for the missing mass in the
universe. Analyses of the rotation of visible galaxies have convinced
scientists that as much as 50 percent of the matter that galaxies contain
must be invisible to telescopes. Although WIMPS should be virtually invisible,
scientists calculate that they should occasionally shake up the nuclei in
crystalline material, and TES sensors have been developed to detect the heat
produced by such interactions.
The new optical version of TES, developed with support from the National
Aeronautics and Space Administration, consists of squares of tungsten film
that are 20 microns (about a human hair width) on a side. When the sheets are
cooled down to a temperature of 80 thousandths of a degree above absolute
zero, the tungsten becomes superconducting, able to carry electric current
without resistance. Tungsten's transition between ordinary metal and
superconductor is exceptionally sharp, so extremely small changes in the
material's temperature give rise to large changes in its electrical
"The sharp resistive transition made it potentially an extremely sensitive
calorimeter," says Cabrera, "but it was very difficult to keep it within the
narrow temperature range required."
In 1994, Cabrera and Kent Irwin, who is now at the National Institute for
Standards and Technology in Boulder, solved the control problem by borrowing
a technique that is widely used in the design of stereo amplifiers: negative
feedback. They placed the sensor in a special circuit that produces a weak
electrical current that automatically keeps the material at its critical
transition temperature. The sensor is cooled slightly below the transition
temperature and the electrical current raises its temperature to the critical
value. When the energy from an individual photon reaches the tungsten, it
heats up the electrons in the material. This heating causes a slight increase
in the electrical resistance of the film. The greater resistance, in turn,
causes a decrease in the electrical heating that exactly equals the amount of
energy that the photon deposited. Not only does this keep the film at the
right temperature but it also gives the scientists a precise measurement of
the photon's energy and its arrival time.
The new sensors have a number of potential uses. Irwin and his colleagues at
NIST have customized TES detectors for use in an X-ray spectrometer. Using
this technology, they have created the highest resolution, high-energy
spectrometer in the world. The semiconductor industry is very interested in
using this instrument to locate small-scale surface contamination that is a
barrier to the continued miniaturization of integrated circuitry. According
to current plans, the next generation X-ray satellite, called Constellation-X,
will include a TES spectrometer to aid in the identification of the chemical
compounds that make up the gas clouds that float between stars and galaxies.
One of the most exciting applications for the sensors could come from mounting
them on existing optical telescopes. "By providing us with information about
the energy of each photon and the time when it arrives, these detectors can
provide important information about some of the key questions in astronomy,"
says physics Professor Roger Romani. He is working with Cabrera and graduate
students Aaron Miller, Tali Figueroa and Sae Woo Nam on a trial application
of the system on the 24-inch student telescope at Stanford this fall.
Over the last 25 years, astronomers have converted their telescopes from
photographic film to electronic CCD detectors similar to those used in
camcorders. This conversion has increased the power of the telescopes by 30
to 100 times. But, like film, CCDs only provide information about the position
of photons. As with the human eye or a camcorder, many photons passing through
various filters are needed to get a crude estimate of the color or average
energy. More complicated electronic systems, called microchanneltrons, can
obtain information about photon arrival times but not their energies.
Currently, the physicists can only make TES detectors with a few pixels. Even
with this limitation, however, they should be able to make meaningful new
measurements of time-varying cosmic phenomena such as pulsars and gas-eating
black holes, Romani says.
Once they have a rudimentary TES array attached to Stanford's small student
telescope, the scientists will make trial observations of the powerful pulsar
in the Crab Nebula. A pulsar is a rapidly spinning neutron star that emits
radio waves with clock-like regularity. By recording the way that the energy
of the visible light from the pulsar varies on time scales as short as a
thousandth of a second, the physicists hope to gain new insights into the
outstanding question of how spinning neutron stars produce optical light. By
examining how the distortion of the light pulses vary at different energies,
it might also be possible to see evidence of the relativistic twisting of
space that should take place in the neutron star's vicinity, Romani
If the experiment with the small telescope is a success, the scientists hope
to put a larger array of optical TES sensors on the 10-meter Hobby Eberly
telescope in Texas. In addition to studies of faint black holes and neutron
stars, the team also hopes to demonstrate that the device will be a powerful
tool for measuring cosmic distances. Because the universe is expanding, the
farther away objects are the faster they are receding. This motion causes
redshift, the apparent reddening of light coming from receding objects. The
larger an object's redshift the further away it must be. Because the speed
of light is constant, objects with the highest redshifts are also the oldest
objects in the visible universe. An array of TES devices could in principle
obtain the redshift of every object in each image that a telescope makes.
Currently, astronomers must follow up their initial observations of a new
object with a lengthy spectrographic analysis to measure its redshift.
An ultimate application of this new technology would be to equip the next
generation of space telescope with a thousand-by-thousand element array of
TES sensors. Such a system would allow astronomers the measure the redshift
of even the most distant objects, those too faint for even the biggest
telescopes on Earth to resolve. In its deep field mode, for example, the
Hubble space telescope has produced images of objects that are a thousand
times fainter than the glow of the dark night sky and are invisible to Earth-
based telescopes. Redshift information about these and other similar objects
could provide astronomers with a more complete picture of the size and shape
of the universe, the distribution of galaxies within it, and how this has
changed over time.
* Blas Cabrera group home page,
* Roger Romani's home page,
* Cryogenic Dark Matter Search web page,
* Kent Irwin's home page,
Two small white cylinders sitting on the horizontal shelf are new optical
detectors so sensitive that they can measure the heat from a single photon
that have been developed in Blas Cabrera's physics lab. The detectors are
connected to a light source by optical fibers. Because they require an
extremely low temperature to operate, they are immersed in a bath of liquid
helium. Source: Cabrera laboratory.
One of the first uses of the new detector will be to study the pulsar that
sits at the heart of the Crab Nebula. The image on the left is a view of the
Crab taken with the Palomar telescope and the image on the right is a blow up
of the center of the Crab where the pulsar resides taken by the Hubble Space
Telescope. Source: NASA.