Craft uses new technology with many potentials
Posted: October 22, 2001

Artist's concept of Swift spacecraft and gamma ray blast. Photo: Sonoma State University
Big things come in tiny packages ... 40,000 tiny packages of gamma ray detectors to be exact. The 40,000 thumbtack-sized detectors were recently delivered to the scientists building NASA's Swift Gamma-Ray Burst Explorer. The detectors may allow advances in medical and security imaging, also.

Swift, scheduled for a 2003 launch, will detect and accurately position gamma ray bursts -- the most energetic events seen in today's Universe. These cadmium-zinc-telluride (CZT) detectors are the heart of Swift's Burst Alert Telescope (BAT) that will enable scientists to detect and accurately position these mysterious gamma ray bursts, which disappear within seconds, never to appear in the same spot in the sky. A "swift" response is therefore the only way to track down these elusive bursts, and this is the primary goal of NASA's Swift mission.

"This delivery is quite a milestone for both the Swift mission and the development of CZT detector technology," said Dr. Ann Parsons, BAT Detector Scientist at NASA's Goddard Space Flight Center, Greenbelt, Md. "Since CZT detectors are very compact and do not require expensive cryogenic cooling systems to operate, large, densely packed arrays can be built for many applications, including medical and security imaging as well as astronomy. The now-proven ability to acquire such large quantities of CZT will allow us to fly a huge BAT detector array that is sensitive enough to detect the faintest gamma-ray bursts, presumably originating from the farthest reaches of the cosmos."

U.S. Rep. Melissa Hart (R, Penn.) and state officials will attend a ceremony on October 22 marking the completion of the 40,000 detectors. The event is hosted by the detectors' manufacturers, eV Products Inc., in Saxonburg, Penn. The CZT detectors will do for gamma-ray astronomy what CCD detectors have done for X-ray and optical astronomy -- that is, create high-resolution images from high-energy photons, particles of light far more energetic than visible light.

BAT will observe and locate hundreds of bursts per year to better than 4-arcminute accuracy and remain sensitive to faint bursts that earlier detectors would have missed. BAT's large size makes it the most sensitive Gamma-ray burst detector ever made. BAT will detect bursts that are five times fainter than the dimmest ones recorded by the trailblazing Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory. Data from the BAT will also produce a sensitive hard (high-energy) all-sky survey about 100 times deeper than previous surveys; astrophysicists estimate this capability will uncover over 400 new supermassive black holes.

Fabricating the CZT detectors was a challenge, since the materials science technology required to produce large volumes of high-quality CZT detectors is still maturing. Various crystallography techniques are used, and it is difficult to grow the detectors without allowing cracks to form. There was some concern whether the large number required by BAT could be fabricated on time, but eV Products actually delivered them ahead of schedule.

Each CZT detector is 4 millimeters (mm) by 4 mm and 2 mm thick and will be fitted onto the BAT array in the form of 128 modules each containing 256 detectors, totaling 32,768 (leaving about 7,000 spare detectors for replacement.) One millimeter is approximately 0.04 inch.

BAT will relay the positions of bursts within 15 seconds to ground- based and other space-based observatories. Swift's other main instruments, the X-Ray Telescope (XRT) and Ultraviolet/Optical Telescope (UVOT), will zoom in on the BAT detection location and provide arcsecond positioning. (An arcsecond is an area of the sky about the apparent size of a penny viewed from two miles away.) This fine positioning information will provide for crucial follow-up observations of the burst's lower-energy afterglow, which can last for days to weeks.

Swift was selected in October 1999 as a medium-class explorer mission (MIDEX). Swift, an international collaboration with partners in Italy and Great Britain, will launch in 2003. Swift will operate for three years following launch, for a total cost of $163 million. Dr. Neil Gehrels from NASA's Goddard Space Flight Center is the Principal Investigator for the Swift Mission.

Swift is a key component of NASA's Structure and Evolution of the Universe theme, dedicated to unraveling the mysteries of the cosmos and ensuring future cosmic journeys.