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Major League PuzzleA
Major League Puzzle
Foreword: Spring Training
Chapter 1: The Playing Field
Chapter 2: The Gamma-Ray Burst Baseball Card
Chapter 4: Pre-Game Analysis Chapter 5: The First Pitch Chapter 6: What Game Are We Playing? Chapter 7: Extra Innings Glossary
Chapter 3 - Watching the Game
Every
baseball fan has "standard equipment" that he or she must have
to watch a game. Of course there are peanuts and cracker-jack, as well as
popcorn. Many youngsters attending the game wear the hat of their favorite
team and a baseball glove to field any potential souvenir foul balls that
might come their way. Depending on the quality of your seat, you might also
bring a pair of binoculars to improve your view of the playing field.
Astronomers, too, have their equipment that they use to watch the events occurring in the universe. Somewhat more technical than popcorn and peanuts, the astronomer uses film, computers, cameras, and telescopes to observe the universe.
A telescope is a familiar object to most of us. It usually is long and cylindrical, sitting inside a large dome on a mountaintop or in some other remote location. Usually, telescopes employ mirrors or lenses (or both) to focus the incoming light from distant sources onto our eyes or onto a piece of photographic film. Telescopes can be almost any size, ranging from small hand-held models to giant 200-inch diameter instruments that can peer to the deepest corners of the universe. The telescope we are familiar with is well-designed to meet the needs of astronomers who observe the universe in the optical portion of the electromagnetic spectrum.
Unlike visible light, gamma-ray photons are so energetic that they are not focused or reflected by mirrors or lenses. Instead, they will penetrate right through a mirror, a lens, or most any other object you care to put in their path. Therefore techniques other than reflection or optical focusing must be used to collect these highly energetic photons. These techniques must furthermore be applied to instrumentation that is lifted above the Earth's atmosphere to detect the gamma-rays before they are absorbed by the thick, obscuring, gaseous layer surrounding the Earth. Instead of using techniques of reflection and focusing to detect gamma-rays, detectors must first stop the gamma-ray photon and convert its energy into a different form that can more easily be collected. These detectors require a large amount of mass to stop the incoming gamma-rays, however the rockets and balloons on which the detectors must ride usually require light-weight payloads. These competing factors are part of the reason that instrumentation used to observe gamma-ray bursts has historically been limited in its sensitivity.
One common gamma-ray detector used on a majority of spacecraft and scientific experimentation is called a scintillator. In these types of detectors, a salt-crystal, usually sodium-iodide, is used to stop the incoming gamma-radiation. Sodium-iodide is commonly found in table salt that is "iodized" and is also a component of sea-salt. The sodium-iodide crystal has the property that when a gamma-ray is stopped by the crystal, the gamma-ray energy is converted into a short pulse of light, or a scintillation. The brightness of this optical scintillation pulse is related to the energy of the incoming gamma-ray photon that initiated the pulse. These scintillation pulses are then collected and changed into an amplified electric signal by a device called a photo-multiplier tube. These electrical signals can then be stored, digitized, processed, and analyzed by computers.
The rather involved detection of a gamma-ray photon is not unlike catching baseballs with your bare hands while wearing a microphone attached to a tape-recorder. If a really hard-hit ball (a gamma-ray) hits your hands (the sodium-iodide crystal), you scream "OUCH!!" very loudly (the scintillation pulse). The sound of your agony is picked up by the microphone (the photo-multiplier tube) and is changed into an electric signal. This signal is then sent by wire to the tape-recorder where it is recorded for later analysis. If the ball was hit a little softer, you might not scream "OUCH!!" so loudly. Like the gamma-ray detector, the loudness of your scream (the brightness of the scintillation pulse) is related to the energy of the incoming line-drive (the incident gamma-ray). If the tape is played back and we hear you scream "OUCH" quite frequently, we infer that the line drives are coming rapidly. If you only scream "OUCH" infrequently, we infer that the rate of incoming baseballs is rather low. Similarly, the number of pulses recorded by a gamma-ray detector in a given period of time tells the observer the gamma-ray counting rate or the brightness of the detected source.
NASA's
Burst and Transient Source Experiment (BATSE) on the Compton Gamma
Ray Observatory, utilizes sodium-iodide scintillation detectors to collect
and analyze incoming gamma-rays. The experiment consists of eight individual
detector modules deployed on each of the eight corners of the box-car sized
satellite. Each of the detector modules weighs about 250 pounds and is about
the size of a large color television set. At first, this may seem to be
a very unusual way to design a telescope. After all, most telescopes we
think of are long and cylindrical, pointing at the particular object we
wish to study. The BATSE design is critical, however, to the efficient detection
of gamma-ray bursts.
Gamma-ray bursts are somewhat quixotic in nature, occurring at unpredictable times and locations in the sky. If we were to build a telescope that were more "conventional" and point it at a small region of the sky, we would only be able to detect those bursts which occurred in the telescope's limited field of view. Most bursts would be missed. The eight detector array of BATSE allows for the entire sky to be viewed simultaneously, like having eyes in the back of your head. In this way, the whole sky is constantly being monitored, and BATSE is ready to detect a gamma-ray burst wherever and whenever it might occur.
Each BATSE detector module consists of two different sodium-iodide gamma-ray detectors, along with an assortment of signal processing electronics. The principal gamma-ray detector is called the Large Area Detector (LAD) and is 20 inches in diameter. The eight LADs are used to constantly monitor the sky for gamma-ray bursts. Its large area allows the LAD to collect many more gamma-rays than smaller detectors flown aboard previous satellites, thereby making it much more sensitive to weak gamma-ray bursts. The second gamma-ray detector is called the Spectroscopy Detector (SD). This detector is used to probe detected bursts in detail by analyzing spectral information from the event.
The faces of the eight BATSE LADs form an octahedron. By arranging the LADs in this manner, any position on the sky is viewed by four detectors simultaneously. Furthermore, no two positions on the sky are viewed by a particular set of detectors with the same perspective. Thus a gamma-ray burst detected from any point on the sky will produce a unique combination of detector illumination signatures. The detector most nearly face-on to the burst, for example, will be the most brightly illuminated, while those detectors with only a small amount of projected area will only be slightly illuminated by the incoming gamma-rays. These unique detector response combinations can therefore be used to determine the bursts' position on the sky. BATSE's ability to independently localize each burst detected, even those bursts that are so weak that they would not have been visible to previous instrumentation, is one major advantage of this instrument.
BATSE was designed and built by a team of dedicated scientists and engineers at NASA's Marshall Space Flight Center in Huntsville, Alabama. Construction, testing, and calibration of the experiment occurred there between 1987 and 1989. In 1989, the experiment was shipped to TRW, Inc. of Redondo Beach, California, the prime contractor for construction of the Gamma Ray Observatory. While in California, BATSE and the three other scientific experiments on the satellite, OSSE, EGRET, and COMPTEL were integrated into a functional observatory and spacecraft. In January 1990, the entire observatory was shipped on a U.S. Air Force C-5 cargo jet to the Kennedy Space Center (KSC) in Florida. While at KSC, the observatory was prepared for its April 1991 launch into space aboard the space-shuttle Atlantis. In all respects, the experiment and spacecraft have lived up to their pre-launch scientific expectations. Like a pair of binoculars for a fan in the upper deck, BATSE has provided a whole new window on the field of gamma-ray bursts, revealing information on these mysterious events that scientists could not have imagined prior to BATSE's development.
SUPPLEMENT III.
The Compton Gamma Ray Observatory (GRO) is the
second in a series of four spacecraft NASA calls "The Great Observatories".
The first and the only other currently operating satellite, in this series
of orbiting observatories is the Hubble
Space Telescope (HST). The other two, the Advanced
X-Ray Astrophysics Facility (AXAF) and Space Infrared Telescope Facility
(SIRTF), are currently still in the development stages. These four spacecraft,
when deployed into Earth orbit, will give astronomers unprecedented views
of the universe over a wide range of the electromagnetic spectrum. Each
of the four spacecraft offer order-of-magnitude improvements in observational
capability over previous ground-based observatories.
The GRO is the most massive science payload ever launched into orbit by the space shuttle. On the ground, the satellite weighs nearly 17 tons and is approximately the size of a large school bus. The spacecraft orbits the Earth every 90 minutes at an altitude of approximately 450 kilometers or 280 miles. From this vantage point high above the Earth's atmosphere, gamma-ray observations of the sky are carried out by four scientific experiments, BATSE, OSSE, COMPTEL, and EGRET.
The Oriented Scintillation Spectrometer Experiment (OSSE) was built by Ball Aerospace Company of Boulder, Colorado for use by investigators at the United States' Naval Research Laboratory in Washington, DC. OSSE consists of four scintillation detectors that can be pointed at various locations on the sky. This experiment operates in a slightly higher gamma-ray region of the electromagnetic spectrum than BATSE, however there is significant overlap in the energy ranges that the two instruments cover.
The Compton Telescope (COMPTEL) was supplied to GRO by the European Space Agency (ESA) and is operated by groups of scientists at the Max Planck Institute near Munich, Germany, as well as at the University of New Hampshire in the United States. COMPTEL's mode of operation is similar in nature to our notion of how a conventional telescope operates. Although it cannot use mirrors or lenses to focus or reflect gamma-rays, the telescope's design is basically cylindrical. Gamma-rays enter the telescope at the top and are detected by instrumentation near the bottom. COMPTEL has the advantage that it can actually make gamma-ray images of the sources and regions of the sky that are under observation.
The Energetic Gamma-Ray Experiment Telescope (EGRET) was built at NASA's Goddard Space Flight Center in Greenbelt, Maryland. EGRET operates in the highest energy regions of the gamma-ray spectrum. It utilizes a large sodium iodide detector, as well as other methods of gamma-ray detection, but is also largely cylindrical in nature with a limited field of view, similar to a conventional telescope. EGRET has performed numerous important observations of celestial objects in the farthest reaches of the gamma-ray region of the electromagnetic spectrum.
Each of these three experiments has some gamma-ray burst capability. When a gamma-ray burst is detected by BATSE, the instrument automatically sends a signal to OSSE, COMPTEL, and EGRET informing them that a burst has been detected. The other three experiments can be pre-programmed to respond to this signal from BATSE in order to enter special data-acquisition modes or otherwise interrupt their normal observational routine. In several instances during the lifetime of GRO a gamma-ray burst has been detected by BATSE that actually occurred in the field of view of these other instruments. In these rare cases, all four experiments are capable of observing the same gamma-ray burst as it occurs. The COMPTEL experiment, for example, has actually "imaged" a gamma-ray burst (it looks like a point-source), and EGRET has detected extraordinarily high-energy photons from several gamma-ray bursts. Most gamma-ray bursts, however, occur outside the limited field of view of these other experiments and are generally too faint to be observed by any instrument other than BATSE.
Foreword: Spring Training
Chapter 1: The Playing Field
Chapter 2: The Gamma-Ray Burst Baseball Card
Chapter 4: Pre-Game Analysis Chapter 5: The First Pitch Chapter 6: What Game Are We Playing? Chapter 7: Extra Innings Glossary