Compton Gamma Ray Observatory Education

Basic results

• The EGRET instrument conducted the first all sky survey above 100 MeV. Using four years of data it discovered 271 sources, 170 of which were unidentified.
• The COMPTEL instrument completed an all sky map of ²⁶Al (a radioactive isotope of aluminum).
• The OSSE instrument completed the most comprehensive survey of the galactic center, and discovered a possible antimatter "cloud" above the center.
• The BATSE instrument averaged one gamma ray burst event detection per day for a total of approximately 2700 detections. It definitively showed that the majority of gamma-ray bursts must originate in distant galaxies, not nearby in our own Milky Way, and therefore must be enormously energetic.
• The discovery of the first four soft gamma ray repeaters; these sources were relatively weak, mostly below 100 keV and had unpredictable periods of activity and inactivity
• The separation of GRBs into two time profiles: short duration GRBs that last less than 2 seconds, and long duration GRBs that last longer than this.

GRB 990123

Main article: GRB 990123

Gamma ray burst 990123 (23 January 1999) was one of the brightest bursts recorded at the time, and was the first GRB with an optical afterglow observed during the prompt gamma ray emission (a reverse shock flash). This allowed astronomers to measure a redshift of 1.6 and a distance of 3.2 Gpc (10 Gly). Combining the measured energy of the burst in gamma-rays and the distance, the total emitted energy assuming an isotropic explosion could be deduced and resulted in the direct conversion of approximately two solar masses into energy. This finally convinced the community that GRB afterglows resulted from highly collimated explosions, which strongly reduced the needed energy budget.

Miscellaneous results

• The completion of both a pulsar survey and a supernova remnant survey
• The discovery of terrestrial gamma ray sources in 1994 that came from thunderclouds

Education & Outreach

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Compton Gamma Ray Observatory Instruments

Website	NASA Compton Gamma Ray Observatory

CGRO carried a complement of four instruments that covered an unprecedented six decades of the electromagnetic spectrum, from 20 keV to 30 GeV (from 0.02 MeV to 30000 MeV). In order of increasing spectral energy coverage:

BATSE

• The Burst and Transient Source Experiment, (BATSE) by NASA's Marshall Space Flight Center searched the sky for gamma ray bursts (20 to >600 keV) and conducted full sky surveys for long-lived sources. It consisted of eight identical detector modules, one at each of the satellite's corners (left, right; front and back; top and bottom). Each module consisted of both a NaI(Tl) Large Area Detector (LAD) covering the 20 keV to ~2 MeV range, 50.48 cm in dia by 1.27 cm thick, and a 12.7 cm dia by 7.62 cm thick NaI Spectroscopy Detector, which extended the upper energy range to 8 MeV, all surrounded by a plastic scintillator in active anti-coincidence to veto the large background rates due to cosmic rays and trapped radiation. Sudden increases in the LAD rates triggered a high-speed data storage mode, the details of the burst being read out to telemetry later. Bursts were typically detected at rates of roughly one per day over the 9-year CGRO mission. A strong burst could result in the observation of many thousands of gamma rays within a time interval ranging from ~0.1 s up to about 100 s.

OSSE

• The Oriented Scintillation Spectrometer Experiment, (OSSE), by the Naval Research Laboratory detected gamma rays entering the field of view of any of four detector modules, which could be pointed individually, and were effective in the 0.05 to 10 MeV range. Each detector had a central scintillation spectrometer crystal of NaI(Tl) 12 in (303 mm) in diameter, by 4 in (102 mm) thick, optically coupled at the rear to a 3 in (76.2 mm) thick CsI(Na) crystal of similar diameter, viewed by seven photomultiplier tubes, operated as a phoswich: i.e., particle and gamma-ray events from the rear produced slow-rise time (~1 μs) pulses, which could be electronically distinguished from pure NaI events from the front, which produced faster (~0.25 μs) pulses. Thus the CsI backing crystal acted as an active anticoncidence shield, vetoing events from the rear. A further barrel-shaped CsI shield, also in electronic anticoincidence, surrounded the central detector on the sides and provided coarse collimation, rejecting gamma rays and charged particles from the sides or most of the forward field-of-view (FOV). A finder level of angular collimation was provided by a tungston slat collimator grid within the outer CsI barrel, which collimated the response to a 3.8° x 11.4° FWHM rectangular FOV. A plastic scintillator across the front of each module vetoed charged particles entering from the front. The four detectors were typically operated in pairs of two. During a gamma-ray source observation, one detector would take observations of the source, while the other would slew slightly off source to measure the background levels. The two detectors would routinely switch roles, allowing for more accurate measurements of both the source and background. The instruments could slew with a speed of approximately 2 degrees per second.

COMPTEL

• The Imaging Compton Telescope, (COMPTEL) by the Max Planck Institute for Extraterrestrial Physics, the University of New Hampshire, Netherlands Institute for Space Research, and ESA's Astrophysics Division was tuned to the 0.75-30 MeV energy range and determined the angle of arrival of photons to within a degree and the energy to within five percent at higher energies. The instrument had a field of view of one steradian. For cosmic gamma-ray events, the experiment required two nearly simultaneous interactions, in a set of front and rear scintillators. Gamma rays would Compton scatter in a forward detector module, where the interaction energy E₁, given to the recoil electron was measured, while the Compton scattered photon would then be caught in one of a second layer of scintillators to the rear, where its total energy, E₂, would be measured. From these two energies, E₁ and E₂, the Compton scattering angle, angle θ, can be determined, along with the total energy, E₁ + E₂, of the incident photon. The positions of the interactions, in both the front and rear scintillators, was also measured. The vector, V, connecting the two interaction points determined a direction to the sky, and the angle θ about this direction, defined a cone about V on which the source of the photon must lie, and a corresponding "event circle" on the sky. Because of the requirement for a near coincidence between the two interactions, with the correct delay of a few nanoseconds, most modes of background production were strongly suppressed. From the collection of many event energies and event circles, a map of the positions of sources, along with their photon fluxes and spectra, could be determined.

EGRET

Main article: Energetic Gamma Ray Experiment Telescope

• The Energetic Gamma Ray Experiment Telescope, (EGRET) measured high energy (20 MeV to 30 GeV) gamma ray source positions to a fraction of a degree and photon energy to within 15 percent. EGRET was developed by NASA Goddard Space Flight Center, the Max Planck Institute for Extraterrestrial Physics, and Stanford University. Its detector operated on the principle of electron-positron pair production from high energy photons interacting in the detector. The tracks of the high-energy electron and positron created were measured within the detector volume,and the axis of the V of the two emerging particles projected to the sky. Finally, their total energy was measured in a large calorimeter scintillation detector at the rear of the instrument.

Mengenal James Edwin Webb

James Edwin Webb (October 7, 1906 – March 27, 1992) was an American government official who served as the second administrator of NASA from February 14, 1961 to October 7, 1968.

Webb oversaw NASA from the beginning of the Kennedy administration through the end of the Johnson administration, thus overseeing all the critical first manned launches in the Mercury through Gemini programs, until just before the first manned Apollo flight. He also dealt with the Apollo 1 fire.

In 2002, a planned replacement for the Hubble Space Telescope Earth-orbiting astronomical observatory, originally called the Next Generation Space Telescope (NGST), was renamed the James Webb Space Telescope as a tribute to Webb.

James E. Webb
2nd Administrator of the National Aeronautics and Space Administration
In office February 14, 1961 – October 7, 1968
President	John F. Kennedy Lyndon Johnson
Deputy	Hugh Latimer Dryden Robert Seamans Thomas O. Paine
Preceded by	T. Keith Glennan
Succeeded by	Thomas O. Paine
16th Under Secretary of State
In office January 28, 1949 – February 29, 1952
President	Harry S. Truman
Preceded by	Robert A. Lovett
Succeeded by	David K. E. Bruce
7th Director of the Bureau of the Budget
In office July 13, 1946 – January 27, 1949
President	Harry S. Truman
Preceded by	Harold D. Smith
Succeeded by	Frank Pace
Personal details
Born	James Edwin Webb October 7, 1906 Tally Ho, North Carolina United States
Died	March 27, 1992(1992-03-27) (aged 85) Washington, District of Columbia United States
Resting place	Arlington National Cemetery
Nationality	USA
Political party	Democratic
Alma mater	University of North Carolina at Chapel Hill, B.A. George Washington University Law School, J.D.
Occupation	Politician and bureaucrat
Military service
Allegiance	United States
Service/branch	United States Marine Corps
Years of service	1930–1932, 1944 – 1945
Rank	Lieutenant Colonel

Biography

 

 

Early years

 

 

Webb was born in the hamlet of Tally Ho in Granville County, North Carolina. His father was superintendent of the Granville County public schools. He completed his college education at the University of North Carolina at Chapel Hill, where he received an A.B. degree in education in 1928. He was a member of the Acacia Fraternity. Webb became a second lieutenant in the United States Marine Corps, and he served as a Marine Corps pilot on active duty in 1930–32. Webb then studied law at the The George Washington University Law School where he received a J.D. degree in 1936. In the same year he was admitted to the Bar of the District of Columbia.

Career

 

 

Webb enjoyed a long career in public service in Washington, D.C., first serving as a secretary to US Rep. Edward W. Pou of North Carolina in 1932–34. He next served as an assistant in the office of O. Max Gardner, an attorney and former governor of North Carolina, in 1934–36. In 1936, Webb became the personnel director, the secretary-treasurer, and later the vice president of the Sperry Gyroscope Company in Brooklyn, New York, before he re-entered the Marine Corps in 1944 during World War II.

After World War II, Webb returned to Washington and served as executive assistant to O. Max Gardner, by then an undersecretary in the U.S. Department of the Treasury, before being named as the director of the Bureau of the Budget in the Office of the President of the United States, a position that he held until 1949. President Harry S. Truman next nominated Webb to serve as an undersecretary of state in the U.S. Department of State. 

 Serving as Undersecretary of State during the Era of McCarthy, Webb was instrumental in implementing stringent personnel requirements intended to prevent suspected anarchists, communists and homosexuals from employment with the Department of State. Webb singled out homosexuals in a later report, noting "It is generally believed that those who engage in overt acts of perversion lack the emotional stability of normal persons."When the Truman administration ended in January 1953, Webb left Washington for a position in the Kerr-McGee Oil Corp. in Oklahoma City, but he was still active in government circles, for instance serving on the Draper Committee in 1958.

NASA

Webb presents NASA's Group Achievement Award to Kennedy Space Center Director Kurt H. Debus, while Werner von Braun (center) looks on.

 

Webb returned to Washington in 1961 when he accepted the position of the Administrator of NASA. Under his direction, the agency undertook to achieve the goal set by President John F. Kennedy of landing an American on the Moon before the end of the 1960s through the Apollo program.

Webb, Vice President Lyndon Johnson, Kurt Debus, and President John F. Kennedy receive a briefing on Saturn I launch operations during a tour of Launch Complex 34, September 1962.

For seven years after Kennedy's May 25, 1961, announcement of the goal of a manned lunar landing, through October 1968, Webb lobbied for support for NASA in Congress. As a longtime Washington insider, and with the backing of President Lyndon B. Johnson, he was able to produce continued support for and resources for Apollo.

During his administration, NASA developed from a loose collection of research centers into a coordinated organization. Webb had a key role in creating the Manned Spacecraft Center, later, the Johnson Space Center, in Houston, Texas. Despite the pressures to focus on the Apollo program, Webb ensured that NASA carried out a program of planetary exploration with the Mariner and Pioneer space programs.

After the Apollo 1 accident in 1967, Webb told the media, "We've always known that something like this was going to happen sooner or later. ... Who would have thought that the first tragedy would be on the ground?" Webb went to President Johnson and asked that NASA be allowed to handle the accident investigation, and to direct its recovery. He promised to be truthful in assessing blame, and he pledged to assign that to himself and NASA management, as appropriate. The agency set out to discover the details of the tragedy, to correct problems, and to continue progress toward the Apollo 11 lunar landing.

Webb reported his investigation's findings to various congressional committees, and he took a personal grilling at nearly every meeting. Whether by happenstance or by design, Webb managed to deflect some of the backlash over the accident away from both NASA as an agency and from the Johnson administration. As a result, NASA's image and popular support were largely undamaged.

However, Webb was a Democrat tied closely to Johnson, and, with Johnson choosing not to run for reelection, he decided to step down as administrator to allow the next president to choose his own administrator.

Webb was informed by CIA sources in 1968 that the Soviet Union was developing its own heavy N1 rocket for a manned lunar mission, and he directed NASA to prepare Apollo 8 for a possible lunar orbital mission that year. At the time Webb's assertions about the Soviet Union's abilities were doubted by some people, and the N-1 was dubbed "Webb's Giant". However later revelations about the Soviet Moonshot – after the collapse of the USSR – have given support to Webb's conclusion. Webb left NASA in October 1968, just before the first manned flight in the Apollo program.

In 1969, Webb was presented with the Presidential Medal of Freedom by President Johnson. He is a 1976 recipient of the Langley Gold Medal from the Smithsonian Institution.

Sumber: Wikipedia