The Compton Gamma Ray Observatory

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Compton Gamma Ray Observatory (CGRO)

Deployment of CGRO from the space shuttle

The Compton Gamma Ray Observatory (CGRO) was launched on April 5, 1991. The second of NASA’s great observatories, CGRO has four instruments that cover an unprecedented six orders of magnitude in energy, from 30 keV to 30 GeV. Over this energy range CGRO has an improved sensitivity over previous missions of a full order of magnitude. It operated for almost 9 years and the mission ended on June 4 2000. Unlikely most satellites, CGRO was too large to burn up entirely in the atmosphere during re-entry. To ensure safety on the Earth’s surface, NASA redirected the spacecraft into Earth’s atmosphere with a controlled re-entry.

The Observatory was named in honor of Dr. Arthur Holly Compton, who won the Nobel prize in physics for work on scattering of high-energy photons by electrons — a process which is central to the gamma-ray detection techniques of all four instruments.

Mission Characteristics

Lifetime	5 Apr 1991–4 Jun 2000
Special Features	First “Great Observatory” at gamma-ray wavelengths

Lifetime

5 Apr 1991–4 Jun 2000

Special Features

First “Great Observatory” at gamma-ray wavelengths

Payload

Instrument	Characteristic	Details
Burst and Transient Source Experiment (BATSE)	Energy Range	20–1000 keV
	Effective Area	1000 cm² at 0.03 MeV 550 cm² at 0.66 MeV (each)
	Field of View	4π sr (all-sky, minus Earth-occulted portion)
	Angular Resolution	3° (strong burst)
	Energy Resolution	32% at 0.06 MeV, 20% at 0.66 MeV (LAS) 8.2% at 0.09 MeV, 5.8% at 1.17 MeV (SD)
	Time Resolution	few milliseconds for burst events
	8 BATSE detectors mounted on each corner of the satellite, providing all-sky coverage to detect and locate gamma-ray bursts events and monitor transient and variable sources. Each BATSE detector module contained a Large Area Detector (LAD) which was made of a 50.8 cm diameter × 1.27 cm thick NaI(Tl) disk, and a Spectrocscopy Detector (SD) with a smaller NaI crystal with CsI backing for higher spectral resolution and extended energy range. Each individual detector would see roughly 2π sterdians. Relative count rate differences between detectors was used to triangulate on sources.
Oriented Scintillation Spectrometer Experiment (OSSE)	Energy Range	0.05–10 MeV
	Effective Area	2013 cm² at 0.2 MeV 569 cm² at 5.0 MeV
	Field of View	3.8° × 11.4° FWHM
	Angular Resolution	10′ square error box
	Energy Resolution	12% at 0.2 MeV 4.0% at 5.0 MeV
	The instrument used four large phoswich detectors (front NaI(Tl) scintillators optically coupled to rear CsI(Na) scintillators) read out by photomultiplier tubes. The phoswich configuration let the electronics discriminate events in the front crystal (gamma rays) from background or charged-particle events interacting in the rear crystal by pulse-shape differences.
Compton Telescope (COMPTEL)	Energy Range	0.8–30 MeV
	Effective Area	25.8 cm² at 1.27 MeV 29.4 cm² at 4.43 MeV
	Field of View	1 steridan imaging capability
	Angular Resolution	0.5–1.0° (90% confidence w/ 0.2 Crab spectrum)
	Energy Resolution	8.8% at 1.27 MeV 6.3% at 4.43 MeV
	COMPTEL performed gamma-ray imagery and spectroscopy in the medium-energy range between OSSE and EGRET. It was a two layer Compton telescope in which the upper detector layer made of a 7×7 array of ∼28 cm deep liquid scintillator modules, and the lower layer consisted of a 14×14 array of ∼7.5 cm thick NaI(Tl) crystals space about 1.5 m apart from the upper layer. An incident gamma ray would Compton scatter off an electron in the upper layer, and recording the energy of the electron and less energetic scattered photon allowed reconstruction of the original gamma ray energy and the cosine of its incident angle. Multiple events from the same source allowed direction to be determined by the overlap of each of the cosine-defined cones.
Energetic Gamma Ray Experiment Telescope (EGRET)	Energy Range	30 MeV – 10 GeV
	Effective Area	1200 cm² at 100 MeV 1400 cm² at 3000 MeV
	Field of View	∼0.6 sr
	Angular Resolution	5-10′ (1 s radius; 0.2 Crab spectrum)
	Energy Resolution	∼20% at 100–2000 MeV
	EGRET was a pair-conversion telescope designed to detect gamma rays by converting them into electron-positron pairs with the 28 layers tantalun conversion foils. Interleaved wire spark-chamber planes measured track direction (with time-of-flight discrimation to select only downward travelling events), and absorbed by a NaI(Tl) total absorption calorimeter to measure their combined energy. It was equipped with an anti-coincidence scintillator dome to veto charged-particle events

Energy Range

20–1000 keV

Effective Area

1000 cm² at 0.03 MeV
550 cm² at 0.66 MeV (each)

Field of View

4π sr (all-sky, minus Earth-occulted portion)

Angular Resolution

3° (strong burst)

Energy Resolution

32% at 0.06 MeV, 20% at 0.66 MeV (LAS)
8.2% at 0.09 MeV, 5.8% at 1.17 MeV (SD)

Time Resolution

few milliseconds for burst events

8 BATSE detectors mounted on each corner of the satellite, providing all-sky coverage to detect and locate gamma-ray bursts events and monitor transient and variable sources. Each BATSE detector module contained a Large Area Detector (LAD) which was made of a 50.8 cm diameter × 1.27 cm thick NaI(Tl) disk, and a Spectrocscopy Detector (SD) with a smaller NaI crystal with CsI backing for higher spectral resolution and extended energy range. Each individual detector would see roughly 2π sterdians. Relative count rate differences between detectors was used to triangulate on sources.

Energy Range

0.05–10 MeV

Effective Area

2013 cm² at 0.2 MeV
569 cm² at 5.0 MeV

Field of View

3.8° × 11.4° FWHM

Angular Resolution

10′ square error box

Energy Resolution

12% at 0.2 MeV
4.0% at 5.0 MeV

The instrument used four large phoswich detectors (front NaI(Tl) scintillators optically coupled to rear CsI(Na) scintillators) read out by photomultiplier tubes. The phoswich configuration let the electronics discriminate events in the front crystal (gamma rays) from background or charged-particle events interacting in the rear crystal by pulse-shape differences.

Energy Range

0.8–30 MeV

Effective Area

25.8 cm² at 1.27 MeV
29.4 cm² at 4.43 MeV

Field of View

1 steridan imaging capability

Angular Resolution

0.5–1.0° (90% confidence w/ 0.2 Crab spectrum)

Energy Resolution

8.8% at 1.27 MeV
6.3% at 4.43 MeV

COMPTEL performed gamma-ray imagery and spectroscopy in the medium-energy range between OSSE and EGRET. It was a two layer Compton telescope in which the upper detector layer made of a 7×7 array of ∼28 cm deep liquid scintillator modules, and the lower layer consisted of a 14×14 array of ∼7.5 cm thick NaI(Tl) crystals space about 1.5 m apart from the upper layer. An incident gamma ray would Compton scatter off an electron in the upper layer, and recording the energy of the electron and less energetic scattered photon allowed reconstruction of the original gamma ray energy and the cosine of its incident angle. Multiple events from the same source allowed direction to be determined by the overlap of each of the cosine-defined cones.

Energy Range

30 MeV – 10 GeV

Effective Area

1200 cm² at 100 MeV
1400 cm² at 3000 MeV

Field of View

∼0.6 sr

Angular Resolution

5-10′ (1 s radius; 0.2 Crab spectrum)

Energy Resolution

∼20% at 100–2000 MeV

EGRET was a pair-conversion telescope designed to detect gamma rays by converting them into electron-positron pairs with the 28 layers tantalun conversion foils. Interleaved wire spark-chamber planes measured track direction (with time-of-flight discrimation to select only downward travelling events), and absorbed by a NaI(Tl) total absorption calorimeter to measure their combined energy. It was equipped with an anti-coincidence scintillator dome to veto charged-particle events

Science Highlights

Discovery of an isotropic distribution of the gamma-ray burst events
Mapping the Milky Way using the ²⁶Al gamma-ray line
Discovery of Blazar Active Galactic Nuclei as primary source of the highest energy cosmic gamma-rays
Discovery of the “Bursting Pulsar”

NASA | GSFC | Sciences and Exploration

HEASARC

High Energy Astrophysics Science Archive Research Center

Compton Gamma Ray Observatory (CGRO)

Mission Characteristics

Payload

Burst and Transient Source Experiment (BATSE)

Oriented Scintillation Spectrometer Experiment (OSSE)

Compton Telescope (COMPTEL)

Energetic Gamma Ray Experiment Telescope (EGRET)

Science Highlights

Archive

Active Missions

Past Missions

Compton Gamma Ray Observatory (CGRO)

Mission Characteristics

Payload

Burst and Transient Source Experiment (BATSE)

Oriented Scintillation Spectrometer Experiment (OSSE)

Compton Telescope (COMPTEL)

Energetic Gamma Ray Experiment Telescope (EGRET)

Science Highlights

Archive