Editing Compton Gamma Ray Observatory (section)

==Instruments==
[[File:Compton Gamma Ray Observatory cutaway (labelled).jpg|thumb|left|Compton Gamma Ray Observatory cutaway]]
CGRO carried a complement of four instruments that covered an unprecedented six orders of the [[electromagnetic spectrum]], from 20 [[electronvolt|keV]] to 30 GeV (from 0.02 MeV to 30000 MeV). Those are presented below 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 burst]]s (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.<ref name=nasa-cgro-nra-g>[https://heasarc.gsfc.nasa.gov/docs/cgro/nra/appendix_g.html#V. BATSE GUEST INVESTIGATOR PROGRAM]</ref> Each module consisted of both a [[Gamma spectroscopy#Scintillation detectors|NaI(Tl)]] Large Area Detector (LAD) covering the 20 keV to ~2 MeV range, 50.48&nbsp;cm in dia by 1.27&nbsp;cm thick, and a 12.7&nbsp;cm dia by 7.62&nbsp;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 [[Gamma spectroscopy#Scintillation detectors|NaI(Tl)]] 12 in (303&nbsp;mm) in diameter, by 4 in (102&nbsp;mm) thick, optically coupled at the rear to a 3 in (76.2&nbsp;mm) thick [[Caesium iodide|CsI]](Na) crystal of similar diameter, viewed by seven [[photomultiplier tube]]s, operated as a [[Phoswich Detector|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 [[Electronic anticoincidence|anticoincidence]] 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 finer level of angular collimation was provided by a tungsten 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 (spacecraft)|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 scattering|Compton scatter]] in a forward detector module, where the interaction energy ''E<sub>1</sub>'', given to the recoil electron was measured, while the Compton scattered photon would then be caught in one of the second layers of scintillators to the rear, where its total energy, ''E<sub>2</sub>'', would be measured. From these two energies, ''E<sub>1</sub>'' and ''E<sub>2</sub>'', the Compton scattering angle, angle θ, can be determined, along with the total energy, ''E<sub>1</sub> + E<sub>2</sub>'', of the incident photon. The positions of the interactions, in both the front and rear scintillators, was also measured. The [[Euclidean vector|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===
{| class=wikitable style="text-align:center; font-size:11px; float:right; margin:2px"
|- style="font-size: smaller;"
| colspan=8 align=center|'''Instruments'''
|-
! Instrument || Observing
|-
| BATSE || 0.02 – 8 MeV
|-
| OSSE || 0.05 – 10 MeV
|-
| COMPTEL || 0.75 – 30 MeV
|-
| EGRET || 20 – 30 000 MeV
|-
|}
{{Main|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 (particle physics)|calorimeter]] [[scintillation detector]] at the rear of the instrument.