Editing Compton Gamma Ray Observatory (section)

===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.