The Mark I, also known as the SLAC-LBL Magnetic Detector, was a particle detector that operated at the interaction point of the SPEAR collider from 1973 to 1977. It was the first 4π detector, i.e.
As the first 4π detector operating at the SPEAR collider's interaction point between 1973 and 1977, the Mark I, or the SLAC-LBL Magnetic Detector, served as a particle detector.