Hermetic Detector

In particle physics, a hermetic detector (also called a 4π detector) is a particle detector designed to observe all possible decay products of an interaction between subatomic particles in a collider by covering as large an area around the interaction point as possible and incorporating multiple types of sub-detectors. They are typically roughly cylindrical, with different types of detectors wrapped around each other; each detector type specializes in particular particles so that almost any particle will be detected and identified. Such detectors are called “hermetic” because they are designed to let as few particles as possible escape; the name “4π detector” comes from the fact that such detectors are designed to cover nearly all of the 4π steradians of solid angle around the interaction point. —http://en.wikipedia.org/wiki/Hermetic_detector