Crystal Ball


The experimental apparatus consists of the famous Crystal Ball together with the TAPS detector as a forward wall, and a central tracker. This configuration provides a geometrical acceptance close to 4π combined with good energy and angular resolution, in particular for neutral final states. After the energy upgrade of the Mainz Microtron (MAMI-C), the Crystal Ball and TAPS setup will be equipped with a frozen-spin polarized target filled with butanol, or D-butanol to perform new high precision, high statistics measurements of neutral meson production and polarisation observables on the nucleon up to 1.5 GeV incident photon energies.



The Crystal Ball Detector

The Crystal Ball, CB, spectrometer consists of a highly segmented sphere made of NaI. The sphere has an entrance and exit tunnel for the beam and a spherical cavity for the liquid hydrogen target and a vertex detector. The target is surrounded by a cylinder of scintillation counters that function as the charged particle detector. The solid angle of the CB is 93% of 4π steradian. The Crystal Ball was build at SLAC and used in J/ψ measurements at SPEAR and b quark physics at DESY. The CB is constructed of 672 optically isolated NaI(Tl) crystals, 15.7 radiation lengths thick. The counters are arranged in a spherical shell with an inner radius of 25.3 cm and an outer radius of 66.0 cm. The hygroscopic NaI is housed in two hermetically sealed evacuated hemispheres. The CB geometry is based on that of an icosahedron. Each of the 20 triangular faces (major triangles) is divided into four minor triangles, each consisting of nine separated crystals. Each crystal is shaped like a truncated triangular pyramid, 40.6 cm high, pointing towards the center of the Ball. The sides on the inner end are 5.1 cm long and 12.7 cm on the far end. Each crystal is individually wrapped in reflector paper and aluminized mylar; it is viewed by its own a 5.1 cm diameter SRC L50 B01 photomultiplier, selected for linearity over a wide dynamic range. The phototube is separated from the crystal by a glass window and a 5 cm gap. The crystals have been stacked so as to form two mechanically separated top and bottom hemispheres. The boundary between the two hemispheres is called the equator region. It is about 0.8 cm thick, consisting of two 1.6 mm stainless steel plates separated by 5 mm of air. This introduces an inactive space amounting to 1.6% of the solid angle. The inner wall of the hemisphere is 1.5 mm stainless steel or 0.09 radiation lengths. The Ball has an entrance and exit opening for the beam which results in a loss of 4.4% of angular acceptance.