The Karlsruhe 4π barium fluoride detector

The Karlsruhe 4π barium fluoride detector is the first spherically symmetric detector made from BaF₂ crystals with ∼100% efficiency for gamma-rays up to 10 MeV. It consists of 42 crystals shaped as hexagonal and pentagonal truncated pyramids forming a spherical shell with 10 cm inner radius and 15 cm thickness. All crystals are supplied with a reflector and photomultiplier, thus representing independent gamma-ray detectors. The energy resolution of the individual detector modules is 10-11% at 662 keV and 4-5% at 6.13 MeV gamma-ray energy, while the time resolution as measured with a ⁶⁰Co source is 400-500 ps. The energy resolution of the sum energy signal of the 4π detector is 14% at 662 keV and 7% at 2.5 MeV gamma-ray energy, the overall time resolution is 500 ps and the peak efficiency 90% at 1 MeV. This is a quite unique combination for a gamma-ray detector that became possible by the recent availability of large barium fluoride crystals.