THE KARLSRUHE 4-PI BARIUM FLUORIDE DETECTOR

A new experimental approach has been implemented for accurate measurements of neutron capture cross sections in the energy range from 5 to 200 keV. The Karlsruhe 4π barium fluoride detector 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 reflector and photomultiplier, thus representing independent gamma-ray detectors. The energy resolution 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. The detector allows to register capture cascades with 95% probability above a threshold energy of 2.5 MeV. Neutrons are produced via the 7Li(p, n)7Be reaction using the pulsed proton beam of a Van de Graaff accelerator. A collimated neutron beam is passing through the detector and hits the sample in the centre. The energy of captured neutrons is determined via time of flight, the primary flight path being 77 cm. The combination of short primary flight path, a 10 cm inner radius of the spherical BaF2 shell, and the low capture cross section of barium allows to discriminate background due to capture of sample-scattered neutrons in the scintillator by time of flight, leaving part of the neutron energy range completely undisturbed. This feature, together with the high efficiency and good energy resolution for capture gamma-rays, allows to separate the various background components reliably enough, that the capture cross section ratio of two isotopes can be determined with an accuracy of ≤ 1.0%. The detector will be used for nuclear astrophysics to investigate the origin of the heavy elements in the slow neutron capture process. © 1990.