A position sensitive high-efficiency NaI(Tl) photon-detection system for use with intermediate energy radioactive ion beams

We have built an array of position-sensitive NaI(Tl) photon detectors for use in radioactive beam experiments at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University. The array consists of 38 cylindrical NaI(Tl) detectors oriented co-axially in three concentric rings around the particle-beam axis. Each crystal has a diameter of 5 cm, is 17 cm long and is read out by two photomultiplier tubes, one at each end of the crystal. Because of the position-dependent attenuation of the scintillation light in the crystal it is possible to determine both the photon energy and the interaction point of the photon in the detector. This photon detection system is very well suited for the investigation of exotic nuclei which, at the NSCL, are produced in fragmentation reactions. Exotic beams produced by this method have high velocities and potentially low particle rates. Both of these properties are addressed in the described setup. The Doppler shift due to the large beam velocities is corrected on an event-by-event basis using the known interaction positions of the photons in the detector. Low particle rates can be accommodated with particle–γ coincidence measurements and the high efficiency of the array.