A high-efficiency NaI(Tl) detector array with position sensitivity for experiments with fast exotic beams

The twenty-four trapezoidal NaI(Tl) detectors previously used as part of the trigger detector for the APEX experiment at Argonne National Laboratory have been adapted for use in experiments with beams of fast exotic isotopes at the National Superconducting Cyclotron Laboratory at Michigan State University. A high-efficiency position-sensitive detector is ideal for inelastic scattering studies with exotic beams produced at projectile- fragmentation facilities. The γ rays emitted from fast-moving projectiles (v/c=0.3–0.5) are Doppler-shifted considerably. The energy in the projectile frame can be reconstructed by simultaneously measuring the emission angle and energy of the lab frame photon. Position sensitivity (3 cm FWHM) is achieved by reading out photo-multiplier tubes at both ends of each detector in coincidence. The large solid-angle coverage (75% of 4π) and high intrinsic efficiency result in a total photopeak detection probability of 17% at 1332 keV with an energy resolution of about 13%.