3-d Measurement of the Nscl Position-sensitive Gamma Ray Detector Array

The National Superconducting Cyclotron Laboratory (NSCL) is a national nuclear physics research laboratory on the campus of Michigan State University. In a typical experiment, heavy ions with masses ranging from hydrogen to uranium are created in an Electron Cyclotron Resonance (ECR) ion source and injected into the K500 cyclotron. The accelerated beam is transported to the K1200 cyclotron, where its electrons are stripped to a higher charge state before additional acceleration. After extraction, the beam has an energy of up to 200 MeV per nucleon. The extracted beam is focused onto a production target and the secondary beam from the fragmentation reaction is analyzed in a series of dipoles and quadrupoles/multipoles. A beam switchyard focuses the beam into one of nine experimental stations. Superconducting magnets are found in the main coils of the two cyclotrons, one of the ECR ion sources, and in all of the high energy beam transport dipoles and quadrupoles/multipoles. The research programs at NSCL include nuclear astrophysics, nuclear structure, and the production of isotopes far from the valley of stability. One of the newest pieces of experimental equipment is the Segmented Germanium Array (SeGA). SeGA is an array of eighteen 32-fold segmented germanium detectors (see Figs. 1 & 2). These detectors are designed to measure gamma rays emitted from heavy-ion beams with energies up to 140 MeV per nucleon. At high beam velocities the detected energy of a gamma ray emitted from the beam nucleus will depend on the angle at which the gamma ray was detected. For example, a 1-MeV gamma ray emitted by a Mg beam with a velocity of 0.4c will have an apparent energy of 1.4 MeV when detected at 30 degrees with respect to the beam; while, at 150 degrees this same gamma ray will have an apparent energy of 0.68 MeV. The energy of the gamma ray must be measured in the reference frame of the beam, so the detected energies must be transformed based on the angle at which the gamma ray was recorded. The intrinsic gamma-ray energy resolution of our SeGA detectors is better than 0.3%. For the example presented above a 0.3% change in the gamma ray energy would correspond to a three millimeter change in the position of the detector. Thus in order to achieve the maximum resolution of our detector array, the detector positions must be known with respect to the emission point of the gamma rays (which is assumed to be a target placed in the center of the array) to an accuracy of better than one millimeter.