A novel and extremely high-efficiency photon detector was just completed at the National Superconducting Cyclotron Laboratory (NSCL) with funding by a Major Research Instrumentation (MRI) grant from the NSF. The array of 192 detectors, called CAESAR (for CAESium iodide ARray), incorporates a special scintillator material to detect the gamma rays emitted in nuclear reactions by atomic nuclei that move at more than 30% of the speed of light. As in atomic systems, determining the spectrum of photons from excited nuclear systems provides unprecedented and unique information on their properties. The new detector system will significantly enhances the world-leading scientific program that the NSCL. After the initial tests, the array was used in two experiments and demonstrated its ability to collect dramatically more data on the collision products in significantly less beam time.
MSU graduate student Andrew Ratkiewicz places a gold target in the beam pipe at the NSCL. The rare-isotope beams available at the NSCL impinge on the target and induce a variety of nuclear reactions. The photons are detected with CAESAR, which allows the researchers to unravel the structure of the rare isotopes.
The structure of stable atomic nuclei is know in great detail and well-described by theoretical models after many years of work. However, the structure of so-called exotic nuclei or rare isotopes with unusual combinations of neutrons andprotons has been found to be significantly different and new results continue to surprise researchers. The NSCL on the campus of Michigan State University (MSU), sponsored by NSF, is the world-leading facility to produce rare isotope beams moving at nearly the speed of light. A beam of a rare isotope is itself studied or more often reacted with stable targets to elucidate its structure. The photons emitted during and after the reaction provide invaluable information on the energy levels of the exotic nuclei and allow detailed studies of their properties. CAESAR is a very efficient detector that is tuned to collect and measure these photons.
The CAESAR device consists of 192 individual CsI(Na) scintillation crystals that cover 95% of the solid-angle surrounding the target. Higher coverage is not possible since space is needed for the beam to enter and exit the target. The large number of detector elements is needed since the photons emitted by moving nuclei are subject to the well-known Doppler phenomena that can only be corrected if the relative direction of emission is known. Each individual CAESAR detector that responds to a photon has a specific angle relative to the beam direction that is used in the Doppler reconstruction to calculate the spectrum of emitted photons in the rest frame of the moving nucleus. CAESAR was successfully commissioned in May 2009 and subsequently used in two nuclear structure experiments in July 2009 at the NSCL. Both experiments will be part of the PhD dissertations of graduate students at MSU and Washington University in Saint Louis. Many more experiments – proposed by a wide variety of users of the NSCL facility – have been accepted at the most recent meeting of the Program Advisory Committee and are expected to provide exciting insights into the structure of rare isotopes during the coming years.