MoNA and initial measurements with 7 He Resonance

MoNA and initial measurements with He Resonance * Of recent interest in the study of exotic nuclei are neutron rich nuclei near and beyond the drip line. Using radioactive beams produced at the National Superconducting Cyclotron Laboratory (NSCL), the Modular Neutron Array (MoNA) detector, and a 4 Tesla sweeper magnet it is possible to study these nuclei. MoNA is a large area, high-efficiency neutron detector consisting of 144 plastic scintillating bars. When used in tandem with the series of detectors and scintillators in the sweeper setup MoNA is capable of detecting neutrons in coincidence with reaction fragments detected at the sweeper. As an initial experiment to test the tandem setup of MoNA and the sweeper a secondary beam of Li was produced in the coupled cyclotron facility and the reaction of interest, stripping off a proton to observe the resonance of He immediately decaying to He+n, was studied with MoNA and the sweeper. The ground state of He resonance was observed in the relative velocities of He and the neutron. *MoNA collaboration is supported by the NSF. TP, RP acknowledge the support from the NSF Research Experience for Undergraduates program. Introduction Studying the properties of exotic nuclei can provide insight to fundamental laws and nuclear models of interest to the physics community. Of recent consideration is the study of neutron rich nuclei near and beyond the drip line. Bound systems are found in the valley of stability on the chart of the nuclides (see figure 1). Stability weakens while moving away from this valley of stability and unbound nuclear systems are found beyond the neutron and proton drip lines. Moving away from the valley of stability nuclei decay to those nuclei on the valley of stability. β unstable nuclei are those near the valley of stability that decay by β emission to the stable nuclei on the valley of stability. Proton and neutron unstable nuclei are those where the proton and neutron separation energies become negative and the last proton or neutron is said to be unbound. These are the nuclei beyond the proton and neutron drip lines. [1] As the stability of nuclei decreases the level of difficulty to experimentally study them increases. However, using radioactive beams created at the National Superconducting Cyclotron Laboratory (NSCL), a Modular Neutron Array (MoNA) detector, and a sweeper magnet it is possible to study these exotic nuclei near and beyond the neutron drip line. It is necessary to study the detected neutrons in coincidence with detected charged fragments from reactions that are swept aside by the sweeper magnet for a more comprehensive understanding of the exotic nuclei studied. MoNA is a large-area, high-efficiency neutron detector comprised of 144 plastic scintillating bars. (See figure 2). The scintillating material is sensitive to electrons, protons, alpha particles, gamma rays and Figure 1. The chart of the nuclides. The black nuclei are stable and make up the valley of stability. Stability decreases moving away from this valley starting with β unstable nuclei close to the valley and ending with the proton and neutron drip lines on the outer most edges. http://www.nscl.msu.edu neutrons. Because a neutron has no charge it is detected indirectly. It must interact with a charged particle within the bar to cause scintillation in order to be detected. Each bar measures 200x10x10 cm and has two photomultiplier tubes, one at each end, which converts the internally reflected light into an electric signal. Currently, MoNA is arranged in nine columns each consisting of 16 detector bars making an array of 144 detectors covering an area of 2.0 m wide by 1.6 m high. Figure 2. A picture of MoNA it its current position and configuration MoNA does, however, have the ability to be rearranged to meet the needs of various experimental setups and is designed to have 70% efficiency at detecting single neutron events and 49% efficiency at detecting a double neutron hit. MoNA also has the capability of using passive iron converters, which increase nuclear interactions in the detector volume, to increase the efficiency of detecting neutrons of above 100 MeV. [2] The sweeper magnet is a 4 Tesla large gap magnet that bends the charged reaction fragments 43 degrees to a vacuum chamber containing multiple detectors. The vacuum chamber contains two Cathode Readout Drift Chambers for position measurements, an ion chamber for energy loss measurements, and thin and thick plastic scintillators for additional energy measurements. As an initial experiment a Li beam created by the coupled cyclotron facility was used to bombard a carbon target to test MoNA’s ability to detect neutrons from the reaction in coincidence with charged fragments from the reaction at the sweeper magnet. It was found that MoNA was, in fact, able to detect neutrons in coincidence with charged reaction fragments detected at the sweeper magnet. R. Pepin, August 2004 Experiment A primary beam was produced by creating O in the Electron Cyclotron Resonance (ECR) ion source and injecting it into the K500 cyclotron via the K500 Injection Line. An ion must be used because electromagnetic fields are used to accelerate the particles in the cyclotron and electromagnetic fields only accelerate charged particles. Once in the K500 the O was accelerated in a circular path and the energy was increased as it passed the same radio frequency and high voltage several times. At 10.91 MeV/nucleon theO was transferred to the K1200 cyclotron via the K500-to-K1200 Coupling Line. [3] Before entering the K1200 the O was ionized to O . The O beam was then accelerated in the K1200 cyclotron using the same radio frequency and high voltage method of acceleration as that of the K500. Upon exiting the K1200 cyclotron at 120 MeV/nucleon the secondary beam was produced by projectile fragmentation when the primary beam (O) struck a beryllium production target of 3,526 mg/cmcreating a large range of isotopes. After striking the production target the beam immediately proceeded to the A1900 fragment separator where the desired isotope, Li, was selected with a series of bending and focusing magnets. The beam was then directed to the N4 vault with another series of focusing and bending magnets in the transfer hall. Figure 3. A schematic of the cyclotron indicating various parts of the cyclotron. In the N4 vault the Li beam was directed toward the sweeper magnet. While entering the sweeper magnet the O beam bombarded a carbon target of 75 mg/cm creating charged fragment reaction products and neutrons. The reaction of interest was the proton stripping of the Li to unbound He decaying to He+n. The sweeper magnet is a 4 Tesla magnet that “sweeps” the charged fragments from the reaction away to the side while allowing the neutrons to continue at zero degrees toward MoNA. The sweeper magnet bends charged particles different amounts according to their mass-to-charge ratio and velocities. Once bent MoNA