RA3. Experimental Equipment -- The Low-Energy Radioactive Ion Beam Spectroscopy Station (LeRIBSS)
(K. Rykaczewski for the LeRIBSS collaboration)
The Low-energy Radioactive Ion Beam Spectroscopy Station (LeRIBSS) was designed to study the decays of exotic neutron-rich nuclei produced in the proton-induced fission of 238U at the HRIBF. The physics motivations of these studies are related to the origins of nuclear structure evolution, to the properties of very neutron-rich nuclei affecting nucleosynthesis within the rapid neutron capture process, and to the decay properties of fission products relevant to the operation of power reactors and nuclear fuel processing.
Figure RA3-1: The main components of LeRIBSS setup: The beam steering, focussing and diagnostic chamber at the left side is separated with a remotely operated valve from the detection chamber surrounded by the detectors supported by the CARDS ring.
The setup is located downstream from the high-resolution magnet and under the HRIBF tandem. It is suspended from the ceiling below which the tandem's energy-analyzing magnet rotates to the various high-energy beamlines. LeRIBSS has two separate vacuum sections, the ion optics and diagnostic chamber and the detection section, see Fig. RA3-1. The first chamber houses the beam X-Y steerer, focussing quadrupole and beam diagnostic equipment which includes a high-sensitivity Faraday cup and a fluorescent beam viewer made out of Alumina plate. There is an additional fluorescent beam viewer in the second vacuum section, only 2 inches upstream from the radioactivity collection spot on the moving tape collector. These viewers die rapidly under the impact of such low-energy ions. We hope to avoid requiring their use once the system is better understood. The beam intensity diagnostics are complemented by a Microchannel Plate detector at the exit of the high-resolution injector magnet, before the LeRIBSS setup.
Figure RA3-2: The Moving Tape Collector (MTC) at LeRIBSS was built by Ed Zganjar at Lousiana State University. The half-inch-wide tape can move collected activity 20-inches far in less than 200 ms.
The radioactive samples collected at the moving tape collector (MTC) in the 2-inch-diameter aluminium beam pipe (0.9 mm thick) can be viewed by detectors supported in a CARDS ring, see Fig. RA3-2. The MTC and CARDS ring were designed and built by Ed Zganjar at Louisiana State University (LSU). An old-fashioned, half-inch-wide computer tape is used for collecting the source for a preselected time (within ~200 ms) and transporting it behind the 2-inch-thick lead wall separating CARDS detectors from the main MTC chamber. The beam-on/beam-off capability is provided by using a surplus magnetic steerer from the Tandem, allowing us to measure the grow-in and decay-out of the radioactivity samples deposited in front of the detector setup. The steerer is implemented upstream from the the high-resolution magnet and will be replaced by an electrostatic steerer.
The field of the HRIBF RIB injector can be set to allow either positive or negative ions to be separated and transmitted to LeRIBSS. We used positive ions of noble gases, e.g., krypton and xenon, to calibrate and verify the ion optics before the first experiment with radioactive ions.
The LeRIBSS Faraday cup and two fluorescent viewers helped to adjust the beam positon during the first experiments. Additionally, we have used a thin window, 10 mm diameter Silicon detector to observe 200-keV ion signals, about 2 inches upstream from the tape collector. For future experiments, this Si detector will be replaced by an small MCP detector currently under construction (Dan Shapira and Steven Padgett). We are planning to use a very thin (~microgram/cm2) carbon foil in this MCP setup, to allow the transmission of the 200-keV ions to the collection spot without a substantial intensity loss.
The LeRIBSS collaboration would like to express their gratitude to the
following people for their important contributions to the design and
construction of this apparatus: Charles Reed (ORAU), Tony Mendez
(ORNL), Jim Johnson (JIHR/UT), Ray Juras (ORNL),
Daryl Dowling (ORNL), and the professional staffs
in the LSU machine and electronics shops.