|Edition 6, No. 1||March 27, 1998||Price: FREE|
Editor: Carl J. Gross
Contributors: J. R. Beene, J. D. Garrett, C. J. Gross, M. J. Meigs, G. D. Mills, A. Piechaczek, M. S. Smith, R. F. Welton
The long-standing discrepancy between various measurements of the ionization efficiency of the electron-beam-plasma source (EBP) when placed on the ion source test facility (ISTF), UNISOR, and the RIB platform has been resolved. The understanding of these measurements is critical to ensure peak ion source performance over the extreme range of operational conditions to which the on-line target/ion source is exposed. In addition, the measured ionization efficiencies are an indicator of good coupling between the source and the early stages of the RIB injector ion-optics. The latest measurements yielded a record Xe+ ionization efficiency of ~20% for this version of the EBP source on the RIB injector which is consistent with the ISTF measurements. We have recently developed techniques that provide reasonably accurate on-line measurements of beam emittance on the RIB injector platform. These measurements also shed light on ion source performance, and are of considerable importance in understanding and optimizing the performance of our isobaric separation system.
Over the last year, a new type of surface ionization source for the direct production of negative beams of radioactive fluorine has been developed at the ISTF. The source is based on a two-step ionization principle where Cs ions are formed on a heated surface, accelerated to a few hundred volts, and directed onto a much cooler surface. Due to the abundance of Cs and other metals such as Al in the ion source, the radioactive fluorine is transported through the ion source from the production target bound in molecular form. Both the fluorine molecules and Cs atoms collect on the cooler surface. During Cs ion impact, the adsorbed fluorine molecules are both dissociated and ejected from the surface as negative F ions. Development of this ion source has progressed through the off-line testing period to an on-line development phase using UNISOR. After resolving several early problems associated with altering UNISOR to accommodate negative ions and dealing with the problems peculiar to negative ion sources that operate with Cs vapor, we were able to achieve an ionization efficiency of 1% for stable fluorine, in very good agreement with measurements at the ISTF. Over the last several weeks, two on-line tests of this negative fluorine source were undertaken. These experiments were similar to those already carried out with electron-beam plasma sources. A low-intensity deuteron beam (~20 nA at 22 MeV) bombarded fibrous Al2O3 target material from Zircar Inc. allowing an effective overall efficiency of 17F production, release, and ionization to be measured directly. The resulting data showed yields of 17F- to be as large as 2.6x105 ions/s/microampere in the first experiment and 6.6x105 ions/s/microampere in the second experiment. These yields of 17F should be compared with those of the positive electron-beam-plasma ion source which was previously tested at UNISOR with the same target material and found to be roughly one order of magnitude larger. Since the tandem accelerator requires negative ions, positive beams must first be charge exchanged in an in-line cell, a process which has been shown to be approximately 10% efficient. Thus we are beginning the on-line development phase of the negative surface ionization source at the same effective yield as the electron-beam-plasma source plus charge exchange cell.
In parallel with the on-line development effort, a closely coupled off-line development program is underway which has also made some important strides. Apparatus has been developed to allow the injection of a well-defined trace flow of the stable homologues of radioactive species into the target/ion source. This allows evaluation of candidate target material/transport vapor/ion source combinations for producing a given RIB species to occur on a much shorter time scale than would be possible with on-line testing. Also, optimization and parametric tuning of a given ion source can be carried out for the same chemical species it will produce on-line. For example, in recent weeks, stable fluorine ionization efficiency measurements have been performed on a source of similar design to that currently being tested at UNISOR in such a fashion as to isolate specific design parameters. It was found that by adding a Re liner to the transfer line of the source, F efficiency was increased from ~1% to ~2%. In addition, adding a repulsive bias to the Cs acceleration electrode of the source further increased the F ionization efficiency to ~4%.
A key element of our ion source design and development program is a strong ion source modeling program using Monte Carlo simulations in conjunction with thermochemical databases to describe thermal transport phenomena, finite element methods to model the heating aspects of the source structure, Poisson-Vlasov equation-based software to understand the ion optics of the source, and particle/plasma dynamic balance models to understand the ion formation processes.
The Batch Mode RIB Ion Source will provide direct extraction of negatively charged ion beams of long-lived radioactive species such as 56Ni (5.9-day half-life). A multiple-position, indexing target system will allow simultaneous target bombardment by the cyclotron beam and sputter-ion-source extraction of the desired radioactive negative ion beam. The target system can be operated remotely so that newly irradiated production targets may be quickly placed within the sputter ion source section of the assembly. The design is compatible with the existing RIB platform and the robotic handling and storage systems. Initial estimates for the approved design are within budget with delivery, installation, and testing to start early this summer.
Recent developments in plans for constructing the National ISOL Facility at the Spallation Neutron Source (SNS) in Oak Ridge include:
Our efforts are currently centered on the design of an ISOL Facility based on the use of a small fraction of the 1-GeV proton beam from the SNS linac. The key issues of this scheme include:
More details of these plans will be given in a future update of the HRIBF Web Site.
Brad Sherrill was unanimously elected vice-chairperson for 1998. He will become chairperson on January 1, 1999, and serve for 1 year. A HRIBF Users Meeting is being planned to take place some time during this summer. The primary topic of this meeting will be the new RIB Facility that is being planned at ORNL. Should you have any suggestions or comments regarding the Users Meeting or any aspect of HRIBF, you may use the new "comment card" found on our web site at http://www.phy.ornl.gov/hribf/Users/comment_card.html. The HRIBF Calendar is now up on the web site. The schedule for this quarter may be found at http://www.phy.ornl.gov/hribf/Users/01-03-1998.html. The schedule for the next quarter (http://www.phy.ornl.gov/hribf/Users/04-06-1998.html) will be available in the near future.
A new set of charging chains was installed in the Tandem accelerator during the January tank opening. These chains replaced the defective chains that were installed in October 1996. The new chains were tested for hardness of the nickel/chromium finish and were found to be satisfactory. Before installation, the charging system was thoroughly cleaned, and new idler wheels, pickoff wheels, and contact bands were installed. Cleaning and replacing these parts was done to prevent any leftover metallic residue from abrading the new chains. Since installation of the new chains, the tandem was operated for approximately two weeks with 22.5 MV terminal voltage.
The 12C(p,gamma)13N was measured in December 1997 with the DRS in inverse kinematics: a 12C beam of energy 0.66 MeV/u bombarded a thin CH2 foil, and 13N capture recoils were detected at the DRS focal plane. Rejection of scattered beam by the separator itself was measured to be greater than 1011, and the gas ionization counter cleanly separated the remaining beam particles from the recoils. This rejection ratio agrees with that obtained by 12C(14N,14N)12C elastic scattering measurements in October 1997 and February 1998, and is consistent with expectations. However, transmission of the 13N recoils was measured to be 7%, about a factor of 7 lower than expected. The acceptance of the DRS was also measured in the elastic scattering experiments and found to be 2 msr, a factor of 3 less than achieved at Daresbury. Work is underway to improve the diagnostics and control over the beam delivered to the DRS, and to optimize the DRS optical solution for maximum transmission. We are also investigating ways to increase the acceptance of the DRS focal plane.
The Moving Tape Collector (MTC), built for the RMS by UNIRIB collaborator LSU as a tool for the spectroscopy of radioactive nuclei with halflives over several hundred milliseconds, was commissioned at the end of January. The MTC was used to transport mass-separated nuclei from reactions with a 200 MeV 58Ni beam and a 500 ug/cm2 target of 24Mg. Reaction products were implanted onto a 35 mm aluminized tape and transported to a detector array consisting of 2 Clover Ge detectors in a close 180 degree geometry and three, 5 mm thick Si(Li) detectors located in front of one of the Clover detectors. The time between two consecutive tape movements of approximately 0.5 m was varied between 10 and 60 s.
Gamma rays following the beta-decay of the ground state and the 228 keV, 5 s
isomer in 80Y were observed with the
Clovers operating in singles and in coincidence with each other or with
the Si(Li) detectors.
Furthermore, the Si(Li) detectors allowed detection of conversion
electrons from the 228 keV isomer, to determine the multipolarity
of its decay. The data from this 40-hour commissioning run,
which may lead to an extension of the
Improvements to the system will include the use of photodiodes on either side of the transport tape at the RMS focal plane to facilitate positioning the mass of interest on the tape. An option to use a 90 degree geometry for the Clover Ge detectors to minimize the detection of 511 keV annihilation radiation is also being planned. Furthermore, we plan to use a multiscaling memory to record gamma singles data without excessive deadtime losses in the data acquisition as well as an active BGO or passive lead shielding for the Clover detectors.
Angus MacNab, a Science and Engineering Research Semester (SERS) undergraduate student assigned to the Physics Division of ORNL, has been selected to present a poster on his work at the Council on Undergraduate Research's (CUR) "Second Undergraduate Research Poster Session on Capitol Hill." MacNab, a junior at Evergreen State University located in Olympia, Washington, did the research to be reported during his August to December 1997 assignment to the Physics Division at ORNL. MacNab worked with Felix Liang, a postdoc at ORNL, on a project to construct and test a new detector system for recording heavy ions located at the focal plane of a magnetic spectrometer. This instrument will be used for experimental studies at the Holifield Radioactive Ion Beam Facility at ORNL. The title of MacNab's poster is "Measuring Fusion Cross Sections with the Enge Split-Pole Magnetic Spectrometer."
The CUR poster session will be held in Room 106 of the Dirksen Senate Office Building from 5:00 to 7:00 p.m. April 12, 1998.
|Jerry D. Garrett||Carl J. Gross|
|Scientific Director||Scientific Liaison|
|Mail Stop 6368||Mail Stop 6371|
|Holifield Radioactive Ion Beam Facility|
|Oak Ridge National Laboratory|
|Oak Ridge, Tennessee 37831 USA|