Physics Division 1996 to 1998
Physics Division 1996 to 1998In December 1996, the Holifield Radioactive Ion Beam Facility (HRIBF) received authorization to commence routine operation. During the past two years, the HRIBF staff has made significant progress toward the goal of providing high quality radioactive ion beams (RIBs) for the users of the facility. There have also been disappointments. The task of producing RIBs of sufficient intensity to do research has proven to be extremely challenging and has required extensive research and development on target materials and ion source design. Although many advances have been made, new challenges lie ahead since each new beam will present new problems. Over the past two years, the target/ion source and RIB development teams have developed the tools and the expertise which, we feel, will allow them to solve these problems.
The HRIBF includes the Oak Ridge Isochronous Cyclotron (ORIC), a K=100 cyclotron which provides high-intensity light-ions for producing radioactive atoms, the 25 MV tandem electrostatic accelerator which is used to accelerate the radioactive-ions for nuclear structure and nuclear astrophysics research, the radioactive ion beam injector, beam transport lines and experimental apparatus. The HRIBF also supports research and development on ion sources and targets suitable for the production of various RIBs. In addition, the HRIBF operates the Oak Ridge Electron Linear Accelerator (ORELA) in support of the nuclear astrophysics research program and, on a cost-recovery basis, other programs requiring pulsed neutron, photon, and positron beams.
Several accelerator improvement projects have been carried out, or are in progress, which are designed to improve the performance and reliability of the ORIC. Improvements include replacement or upgrading of the most troublesome power supplies, procurement of new trim coils, several sections of which are no longer operable due to water leaks, and moving most of the controls to a new control system. In collaboration with the cyclotron group at Michigan State University, the central region was modified to improve the extraction efficiency and values of 85% have been achieved for both proton and deuteron beams. This can be compared to typical values of 40%-50% prior to modification. Extracted proton and deuteron beam currents in excess of 10 uA have been delivered to the RIB injector.
The tandem continues to be quite reliable, but several additions and improvements have been made to meet the needs of the research programs and improve reliability. Since the intensity of the injected radioactive beam is very low, low intensity beam diagnostics devices have been developed and installed at focal points inside the tandem and beam transport lines. In addition, several aging power supplies have been replaced and controls have been moved to a new control system and the old control computers replaced.
A major accomplishment over the past two years, requiring a significant fraction of the tandem accelerator operating hours, was the completion of commissioning of the Recoil Mass Spectrometer (RMS) and Daresbury Recoil Separator (DRS). These devices are the primary research tools for the nuclear structure and astrophysics research programs, respectively, and required a variety of stable ion beams and beam energies to thoroughly test the devices and associated detector systems. Stable ion beams from the tandem were also provided for development of the Enge spectrograph and new beam imaging devices.
The target/ion source assembly is the "heart" of any ISOL-type RIB facility and much of our efforts over the past two years have been devoted to design, fabrication, and testing of target materials and ion sources suitable for producing RIBs requested by the research community. This has been a difficult and challenging task since these targets and ion sources must be capable of long term operation in a harsh environment of high temperature and high radiation fields while maintaining high voltage and high vacuum integrity. The ion source presently in use is the Electron-Beam-Plasma (EBP) positive-ion source that, although based on a proven design used at ISOLDE, has required many revisions to meet our specific needs.
Extensive testing is essential for any ion source prior to its use for the production of radioactive ion beams, and facilities have been built or modified for both off-line and on-line testing. Initial off-line tests are carried out at the Target Ion Source Test Facility (TISTF). The TISTF allows convenient measurement of ion source performance over a wide range of operating parameters in a controlled, radiation-free, environment. For on-line testing, the UNISOR isotope separator has been modified to allow target/ion source assemblies to be installed and tested with low intensity proton and deuteron beams from the tandem electrostatic accelerator and a significant fraction of the tandem operating time has been devoted to on-line ion source and target tests. An ion source test stand has also been constructed to assemble ion source components, check dimensional tolerances, ensure the integrity of vacuum seals and cooling water circuits, and measure electrical parameters prior to installation at either the on-line test facility or the RIB Injector. The stand also includes target-heating capabilities for outgassing target materials and for efficiency and release time measurements of various radioactive species from irradiated targets.
The first PAC-approved RIB experiment was carried out in May-June 1997 using a liquid germanium target developed for the production of arsenic and gallium RIBs. The target consisted of enriched 70Ge contained in a graphite target cell and radioactive atoms were produced by bombardment of the target with 42 MeV proton beams from ORIC to produce 69,70As from 70Ge(p,xn) reactions and 67Ga from 70Ge(p,alpha) reactions. In this first experiment, the ORIC beam current was limited to about 4 uA that resulted in a maximum of 1.4x106 ions/s of 69As beam delivered to the experimental station. This corresponds to a total efficiency, defined as the number of accelerated radioactive ions delivered to the experimental station divided by the number of radioactive atoms produced, of about 6x10-5. Following studies with the 69As beam, the beam line was tuned for mass 67 and approximately 105 ions/s of 67Ga beam was delivered to the experimental station.
A second experiment was carried out in September 1997 with the intent of increasing the ORIC beam intensity and yield of the 69As beam. A beam of 69As was successfully transported to the Recoil Mass Separator but the maximum beam intensity which could be achieved was only 105 ions/s for proton beam intensities up to about 6 uA. As the ORIC beam current was increased to 10 uA it was discovered that the yield of 69As decreased rapidly and post-run examination of the target revealed that most of the Ge had been vaporized. This has led to efforts to design a new liquid metal target that will condense and recirculate the target material.
Following these initial experiments with arsenic and gallium beams, the RIB development efforts concentrated on developing targets and ion sources suitable for producing 17F beams using the 16O(d,n) reaction. Low-density fibrous aluminum oxide targets were found to provide acceptable fluorine release rates but since fluorine is chemically active, it must be extracted from the target and transported to the ion source in the form of AlF molecules and AlF+ extracted from the ion source. Fortunately, charge exchange studies carried out at UNISOR showed that the AlF+ molecule could be dissociated, and F- ions formed, with a combined molecular-breakup/charge-exchange efficiency possibly as high as 10%.
The first high intensity study of a fibrous alumina target coupled to an EBP ion source was completed in March-April 1998 using 28 MeV deuterons from the ORIC. The maximum ion source efficiency was obtained at a deuteron beam current of about 2 uA. At higher deuteron beam currents, the source efficiency decreased due, at least in part, to a rapid increase in the aluminum vapor load in the ion source. The maximum yield of Al17F+ observed was 3x106 ions/s at a deuteron beam current of 4 uA. Further increases in the deuteron beam current resulted in reduced yields and, because of the decline in source output, we were unable to produce a F- beam. Post-run examination revealed that the target material had suffered significant shrinkage and deterioration.
A second high intensity test, again using a fibrous alumina target, was made in August 1998. In this test, the deuteron beam current was limited to 2 uA to avoid target degradation. The yield of Al17F+ was 2x106 ions/s and after breakup of the AlF molecule and charge exchange, the yield of 17F- was measured to be only 5x104 ions/s and attempts to transport this small amount of 17F- beam through the tandem were unsuccessful. At least part of the problem encountered in transporting the fluorine beam can be attributed to the large energy spread, 400 eV, introduced by the molecular breakup process. Studies of other metal-oxides having higher operating temperatures are in progress in an effort to develop targets capable of withstanding higher deuteron beam currents. A major part of the development program for these target materials is the addition of a vapor feed system to provide a reliable source of aluminum or other metals to form metal-fluoride molecules suitable for release and transport in the target/ion source. A negative-ion source is also under development that would avoid the problems associated with the molecular breakup and charge-exchange processes.
A "batch-mode" target/ion-source is presently being fabricated for use with longer lived radioactive species. This source will allow a target to be irradiated with beam from the ORIC, and then rotated to a negative-ion cesium-sputter source for production of a RIB beam while a new target is being irradiated by the ORIC beam. The first tests of this source, using Ni targets for the production of a 56Ni beam, are expected to be made in the near future.
Low-density uranium carbide targets are being investigated for the production of neutron-rich RIBs through proton induced fission of 238U. Low-intensity studies are being carried out at the UNISOR facility to measure fission fragment release from targets consisting of thin UC2 coatings on carbon fibers. Use of uranium targets for production of RIBs will require changes in the facility authorization basis documents and these changes have been initiated.