Contents:
Edition 4, No. 5 September 20, 1996 Price: FREE
Editor: Carl J. Gross
Contributors: C. Baktash, J. R. Beene, J. D. Garrett,
P. E. Mueller, M. S. Smith
On August 30, at about 1:00 p.m., the HRIBF successfully accelerated its first radioactive ion beam. The initial demonstration beam was 70-As, produced with 70-Ge(p,n) using 42 MeV protons from ORIC on a liquid germanium target, and accelerated to 140 MeV by the 25 MV tandem accelerator. The demonstration was carried out under the rules for "low-intensity commissioning" which limit the driver proton beam to approximately 100 nA. Unfortunately, the actual proton intensity incident on the production target was only 10 nA during the demonstration as a result of various difficulties with ion-optical and diagnostic equipment on the beam line from ORIC. Consequently, the accelerated 70-As beam current was very low (2000 ions/sec), but easily detected by observing gamma-radiation following beta-decay. This would correspond to an accelerated beam current of 10**6 ions-per-second-per-microampere of proton beam, under production conditions using an enriched 70-Ge target. A natural (20% 70-Ge) target was used in the demonstration run.
A beam of 69-As was also produced, accelerated off the RIB injector platform, accumulated on a moving tape system, and detected by observing its decay.
We refer to the combination of all equipment on the 300 kV platform (target, ion source, first-stage mass separator, electrostatic lenses, and charge-exchange cell) and the transfer beam line (electrostatic lenses and second stage mass separator) to the tandem as the RIB injector system. This is obviously the heart of the HRIBF. We have made great progress over the past several months in understanding and optimizing the performance of the elements of this system. During preparation for the demonstration run, we observed a total transmission of 28% for 70-Ge ions through the injector system from the ion source to the tandem accelerator. Essentially all loss was in the Cs vapor charge-exchange cell; transmission of positive ions through the cell with no Cs vapor was 70%, and the measured negative ion fraction was 40% under demonstration run conditions. All other elements, including the second-stage mass separator, transmitted the beam without measurable loss. These measurements were made with a 50 keV 70-Ge beam on the high-voltage platform, accelerated to 200 keV for second stage analysis and injection into the tandem. The transmission of the entire system from the ion source through the tandem was measured to be about 6%, corresponding to a transmission through the tandem itself of 20%.
The design mass resolution of the second-stage mass separator is 1 part in 20,000. The resolution is limited by the size of the beam at the object of the separator in the dimension corresponding to the bending plane of the separator. We achieved 100% transmission with a size corresponding to a resolution of 1 part in 7500 (FWHM). No effort was expended to tune for a smaller spot. With this tune we could achieve 1 part in 10,000 resolution with 60% transmission or 1 part in 20,000 with approximately 20% transmission.
The resolving power actually achieved with the second-stage separator is available from a preliminary analysis of an experimental study of the energy-loss straggling in the Cs vapor charge-exchange cell. This study measured a resolution of 1 part in 10,000.
The annual meeting of the HRIBF Users Group will be held in conjunction with the Division of Nuclear Physics (DNP) Meeting in Cambridge, MA. This year's meeting will be held in Room W11, Baxter Hall (on the MIT campus) from 4 to 5 p.m. on Thursday, October 3.
Drop by for refreshments and to hear the latest news from the HRIBF.
Sixteen proposals (representing 54 researchers from 22 institutions) were received in June. The HRIBF Program Advisory Committee (PAC) met at ORNL on August 6. Of the 123 eight-hour shifts of radioactive 69-As and 70-As beams and 239 shifts of stable beams (SIBs) requested, 72 shifts of radioactive beam and 106 shifts of stable beam were granted. Many RIB experiments are aided by cross bombardment with SIBs and most of the experimental equipment is to be commissioned with SIBs. The ten projects accepted are summarized in the following table.
In the fall, the HRIBF will undergo an accelerator readiness review. Successful completion of this review will permit us to proceed with high-intensity RIB commissioning and routine user operation.
Until October 11 the HRIBF will provide beams for RIB development and facility and RMS commissioning. From October 14 - November 8 the tandem will be open for installation of new charging chains and other routine maintenance. During November and December it is hoped that the first of the approved experiments can be run. Spokespersons of accepted experiments will soon be contacted for information to fix the running schedule.
A dedication of the HRIBF is planned for Thursday afternoon, December 12. A mini-symposium on RIB physics will be held on Friday, December 13. Please reserve these days to attend. A program for this festive event will be circulated in the next HRIBF News.
Several reactions have been used to commission the RMS:
32-S + 58-Ni at 120 MeV 58-Ni + 98-Mo at 250 MeV 58-Ni + 92-Mo at 250 MeV 58-Ni + 60-Ni at 220 MeV.To date, the best mass resolution achieved has been M/dM = 450. The energy resolution is approximately +/- 12.5%, and the mass-over-charge acceptance is in excess of +/- 4.5%. These numbers translate to an overall efficiency of some 1 to 2 percent. One common feature of all these reactions has been the lack of beam observed on the focal plane detector PSAC. This feature has allowed us to routinely run with 15 pnA of beam on target, and higher currents should be possible. A test of a double-sided silicon-strip detector placed behind the PSAC was made using the molybdenum reactions to produce nuclei which decay via particle emission. Protons from the ground and isomeric states in 147-Tm were observed using the 92-Mo target.
In the coming months we will increase the number of Ge detectors from four to twelve, add four neutron detectors, commission the ionization chamber, test the finger system for inverted reactions, optimize the strip detector setup, and test the LSU moving tape collector (MTC).
The Daresbury Recoil Separator (DRS), which will be utilized to directly measure nuclear reactions occurring in stellar explosions, is currently being installed at HRIBF. The system is under vacuum, and we have put +/- 60 kV on the electrostatic plates in the velocity filters. We will soon perform high-voltage conditioning of the electrostatic plates. We are currently focusing on the development of the computer control system for the high-voltage and magnet power supplies, as well as the vacuum control system. The target chamber and focal plane assembly will be installed soon, pending their arrival from the University of North Carolina and Yale University, respectively.
The world's first two segmented clover germanium detectors have been delivered to the HRIBF and have met their design specifications. These segmented clover detectors, constructed by Eurysis Mesures in France, are designed to provide the most cost-effective solution for the detection of high-energy gamma rays emitted from the weakly populated nuclei which lie far from the valley of stability. Each detector consists of four individual Ge crystals, sharing a common cryostat. The four crystals may be operated either as four independent detectors, or as one large detector by summing their signals. Each Ge crystal is in turn electronically subdivided ("segmented") into halves, providing a lateral spatial resolution of less than 2 cm. Therefore, in addition to providing excellent efficiency for detecting high-energy gamma rays (150% relative to a 3"x3" NaI crystal), these detectors maintain a superior energy resolution even for gamma rays emitted from fast-moving recoiling nuclei. They also allow a measurement of the linear polarization of the detected gamma rays.
Jerry D. Garrett, Scientific Director |Email: garrett@orph01.phy.ornl.gov Mail Stop 6368 |Tel: (423) 576-5489 Carl J. Gross, Scientific Liaison |Email: cgross@orph01.phy.ornl.gov Mail Stop 6371 |Tel: (423) 576-7698 Holifield Radioactive Ion Beam Facility |Tel: (423) 574-4113 Oak Ridge National Laboratory |Fax: (423) 574-1268 Oak Ridge, TN 37831