|Edition 6, No. 4||November 19, 1998||Price: FREE|
- 1. First 17F Beams for Research
- 2. Recent HRIBF Research - New Proton Emitters
- 3. Production of Neutron-rich Radioactive Species with ORELA
- 4. Commissioning Schedule of the Clover Ge Array
- 5. Reminder of Call for Proposals and Letters of Intent for Neutron-rich Beams
- 6. International Symposium on Proton-emitting Nuclei
- RA1. RIB Development
- RA2. Accelerator Systems Status
- RA3. Experimental Equipment - Enge Spectrometer
- RA4. HRIBF Users
- RA5. Experiments, Spokespersons, and Dates Run During the Past Quarter
Editor: Carl J. Gross
Feature contributors: J. C. Batchelder, C. Baktash, J. R. Beene,
J. D. Garrett, A. Galindo-Uribarri, K. Rykaczewski
Regular contributors: J. Gomez del Campo, F. Ervin, J. D. Garrett, C. J. Gross, F. Liang, M. J. Meigs, P. E. Mueller, D. W. Stracener, B. A. Tatum
One of the highest priorities of the HRIBF is the production of an experimentally useful 17F beam. A major milestone in this difficult undertaking was achieved at the end of October. A beam of ~8 uA of 44 MeV deuterons incident on a target of thin fibers of HfO2, together with an aluminum vapor feed, produced a beam of 3x108 Al17F+ ions per second (ips) after first-stage mass analysis and ~2x107 17F- ips after breakup/charge exchange in Cs vapor and acceleration off the RIB injector platform. A short time later, the first accelerated 17F beam of about 3x105 ips was delivered from the tandem to an experiment mounted at the astrophysics endstation.
The development of a target system that would release 17F in a time commensurate with its half-life and stand up under high-intensity driver-beam bombardment was an absolute necessity for our beam development program. This has now been achieved. Considerable scope remains for optimizing operating parameters of the target and improving the fraction of the 17F beam transmitted through the tandem. We expect and intend to make rapid progress toward reliable delivery of 17F beams at usable intensity.
It has been about two years since our first major milestone in fluorine beam development: the off-line demonstration of release of radioactive fluorine from Al2O3 fibers. The testing of high-power fluorine production targets was delayed considerably by intensive development effort on ORIC. While it was frustrating to all of us to spend so much time and effort on work that produced little externally visible result, I strongly believe these efforts were essential, and will pay future dividends. The HRIBF engineering and technical staffs have done an excellent job under extreme pressure. ORIC is a vastly improved machine - an improvement that was absolutely necessary to effective operation of HRIBF.
There have been other results in our beam development program in recent weeks. One of considerable importance for the future of the facility was the first successful on-line run for the development of neutron-rich beams. An important development in our effort to produce a beam of 56Ni was the delivery of the first multi-sample "batch-mode" sputter source, which will be used in 56Ni production. We will continue a two-front effort in our fluorine development program. The second front is a negative ion source for direct production of F-, thus eliminating the molecular-breakup/charge-exchange step. These and other developments are discussed in the technical articles that follow.
Within the last year, physicists from 11 American and European laboratories identified five new proton emitters at the HRIBF. The observed activities were assigned to 140Ho, 141mHo, 145Tm, 150mLu, and 151mLu proton-emitting ground and isomeric states.
These discoveries help to define the limits of the proton drip line and test, under extreme conditions, models of nuclear structure, including the recent predictions of the proton emission rates made by a group of theoreticians from the United States, Hungary, Romania, Poland, and Sweden. Such verifications are crucial not only for the description of the energy and shape of the nucleus itself, but also for the understanding of the processes characterized by much larger energy, time, and space coordinates like the evolution of stars and origin of elements.
The first new proton line at 1.73 MeV observed at HRIBF was assigned to the decay of 145Tm (see J. C. Batchelder et al, Phys. Rev. C 57, 1998, R1042). The proton radioactivity of 145Tm, T1/2 = 3.5 microseconds, represents the fastest proton decay yet observed. The success of this experiment is related to the optimization of the experimental setup for the detection of very short-lived particle activity followed by an on-line calibration procedure using the known 113Cs proton decay. The study of 145Tm is important for the determination of a spectroscopic factor for the Z=68 nuclei, since the error bars on the earlier value deduced from 147Tm proton decay were too large for a meaningful comparison with theory. It indicates also that the h11/2 ground-state of 145Tm has spherical properties, in contrast to some predictions suggesting a deformed prolate shape.
The experimental spectroscopic factors for the proton emission from the h11/2 orbital compared to the predictions of Aberg, Semmes, and Nazarewicz, Phys. Rev. C 56 (1997) 1762, indicate that these relatively high-spin proton states have a rather pure configuration. This is in contrast with the spectroscopic factors measured for the d3/2 proton emitters, indicating the configuration mixing of proton emitting states with the proton s1/2 orbital. The proton emission from the d3/2 isomeric state (T1/2 = 16 microsecond) in 151Lu recently observed at HRIBF (C. Bingham et al., to be published) contributed to the above conclusion.
The spectroscopic factors for proton emitters above Z=68 can be understood within spherical model of nuclear states. However, such a description is not applicable for the proton drip line nuclei 54 < Z < 68. This was recently demonstrated with discovery of two new, deformed proton emitters 131Eu and 141Ho with FMA setup in Argonne, see C. Davids et al., Phys. Rev. Lett. 80 (1998) 1849. Recent experiments at HRIBF contributed to the subject of proton emission from deformed states via the identification of two new proton emitters in the deformed region, 140Ho and 141mHo (K. Rykaczewski et al., to be published). The observation of 140Ho was made for the first time using the very rare p5n reaction channel having a cross section of about 10 nanobarns.
We are studying the feasibility of using the 60-kW, 150-MeV Oak Ridge Electron Linear Accelerator (ORELA) to test the performance of actinide targets designed for use in high-power beams such as those at an advanced RIB facility. We are also exploring the use of ORELA for production of radioactive species from the fission of actinide targets. Simulations of the power deposition and estimates of the fission rate of actinide targets are very encouraging. Calculations indicate that we can produce fission fragments using the ORELA electron beam at a rate of one or two orders of magnitude larger than that at the current HRIBF. Initial tests are focussed on production rates with very low-power beams from ORELA. Targets of Ta, U, and UC have been exposed to bremmstrahlung radiation generated when the accelerated electron beam struck an air-cooled Ta radiator foil. After bombardment, the targets were removed to a remote area for off-line yield measurements with Ge detectors. Preliminary results for the yields of radionuclides produced are very encouraging. This proof-of-principle initiative might lead to the development of a high-power target test facility and a production facility for neutron-rich RIBs.
Below is a brief report of the status of the Clover Ge Array which we have named CLARION, an acronym for "CLover Array for Radioactive ION beams."
All daughter boards and one of the mother boards of this module have been fabricated. The second mother board will be fabricated in December. The assembly and testing of all 11 modules are expected to be completed by mid to late February 1999.
Users are reminded that proposals are now being accepted for the next series of experiments at the HRIBF. Proposals are solicited for experiments with beams of radioactive 66Ga and 67Ga with energies between 3.4 and 360 MeV and 69As and 70As with energies between 3.4 and 380 MeV. Users desiring higher or lower beam energies or beams of heavier gallium or arsenic isotopes or beams of radionuclides of selenium should contact Jerry Garrett.
About 106 ions per second (i/s) of 66Ga, 67Ga, 69As, and 70As are now available. Somewhat more intense beams of these isotopes might be available during the scheduling period. The 70As beam is highly contaminated with its isobar 70Ge. Fluorine and 56Ni beams are still being developed. Users should anticipate up to about 107 i/s of 56Ni and 106 i/s of 17F or 18F. The 56Ni beam will be contaminated with 56Co.
Requests for measurements using stable beams will be considered for:
Arguments substantiating that stable beam work falls into one of these categories should be contained in the experimental write-up. However, projects involving radioactive beams will be given preference over those based on purely stable beams. A list of stable beams which have been accelerated is given on the HRIBF web site. If other stable beams are desired, please contact Jerry Garrett or Carl Gross.
The development of neutron-rich radioactive beams produced as fission fragments from the reaction of 40-MeV protons and 238U has started. It may be possible to provide RIBs for isotopes of volatile elements with halflives greater than a few seconds. Energies near the Coulomb barrier may be available for fission fragments up to A ~ 90. Additional information on the possible availability of these neutron-rich beams is given in this and our October 27 newsletter. Users are invited to submit letters of intent for experiments based on these neutron-rich RIBs. These letters of intent will be used to help determine which of these beams to develop. The minimum intensity requirements for obtaining useful experimental results should be explicitly stated.
THE DEADLINE FOR THE RECEIPT OF THESE PROPOSALS AND THE LETTERS OF INTENT IS FRIDAY, DECEMBER 4, 1998. Twelve copies of each proposal or letter of intent should be sent to:
|U.S. Mail Address||Courier Service Address|
Jerry Garrett |
Oak Ridge National Laboratory
Bldg. 6000, Mail Stop 6368
P.O. Box 2008
Oak Ridge, TN 37831-6368
Jerry Garrett |
Oak Ridge National Laboratory
Bldg. 6000, Mail Stop 6368
Bethel Valley Road
Oak Ridge, TN 37831-6368
More information on this Call for Proposals as well as the cover pages of the HRIBF proposals and guidelines for their preparation are available on the HRIBF web site (http://www.phy.ornl.gov/hribf/hribf.html).
An International Symposium on Proton-emitting Nuclei is being planned for October 7-9, 1999, at the Joint Institute for Heavy Ion Research, Oak Ridge National Laboratory.
The past few years have seen an explosion of work both experimentally and theoretically on the topic of proton emitters. This symposium aims to review the current situation and to discuss directions for the future.
Topics to be covered include:
The activities reported here focus on the RIB development activities at the UNISOR separator. This facility is used to test various target materials for the release of radioactive ions, to perform on-line testing of various ion sources, and to supply the RIB Injector with target/ion sources for the production of radioactive ion beams. In the past six months we have had 8 on-line runs to test various target materials for release of 17F and neutron-rich isotopes.
The on-line target/ion source tests with Al2O3 fibers from different manufacturers showed that the efficiency for 17F release depended on the fiber diameter and the purity of the sample. The best results achieved at UNISOR with an Al2O3 target was 9x106 17F ions/s/uA which corresponds to a source efficiency of 0.2% at a target temperature of 1500 C. High-intensity tests with Al2O3 fibers showed that the target material sintered quite rapidly at >5 uA and the maximum operating current was about 3 uA on target.
This limitation of Al2O3 required us to look for more refractory oxide materials which were available in a fiber form. Hafnium oxide (hafnia) and zirconium oxide (zirconia) stabilized with yttrium oxide seemed to be good candidates, and hafnia has been tested, while we hope to test zirconia/yttria in the near future.
The hafnia target, which performed well in the ion source at temperatures up to 2300 C, required a modification of our target heater. The "old" design consisted of a 0.010-inch Ta filament with Ta supports and could raise the target temperature up to 1800 C without mechanical failure. The "new" heater has a 0.005-inch Re filament with W supports and has heated the target to 2300 C without a failure.
The initial test of the HfO2 material was on a sample which showed significant levels of contamination, most notably Al. This was fortunate because the majority of the radioactive fluorine observed (>90%) was found in the molecule AlF+. No HfF+ was observed. Since the batch of hafnia that we subsequently ordered contains a much lower level of Al contamination, it is necessary to supply Al vapor to the system to transport the fluorine to the ionization region. Other elements such as Be, B, Li, or Ba may also be efficient in transporting the fluorine, and we plan to test some of these soon.
The best yield obtained thus far with a HfO2 target is 8x106 17F ions/s/uA, which corresponds to an ion source efficiency of 0.2%. This efficiency was achieved at a target temperature of 2100 C, and it has been shown that the hafnium oxide target can withstand higher beam currents than possible with the Al2O3 targets (see article for the RIB Injector).
Presently, we are in the process of installing and testing a negative ion source to produce negative ions of fluorine directly. A similar source was tested at UNISOR early this year (see HRIBF News 3/27/98). Several improvements have been made which have increased the efficiency for stable F up to 5%. This represents a factor of 5 increase in the efficiency over the source previously tested at UNISOR which yielded 6x105 17F- ions/s/uA of 2H. This yield of 17F- is comparable to the 17F- yield achieved from the positive-ion EBP source and a 10% charge exchange efficiency. An on-line test of this source is scheduled for next week.
Machine start-up and reliability have greatly improved, largely due to funding of the ORIC Accelerator Improvement Project (AIP). Startup has typically been reduced from one shift to two or three hours. Diagnosis of problems requires considerably less time due to modernization of the control system and improved diagnostics. Most recently, the ORIC radiofrequency (RF) and vacuum system controls were converted to programmable logic controller (PLC) and Vista system operation.
Many of the older power supplies have been replaced including the 20-kV, 30-A RF power amplifier anode power supply and the 6000A coaxial magnetic extraction channel power supply; four new trimming coil power supplies are being delivered this month. All new power supplies are designed to be directly compatible with the HRIBF control system by the inclusion of ethernet and ControlNet compatible PLCs.
A new set of circular trimming coils is being fabricated, and several other power supplies will be replaced in the months ahead. Control system upgrades will continue.
The beam development program will continue to emphasize expanding energy ranges, attaining higher intensities, and greater extraction efficiency. With recent advancements and near-term planned improvements, ORIC appears to be positioned to reliably operate for many years to come.
The tandem can only accelerate negative ions. Positive AlF ions are converted to negative ions in a charge-exchange process which also results in breakup of the molecular ion. The resulting 17F- ions have a large (+/- 250 eV) energy spread as a result of the kinematics of the molecular breakup. This energy spread precludes effective use of the second-stage mass separator to eliminate contamination from the 17O isobar. The high beam purity achieved results from effective suppression of AlO+ ions produced by the ion source as a result of the properties of HfO2 and the temperature distribution in the source. The large emittance of the 17F- beam after breakup probably results in some transmission loss through the tandem.
An order of magnitude higher yield of F- beams was achieved by raising the operating temperature of the charge exchange cell from 160 C to 200 C. The depletion rate of Cs at this temperature is large (5 gm/day with a 25 gm total load), and design modifications are under way to increase the recirculation performance of the cell. Sodium has also been tested as a possible alternative to Cs and shown to yield similar results. The much higher operating temperature (400 C) resulted in a failure of the heater. In addition, for our application, Na is much more difficult to handle than Cs. Carbon foil stripping was also attempted but resulted in extremely poor efficiency.
Hardware improvements continue to be made to the RIB injector and Remote Handling System (RHS). A new water-cooled DANFYSIK power supply for the first-stage mass analyzer has replaced the very problematic air-cooled INVERPOWER power supply. The motor-generator system has been completely overhauled. The generator bearings and the pillow-block spherical bearings at the motor end of the drive shafts were all replaced after vibration analysis indicated bearing failure was in progress. The flange-mounted spherical bearings inside the east shield wall of C111N were replaced due to excessive movement of the shafts. The motors, generators, and shafts were aligned with a laser system. Two 3.5-inch mild steel shielding casks have been received to house each of the two Contamination Control Box(es) (CCB) of the RHS. Under manual control, the RHS opened a cask, placed a CCB in the cask, and closed the cask. An aluminum enclosure has been successfully tested with a target ion source at maximum operating temperature (with no internal heat shields) and nominal cathode emission.
In the immediate future, NaI(Tl) detectors will be installed near various Faraday cups along the RIB injector and after the tandem electrostatic accelerator. For short-lived isotopes producing only one gamma ray, e.g. 17F, these detectors are an excellent alternative to tape systems for measuring the absolute intensity of a radioactive beam. Nonetheless, the beam line at the image slits of the second-stage mass separator will soon be reconfigured to accommodate a second tape system that is presently being fabricated at LSU. This tape system will allow the measurement of radioactive beams with longer halflives and/or multiple gamma rays.
The Enge split-pole magnetic spectrograph has been reconfigured for measuring evaporation residue (ER) cross sections. A foil window is installed at the entrance to the spectrograph to separate the vacuum between the target and magnet chambers. The magnet chamber can be filled with gas up to pressures of 10 Torr while leaving the target chamber under high vacuum.
A focal plane detector, position-sensitive avalanche counter (PSAC), has been constructed for ER identification. The active area of the PSAC is 36x7.5 cm2. It is similar to the PSAC used at the focal plane of the Recoil Mass Spectrometer. The exit of the PSAC is backed by a large-area plastic scintillator to measure the energies of the particles. The scintillation photons are directed to the sides by an acrylic light guide. Photomultiplier tubes attached to each end of the light guide are used to count the scintillation photons.
The ERs are identified by energy, position, and time-of-flight. A micro-channel plate installed in the target chamber provides timing start signals and can be moved to angles corresponding to the spectrograph. The device has been tested for identifying ERs from Ge+Al and As+Ti reactions at energies near and below the Coulomb barrier. The excitation function of As+Ti using the stable As beams has been measured as a part of the RIB012 experiment.
Improvements to the scattering chamber of the Enge Spectrograph have been made in order to accommodate various types of silicon detector systems that could be used in coincidence studies (transfer reactions and projectile breakup) with the focal plane detector. In particular, we installed one 4 cm by 4 cm, 16 strip by 16 strip silicon detector. The strip detector is mounted on a rotatable arm that can cover any scattering angle. In addition, a new monitor detector at 10 degrees with respect to the beam direction has been added. Future plans include improvements to the vacuum system such as a new cryopump and more electronics to support a microstrip array.
The election for the HRIBF Users Executive Committee was held during the month of October and the results announced at the Users Group Meeting held at the DNP Fall Meeting in Santa Fe. The election was held by electronic mail for the first time. Although fast, the voter response was significantly less than last year. We will review this unexpected result when planning for future elections. The committee for next year will consist of:
Brad Sherrill (Chairperson)
We wish to thank John D'Auria and Mark Riley for their past service as members of the Users Executive Committee.
The present members of the Program Advisory Committee are given below. We wish to thank J. R. Beene, R. F. Casten, R. M. Diamond, and W. C. Haxton for their past service as PAC members. The PAC members retiring in 1998 have agreed to extend their service to include the upcoming PAC meeting planned for January 1999.
|S. M. Austin||Michigan State University||1994-1998|
|N. Benczer-Koller||Rutgers University||1998-1999|
|W. Gelletly||University of Surrey||1994-1998|
|J. C. Hardy||Texas A&M||1998-1999|
|J. J. Kolata||Notre Dame University||1998-1999|
|I. Y. Lee||Lawrence Berkeley National Laboratory||1998-1999|
|W. Nazarewicz||University of Tennessee||1994-1998|
|P. D. Parker||Yale University||1994-1998|
|J. R. Beene||Oak Ridge National Laboratory||ex officio|
The annual HRIBF Users Group Meeting was held in a combined session with the users of ATLAS, 88-Inch, and GAMMASPHERE at the DNP Meeting in Santa Fe. The meeting, chaired by Michael Smith, was very well attended. HRIBF presentations included a RIB status report by Jim Beene highlighted by the announcement of a successful 17 extraction of a beam of more than 107 negatively charged ions per second off the RIB injector platform which, accelerated by the tandem, was transported to the target station of the DRS for an elastic scattering experiment. Jerry Garrett briefly presented our plans for placing the proposed NISOL facility at the Spallation Neutron Source ( http://www.phy.ornl.gov/nisol/) and Cyrus Baktash spoke about our Letter of Intent requesting that GAMMASPHERE be moved to the HRIBF in the year 2002.
|RIB-012 - As + Ti Sub-barrier Fusion||Liang/ORNL||9/15/98|
|RIB-018 - Proposal to Measure the p(17F,17F)p Reaction at the Holifield Radioactive Ion Beam Facility||Bardayan/Yale||
|RIB-026 - Study of the Decay of Nuclides with N Close to or Equal to Z that are Crucial Bottlenecks in the Astrophysical Rp-process. I. Half-life Measurement of the N = Z Nuclide Zr-80||Walters/Maryland||9/28-10/1/98|
|RIB-031 - States in 18Ne Populated by Resonance Scattering of 18F on 1H Using Thick Targets||Galindo-Uribarri/ORNL||
|RIB-032 - Studies of the Isomers in Tz = 1 Nuclei Near Doubly-magic 100Sn with the Recoil Mass Spectrometer||Grzywacz/Tennessee||9/16-22/98|
|RIB-035 - Uranium Carbide Target||Stracener/ORNL||
|RIB-036 - Identification of the Beta Decay of 66Se||Piechaczek/Louisiana State||10/14-15/98|
|RIB-039 - High Voltage Injector Development||Mueller/ORNL||
|RIB-040 - Beam Diagnostics Development||Shapira/ORNL||10/27-28/98|
|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|