| HRIBF NEWS |
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| HRIBF NEWS |
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| Edition 10, No. 1 | Winter Quarter 2002 | Price: FREE |
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Feature ArticlesRegular Articles
- 1. HRIBF Update and Near-Term Schedule
- 2. Recent HRIBF Research - Study of the 18F(p,a)15O Reaction at Energies Relevant for Nova Nucleosynthesis
- 3. Recent HRIBF Research - Coulomb Excitation of the N=80,82 Te Isotopes at the HRIBF
- 4. The Results from PAC-7
- 5. First Annual RIA Summer School to be held August 12-18 in Oak Ridge
- 6. Ron Auble's Retirement and HRIBF Reorganization
Editors: C.-H. Yu, W. Nazarewicz, and C. J. Gross
Feature contributors: D. W. Bardayan, C. J. Barton, J. R. Beene,
C. J. Gross
Regular contributors: A. Aprahamian, M. R. Lay, M. J. Meigs, P. E. Mueller,
D. W. Stracener, B. A. Tatum, E. Zganjar
The total number of RIB experiment hours scheduled in these campaigns is about 1300. It is likely that our operating budget will not support the completion of this schedule (which would correspond to 1500 hours of RIB operation for experiments). Nevertheless, we have decided to schedule aggressively and push our resources as far as they will go.
Over the last couple of years, we have reduced the number of scheduled tandem shutdowns for major tank openings from three to two per year. Last year, this resulted in shutdowns in fall and mid-summer. It is obviously a bad practice for a facility that strives to attract users from the university community to schedule extensive maintenance during the period when it is most convenient for university scientists to do experiments. This year we will attempt to extend the period between tandem maintenance periods so that the next major shutdown will occur no earlier than mid-August. The goal is to shift these regular maintenance periods from ~January and July to roughly March and September by next year.
Despite intensive efforts to understand nova explosions, significant discrepancies exist between model predictions and observations of such global properties such as the ejected envelope mass [St99]. A further constraint on nova models could come from the observations of gamma rays from nova ejecta by orbiting telescopes. During the first several hours after the explosion, the most intense source of gamma rays is the annihilation radiation produced by the positron decay of 18F. The amount of 18F produced (and thus the number of gamma rays emitted) is severely constrained by the 18F destruction rate via the 18F(p,a)15O reaction in the burning shells. To understand gamma-ray emission from novae, we therefore need to know the 18F(p,a)15O stellar reaction rate.
The 18F(p,a)15O reaction rate is believed to be dominated by contributions from resonances at Ec.m. = 330 and 665 keV. The contribution from the 665-keV resonance was studied in a previous HRIBF measurement [Ba01]. We report here on the first significant measurement of the contribution from the important 330-keV resonance. A radioactive 18F beam was used to bombard a thin (57 mg/cm2) polypropylene (CH2) foil. Recoil a-particles and 15O ions were detected in coincidence in the Silicon Detector Array (SIDAR) [Ba00]. The beam was contaminated by the stable isobar 18O (18F/18O ~ 0.2). By reconstructing the total energy of the events observed in coincidence, the 18F(p,a)15O events were readily distinguished from 18O(p,a)15N events and elastic scattering based on the different Q-values of the reactions.
The 18F(p,a)15O cross section was measured on- and off-resonance, and our data are plotted in Fig. 2-1. Our analysis is still preliminary, but initial results indicate that the strength of the 330-keV resonance is about 50% of previous estimates. Further work is planned at the HRIBF to explore the energy region between the two resonances.
Fig. 2-1: The measured 18F(p,a)15O cross section is shown along with a calculation of the contributions from the 330- and 665-kev resonances. The calculated curve has been averaged over the energy loss in the target for direct comparison with the data. The flatness of the curve at 330 keV is the result of the energy loss in the target being much greater than the width of the 330-keV resonance.
[Ba00] D.
W. Bardayan et al., Phys. Rev. C 62, 055804 (2000).
[Ba01] D.
W. Bardayan et al., Phys. Rev. C 63, 065802 (2001).
[St99] S.
Starrfield, Phys. Rep. 311, 371 (1999).
In April 2001, the B(E2;01+ -> 21+) values for the radioactive species 132,134Te were successfully measured via Coulomb excitation in inverse kinematics. The experiment used a high efficiency through-well NaI(Tl) detector (5.7% at 1.3 MeV) with a natC target placed at the center and thin foil Microchannel plate (MCP) detectors for beam particle detection. The availability of the high-intensity beams of neutron-rich radioactive Te nuclei at the HRIBF has allowed the measurement of the B(E2;01+ -> 21+) values for the first time.
The g-ray spectrum resulting from Coulomb excitation of even-even nuclei in inverse kinematics on a low-Z target at energies safely below the Coulomb barrier contains only a single g-ray from the dominating E2 excitation of the 21+ state. A simple analysis extracts the structurally important B(E2;01+ -> 21+) value. Given the simple g-ray spectrum and RIB intensities of <107 ions/s, it can be beneficial to sacrifice detector energy resolution for intrinsic efficiency, motivating the use of the GRAFIK NaI(Tl) detector[1].
Coulomb excitation of 130,132,134Te on 1.0 mg/cm2 natC at beam energies of 350 MeV for 1.6 h, 7.3 h and 15 h, respectively, was performed with intensities on target of ~106 ions/s for 132Te and ~105 ions/s for 134Te. The position-sensitive tilted foil microchannel plate detector [2], illustrated in Fig. 3-1, was used for kinematic selection of scattered beam nuclei where inverse kinematics permits nearly 4p solid angle coverage. An elliptical hole excised from the center of the foil minimized particle background by allowing 99% of Rutherford scattered beam nuclei to pass through undetected. Much of the total yield of Coulomb-excited Te nuclei, scattered mostly near 5 degrees in the laboratory frame, was detected. Two thin-foil microchannel plate timing detectors [3] were used to count the total number of incident beam particles and provided timing signals for particle-g coincident analysis. The particle gated NaI(Tl) energy spectrum is shown in Fig. 3-2. The isobarically contaminated beam purity was determined with a beam assay, which studied the b decay of stopped beam nuclei with an efficiency-calibrated Ge detector. The B(E2;01+ -> 21+) values for 132,134Te, normalized to 130Te, are 0.19(3) and 0.13(4) e2b2.
The expected decrease in B(E2) values toward the N=82 magic number was confirmed. Comparisons with a hydrodynamic approach, treating the 21+ level as a quadrupole vibration, and the shell model, where the spectrum and transition rates of 134Te should be similar to those of two protons in the 50-82 shell interacting with a short range interaction, were made. The resulting B(E2;01+ -> 21+) value of 134Te is weaker than the collective model but is somewhat larger than predicted by the shell model.
[1] C. J. Barton, R. L. Gill, R. F. Casten, D. S. Brenner, N.
V. Zamfir and A. Zilges, Nucl. Instrum. Methods Phys. Res.
A391, 289 (1997).
[2] D. Shapira, et al., Nucl. Instrum. Methods Phys. Res.
A454, 409 (2000).
[3] D. Shapira, et al., Nucl. Instrum. Methods Phys. Res.
(submitted).
The members of PAC-7 met in Oak Ridge on January 18-19, 2002. Twenty-four proposals and letters of intent requesting 176 shifts of RIBs and 266 shifts of SIBs were considered. Of these, 13 received some allocation of beam time and 4 were deferred for more information and discretionary time. The titles and spokespersons may be viewed on our website. A total of 196 shifts (57 RIB and 139 SIB) was accepted requesting beams of radioactive Fluorine and Tellurium. An additional 21 shifts of 7Be was tentatively approved.
The next Call for Proposals will be issued in our May newsletter. The next meeting, PAC-8, is tentatively scheduled for August 19-20, immediately after the First Annual RIA Summer School. The deadline for proposal submission will be around July 3.
HRIBF is proud to host the First Annual RIA Summer School August 12-18. The school is organized by the 88-inch Cyclotron Facility, ATLAS, HRIBF, and NSCL and will rotate among the four laboratories. Approximately 35 students are expected to attend lectures in the morning and take part in hands-on demonstrations of experimental equipment and techniques in the afternoon. Time will also be provided for the students to present their research projects. For more information go to this year's school website at www.orau.gov/ria.
On December 31, 2001, Ron Auble retired after 32 years of exemplary service to the Physics Division and ORNL. The hole Ron's departure leaves in our organization is considerable. Though we plan to make use of his talent and experience through occasional consulting, his solid good sense and physical intuition will be sorely missed in the day-to-day operation of the facility.
Ron provided long and distinguished service to the Holifield facility, beginning as User Liaison for the HHIRF (our previous incarnation as the Holifield Heavy Ion Research Facility). During the construction of HRIBF in the early 1990s, Ron managed Accelerator Improvement Project funds, as well as capital funds for the construction of major experimental equipment. In 1995, he took charge of day-to-day operations of the facility as head of the Facility Operations Group. He continued in this capacity until his retirement, with the additional responsibility of Deputy Director for Operations added in 2000. From August 2000 until August 2001, Ron served as Acting Director of the HRIBF. Ron has served the HRIBF in leadership roles since the conversion to a radioactive ion beam facility began; he deserves a good deal of the credit for the success we have had in recent years.
The organization of the HRIBF has changed somewhat with Ron's retirement. The facility staff is now organized into two "tasks" - Facility Operations and Facility Development. Ray Juras will be responsible for day-to-day operation of the HRIBF as head of Operations, while Alan Tatum, as head of Development, will take on responsibility for planning and executing facility improvements and upgrades. Alan will also have responsibility for R&D activities.
In a couple of the previous HRIBF newsletters we reported the results of tests of the release of 25Al and 26Al from silicon carbide targets and we recently completed two more on-line tests. The SiC target was made of loosely packed powder with a particulate size of 1 micron. The target density was 0.74 g/cm3 and it was thick enough (2 cm) to stop the production beam in the target. The radioactive aluminum atoms were produced in the SiC target using two different production beams from the tandem, 40-MeV protons and 40-MeV deuterons with beam intensities up to 25 nA. An electron-beam-plasma (EBP) ion source, coupled to the SiC target via a heated tantalum tube, was used to ionize the 25,26Al atoms.
The yields from the proton-induced reactions agreed with earlier measurements and were, at best, still rather low at less than 104 positive ions per second per microAmp of production beam. Folding in the charge exchange efficiency (10%) and the transmission losses, this yield would correspond to only a few hundred particles per second on target at a beam energy of 5 MeV per nucleon. Once again we observed that the yields of Al and AlF increased significantly (almost a factor of ten) when fluorine atoms were added in the form of SF6 gas. Also it was observed that the yields from the deuteron-induced reactions were higher than the yields from the proton-induced reactions, as had been predicted. The yield of 25Al increased approximately 50% while the yield of 26Al increased by a factor of three.
The next target geometry that we plan to test consists of a thin layer (3-5 microns) of SiC deposited onto the fibers of a low-density carbon matrix. This gives us the advantage of short diffusion paths in the SiC coupled with a high porosity target through which the aluminum atoms may effuse. The carbon matrix is the same as the one used successfully in the uranium carbide targets.
ORIC was in scheduled shut-down during most of the reporting period. However, numerous development runs were conducted. First, tests were conducted in a attempt to increase cathode lifetimes for 4He production. These tests have included replacement of the tantalum cathodes with tungsten, cleaning source parts ultrasonically, and installing tighter-fitting source heads which contain the gas better and alter the pressure distribution inside the source. Although these tests have yielded useful information, the cathode lifetimes have not appreciably increased and remain in the 40 to 60 hour range. Secondly, development runs were conducted to increase extraction efficiencies for 4He. Last year, it was observed that circulating beam had been striking the exit portion of the deflector septum. Thus, a new carbon septum was machined with a milled groove in that region. The new septum, combined with tuning the RF system at slightly lower dee voltage has proven beneficial, with 4He extraction efficiencies increased from the 40-45% range up to the 50-55% range. This septum has also been tested with deuteron beam with similar efficiencies. Additional tests will be conducted with the septum groove milled even deeper, and the effect on proton extraction and beam energy will also be studied.
The main magnet coils were also passivated in early January, a process which involves circulating a solution of potassium dichromate through the coils. The solution coats the interior to reduce susceptibility to pitting of the aluminum which is inherent from long-term exposure to water.
The Tandem Accelerator has operated more than 660 hours since the last report with all beams provided from the stable injector. The machine ran at terminal potentials of 3.55 to 21.225 MV and 124Sn, 124Te, 58Ni, 28Si, 17O, 13C, 2H, and 1H were provided. The tank was opened three times during this period with the majority of time spent in the first opening, which had been scheduled for some time. Routine maintenance was done in this long opening, which included replacing terminal foils, terminal sublimator pump elements, rotating shaft bearings, Georator bearings and the repair and cleaning of many other parts of the accelerator. The second opening was required by the failure of a terminal quadrupole lens power supply, and the third opening was necessary because one of the Georator bearings, replaced in the first tank opening, was defective.
During this reporting period, the platform control system was successfully ported from VISTA running under VMS on a single VAX ELN at platform potential to EPICS running under VxWorks on one PC at platform potential and on another PC at source potential. Interlocks have been implemented for all vacuum valves in C111S. The control system windows and the control system documentation have been vastly improved. Based on previous experiences, we expect a much better performance from the new control system.
Both platform and source motor-generator shaft pillow block bearings in C109 were overhauled. The source motor-generator shaft flanged bearing and pilot bearing in the wall in C111N were also overhauled. Two of the three cryopumps in beam line 12 were sent to a vendor for overhaul.
Unfortunately, the HfO2 target/kinetic ejection negative ion source that performed so well last fall (about 380 hours of successful running) finally failed; the target heater opened up. A second target/ion source is being prepared in anticipation of the start of another fluorine campaign at the end of February.
CARDS, an acronym for Clover Array for Recoil Decay Spectroscopy, was originally designed and constructed for use at the focal plane of the recoil mass spectrometer (RMS). It has, however, also been used in a number of different applications and locations at HRIBF. It was designed by Ed Zganjar (LSU) and Jim Johnson (HRIBF), and constructed at LSU. CARDS consists of two octagonal rings, each of which can hold up to 8 detector systems. It includes an array of gadgets designed specifically to mount a variety of detector systems such as bare clover detectors, clover detectors with BGO shields, other Ge detectors, X-ray detectors, or conversion-electron detectors. To date, however, a maximum of 4 detector systems per ring have been employed and, in order to maximize the solid angle subtending the activity, a fifth detector has been mounted at the rear of the ring (on the beam axis). The focus of the design of CARDS was to enable one to perform decay spectroscopy with maximum versatility. Included in this versatility is the selection of catcher for the radioactive products. One can set up a static catcher or a dynamic catcher such as a moving tape collector (MTC) as appropriate to the investigation.
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Shown in Fig. RA3-1 is one CARDS ring mounted at the RMS focal plane for an experiment on 140Dy decay using 4 unshielded clover detectors and a static catcher. Note the notch in the ring at the top. This allows for the use of CARDS at the RMS acromat position. To date it has not been used at that location.
Fig. RA3-2 shows both rings mounted at the RMS focal plane for an experiment on the 84Mo decay. Here, 7 bare clover detectors and two x-ray detectors (one mounted at the rear of the last ring) are utilized along with the MTC which is set up as a dynamic catcher at the first ring, thus enabling a collect/move/count experiment at the second ring.
Fig. RA3-3 shows one CARDS ring mounted at the former UNISOR isotope separator for a recent experiment on neutron-rich Ag isotopes. The MTC can be seen to the far left.
Fig. RA3-4 shows one ring (mounted on the CARDS storage and transportation cart being used off-line to hold two bare clover detectors for a test of a helium-jet system.
Plans are currently underway to provide a mount for CARDS to enable one to study products exiting the RIB high voltage platform prior to further separation and acceleration.
As new chair of the Users Executive Committee for the year 2002, I was invited to attend PAC-7 held on Jan. 18-19, 2002, in Oak Ridge. Shortly after that, a telephone conference of the Users Executive Committee was held, and I am pleased to announce that Paul Mantica was elected as the vice chair and he will become the chair of the Users Executive Committee next year. In addition, a number of suggested changes to the users charter have been proposed. One of these changes is to include a member of the Users Executive Committee at all future PAC meetings. I can only applaud this change as a way of having additional user input to the process. Some of the other changes include the exclusion of ORNL staff from being the chair of the Users Executive Committee, the viability of using electronic communication as a valid decision making tool in addition to a meeting of the users, and to limit the renewal of the users list to a process carried out every five years instead of three as indicated in the present charter.
Now that the HRIBF facility is successfully running, the Users Executive Committee, along with ORNL management, is planning a number of workshops on special topics as a way of getting outside users as well as in-laboratory staff together to discuss feasibility and developments for new experimental tools. The concept is to hold workshops of interested parties and present users hosted by ORNL in order to enhance the available capabilities and desirable developments at HRIBF. There are two that are envisioned immediately: One on transfer reactions and a second one on decay experiments. Although no exact dates have yet been chosen, the idea is to hold the transfer reactions workshop in June of 2002 to enable university faculty, students, and postdocs to attend as well.
All nuclear physics and nuclear chemistry community participants are strongly encouraged to register their graduate and senior undergraduate students as well as postdoctoral fellows to attend the first RIA summer school on Exotic Beam Physics to be held in August 12-17, 2002, at ORNL. The program is broad and includes topics from studies of nuclei far from stability to collective excitations in exotic nuclei, to origin of the elements, reactions, structure, and applications of RIA. Part of the summer school is also going to include hands-on experiments and demonstrations as well as an afternoon on writing proposals that will be evaluated by a "practice PAC" to give an idea of the process of selecting experiments!
Please feel free to bring your concerns, comments, and questions to your Users Executive Committee! We are here to serve you!
Ani Aprahamian, Chair
Users Executive Committee
HRIBF welcomes suggestions for future radioactive beam development. Such suggestions may take the form of a Letter of Intent or an e-mail to the Liaison Officer at liaison@mail.phy.ornl.gov In any case, a brief description of the physics to be addressed with the proposed beam should be included. Of course, any ideas on specific target material, production rates, and/or the chemistry involved are also welcome but not necessary. In many cases, we should have some idea of the scope of the problems involved.
Beam suggestions should be within the relevant facility parameters/capabilities listed below.
| Experiment | Title | Spokesperson/Institute | Dates |
| RIB-012 | Enge development | P. Hausladen/ORNL | 1/18 |
| RIB-013 | DRS development | M. Smith/ORNL | 12/19-12/21 1/23-1/25 |
| RIB-014 | RIB development on UNISOR | D. Stracener/ORNL | 1/16-1/17 |
| RIB-046 | Beta decay half-life of 84Mo | W. Walters/Univ. Maryland | 12/10-12/15 |
| RIB-051 | Search for the decay of 16+ isomer in 96Cd | R. Gryzwacz/ORNL | 1/7-1/15 |
| RIB-062 | Search for new 1p and 2p resonances in 18Ne | J. Gomez del Campo/ORNL | 1/22 |
| RIB-067 | A study of shape coexistence in the neutron-rich cadmium isotopes: Decays of 116,118,120,122Ag | J. Batchelder/ORNL | 1/28-1/31 |
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| Witek Nazarewicz | Carl J. Gross | Chang-Hong Yu |
| Deputy Director for Science | Scientific Liaison | Newsletter Editor |
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