Edition 8, No. 2 Spring Quarter 2000 Price: FREE


Feature Articles Regular Articles

Editors: C. J. Gross and W. Nazarewicz

Feature contributors: J. R. Beene, D. Bardayan, J. Blackmon, A. Galindo-Uribarri
Regular contributors: F. P. Ervin, M. J. Meigs, D. W. Stracener, B. A. Tatum, R. F. Welton

1. Update of RIB Delivery Plans

The near-term plans for delivery of radioactive ion beams at the HRIBF remain unchanged since the last newsletter, except for timing. The start of neutron-rich beam development has been delayed by two circumstances. The remarkably successful 17,18F campaign was extended by one month beyond our initial expectations because of the performance and robustness of the kinetic ejection negative ion source (KENIS) and the strong demand for the fluorine beams we were producing. A second, less positive, reason for delay is the failure of the ORIC power amplifier tube in early May, as we were preparing to begin bombardment of the uranium carbide target system on the RIB Injector. Replacing the tube requires several weeks; as a consequence, we moved forward the scheduled tandem maintenance period to maximize overlap with ORIC downtime. We expect to resume initial testing of the uranium carbide target system in mid June, with accelerated neutron-rich beam available in mid July. After about one month of neutron-rich RIB operation, we will proceed with batch-mode source operation (11C test beam and 56Ni production beam) as outlined in the winter newsletter.

2. Recent HRIBF Research - Measurement of the 17F(p,a)14O Cross Section at HRIBF

In stellar explosions such as novae and X-ray bursts, 14O is produced by two successive proton captures on 12C and 13N, respectively. Large quantities of 14O are produced from the substantial carbon fuel that is mixed from the core in novae or produced from the triple-a process in X-ray bursts [1,2]. Hydrogen burning of 14O is inhibited because 15F is proton unstable, and the fate of the 14O is uncertain due to the uncertain rate of the 14O(a,p)17F reaction. It is believed that the rate of the 14O(a,p)17F reaction at stellar-explosion temperatures is dominated by contributions from a single resonance and the direct reaction cross section [3,4]. The crucial resonance arises from the second-lowest-energy 1- state in 18Ne, the mirror to the Ex=6.20 MeV state in 18O. A state only recently observed in 18Ne at Ex=6.15 MeV has been tentatively identified as the 1- state, but the only property of the state that could be accurately determined in the measurement was the excitation energy [3]. Other states observed in 18Ne in the energy range Ex=6.0-7.5 MeV could make significant contributions to the reaction rate [5], but their relative importance has been difficult to determine due to the experimentally unconstrained contributions from the important 1- state and direct capture.

To address the large uncertainties in the 14O(a,p)17F reaction rate, the cross section for the time-inverse reaction, 1H(17F,a)14O, was measured at the HRIBF in January and February of 2000. The cross section was measured at 21 beam energies spanning the energy region important for explosive hydrogen burning, E(17F)= 40.1-68.2 MeV (Ex=6.1-7.7 MeV). The beam was post-stripped following the tandem accelerator, and the HRIBF tandem analyzing magnet was used to select the 9+ charge state which eliminated virtually all 17O from the beam. Post-stripping significantly reduced the 17F beam intensity on target, but a high-purity beam was crucial to eliminate background from the 17O(p,a)14N reaction. Thin (0.1-0.3 mg/cm2) polypropylene targets were used, and the reaction products were detected in coincidence in an array of silicon detectors. Alpha particles were detected in the SIDAR array [6] covering theta(lab)=10.5-25.8 degrees, while recoiling 14O ions were detected in a smaller annular silicon detector (96 mm O.D.) spanning theta(lab)=3.4-6.7 degrees. This arrangement allowed detection of about 70% of the 17F(p,a)14O events in coincidence. The measurement of coincident particles and excellent energy resolution achieved (about 0.6% for the summed energy of the particles) allowed events from the 17F(p,a)14O reaction to be easily distinguished from other events (primarily fusion-evaporation reactions on carbon in the target) on the basis of kinematics alone with effectively no background.

Data were collected at the lowest energy, E(17F)=40.1 MeV, for 88 hours with an average current of 5x105 17F/s, and 10 a-14O coincidences were observed. At the higher energies, e.g., E>60 MeV, more than 50 coincident events were typically collected at each energy in less than 2 hours of running with average beam currents of 1-2x106 17F/s on target. Evidence for a resonance is seen in the lowest energy measurements, which corresponds to a state in 18Ne at Ex=6.2 MeV. At least three other resonances are observed in the higher energy measurements, in the energy range Ex=7.0-7.8 MeV. Data analysis to extract the resonance parameters is still in progress. In addition, measured cross sections at energies E(17F)=48-55 MeV (Ex=6.6-7.0 MeV), where no states in 18Ne are known or expected, accurately establishes the direct reaction cross section and its interference with 1- states in 18Ne.

The 17F(p,a)14O cross section has now been measured for the first time over the range of energies important for determining the 14O(a,p)17F reaction rate in stellar explosions. The contribution from the most important resonances and the direct reaction cross section were determined. This measurement allows the 14O(a,p)17F reaction rate to the ground state in 17F to be determined from detailed balance to about 30% accuracy over most temperatures of interest to astrophysics. However, further measurements are required to better constrain the 14O(a,p)17F* reaction to the 1/2+ first-excited state in 17F.


[1] S. A. Glasner, E. Livne, and J. W. Truran, Ap. J. 475, 754 (1997).
[2] M. Wiescher, H. Schatz, and A. E. Champagne, Philos. Trans. R. Soc. London A356, 2105 (1998).
[3] K. I. Hahn et al., Phys. Rev. C 54, 1999 (1996).
[4] C. Funck and K. Langanke, Nucl. Phys. A480, 188 (1988).
[5] B. Harss et al., Phys. Rev. Lett. 82, 3964 (1999).
[6] D. W. Bardayan et al., Phys. Rev. Lett. 83, 45 (1999).

3. Recent HRIBF Research - Measurement of the 18F(p,p)18F and 18F(p,a)15O Excitation Functions for the Astrophysically-Important 7.075-MeV State in 19Ne

The observation of gamma rays from nova explosions would provide a rather direct test of nova models [1]. The most powerful emission in gamma rays immediately after the explosion comes at energies of 511 keV and below, originating from electron-positron annihilation following the positron decays of proton-rich radioactive nuclei produced in the explosion [2]. The main sources of positrons in nova envelopes are expected to be 13N and 18F. When 13N (T1/2 = 9.97 m) decays, the envelope is most likely too opaque for gamma-ray transmission; therefore, the decay of 18F (T1/2 = 109.8 m) is the most significant for observations within the first several hours after the explosion. The amount of 18F that is mixed into the cooler outer layers where it can only decay is severely constrained by its destruction rate in the burning shells which is dominated by the 18F(p,a)15O reaction. Unfortunately, it has been found that the current uncertainties in the 18F(p,a)15O rate result in a factor of 300 variation in the amount of 18F produced in models [3]. It is impossible to determine whether gamma-ray observations by orbital detectors are feasible without a more precise value of the 18F(p,a)15O stellar reaction rate.

The 18F(p,a)15O rate is thought to be dominated at high temperatures by a resonance near 660 keV (Ex = 7.07 MeV) in 19Ne [5]. This state is thought to have Jp = 3/2+ and would be an s-wave resonance for the 18F+p system since the ground state of 18F has Jp = 1+. The properties of this state were uncertain because of discrepant results from previous measurements [5,6,7]. These discrepancies (as much as a factor of 3 in the width and 21 keV in the resonance energy) result in up to a factor of 3 variation in the 18F(p,a)15O rate.

To resolve these discrepancies, we have simultaneously measured the 1H(18F,p)18F and 1H(18F,a)15O excitation functions using a radioactive 18F beam at the HRIBF. Our method utilized a thin (35-mg/cm2) polypropylene target which, along with the excellent energy resolution of the beam, allowed for a more precise measurement of the resonance properties. Because the beam was contaminated by 18O (18O/18F ~ 10), coincidence measurements were required to distinguish the events of interest from background events induced by 18O projectiles. For the 1H(18F,p)18F measurement, protons were detected in the Silicon Detector Array (SIDAR) [8] in coincidence with recoil 18F ions detected by an isobutane-filled ionization counter which provided energy loss information for particle identification and allowed us to readily distinguish the 18F+p scattering events from the more intense 18O+p events. For the 1H(18F,a)15O measurement, both the recoil 4He and 15O ions were detected in the SIDAR. The total energy of the event was reconstructed, and this allowed the 1H(18F,a)15O events to be distinguished from the 1H(18O,a)15N events based on the different Q-values of these reactions. The excitations functions were measured at 15 beam energies between 10 and 14 MeV over the course of 3 days. A simultaneous fit of the two data sets has been performed, and the preliminary best-fit resonance properties are shown in Table 1 along with the previously measured values.

While our measurement has resolved the discrepancies in the location and width of this state, the 18F(p,a)15O rate is still uncertain at lower temperatures owing to the unknown properties of lower-energy states in 19Ne. Further work with 18F beams is planned at the HRIBF in order to address these uncertainties.

Table 1
Ref. [5] Ref. [6] Ref. [7] HRIBF (a)
Er (keV) 659 +/- 9 638 +/- 15 652 +/- 4 663.7 +/- 0.5
Width (keV) 39 +/- 10 37 +/- 5 13.6 +/- 4.6 36.9 +/- 1.8
Proton Width/Total Width 0.37 +/- 0.04 0.4 - 0.6 0.37{b} 0.40 +/- 0.02

{a} Preliminary analysis. Quoted uncertainties are statistical.
{b} Assumed from Ref. [5].


[1] M. D. Leising and D. D. Clayton, Astrophys. J. 323, 159 (1987).
[2] M. Hernanz et al., Astrophys. J. 526, L97 (1999).
[3] A. Coc et al., Astron. Astrophys. (astro-ph/0003166) (to be published).
[4] M. J. Harris et al., Astrophys. J. 522, 424 (1999).
[5] S. Utku et al., Phys. Rev. C 57, 2731 (1998).
[6] R. Coszach et al., Phys. Lett. B353, 184 (1995).
[7] K. E. Rehm et al., Phys. Rev. C 53, 1950 (1996).
[8] D. W. Bardayan et al., Phys. Rev. Lett. 83, 45 (1999).

4. RIA Workshop Scheduled for July 24-26

In preparation for the upcoming NSAC Long Range Plan in the U.S., there will be a workshop for the Rare Isotope Accelerator (RIA) scientific community, July 24-26, 2000. The location of the workshop will be Dyrham, NC. For up-to-date information, see the RIA web site at "" or send email to We urge you to attend and support this meeting and to inform your colleagues about it so that as broad input as possible is obtained. Deadlines are rapidly approaching, so please do not delay.

This is an extremely important time to sharpen the scientific case for RIA in light of the expanded scope of the project that was the recommendation of the 1999 NSAC ISOL Task Force. As currently envisioned, RIA will consist of a 400-MeV/u primary accelerator capable of providing beams of heavy ions up to uranium. RIBs may be produced either by fragmentation or conventional ISOL techniques. Heavy beams, striking a light-ion target such as liquid lithium, will produce a wide range of fragments far from stability. These fragmentation products will be separated by momentum analysis and may be used as they are produced, or they may be cooled in gas and reaccelerated up to 12 MeV/u. These techniques will require much R&D but promise to provide high-quality beams of the shortest-lived isotopes.

The suggested format of the Workshop will consist of plenary talks and working group sessions. The plenary talks will review the current ideas for RIA physics and capabilities. Much of the workshop will be devoted to active working groups to clearly establish what scientific objectives can be achieved with RIA and how these advance our field.

5. Joe Hamilton named ORNL's First Visiting Distinguished Laboratory Fellow

Professor Joseph H. Hamilton of Vanderbilt University was named the Oak Ridge National Laboratory's first Visiting Distinguished Laboratory Fellow this past February. This program recognizes extraordinary contributions to ORNL of scientists from universities, industry, and other institutions through sustained leadership in scientific and programmatic activities.

For more than 30 years, Joe has been a powerful force for nuclear physics at ORNL. Joe was a leading force in the creation of the University Isotope Separator at Oak Ridge (UNISOR) consortium in 1971 and the Joint Institute for Heavy Ion Research in 1981. He continues to aid nuclear structure research at the HRIBF through his initiative to build and support the HRIBF Recoil Mass Spectrometer and many of its detector systems.

6. HRIBF's Robert Welton Wins Best Contributed Talk Award at RNB2000 Conference

The 5th International Conference on Radioactive Nuclear Beams (RNB2000) was held at Divonne, France, on April 3-8, 2000. The conference was attended by about 200 participants, including 12 participants from the U.S.

It is a great pleasure to inform you that Robert Welton's contributed presentation on the HRIBF 17F beam development and on-line performance was awarded the prize for the best contributed talk. This is not only a tribute to the quality of Robert's presentation, but also to all the work done by the members of the Advanced R&D Projects Group and the Operations Group.

7. Feedback Sought for Proposed Newsletter Distribution Change

In our continuing efforts to improve the quality of information we deliver to our users, we propose to change the distribution method for the HRIBF Newsletter and request your opinion on the proposed changes. We propose to send you notification via email that the Newsletter is available for viewing on the web. This notification will contain the web address and the table of contents of the current issue. There are many advantages supporting this technique:

Please send your comments about the proposed change to Carl Gross at Depending upon the response, the proposed change will take effect with the next issue.

RA1 - RIB Development

UNISOR Facility - Charge Exchange Cell
The testing phase of the new-design charge exchange cell has been completed and this cell has been installed and aligned on the RIB Injector Platform. Charge exchange efficiency in a Cs vapor was measured for stable isotopes of several elements at energies from 20 keV to 50 keV. At UNISOR, the Cs reservoir was operated at 250 C for four weeks with no measurable loss of Cs vapor, and then over the next three weeks the maximum charge exchange efficiency dropped slowly to about 35% of the original value. This cell is designed to circulate the Cs vapor and has cooler surfaces where the Cs vapor condenses and drips back into the reservoir. These cooler surfaces (copper and stainless steel) tend to collect Cs over time and must be periodically heated to drive the Cs back into the reservoir. After such a rejuvenation process, the optimum charge exchange efficiency was recovered, so the cell should be able to operate for long periods of time with occasional heating cycles to recover Cs from the walls of the cell.

Neutron-rich RIB Development
The beam species that we chose to measure were dictated by the beams extracted from the UC target that is now ready for operation on the RIB Injector Platform. Also, we concentrated on beams with mass less than 100 amu since these beams can be accelerated in the Tandem to energies above the Coulomb barrier. Charge exchange efficiencies in Cs vapor were measured for the following elements: As (42%), Se (22%), Br (1.2%), Rb (0.5%), Sr (0.9%), and Sn (40%). The energy for maximum charge exchange varied for each element, but tended to be lower (20 keV) for As, Se, and Sn ions and higher (50 keV) for Br, Rb, and Sr ion beams. We will continue to use this test facility for charge exchange measurements in support of the RIB Facility.

As, Ga, and Se RIB Development
We have started a couple of projects that will keep us busy through the summer, namely (i) testing of a newly designed circulating Ge target/ion source for the production of As, Ga, and Se ion beams and (ii) production and on-line testing of uranium carbide targets on a highly-conductive graphitic foam matrix. These will be discussed in the next newsletter.

RA2 - Accelerator Systems Status

ORIC Operations and Development
During the first weeks of the reporting period, ORIC continued to provide 44.5-MeV deuterons to the RIB Injector for 17F production. Subsequently, a helium cylinder was installed and an 85-MeV 4He beam was produced. Several days of development time were required since 4He had not been extracted without the availability of trimming coil T9. The beam was then transported to the target/ion source on the RIB injector platform where 18F was successfully produced. Following the 4He run, ORIC was shut down and three new trimming coil power supplies were installed along with enhanced thermal protection of the coaxial extraction channel.

Tandem Operations and Development
During the period since the last report, the tandem accelerator has provided 1018 hours of beam on target with beams of 17F, 18F, 32S, 40Ca, 54Fe, and 58Ni. One tank opening was required during this time to take measurements for the upcoming installation of the recirculating gas stripper. Sixty-seven hours were spent conditioning the machine during this period, and the range of terminal potentials was 3.42 to 19.46 MV. Typically, the machine ran well with only one period of instability, which was traced to an incorrect setting for the terminal potential stabilizer.

RIB Injector Operations and Development
Over the month of February the RIB injector had been engaged in delivery of beams of 17F. Physics experiments described in the previous newsletter were completed during this period. In early March we developed an 18F beam by bombarding the same target/ion source (kinetic ejection negative ion source and HfO2 fiber target) with 0.5-1.5 particle uA of 85-MeV 4He ions from ORIC. Beams of approximately 107 ions/s of 18F were produced from the 16O(4He,pn) reaction. Since the target/ion source was already designed and optimized for the production of fluorine (17F), development of the 18F beam simply amounted to tuning 4He through the cyclotron. Since the 4He beam had already been developed using this machine, the effort took only a few days. Beams of 18F were on target for the first half of March allowing several astrophysics experiments to be completed. A description of these experiments can be found elsewhere in this newsletter.

After completing these experiments, our attention became focused on upgrading the injector to accommodate a uranium carbide target which will be used for producing various neutron-rich beams. During the late March-April period, a UC target ion/source, a newly designed charge exchange cell, and a tape system were installed on the RIB injector. The ion source was our standard positive electron beam plasma source, and the target consisted of a low-density reticulated vitreous carbon matrix coated with ~12 mm of UC2, forming a total target thickness of approximately 2 g/cm2. The new charge exchange cell is based on the vapor fountain, re-circulation principle which has been shown to offer substantially better operational lifetime and overall reliability than our existing cell which was deemed inadequate for long-term operation. Tests of this charge exchange cell were conducted at UNISOR and are described in section RA1 of this newsletter. The new retractable tape station is being installed at the image of the second stage (isobar) magnet and will allow a direct measurement of the intensity of the RIB exiting the injector.

In conjunction with the Metals and Ceramics Division at ORNL, we have also developed a process of fabricating UC targets using bench top chemistry. We have employed this technique to produce targets which are extremely thermally conductive (s~10-50 W/mK) while, simultaneously, being quite U rich (typically 5:1 U:C ratio by weight). Calculations show these targets will be able to withstand nearly all of the available cyclotron beam and not be limited to only a few microamperes as are the current generation of targets. Microscopic inspection of the central region of UC targets made in this fashion show remarkably uniform coatings can be achieved. We plan to initiate on-line tests of this material during the summer of 2000.

RA3 - Experimental Equipment - HYBALL Charged-Particle Detector System

The advent of radioactive ion beam facilities has brought new opportunities for research in nuclear physics. However, there are technical challenges associated with the low beam intensities and high background produced from the decay of the scattered beam. Large solid-angle, charged-particle detection systems with good energy and angular resolution are necesary for both nuclear spectroscopy and nuclear reactions.

There is currently a vigorous effort at HRIBF in the development of a dedicated array of charged-particle detectors for use with radioactive ion beams. The first phase of the project has been completed with the construction and commissioning of a small, Hybrid-4p (HYBALL) array of charged-particle detectors. This phase of the project consists of an array of 95 CsI(Tl)-crystals coupled to photodiodes to operate as an inner ball inside the CLARION array of clover Ge detectors. Additional information may be found on our web site at and

The phase II of this array (HYBALL) requires a Silicon Double Sided Strip Detector (DSSD) forward array and is currently under construction. The mechanics of this array are modular to adapt to various experimental requirements. The modular forward array consists of 2 layers of very large silicon DSSD and is based on a 6-inch wafer technology. Each layer consists of 6 sectors; the front of each sector consists of 16 annular p+ strips and the back consists of 8 n+ strips at 7.5 deg pitch. This results in 768 pixels of detection. The silicon DSSDs will be able to deal with the high backgrounds associated with RIB's. In one configuration, for nuclear spectroscopy, the DSSD modular array will operate in conjunction with 79 of the 95 CsI detectors to cover the forward angles between 6 and 25 degrees. It will operate in the large forward fantail section of the target chamber thus removing scattered RIBs from the focus of CLARION. The modular design of the forward array will allow its use in other experiment endstations (e.g., for the Reactions Program). Proton-a discrimination in the Si-detectors may be achieved using only energy loss in a single layer of silicon (transmission mode) or energy loss-total energy (dE-E) in two layers of Si-detectors (dE-E mode).

The various components of the CHARMS system - CLARION and HyBall Arrays with the Recoil Mass Spectrometer - have been successfully tested together. Different triggers have been tested including CLARION+HyBall (CHA) and CLARION+HyBall+RMS (CHARMS). The first two experiments with the full array of CsI detectors of the HyBall both to explore spectroscopy near the drip line in the A=100 and A=130 mass region have been completed. Analysis of these experiments is in progress. The data of the mass 130 region experiment will serve as part of the Ph.D. dissertations of E. Padilla (UNAM, Mexico) and O. Zeidan (U. of Tennessee). For more information regarding HYBALL, please contact Alfredo Galindo-Uribarri at

RA4 - Experiments, Spokespersons, and Dates Run During the Past Quarter

February 1, 2000 - April 30, 2000




RIB-019 - Proposal to Determine the 14O(a,p)17F Reaction Rate



RIB-031 - States in 18Ne Populated by Resonance Scattering of 18F on 1H Using Thick Targets



RIB-000 - Commissioning of the RMS





RIB-009 - Population Systematics of Light Rare Earth Nuclei



RIB-020 - Determination of the Rates of the 18F(p,g)19Ne and 18F(p,a)15O Reactions in Nova and X-ray Burst Explosions

Parker/Yale University


RIB-040 - Beam Diagnostics Development



RIB-024 - Decay Studies at the Proton Drip Line in the 100Sn Region with a 56Ni Radioactive Beam



RIB-006 - Study of Low-lying Transitions in 103Sn



RIB-037 - Tandem Development



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Unscheduled Maintenance


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Witek Nazarewicz Carl J. Gross
Deputy Director for Science Scientific Liaison
Mail Stop 6368 Mail Stop 6371
+1-865-574-4580 +1-865-576-7698

Holifield Radioactive Ion Beam Facility
Oak Ridge National Laboratory
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Telephone: +1-865-574-4113
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