HRIBF NEWS

1. First Scientific Publication Based on Data Taken with Radioactive Beams at the HRIBF and Present Status of ¹⁷F Beams

A letter reporting the observation of the 3+ state at an excitation of 4.523 MeV in ¹⁸Ne in the elastic scattering of radioactive ¹⁷F from ¹H has been accepted for publication in Physical Review Letters. This state dominates the rate of the ¹⁷F(p,gamma)¹⁸Ne reaction, which is important in stellar explosions. In this study a nuclear resonance is observed using an accelerated beam of radioactive ions with "tandem" energy resolution.

Not only is this the first scientific publication using radioactive ion beams from the Holifield Radioactive Ion Beam Facility (HRIBF) at ORNL, it also is the first publication based on the Isotope Separator On-Line (ISOL) technique with accelerated radioactive beams in the U.S. The scientific work is a collaboration of scientists from ORNL, Yale University, the University of North Carolina, Duke University, Tennessee Technological University, the University of Edinburgh, Colorado School of Mines, and the Chinese Institute of Atomic Energy in Beijing.

At present, the HRIBF has been using a new negative surface ionization source (see Articles 6 and RA2 for details) to deliver ¹⁷F beams of up to 0.5 M ions per second at energies from 11-32 MeV. The source has currently operated for a period of more than two weeks. Although more intense beams of ¹⁷F are possible with this source, it can be highly contaminated with more than ten ¹⁷O for every ¹⁷F. At sufficiently high energies, a pure ¹⁷F beam may be obtained by adding a second stripper foil and producing fully stripped fluorine ions. This technique has been used in the 32 MeV experiment. Two additional experiments using ¹⁷F beams have been completed.

2. Grunder Committee Preliminary NISOL Report Given to NSAC

In the fall of 1998 a Task Force was organized through NSAC and given the charge to assess technical opportunities and identify R&D needs for an Advanced ISOL Facility to be funded by DOE. The scientific case for such a facility was provided by the 1997 white paper "Scientific Opportunities with an Advanced ISOL Facility." The Task Force is due to make a final written report in October 1999, but it provided an interim report to NSAC on April 30 in the form of presentations by Task Force members. Dick Boyd provided members of NSAC with a reminder of the scientific justification, largely taken from the 1997 white paper, while Herman Grunder, the Chairman of the Task Force, presented the interim technical conclusions. The members of the Task Force came to the conclusion that in order to justify the effort and expenditure which will be involved in an advanced ISOL facility, we should begin with an approach that is likely to lead to major advances beyond existing and planned ISOL facilities. The framework arrived at to achieve this goal is a facility based on a flexible multi-beam driver which relies heavily on a projectile-fragmentation-ISOL concept. This hybrid concept uses fragmentation of heavy projectiles as the means of production of many of the radioactive species. In the full implementation, the species of specific interest are selected in a fragment separator, slowed in a foil, and stopped in a He gas cell from which they are extracted as 1⁺ ions by ion-guide techniques and re-accelerated. A driver accelerator capable of producing ~1 puA of 200-400 MeV/u U is required. Implementation of the full facility concept requires a variety of driver beams, ranging down in mass to 400 MeV deuterons with an intensity of ~100 kW.

There was essentially no discussion of this ambitious concept at the meeting, either by NSAC members or observers - only a question about cost. Dr. Grunder offered the guess that in "units of billions of dollars the cost is about 1/2."

The interim Task Force conclusions were presented to Martha Krebs, Director of the DOE Office of Science, in a briefing several days prior to the NSAC meeting. In her remarks to NSAC Dr. Krebs expressed strong enthusiasm for the concept presented by the Task Force.

The Task Force began its deliberations by considering ISOL facility concepts presented by ORNL and ANL. The ORNL proposal was based on a single-beam driver, utilizing a part of the 1 GeV proton beam produced by the linac of the Spallation Neutron Source, while the ANL proposal was based on a 100 MeV/u multi-beam accelerator capable of providing beams from protons through mass ~100. Dr. Grunder took pains to point out that the Task Force has not endorsed either proposal; in fact, it has endorsed an enhanced and expanded version of the ANL proposal.

A second aspect of the Task Force's charge was to identify R&D crucial to the success of the advanced ISOL facility, which DOE should fund as expeditiously as possible. A preliminary list was provided to NSAC which contains a number of areas in which users and staff of HRIBF will be well placed to make important contributions.

3. Recent HRIBF Research - Theoretical Description of Deformed Proton Emitters

Proton radioactivity is an excellent example of elementary three-dimensional quantum-mechanical tunneling. The lifetimes of proton emitters and the energies of emitted protons provide direct information on the wave functions of narrow proton resonances and on shell structure beyond the proton drip line. More than twenty ground-state proton emitters and more than ten proton-decaying excited states have already been found. For most of them, proton emission rates can be well understood within a spherical picture in which a single proton tunnels through the Coulomb-plus-centrifugal barrier. However, it was realized early-on that in some cases, such as ¹⁰⁹I and ¹¹³Cs, deformation effects can play a role.

A new avenue in the spectroscopy of proton emitters has opened up with the discovery [1] of the ground-state proton radioactivity of ¹³¹Eu and ¹⁴¹Ho. In these nuclei, the associated quadrupole deformations are large, beta2~0.27; hence the standard theoretical methods based on the spherical picture cannot be applied. Recently, two new proton-emitting states in deformed nuclei, ¹⁴⁰Ho and ^141mHo, were identified [2] at HRIBF. The proton energies and half-lives measured to be 1086(10) keV and 6(3) ms, and 1230(20) keV and 8(3) us were assigned to the emitters ¹⁴⁰Ho and ^141mHo, respectively. The half-life of the previously known proton-emitting state ^141gsHo was observed to be 3.9(5) ms, in agreement with the previously reported [1] value.

In order to interpret experimental data for ¹⁴¹Ho, a theoretical group involving physicists from Tennessee, Hungary (Debrecen), and Romania (Bucharest) performed calculations employing the recently developed method based on the coupled channel Schroedinger equation with outgoing boundary conditions. Bound and resonant energies correspond to poles of the scattering matrix on the complex energy sheets. Normalization of the resonant wave functions was done using the exterior complex scaling method. In the calculations, the deformed proton optical potential was approximated by an average Woods-Saxon field containing the central term and the spin-orbit potential, and the Coulomb potential. Since the resonance widths can be as small as 10**-22 MeV, an unprecedented numerical accuracy is required. The spectroscopic factors were calculated in the BCS approximation.

According to the calculations, a 4 ms state can be associated with the 7/2^-[523] Nilsson orbital, while the only candidate for an 8 us resonance is the 1/2⁺[411] Nilsson level. In the absence of interference effects due to level crossings, the particle decay width of a deformed Nilsson orbital is governed by the partial wave with the lowest value of angular momentum allowed. The low-l components that are not present at the spherical shape appear at large deformations thanks to the multipole coupling. For instance, the decay of the 7/2^-[523] orbital is governed by the f wave, while the main contribution to the decay width of the 1/2⁺[411] level comes from the s wave. In this context, it is interesting to note that for a spherical resonance at Q_p=1.19 MeV, the calculated half-life is about 30 ms for l=5 and ~0.1 ms for l=3. Hence, the calculated half-life of ~8 ms of the deformed resonance comes as a result of an interplay between the large penetrability of the f-wave through the deformed barrier, and the large content of an h-wave in the wave function (86%). Though recent years have brought significant theoretical progress [3] to our understanding of proton emission, more work remains to be done. An obvious extension of the present approach is to take into account the effects of the Coriolis coupling. This will make it possible to make a consistent description of an interplay between gamma emission and proton decay from deformed nuclei.

References

[1] Davids et al., Phys. Rev. Lett. 80, 1849 (1998)
[2] Rykaczewski et al., Phys. Rev. C, 60, in press
[3] Aberg et al., Phys. Rev. C 56, 1762 (1997)

4. SNEAP and Proton Emitter Conferences to be Held in East Tennessee

SNEAP

The 1999 Symposium of Northeastern Accelerator Personnel (SNEAP) will be held in Knoxville, Tennessee, from October 25 to 28, 1999, hosted by the Physics Division and the Surface Modification and Characterization Research Center at Oak Ridge National Laboratory. The purpose of the symposium is to provide a forum for discussion of construction, operation, and use of electrostatic accelerators and boosters. The traditional format of SNEAP includes a large amount of time for discussion and the exchange of ideas. Topics will include ion sources, charging systems, accelerator developments, control systems, equipment suppliers, and other topics suggested by the participants. For more information, please visit the SNEAP '99 web site at www.phy.ornl.gov/sneap/ or contact Martha Meigs at meigs@mail.phy.ornl.gov.

International Symposium on Proton-Emitting Nuclei (PROCON99)

On October 7-9, 1999, the UNIversity Radioactive Ion Beam Consortium (UNIRIB), Oak Ridge Associated Universities (ORAU), the Joint Institute for Heavy Ion Research (JIHIR), and the Physics Division of Oak Ridge National Laboratory (ORNL), will host the International Symposium on Proton-Emitting Nuclei. The Symposium will be held in Oak Ridge, Tennessee, USA. The scope of the conference is to discuss recent results and new ideas related to proton emission from nuclear states. The topics to be covered include:

• Proton Radioactivity - Deformed and Spherical
• Spectroscopic Factors and Orbital Mixing
• Proton Emission From Excited States:
  • Beta Delayed Proton Emission
  • Proton Emission From High Spin States
• Novel Experimental Techniques and New Directions

More details may be found at http://www.orau.gov/unirib/procon99.html or by contacting the symposium secretariat at stewart@mail.phy.ornl.gov.

5. The HRIBF Tandem Still a Record Holder After 20 Years

During the voltage test of the Tandem Accelerator at the Holifield Heavy Ion Research Facility in May 1979, a record voltage of 32 MV was attained without accelerating tubes installed. Twenty years later this voltage is still the highest sustained man-made potential difference. Tandem voltages up to 25.5 MV have been used for experimental studies (i.e., with accelerating tubes installed). This is also a record.

6. Accelerated Beam of 5x10**5 ions/s of ¹⁷F Produced with Negative Ion Target-Ion Source

A kinetic-ejection negative ion source [1] has been developed for use in generating radioactive ion beams for the HRIBF. The source has recently been successfully employed on-line to generate high-intensity ¹⁷F^- beams using the ¹⁶O(d,n)¹⁷F reaction to produce the radioactive species in a refractory HfO₂ target. Chemically active elements are often released from target materials in a variety of molecular forms. In particular, fluorine is released principally as AlF from Al₂O₃ targets. The electron-beam-plasma (EBP) ion source used at HRIBF for positive ion generation is often ineffective for simultaneously dissociating the molecular carriers and ionizing the atomic species of interest. The new negative ion source has shown to be highly efficient in simultaneously dissociating and negatively ionizing the atomic fluorine constituents. To date, mass analyzed ¹⁷F^- beams exceeding 8x10**6 ions/s have been extracted on-line from the negative ion source and injected into the tandem accelerator. Beams of up to 5x10**5 ¹⁷F ions/s have been delivered to several RIB experiments at the HRIBF. Details of the source performance can be found in the Regular Article RA1 in this issue of HRIBF News.

The mechanism for negative ion formation in the source is secondary negative-ion emission based on velocity-dependent surface ionization [2]. The target-ion source assembly consists of a Ta target-material reservoir transversely attached to a Ta vapor-transport tube, and a positive Cs surface ionizer coaxially located at the exit end of the transfer tube. Radioactive species released from the target are transported to the ion source and adsorbed onto the inner surface of a relatively cool conical-geometry Ta cathode. Positive-charged Cs ions, produced by surface ionization, are accelerated through a ~90% transparent Mo-grid which is negatively biased at voltages up to -300V, bombard the inner surface of the metal cathode, dissociate the carrier molecules, and eject particles off the cathode surface. During the sputter ejection process, negative ions are formed by promoting electrons from the Fermi level of metal surfaces to the electron affinity level of the particles. The Cs⁺ beam, in combination with neutral Cs vapor, lowers the work-function of the cathode surface [3] and greatly enhances the negative ion formation. The cathode is biased at a potential more positive then the grid so that negative ions formed during the sputter process are repelled towards the negative-ion extraction region. In off-line characterization studies, SF₆ was used as the source of fluorine. Sulphur hexafluoride molecules were injected at a precisely controlled rate into the target reservoir at high temperature where the fluorine reacted with fibrous Al₂O₃ to form fluorine-rich compounds which were effusively transported to the ion source. The source demonstrated a nominal efficiency of 5%-7% for the generation of F^- beams. The mass spectrum extracted from the source was simple with essentially 100% of the F^- in the A=19 channel. This is in contrast to the EBP positive ion source, which produces F⁺, AlF⁺ as well as other fluoride molecular ions, with only ~13% of the fluorine in elemental form. The normalized emittance of the source has been measured to be ~6.8 mm-mrad-MeV^1/2 at the 80% contour level. The source operates very stably and reliably, and has demonstrated a continuous operational lifetime in excess of one month. In low-intensity, on-line tests conducted at the UNISOR facility with three fibrous targets, Al₂O₃, ZrO₂ and HfO₂, the source has demonstrated the capability of providing 10**7 ¹⁷F negative ions/s/uA of deuterons from the ZrO₂ and HfO₂ targets (HRIBF News, Winter Quarter 1999). The expected ¹⁷F^- beam intensities are about 10 times higher than those obtained after the molecular breakup and charge exchange process following the EBP positive ion source. The kinetic-ejection negative-ionization mechanism is an efficient means for simultaneously dissociating molecules and negatively ionizing species with intermediate to high electron affinities. In combination with low-density, fibrous and composite targets developed at the HRIBF [4], we will be able to provide radioactive ion beams of several other electronegative species using this ion source.

References

[1] G.D. Alton, Y. Liu, C. Williams and S.N. Murray, CP473, Heavy Ion Accelerator Technologies: 8th Int. Conf., Ed. K.W. Shepard, AIP Press, New York, 1999, p. 330
[2] J.K.Norskov and B.I. Lundqvist, Phys. Rev. B 19, 5661 (1979)
[3] G.D. Alton, Surf. Sci. 175, 226 (1986)
[4] G.D. Alton, Proceedings of the Application of Accelerators in Research and Industry, Ed. J.L. Duggan and I.L. Morgan, CP392, AIP Press, New York, p.429; G.D. Alton, Nucl. Instr. And Meth. A382, 204 (1996)

7. Frank Plasil Named Corporate Fellow at ORNL

Frank Plasil is one of three researchers at Oak Ridge National Laboratory that have recently been named corporate fellows of Lockheed Martin Energy Research (LMER) Corporation. This ranking is one of the highest to be earned at LMER. Frank's corporate fellow citation reads: "Plasil was selected for his accomplishments in the field of nuclear physics. He is a senior research staff member in the Physics Division. He is the coauthor of more than 250 professional articles in journals and books. In 1984, he received the Senior Alexander Von Humboldt Award. He also earned the 1998 Honorary Medal for Merit in Physical Sciences of the Academy of Sciences of the Czech Republic."

Plasil received his Ph. D. in nuclear chemistry from the University of California, Berkeley and was a postdoctorial fellow at Brookhaven National Laboratory. He has been a staff member in the Physics Division at ORNL since 1967.

RA1 - RIB Development

In this last quarter, the RIB development group and UNISOR facility have been involved in off-line tests on thin liquid germanium targets and measuring charge exchange efficiencies. The group has also been involved extensively in the efforts to implement a negative ion source on the RIB injector. The performance of this source for the production of ¹⁷F beams for experiments is discussed in article 6 in this newsletter.

Batch Mode Source Development

The highest priority for new RIB development at HRIBF is now the production of a ⁵⁶Ni beam. Because of the long lifetime and the highly refractory nature of Ni, the beam will not be produced by conventional ISOL techniques, but rather in a multi-sample sputter source, in which a target can be bombarded with ORIC beam for a period comparable with its half-life, and then mechanically transported to a position where the activity can be exposed to a sputter beam for production of a ⁵⁶Ni negative-ion beam. The source is designed so that one production target can be exposed to ORIC beams while another is simultaneously being sputtered.

A prototype version of this source is now being tested at the Ion Source Test Facility at HRIBF. Initial tests of the positive-ion (Cs⁺ sputter beam) performance have been successfully completed, and we are moving to tests of negative-ion performance. If all goes well, on-line tests will be carried out in early summer. We hope to produce a beam for experiments before October 1.

Arsenic RIB Development

As noted in our last newsletter, we are interested in developing thin liquid targets (specifically Ge) in order to reduce the diffusion time of the radioactive atoms in the target material and thus increase our efficiency for short-lived nuclides. We have investigated two systems where the molten Ge wets a substrate and results in a thin coating of Ge. The systems studied so far, molybdenum wire and silicon carbide fiber, have not resulted in satisfactory performances.

Efficiencies for forming negative As ions in cesium- and magnesium-vapor charge-exchange cells were recently re-measured by the group from Texas A&M University using As+ beams from the UNISOR separator. The most interesting result of these experiments was the increase in negative-ion formation efficiency when two cells were used. The first cell containing Mg vapor was optimized to yield neutral As atoms. This was followed by a Cs vapor cell which produced the negative As ions. While the efficiency was only slightly higher (35% for 50 keV As ions) than that obtained with a single Cs cell, the total atomic density in the two cells (Cs+Mg) was roughly an order of magnitude smaller than the Cs density required to reach equilibrium in the single cell. This should result in smaller beam emittance and better beam transmission to the experiment.

UNISOR Facility

Two projects are currently underway to modify/upgrade the UNISOR separator to enhance its capabilities to serve as a testing and preparation facility for the RIB injector. The front end of the separator is being modified so that a target/ion source enclosure can be mounted and the ion source can be tested at UNISOR before it is moved to the RIB injector. Also we have begun construction on a charge-exchange test facility on one of the UNISOR beam lines. This facility will be used to develop charge exchange cells for the RIB injector and to determine their optimum operating parameters.

RA2 - Accelerator Systems Status

ORIC Operations and Development

ORIC was shut down for machine upgrades following the ¹⁷F RIB run in December which was driven by 44.5 MeV deuterons. In the previous newsletter, it was noted that two new trim coil power supplies were being installed, harmonic coil control converted to Vista, and a new lower-channel power bus was being installed. These tasks were successfully completed, and the 1970's-vintage Modcomp control computer is no longer in use. In recent weeks, ORIC has resumed production of 44.5 MeV deuterons for ¹⁷F RIB runs and has run quite reliably. Two more trim coil power supplies and a new lower-channel power supply to replace the #2 1938 motor generator will be installed during the next shutdown period.

Tandem Operations and Development

A small, pressure-sensitive leak in the terminal was found by using a new leak-hunting technique which involved placing a turbo pump in series with the leak detector to increase its sensitivity by increasing the pumping speed. The tank was closed, and vacuum recovery and conditioning was begun. The accelerator has subsequently been conditioned so that it should be able to run up to 22 MV. Unfortunately, we have had three more tank openings during this period: one to repair a failed power supply, one to repair a failed actuator, and one to retrieve broken shorting rods from the machine. During the second of these tank openings, the first three tube sections in units 10/11 were shorted. These units had been troublesome during the conditioning and tended to decondition with time. The reasons for these problems will be explored during the next scheduled maintenance. Aside from these problems, the accelerator has run smoothly, providing beams of ³²S, ¹⁷O, ¹H, ⁵⁸Ni, ²⁸Si, ⁴⁰Ca, and ¹⁷F to the RMS, the DRS, UNISOR, and the Enge magnet position.

On May 4, 18,760 pounds of SF₆ was delivered to the tandem accelerator. This amount of gas corresponds to an increase of about 9 psi in the accelerator tank. It should allow the terminal voltage to approach 25 MV with the proper conditioning. The delivery went very smoothly and only interrupted accelerator operation for seven hours.

RIB Injector Operations and Development

After a period of off-line development and testing at the Ion Source Test Facility and low-intensity on-line testing at UNISOR, the negative ionization target/ion source was mounted on the RIB injector in April. One to two microamperes of 44 MeV deuterons from the ORIC cyclotron were directed onto a fibrous HfO₂ target with a supply of Al atoms, and beams of ¹⁷F were produced. After optimizing the target/ion source (TIS) operating parameters, approximately 10**6 ¹⁷F i/s were injected into the tandem accelerator over the course of several weeks. A maximum TIS efficiency of 5x10**-4 was observed. This is about a factor of ~5 lower than low-intensity measurements made at UNISOR.

To increase the ¹⁷F beam available at the HRIBF we have designed several high-power target configurations based on a larger target holder. These target holders can support configurations of the target material which operate with lower beam power densities and more favorable thermal structures for radiative heat transfer. The designs include a partitioned target, an inclined target, and targets with invasive metallic structures. We also plan to reduce the beam power density by increasing the size of the beam by defocusing, rastering, or scattering prior to entrance into the target.

During this period several changes to the infrastructure of the RIB injector have also been implemented. Retrofit of the negative ion source to the injector required installation of several acceleration and ion source power supplies and modifications to the TIS vacuum enclosure to accommodate various new feedthroughs and an external Cs vapor system.

RA3 - Experimental Equipment - A Gas Target for the Daresbury Recoil Separator

A pure gas target has numerous advantages over solid targets, e.g., polypropylene or targets implanted with hydrogen or helium. The yield from a pure gas target is at least a factor of 3 larger than from solid targets, thus substantially reducing the required radioactive beam current and/or running time. Gas targets have few impurities, are stable, and do not experience the precipitous loss of hydrogen common to polymer targets. The background from scattering off high-Z constituents in the target is eliminated, and the target thickness may be easily controlled and monitored.

The windowless gas target consists of 9 chambers connected by apertures. A constant pressure is regulated in a central disk-shaped chamber that has no direct pumping. The chamber has an inner diameter of 15 cm and height of 3 cm. A target thickness of up to 5x10**18 atoms/cm**2 may be obtained with a pressure of 5 Torr maintained in the central chamber. Eight collimated ports are located around the circumference of the chamber that may house silicon surface-barrier detectors to monitor scattering and reactions. The small height of the central chamber allows for gamma-ray detectors to be placed in close geometry for recoil-gamma coincidence measurements.

On each side of the central chamber are 4 differential pumping stages. Large roots blowers on the first pumping stages carry away nearly all of the hydrogen gas. Turbo pumps on the outer three stages reduce the pressure at the ends of the target to high vacuum. The compact design (overall length of 1 meter) allows the target to be placed close to the first quadrupole of the DRS. Optics calculations indicate that transmission of any particular charge state should be 100% due to the extremely narrow opening cone (theta < 0.5 degrees) for capture reactions.

Much of the target was designed and constructed at Ruhr-Universitaet Bochum by our collaborators Uwe Greife and Frank Strieder. All six turbo pumps have been ordered, and the first stage roots blowers are currently going out for bids. Installation of the gas target should be completed around the end of the year, and it will be available for experiments in 2000. Examples of experiments which would benefit from such a target are ¹⁷F(p,gamma)¹⁸Ne and ⁷Be(p,gamma)⁸B.

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

February 1, 1999 - April 30, 1999

Experiment	Spokesperson/Institution	Dates
RIB-000 - Recoil Mass Spectrometer Commissioning	Gross/ORISE, ORNL Paul/Vanderbilt University	3/22-24/99 3/31-4/1/99 4/12-13/99 4/22-24/99
RIB-014 - Target-Ion-Source Development - Arsenic and Fluorine	Stracener/ORNL	4/9/99
RIB-031 - States in 18Ne Populated by Resonance Scattering of 18F on 1H Using Thick Targets	Galindo-Uribarri/ORNL	4/14/99
RIB-039 - High Voltage Injector Development	Mueller/ORNL	4/15-16/99
RIB-027 - Band Structure in 78Y and Question of T = 0 Pairing	Paul/Vanderbilt University	4/26-30/99
RIB-037 - Tandem Conditioning	Meigs/ORNL	3/4-12/99 3/24-26/99 4/19-20/99

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For this quarter's schedule go to http://www.phy.ornl.gov/hribf/users/04-06-1999.html.