HRIBF NEWS

1. Update of RIB Delivery Plans

A detailed discussion of our near-term plans for RIB delivery at HRIBF was presented in the summer issue of the HRIBF Newsletter. The substance of those plans remains unchanged, but we have made some changes in the sequence. The next ¹⁷F run, which has been delayed somewhat by target and tandem problems, will continue through December. Before we proceed to an extended period of batch-mode beam operations, which include tests with ¹⁸F or ¹¹C and a long run with ⁵⁶Ni, we plan an approximately one-month-long run on neutron-rich beams produced with the HRIBF Uranium-Carbide Target System. The intention of this neutron-rich test run prior to the batch-mode experiments is to provide us with experience using the UC target with intense proton beams before we request proposals for neutron-rich experiments.

2. Breakup of Weakly Bound ¹⁷F at 10 MeV/A

The proton separation energy of ¹⁷F is only 600 keV. It is speculated that the breakup of ¹⁷F would influence the fusion rates at energies near and below the Coulomb barrier for ¹⁷F induced reactions [1,2,3]. Fusion excitation functions for ¹⁷F + Pb were measured by Rehm et al. [4] where no fusion enhancement due to breakup was observed.

Measurements have been carried out to study the breakup mechanism of the weakly bound ¹⁷F. The experiment was performed using a 170 MeV ¹⁷F beam incident on a 2 mg/cm**2 Pb target. The ¹⁷F was stripped to charge state 9+ to assure purity. The beam intensity was 200,000 particles per second on target for a total of 5 days. A Si detector mounted at 10 degrees with respect to the beam axis was used to monitor the beam and for normalization. The Enge split-pole spectrograph was placed at 2 degrees on the other side of the beam axis to monitor the beam.

The breakup products, proton and ¹⁶O, were measured in coincidence by a double-sided strip detector (DSSD) located near the grazing angle and a Si surface barrier detector (SBD) mounted on the back of the DSSD. The thickness of the DSSD is 300 um which stops the heavy fragment, ¹⁶O, but allows the light fragment, one proton, to punch through and be detected by the SBD. The DSSD is 5x5 cm**2 with 16x16 strips and spans from 33 to 57 degrees. Protons are identified by a dE-E technique using the energy loss in the DSSD (dE) and the energy deposited in the SBD (E). The coincident ¹⁶O is sought in the events where a proton has been identified.

The data, as detected by the experimental setup, was simulated by Monte Carlo calculations. The energy distribution of the breakup protons was reproduced by the Monte Carlo simulation assuming a direct mechanism. In the simulation, the ¹⁷F was excited to energy between 0.6 and 1.3 MeV and disintegrated at the distance of closest approach. The excitation function of low-energy radiative capture of proton on ¹⁶O [5] was folded in the simulation to generate events. The angular correlation of the breakup fragments was also reproduced by the Monte Carlo simulation. The measured breakup cross section was found to be very small, although further measurements are underway to reduce the uncertainty. It is expected that at energies near the barrier the breakup cross section will be smaller than that measured in this work. This suggests that the influence of the breakup of ¹⁷F on near barrier fusion may be too small to be measurable.

References

[1] C. H. Dasso and A. Vitturi, Phys. Rev. C 50, R12 (1994).
[2] M. S. Hussein et al., Phys. Rev. C 46, 377 (1992).
[3] N. Takigawa et al., Phys. Rev. C. 47, R2470 (1993).
[4] K. E. Rehm et al., Phys. Rev. Lett. 81, 3341 (1998).
[5] R. Morlock et al., Phys. Rev. Lett. 79, 3837 (1997).

3. Shell Model Monte Carlo Calculations in Nuclear Structure and Astrophysics

As facilities, including HRIBF, explore the structure of neutron-rich nuclei, a challenge remains to understand these systems from a shell model perspective. From the technical point of view, the shell-model space needed to treat the relevant degrees of freedom properly grows enormously as one adds neutrons. An equally difficult complication involves developing useful two-body shell-model interactions in regions of the periodic table where little is known experimentally.

One such area is in the sd-pf shell, in which ¹⁶O is taken as a core. For neutron-rich nuclei in this region, neutrons typically occupy the upper part of the sd-shell and lower part of the pf-shell, while protons occupy mainly the sd-shell; however, the proton-neutron interaction across the shells plays an important role in determining the structure of these nuclei, and cannot be ignored. Thus, calculations that include both major-oscillator shells become necessary. Provided that a shell-model interaction and a technical method for computing observables exist, one can study interesting effects in the sd-pf shell model region. For example, studies of shell closures at the N=20,28 neutron magic numbers may be carried out.

While obtaining an effective two-body interaction for a given shell model space has a long history, seldom has the many-body perturbation theory been applied to large, multi-major oscillator shells. Work in this direction yielded an effective interaction that may be applied across the sd-pf shells [1]. Using Shell Model Monte Carlo (SMMC) technology that was specifically enhanced for cross-shell calculations (including center of mass removal), several interesting features of nuclei near the N=20 region were investigated. One conclusion, consistent with other work, was that nuclei such as ³²Mg tend to have two particles in the pf-shell (specifically in the f_7/2 orbital). A further feature of these shell-model interactions in neutron-rich systems is that they exhibit weak J=0 total pairing strengths across the N=20 shell gap [2].

SMMC has also recently been applied to the interesting astrophysical problem of characterizing the Type-Ia explosion mechanism [3]. The Chandrasekhar mass model for Type Ia Supernovae (SNe Ia) has received increasing support from recent comparisons of observations with light curve predictions and modeling of synthetic spectra. It explains SN Ia events via thermonuclear explosions of accreting white dwarfs in binary stellar systems, being caused by central carbon ignition when the white dwarf approaches the Chandrasekhar mass. As the electron gas in white dwarfs is degenerate, characterized by high Fermi energies for the high density regions in the center, electron capture on intermediate mass and Fe-group nuclei plays an important role in explosive burning. Electron capture affects the central electron fraction, which determines the composition of the ejecta from such explosions. Up to the present, astrophysical tabulations based on shell-model matrix elements were only available for light nuclei in the sd-shell. Recently, new SMMC and large-scale shell-model diagonalization calculations have also been performed for pf-shell nuclei. These led, in general, to a reduction of electron capture rates in comparison with previous, more phenomenological, approaches. Making use of these new shell-model-based rates, the first results were presented for the composition of Fe-group nuclei produced in the central regions of SNe Ia and possible changes in the constraints on model parameters like ignition densities and burning front speeds.

References

[1] D. J. Dean et al., Phys. Rev. C 59, 2474 (1999).
[2] P.-G. Reinhard et al., Phys. Rev. C 60, 014316 (1999).
[3] F. Brachwitz et al., submitted to ApJ (1999).

4. SNEAP and Proton-emitting Nuclei Conferences Held in East Tennessee

SNEAP Conference held in Knoxville, Tennessee

The 1999 Symposium of Northeastern Accelerator Personnel (SNEAP) was held in Knoxville, Tennessee, from October 25 to 28, 1999, hosted jointly by the Physics and Solid State Divisions at Oak Ridge National Laboratory. Sixty-nine attendees, representing 13 countries and 41 institutions, participated in discussions concerning development and operation of electrostatic accelerators and associated equipment. The symposium brought together operations personnel from very small machines (1 MV or less) up to very large machines (25+ MV) to discuss topics of interest to all such as ion sources, charging systems, ion optics, etc. In addition to the contributed talks, a tutorial describing ion optics calculations was given by an independent consultant, Dan Larson. Talks concerning ion source modeling and foil stripper lifetimes and transmissions were quite interesting, but perhaps the most useful session was the "open forum" where problems could be brought before the group for general discussion. The next SNEAP meeting will be held at Yale University in 2000. The proceedings of the symposium will be published by World Scientific within the next year.

Oak Ridge International Symposium on Proton-Emitting Nuclei

On October 7-9, 1999, the first conference devoted to the study of proton emitters (International Symposium on Proton-Emitting Nuclei) was held at Pollard Auditorium in Oak Ridge, Tennessee. The conference was jointly supported and hosted by the UNIRIB Consortium, the Joint Institute for Heavy Ion Research, Oak Ridge National Laboratory, and Oak Ridge Associated Universities. The meeting was attended by 87 physicists representing 38 universities and laboratories from 14 countries. The conference featured both theoretical and experimental lectures by 33 speakers on various subjects including proton emission from ground states and low-lying excited states of spherical and deformed nuclei and proton emission from highly excited states (including beta-delayed proton emission, proton emission from high spin states). New physics highlights from the meeting include the first observation of ⁴⁸Ni (a candidate for the yet-unobserved mode of ground-state, two-proton emission) by the GANIL group. The first observation of proton fine structure from an odd-odd nucleus was reported by the HRIBF collaboration. Theoretical results on proton emission from several groups were presented. In addition, recent experimental results were reported from ANL, GSI, INFN, ISOLDE, Jyvaskyla, LBNL, MSU (Michigan), and ORNL. The conference proceedings will be published by the American Institute of Physics.

5. Witek Nazarewicz Appointed HRIBF Deputy Director for Science

Effective October 1, 1999, Witek Nazarewicz assumed the position of HRIBF Deputy Director for Science. His responsibilities include operation of the HRIBF Program Advisory Committee, interaction with the Users Executive Committee, advising the facility on priorities for beam and equipment development, and presenting the HRIBF scientific program to the outside community. Witek will continue in his position as Professor of Physics at The University of Tennessee and will carry out his HRIBF responsibilities as a part-time ORNL Adjunct Staff Member. Carl Gross will continue as HRIBF Scientific Liaison, and Franda Ervin will continue to be responsible for the operation of the User Office.

6. Three New PAC Members Appointed

Three new additions to the HRIBF Program Advisory Committee have been made. J. Aysto, C. Baktash, and M. Wiescher have replaced S. M. Austin, W. Gelletly, W. Nazarewicz, and P. D. Parker. We wish to thank the previous members for their service. The present PAC membership includes:

• J. Aysto (CERN/Jyvaskyla)
• C. Baktash (ORNL)
• N. Benczer-Koller (Rutgers)
• J. C. Hardy, chair (Texas A&M)
• J. J. Kolata (Notre Dame)
• I. Y. Lee (LBNL)
• M. Wiescher (Notre Dame).

Early next year, a Call for Proposals is expected to be issued focussing on neutron-rich RIBs.

RA1 - RIB Development

Neutron-rich RIB Development

Recently we made measurements of several radioactive beams from a negative ion source and a uranium carbide target. The radioactive species were produced using proton-induced fission and ionized in a kinetic-ejector negative ion source. The yields measured were 10 to 200 times lower than the yields measured earlier from the positive ion source. Since the target temperatures were the same for both measurements, the limiting factors appear to be effusion in the Ta transfer tube and/or the ionization efficiency. In the negative ion source the transfer tube temperature ranges from 1500 C to 1100 C, which is significantly cooler than the temperatures of the transfer tube in the positive ion source (1700 C to 2100 C).

Since we have seen a thirty-fold increase in F- yields when aluminum oxide was added to the target, we decided to see if the same was true for Br- ions. When we added Al₂O₃ fibers to the UC target, we found that the yield doubled but it was still about 80 times less than the yield of Br+ ions from the EBP positive ion source. For Sn- ions there was no measurable change in the yield.

Fluorine RIB Development

In the EBP positive ion source we have shown that the addition of aluminum vapor can enhance the yield of AlF+ by two orders of magnitude. However, the addition of Al vapor via an external oven into the negative ion source has little or no effect on the yield of F- until the vapor pressure is so high that the efficiency of the ion source drops dramatically. On the other hand, the presence of Al₂O₃ fibers in the target holder (containing HfO₂ or ZrO₂) can significantly increase the yield of F- from the negative ion source. In recent tests, we have shown this effect conclusively and have also shown that ZrO₂ stabilized with Y₂O₃ is not a good target for fluorine production. This is due to low intrinsic yields and a degradation of the material in the presence of aluminum or aluminum oxide. A target of hafnium oxide fibers in the presence of aluminum oxide fibers still seems to be the most efficient way to produce negative ion beams of radioactive fluorine.

UNISOR Facility

Two construction projects at UNISOR are nearing completion, namely, modification of the front end of the separator to allow the use and testing of the standard HRIBF target/ion source enclosures and construction of a charge-exchange cell test facility on one of the separator beam lines. Most of the necessary modifications to the front end of the separator have been made, but the project is presently on hold while we manufacture an adapter flange. We plan to finish this project early next year but are presently using our original enclosure to measure the yields of neutron-rich isotopes (see above). The mechanical portion of the charge-exchange cell test facility has been completed, and we have begun commissioning this new beam line. We are presently finishing the control program and plan to use this facility next month to measure charge-exchange efficiencies for several elements (eg., Br, Se, Sn) and to characterize the operational parameters.

RA2 - Accelerator Systems Status

ORIC Operations and Development

ORIC was shut down for most of the reporting period for machine upgrades. The primary upgrades were the installation of two new trimming coil power supplies and a new lower channel power supply to replace the 1938-vintage #2 motor-generator. Three additional trimming coil power supplies have been ordered. Numerous control system enhancements were made in preparation for a conversion to EPICs, and the old Modcomp computer system was removed. An rf inspection revealed a damaged drive line capacitor which was repaired along with an inoperable spark detector. On restart of ORIC, a lower channel failure occurred which resulted in the installation of the spare channel. As of this writing, ORIC operation has resumed with 44.5 MeV deuterons again on target for ¹⁷F production from the RIB injector negative target-ion source.

Tandem Operations and Development

The Tandem Accelerator ran smoothly until August 16, when the tank was opened for scheduled maintenance. This period of continuous operation, from April 20 to August 16, was one of the longest since the accelerator was commissioned. Tank closing was scheduled for September 18, but due to problems with the installation of a new recirculating gas stripper, closing was delayed until October 16. The installation of this recirculating system was postponed until the next scheduled tank opening so that problems could be corrected. The foil stripper failed during this opening and was completely rebuilt (new belt, sprocket, and bearings). This failure was another factor causing the prolonged opening, but it would probably have failed during the next run period, resulting in greater schedule disruption.

The control system was changed from VISTA to EPICS during this maintenance period. The EPICS system shows signs of much greater stability than VISTA software with no more crashes in the middle of the night. Work remains to convert all of the utility programs that had been developed for VISTA and to add logging and other functionality which was removed from VISTA software to reduce crash frequency. The next task will be to convert accelerator beam lines.

RIB Injector Operations and Development

During the fall quarter of 1999 the RIB injector has been involved in initial tests of the batch mode source and in testing a high power HfO₂ target configuration for ¹⁷F. In addition, stand-alone, mass spectrometric apparatus has been constructed to investigate yield- limiting, chemical interactions between RIB species and candidate target materials.

In late August and early September, the batch mode source was mounted on the RIB injector, and stable beams of C-, Si- and Ni- were extracted from targets made of these materials. Measured intensities were lower than expected for each of these beams suggesting target misalignment. Further examination of the sputter patterns on the targets confirmed this misalignment. Compound backlash associated with numerous drive couplings resulted in an unacceptable variation in the position of the target wheel. As a result, the target wheel positioning system has been completely redesigned. The drive motor is now close-coupled to the target wheel through a single worm coupling and the target position is also read back directly through a potentiometer. The position of the target can now be set to a 0.005" tolerance (~1% target diameter). This system has been constructed and awaits testing.

In late September, an elongated 5.5" HfO₂ target was installed on the RIB injector. This target configuration allowed more efficient radiative cooling than earlier designs by incorporating spaces between disks of target material for heat to radiate through. The target diameter was also increased to accept a wobbling beam. We hoped this configuration would increase the target's ability to withstand higher ORIC beam intensities and thereby improve ¹⁷F yield. In early November, after ORIC became available, beam was put on target. With 5 uA of ²H beam on target, the ¹⁷F yield increased with target heater temperature to values of 6x10**5 particles/s (factor of 20 less than nominal yields with previous target configuration). Upon inspection, several observations were made which help to explain the lower yield ¹⁷F ions:

• A grid which maintains a 300 V potential in the ion source was broken.
• A 3 mm hole was found in the beam entrance window (thin Ta foil).
• The target material had shrunk by 30% and was no longer held firmly in place.
• The Al₂O₃ disks were no longer visible, which could indicate that there was insufficient carrier atoms for the transport of fluorine.

With these observations in mind, we have installed a negative ion source with a slightly modified target which should increase the lifetime. The target material is HfO₂ cloth inside an Al₂O₃ felt liner which will provide the carrier atoms. This target/ion source will be used over the next few weeks to provide ¹⁷F beams for experiments.

In collaboration with the Metals and Ceramics Division at ORNL, a stand-alone target material characterization apparatus has been constructed and tested. The instrument employs a small residual gas analyzer to measure the time profile of permeation of chemical species through a membrane constructed from candidate target material. This system has been tested with oxygen diffusing through yttria stabilized zirconia. Measured diffusion coefficients for this system show good agreement with literature values. We intend to use this apparatus in initial feasibility studies of the development of new beams and to gain insight into on-line experiments where the measured yields represent a convolution of diffusion, desorption, effusion and ionization. This matrix also allows the direct investigation and development of new ISOL techniques for mass transfer such as electrochemical transport.

RA3 - Experimental Equipment - CLARION

The CLARION clover Ge detector array at the HRIBF will be completed within the next month or two, with the delivery of the last BGO Compton suppressor. The array comprises eleven segmented Clover Germanium detectors, each with a large BGO Compton Suppressor. They are positioned at the target chamber of the HRIBF Recoil Mass Spectrometer, mostly at backward angles. The target-to-detector distance is adjustable between 20.0 and 23.5 cm.

New dedicated electronics (CAMAC, FERA-readout) has been fully tested and debugged. These electronics are similar in many respects to the GAMMASPHERE electronics, but are housed in quadruple-wide CAMAC modules, with ECLine (FERA-type) readout. The modules require a first-level trigger within 1 microsecond of the event and a second-level trigger within 5 microseconds, thus allowing a trigger on delayed coincidences with recoils at the RMS focal plane. Total shaping, conversion, and readout times are typically 20-25 microseconds from the start of the event. Digitized outputs are:

• Four high-resolution Ge energies (14 bits, 0-4 or 0-8 MeV)
• Three low-resolution side-channel (segmentation) energies (12 bits)
• Four Ge times (12 bits, 0-5 microseconds)
• BGO energy and time (12 bits).

Two experiments with all of the eleven Ge detectors, and the new electronics, have been performed.

The performance of the array at a target-detector distance of 21.8 cm and at 1.33 MeV is tabulated below. Pictures of the array being mounted in the support structure can be found at http://www.phy.ornl.gov/hribf/research/gallery

Individual relative photopeak efficiency	154% (with add-back) at 1.33 MeV
Measured Total Absolute Photopeak Efficiency	2.25% at 1.33 MeV
Measured Peak-to-Total Ratio	0.57 at 1.33 MeV

RA4 - Users Group Election Results presented at Annual Meeting

The annual HRIBF Users Meeting was held at the DNP Meeting in Asilomar, California on October 21. Approximately 70 people attended the hour-long meeting which featured reports by Fred Bertrand, Carl Gross, Alfredo Galindo-Uribarri, and Brad Sherrill. Topics covered included:

• previous year's ¹⁷F yields and the latest ¹⁷F beam developments
• neutron-rich beam developments using UC targets at UNISOR
• batch-mode source developments (¹¹C, ¹⁸F, and ⁵⁶Ni beams)
• experimental yields from photo-fission of U using electron beams from ORELA
• summary of recent experiments using ¹⁷F beams
• CLARION & HYBALL detector array status
• Users Executive Committee election results
• new members of the PAC

Many of the above topics have been covered in this and previous newsletters. In brief, ¹⁷F experiments will continue through December; initial tests using ORIC and the UC target to produce accelerated, neutron-rich beams (most probably Br isotopes near A=90) should occur early next year, and will be followed by a series of experiments requiring the batch-mode source. Early next year, a Call for Proposals is expected to be issued focussing on neutron-rich RIBs.

The Users Executive Committee election was won by K. Rykaczewski (ORNL) and L. L. Riedinger (Tennessee). They will join I. Y. Lee (LBNL), B. M. Sherrill (Michigan State), W. B. Walters (Maryland), and M. Wiescher (Notre Dame) in January of 2000. I. Y. Lee will be chairperson next year. We wish to thank N. Benczer-Koller (Rutgers) and M. S. Smith (ORNL) for their past three years of service to the Users Group. As always, the committee members are there to represent the interests of the users. You are encouraged to contact any member to express any concerns, suggestions, or opinions you wish to express and have brought up for discussion.

RA5 - Experiments, Spokespersons, and Dates Run During the Past Quarter

August 1, 1999 - October 31, 1999

Experiment	Spokesperson/Institution	Dates
RIB-050 - Selective Study of Excited States of N=Z Nucleus 66As Using Decay Tagging Technique	Grzywacz/University of Tennessee	8/2-5/99
RIB-000 - Commissioning of the RMS	Gross/ORISE	8/6/99 8/13/99
RIB-040 - Beam Diagnostics Development	Shapira/ORNL	8/9/99 8/11/99 10/20-21/99 10/27-28/99
RIB-037 - Tandem Development	Meigs/ORNL	8/10/99 10/18-20/99 10/21-22/99 10/25-27/99
RIB-014 - Target-Ion-Source Development - Arsenic and Fluorine	Stracener/ORNL	8/12/99 10/28-29/99
Scheduled Maintenance		8/16-9/17/99
Unscheduled Maintenance		9/20-10/15/99

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For this quarter's schedule go to http://www.phy.ornl.gov/hribf/users/10-12-1999.html.

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Additional copies of the newsletter and more information about HRIBF can be found on the World Wide Web at www.phy.ornl.gov. You may contact us at the addresses below.

Witek Nazarewicz	Carl J. Gross
Deputy Director for Science	Scientific Liaison
Mail Stop 6368	Mail Stop 6371
witek@mail.phy.ornl.gov	cgross@mail.phy.ornl.gov
+1-865-574-4580	+1-865-576-7698
Holifield Radioactive Ion Beam Facility
Oak Ridge National Laboratory
Oak Ridge, Tennessee 37831 USA
Telephone: +1-865-574-4113
Facsimile: +1-865-574-1268