HRIBF NEWS

1. HRIBF Passes 1100 Hour RIB Operation Mark

Thus far during FY 1999, the HRIBF has provided over 1100 hours of radioactive ion beam (RIB) operation. The ¹⁷F beam development program, which has been discussed in previous newsletters, culminated in October 1998 in the production of ¹⁷F beams for the nuclear astrophysics research program. The initial operation was carried out using a positive-ion EBP source and using a cesium-vapor charge exchange cell to break up the AlF+ molecule and to produce F- ions. This source was operated for over 390 hours during October-December 1998 to provide ¹⁷F beams for a study of the ¹H(¹⁷F,¹H) reaction. Although the EBP source proved to be quite reliable, the molecular-breakup plus charge exchange process is relatively inefficient and limited the amount of beam which could be provided. As mentioned in previous newsletters, the RIB development program had successfully tested a negative-ion source (NIS) which held promise of higher ¹⁷F beam intensities. This source proved to be extremely robust and was operated for over 720 hours during May-July 1999 for the production of ¹⁷F beams. While this is a major achievement for the HRIBF, it has also demonstrated the limitations of the present target design. The ORIC beam current must presently be limited to about 2-3 microamps of 42 MeV deuterons to avoid excessive target temperatures and rapid target degradation. The next NIS will include a larger target cell with provisions for improved radiant heat transfer which should allow a substantial increase in the ORIC beam current. Preliminary tests of the new target design look promising and, following a final test of the aluminum vapor feed system, is expected to be ready for installation by the end of the August-September maintenance period. RIB operation with ¹⁷F beams is presently scheduled to resume in October.

2. Near-Term Plans for RIB Delivery at HRIBF

Much remains to be accomplished before the HRIBF can be considered a mature, routinely operating facility. In our impatience to reach this goal, it is easy to underestimate how much has been accomplished in accelerator systems improvements, controls, diagnostics, and many other areas. However, the subject of this note is our beam development program. Unless we develop a considerable stable of physically interesting radioactive ion beams (RIBs), we will not survive as a facility.

We have now developed and deployed four RIB production systems.

1. The kinetic-ejection negative ion source (KENIS)/refractory oxide fiber target system for production of ^17,18F.
2. The batch mode cesium-sputter ion source for production of long-lived species, e.g., ⁵⁶Ni ¹⁸F ¹¹C.
3. The uranium carbide/reticulated vitreous carbon fiber (UC/RVCF) target for production of neutron-rich beams by fission.
4. The liquid germanium target system for production of proton-rich isotopes of As, Ga, and Se.

As of early September 1999, all four of these systems are tested and ready for use in producing beam for experiments. The level of development of the four systems is not the same, as will be evident when each is discussed in more detail. Before proceeding to these discussions, we will outline our plans for the use of these systems over the next few months.

As of the first week of September, the HRIBF staff is carrying out scheduled accelerator maintenance. This follows a period during which we provided in excess of 1000 hours of RIB (¹⁷F) on target for experiments during the first eight months of 1999. The batch mode source is currently installed on the RIB injector platform for engineering and for operational testing. It has previously undergone testing at the off-line target/ion source test facility. Before the end of the maintenance period the KENIS will be reinstalled with a refractory oxide target designed to sustain 3 to 5 times larger driver beam currents than have been used so far. This system will be used to produce ¹⁷F beam for approximately four additional weeks of experiments (intensity >10**6 ions per sec). We will then reinstall the batch mode source for in-beam testing. Since the production of ⁵⁶Ni (which is the primary reason for the existence of the batch mode source) will result in the production of relatively large amounts of long-lived radioactive material, the initial in-beam test of the source may be carried out with beams that can be produced relatively cleanly. The best candidates so far identified are ¹¹C and ¹⁸F. Following these tests, which we estimate will last 3 to 5 weeks, we will proceed with production of ⁵⁶Ni. If all goes well, we should be able to provide 50 to 60 eight-hour shifts of ⁵⁶Ni at an accelerated beam intensity approaching 10**7 ions per second before the source must be dismounted for servicing.

By the time the ⁵⁶Ni run is completed, we expect to have approved experiments for neutron-rich RIBs produced in our UC/RVCF target system. Our current preference for a first neutron-rich beam is an isotope of Br; but many possibilities exist, and as always, we will be guided by the preference of our users.

The following paragraphs provide a somewhat more detailed description of each of these target/ion source systems.

We have developed two systems for production of radioactive fluorine beams. Both utilize a high-temperature refractory-oxide target (ZrO₂ or HfO₂). The highest intensity ¹⁷F beam is produced by the KENIS. However, this source produces a substantial ¹⁷O impurity beam (roughly 10/1 ¹⁷O/¹⁷F). A pure ¹⁷F beam can be provided by stripping to the 9+ charge state after acceleration, provided the desired energy is sufficiently high. We have also successfully produced ¹⁷F from an electron beam plasma-positive ion source as a molecular beam, AlF+ that is then dissociated and charge exchanged to form ¹⁷F-. The low efficiency of the dissociation/charge-exchange process and the poor phase space quality of the resulting ¹⁷F- beam results in a 3 to 10 times lower intensity following acceleration than can be produced with the KENIS, but the ¹⁷O contamination is very small (1/100).

The batch mode source was designed for production of a beam of ⁵⁶Ni- by Cs sputtering. Samples containing ⁵⁶Ni are prepared by bombarding a ⁵⁸Ni sample with 20 uA of ~40 MeV protons for a period of ~3 days. The sample is then rotated to a sputter position where the ⁵⁶Ni beam is produced, while another sample is being prepared in the cyclotron beam. A total of eight sample positions are available in the present design. The reaction used to produce ⁵⁶Ni produces ⁵⁶Co with a cross section about 20 times larger. This unfortunate circumstance means that the ⁵⁶Ni beam will have a substantial ⁵⁶Co impurity and that large amounts of long-lived ⁵⁶Co will be produced in the source. It may be desirable to test the source with comparatively clean RIBs before the ⁵⁶Ni production run begins. For this reason, beams of 10**5 to 10**6 ions per second of ¹¹C and/or ¹⁸F may be available for several days during testing of the batch mode source.

The UC/RVCF target system has been tested in-beam and found to produce promising quantities of a variety of neutron-rich beams. These tests have been discussed in earlier issues of the Newsletter. We will be able to provide usable intensities of RIBs of several n-rich isotopes of two or three species early next calendar year. Ba, Rb, Ga, and Sn isotopes are candidates for the first species to be developed, but there are many other possibilities. Let your preferences be known.

The liquid germanium target offers the possibility of reasonably high intensities (10**7 to 10**9 ions per sec) of ^68,69,70As, ^66,67Ga, and ^70,71Se. Because of the modest demand for these beams from our user community, work on improving the performance of this target system has been pursued at a comparatively lower priority.

In summary, we expect to provide approximately four additional weeks of ¹⁷F beam following the present maintenance period. This will be followed by mounting of the batch mode source. We expect to test the source by short runs of relatively clean beams (perhaps ¹¹C and/or ¹⁸F) before we begin a production run of 50 to 60 shifts of ⁵⁶Ni on target for experiments. This will take us up to the end of 1999. Our next beam will be a neutron-rich beam produced with the fission (²³⁸U) target system.

Our longer-term beam development program will be dealt with in a subsequent issue of the Newsletter, but we will end with a few general comments. We have identified a number of target systems that hold significant promise for production of beams of interest to the user community. Most of these have thermal characteristics similar to either the refractory oxide fiber targets used for fluorine production or the UC/RVCF target for neutron-rich beam production. This means that a great deal of our work on beam power handling in these two systems can be carried over directly to the new systems. Likewise, the liquid germanium target can be considered a prototype for a family of liquid metal or liquid metal film targets now under development. Most of these targets will be used in the ion sources already developed. However, we continue a vigorous program of RIB ion source development, currently concentrated on the RIB ECR source. The efficient extraction and acceleration of high currents of higher energy ³He and ⁴He production beams is also a high priority. A reliable beam of ³He with energy greater than 100 MeV and intensity in excess of 10 uA would greatly increase our flexibility.

The development of our facility to its current state has been a difficult and sometimes maddeningly slow process. The HRIBF clearly has shortcomings as a RIB facility: much remains to be done in many areas. We believe, however, that we have finally begun to reap significant rewards for our efforts, and that the pace of our successes will accelerate over the next several months.

3. Recent HRIBF Research - Proton Resonance Reactions Induced in Thick Hydrogen Targets Using ¹⁷F Beams

A collaboration between HRIBF and the Universidad Nacional Autonoma de Mexico has studied proton resonance reactions using a ¹⁷F beam incident on a 40-um-thick polypropylene (CH₂). The target is thick enough to stop the ¹⁷F beam, but thin enough to let the recoil protons escape. This thick target technique and the procedure to extract the excitation function from the measured recoil proton energy spectra has been described in detail in ref [1].

Measurements were done at a ¹⁷F bombarding energy of 33 MeV, which covers a range of center of mass energies from 0 to 1.8 MeV. By the use of a second stripper after acceleration, a 9+ ¹⁷F beam with intensities on the order of 10**4 ions/s was obtained, thus avoiding contamination with the ¹⁷O isobar. A large, solid-angle (180 msr) microstrip detector was used to detect the recoil protons, covering center of mass angles from 180 to 160 degrees. A microchannel plate detector was placed at the entrance to the scattering chamber to monitor the beam and to provide a timing signal used to gate the proton detector. The microchannel plate can stand a large counting rate of at least 10**5 ions/s. This coincidence technique reduced the positron background which results from the decay of the stopped ¹⁷F beam.

In a single exposure of about 16 hours an excitation function was obtained, covering the center of mass energy range of 400 keV to 1400 keV in 5 keV steps. The resonance recently observed [2] at center of mass energy 600 keV and spin-parity 3+ and the gamma width of 18 keV has been confirmed. Addtional resonances at energies of 1184 and 1231 keV were observed having spin and parities consistent with 2+ and 3-, respectively. The data were analyzed using the R-matrix code MULTI described in ref [3]. The strongest resonances observed (3+ at 600 keV and 2+ at 1184 keV ) were fitted with an l_p=0 which results from the coupling of a s_1/2 proton to the d_5/2 proton of the ¹⁷F ground state configuration. The 2+ state agrees with the spin assignment given in ref. [4] in contrast with the disagreement to the 3+ spin assigned to the 639 keV state in ref. [4] and discussed in ref. [2].

References

[1] A. Huerta Hernadez, et al., Nucl. Instrum. Meth. B 143, 569 (1998)
[2] D. Bardayan, et al., Phys. Rev. Lett. 83, 45 (1999)
[3] R. O. Nelson, et al., Nucl. Instrum. Meth. A 236, 128 (1985)
[4] K. I. Hahn, et al., Phys. Rev. C 54, 1999 (1996)

4. Refurbishment of the HRIBF Guest House is in Progress

The HRIBF Guest House, a part of the Joint Institute for Heavy Ion Research, is undergoing refurbishment. The interior rooms will be repainted and recarpetted including the bathrooms, which will be completely remodelled. A new television has also been purchased. The kitchen area was renovated in 1995. In addition, the exterior will be repainted and new landscaping will also be done.

The Guest House is a ten-bedroom facility adjacent to the HRIBF, making it very convenient for users during their experiments. The rooms are available free of charge. Users of ongoing experiments have priority in reserving the rooms.

RA1 - RIB Development

Batch Mode Source Development

Off-line development and testing of a batch-mode, Cs-sputter, negative ion source has been successfully completed at the Ion Source Test Facility (ISTF). Long-lived species such as ⁵⁶Ni and ¹⁸F can be produced in a batch mode, where the target is first bombarded with beams from ORIC for a period comparable with their halflife and then mechanically rotated to a Cs-sputter ion source which utilizes a 1-2 keV Cs beam to sputter the negative ions from the irradiated material. The batch-mode ion source features a water-cooled target wheel that can hold eight targets that can be remotely rotated from the production position to the Cs-sputter position. It is designed so that one target can be exposed to the ORIC production beam while another is being used to generate negative ion beams from the Cs-sputter source. In off-line, stable beam tests at the ISTF, operation of the source was optimized, with the result that more than 20 micro-amperes of ⁵⁸Ni negative ions were obtained from a natural Ni target. This corresponds to ion source efficiencies of 4% for generating Ni, in agreement with design estimates. This source will soon be mounted on the RIB Injector Platform for the production of ⁵⁶Ni beams.

UNISOR Facility

We have completed a series of tests at the UNISOR facility on a new target design for the production of ¹⁷F beams. The maximum production beam intensity that a target can handle is determined by the ability of the material to withstand high temperatures and by the ability to remove heat from the production region in the target. Refractory oxides such as zirconia and hafnia work well at temperatures up to 2000 C and 2300 C, respectively, but they are also good thermal insulators and thus do not efficiently conduct the heat away from the center of the target. This characteristic has limited the production beam intensity to 3 micro-amperes on the RIB Injector Platform. The new design utilizes 2-mm-thick disks of these materials which are separated by a few millimeters (4-5) along the ORIC beam axis to allow the central region of the target to radiate heat out to the wall of the target holder, which is typically a few hundred degrees cooler than the central region. The resulting target holder is more than twice as long and has a volume which is ten times greater than the previous designs. The target diameter was also increased in order to contain the scattered production beam. The recent tests determined that this larger target holder and the redesigned target heater are mechanically sound and do not adversely affect the performance of the ion source. We now plan to implement this new target design on the RIB Injector Platform in September. At that time we will be able to determine if this target will remain viable at production beam intensities up to the design goal of 10-15 micro-amperes.

RA2 - Accelerator Systems Status

ORIC Operations and Development

ORIC has continued to provide 44.5 MeV deuterons for ¹⁷F production. However, problems were encountered with the ORIC ion source during the period. Cathode lifetimes in the source typically reach 300 hours, but they had become reduced by as much as a factor of ten. Following extensive diagnosis of the source, it was determined that the cause was a water leak at a stainless steel to tantalum junction on the west high voltage rod. An additional air leak in the west high voltage rod insulator further compounded problem diagnosis. In the process of diagnosis and repair, an improved thermal design of the rod tips and cathode mounts was developed which is anticipated to increase cathode lifetimes beyond 300 hours. The cathode set presently in use already has attained 269 hours to date.

With regard to machine upgrades, the new lower-channel power supply, which will replace the number two motor-generator set and two more new trim coil power supplies, will be installed during the August shutdown. Several control and rf system upgrades are also in progress.

Tandem Operations and Development

The Tandem Accelerator has run smoothly, providing beams of ¹H, ²H, ⁹Be, ¹⁷O, ¹⁷F ²⁸Si, ³²S, ⁴⁰Ca, and ⁵⁸Ni to the RMS, the DRS, UNISOR, beam line 23, and the Enge split-pole spectrometer. During this period, the machine was conditioned for 23 MV operation, but the highest voltage used was 21.2 MV. A large portion of the time was spent accelerating ¹⁷F, much of which was delivered as ¹⁷F⁹⁺. The postaccelerator foil stripper was filled with carbon foils of varying thickness to produce the fully stripped ions. No tank openings were required during this three-month period.

The tank will be opened on August 16 for normal scheduled maintenance and the installation of a recirculating gas stripper. This recirculating gas stripper should significantly decrease maintenance time because the sublimator pump will not be used as often. It should also allow better equilibrium stripping, due to the use of more gas in the stripper with a lighter gas load seen by the acceleration tubes. This maintenance period is scheduled for four weeks.

RIB Injector Operations and Development

Approximately one to three million particles per second of ¹⁷F- were injected into the tandem accelerator over the period from May to July using a negative ion source and HfO₂ target. In late July the source was removed to allow the installation of the batch-mode, Cs-sputter, negative ion source described in RA1. In order to accommodate this new source and provide a universal target ion source enclosure compatible with all HRIBF ion sources, a new remotely handled enclosure was designed by the engineering group. This design reduces cost while increasing mechanical reliability as is required for long-term operation of this facility. Over the summer, the enclosure has been tested at operating temperatures and appears to function well. The receiving component of this enclosure has been permanently installed on the RIB injector, target material test stand, and ion source test facility to allow rapid interchange of target ion sources.

RA3 - Experimental Equipment - the ORNL-MSU-TAMU Barium Fluoride Array

The BaF₂ array is a joint project of the ORNL Physics Division, the Texas A&M University Cyclotron Institute (TAMU) and the Michigan State University National Superconducting Cyclotron Laboratory (NSCL). These laboratories have combined their inventories of BaF₂ detectors, photomultiplier tubes, cables, and electronics to provide a self-contained detection system suitable for measurements of energetic photons from nuclear reactions in coincidence with a variety of external particle detectors, ranging from silicon arrays to magnetic spectrometers. The array is composed of 152 detectors, all of which are right, hexagonal prisms. Most of the detectors are 20 cm long and 6.5 cm from face-to-face coupled to fast, two-inch photomultiplier tubes. The array has sufficient mounting hardware to arrange the detectors into seven blocks of nineteen detectors, "19-packs," suitable for target decay measurements, and into a wall of about 145 detectors, the "forward wall," suitable for projectile decay measurements.

BaF₂ as a detector material offers the advantages that it is very dense (4.89 g/cm³), has a radiation length of 2.05 cm, and has an extremely fast light emission with a 0.7 ns risetime. The material scintillates with a "fast" component at wavelength 220 nm with the 0.7 ns risetime and with a "slow" component at wavelength 310 nm with a 620 ns risetime. It also has a relatively large light output at 5% that of NaI for the 220 nm component and 20% that of NaI for the 310 nm component. A further advantage of the material is that the ratio of the fast to slow components varies with incident particle type (above ratio given for photons) and so can be used for particle identification.

Most experiments done to date involve measurements of photons from the decay of the giant resonances excited by inelastic scattering or by fusion-fission reactions. The array has also been used to look at photons from bound state decays in radioactive beam reactions. There have been proposals to use the array to investigate (p,gamma) reactions of astrophysical interest. The experiments have been performed at the NSCL, the State University of New York at Stony Brook, and TAMU.

This array is not part of the HRIBF user apparatus nor of any particular accelerator facility. Use of the array is by arrangement with the current collaborators. To discuss use of the array, contact Robert Varner or Jim Beene of the ORNL Physics Division.

RA4 - Upcoming Users Executive Committee Election and Annual Users Meeting at the DNP

The annual Users Executive Committee Election will be held in September and October of this year, with the results announced at the annual HRIBF Users Meeting which will be held during the DNP meeting in Asilomar on October 21-23. A nominating committee has been formed and is chaired by Rick Casten. This committee has selected four members who will stand for election. The two successful candidates will replace Noemie Koller and Michael Smith, and will serve for three years. This year's nominees are Dick Boyd (Ohio State), Peter Parker (Yale), Lee Riedinger (Tennessee), and Krzysztof Rykaczewski (ORNL).

Participation in last year's election was somewhat disappointing despite the convenience of using email to cast the ballots. We encourage you to respond quickly when you receive your ballot. Ballots will be mailed electronically to those on our membership roles as of September 1, providing three weeks to respond. Receipt of this newsletter does not necessarily mean that you are a member of the users group. If you are unsure of the status of your membership, please contact Ms. Franda Ervin at ervin@mail.phy.ornl.gov or register on-line at http://www.phy.ornl.gov/hribf/usersgroup/registration.html. Membership is open to all persons, including non-experimentalists, interested in radioactive ion beam physics.

Details concerning the annual Users Meeting will be sent out along with the election ballots at the end of September.

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

May 1, 1999 - July 31, 1999

Experiment	Spokesperson/Institution	Dates
RIB-018 - Measure the p(17F,17F)p Reaction at the Holifield Radioactive Ion Beam Facility	Bardayan/Yale University	5/3-8/99
RIB-031 - States in 18Ne Populated by Resonance Scattering of 18F on 1H Using Thick Targets	Galindo-Uribarri/ORNL	5/9-14/99 7/21-23/99
RIB-019 - Determine the 14O(a,p)17F Reaction Rate	Blackmon/ORNL	5/17-20/99 5/22-23/99
RIB-041 - Double Strand Breaks in DNA	Datz/ORNL	5/21/99
RIB-000 - Commissioning of the RMS	Gross/ORISE	5/26/99 6/28/99
RIB-034 - Search for Proton Emission from 149Lu	Batchelder/ORISE	5/27-28/99 6/1-3/99
RIB-014 - Target-Ion-Source Development - Arsenic and Fluorine	Stracener/ORNL	6/10/99 7/13/99 7/28/99
RIB-040 - Beam Diagnostics Development	Shapira/ORNL	6/18/99
RIB-017 - 208Pb Breakup Fusion at Near Barrier Energies	Liang/ORISE	6/21-27/99 7/7-9/99 7/14-20/99
RIB-053 - Search for Neutron Single-particle States in 149Yb and 145Er via Proton Radioactivity Studies	Ginter/Vanderbilt University	6/29-7/2/99 7/26-27/99
RIB-050 - Selective Study of Excited States of N=Z Nucleus 66As Using Decay Tagging Technique	Grzywacz/University of Tennessee	7/29-30/99

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For this quarter's schedule go to http://www.phy.ornl.gov/hribf/users/07-09-1999.html.