HRIBF NEWS

Edition 10, No. 2 Spring Quarter 2002 Price: FREE

Feature Articles Regular Articles

Editors: C. J. Gross, W. Nazarewicz, and C.-H. Yu

Feature contributors: D. Bardayan, J. R. Beene, C. J. Gross, B. A. Tatum
Regular contributors: M. R. Lay, M. J. Meigs, P. E. Mueller, D. W. Stracener, B. A. Tatum, P. Hausladen

1. HRIBF Update and Near-Term Schedule

Our recent Fluorine RIB campaign has been plagued by a series of problems which have prevented its completion and will most likely delay the planned neutron-rich campaign. Over the past three months we have had two Kinetic Ejection Negative Ion Sources (KENIS) fail prematurely and a water leak in the coaxial extraction section of ORIC (See related article RA2). While the water leak could be repaired in a straightforward manner, the ion sources require time for the radioactivity to decay before autopsies can be done. These autopsies are now in progress, and initial indications are that the underlying causes of the failures have been identified. If our current understanding proves to be correct, the modifications required to correct the problems are straightforward, and can be implemented quickly.

Presently, we have a short tank opening to replace bearings and prepare for the neutron-rich RIB campaign originally planned for June-July. However, if we are confident that the KENIS failure mode has been corrected, we will continue the Fluorine RIB campaign and move the neutron-rich runs to July-August. There will be a break for the RIA Summer School when the students will be doing hands-on activities with stable tandem beams.

Return to Contents

2. PAC-8 Proposals due July 8; PAC-8 will meet August 19-20, 2002

HRIBF is pleased to issue a Call for Proposals for consideration by the next Program Advisory Committee (PAC-8) meeting. This Call is for any experiment using all available radioactive and stable beams. The deadline for the receipt of the proposals is July 8, 2002. Proposals are only accepted electronically and may be submitted by email attachment to liaison@mail.phy.ornl.gov. Specific instructions may be found on our website. Information on HRIBF equipment may be found on our website which contains specifics about each device and past newsletters. Alternatively, you may contact the liaison officer at the address above. The PAC will meet in Oak Ridge on August 19-20, 2002.

We have two new members on our PAC: Robert Janssens (Argonne National Laboratory) and Walter Loveland (Oregon State University). They replace Noemie Koller (Rutgers University) and Jim Kolata (Notre Dame University) whom we thank for their service to the facility and the RIB community.

Present PAC Membership

J. Aysto CERN & University of Jyvaskyla
C. Baktash Oak Ridge National Laboratory
J. C. Hardy (chair) Texas A&M University
R. V. F. Janssens Argonne National Laboratory
W. Loveland Oregon State University
I. Y. Lee Lawrence Berkeley National Laboratory
M. Wiescher Notre Dame University

Return to Contents

3. RIA Summer School to be held at ORNL August 12-17, 2002

We are pleased to announce that the first annual RIA Summer School on Exotic Beam Physics will be held at Oak Ridge National Laboratory from August 12-17, 2002, at the Holifield Radioactive Ion Beam Facility (HRIBF) on the campus of Oak Ridge National Laboratory, Oak Ridge, Tennessee. The aim of the summer school is to nurture future RIA scientists so that the community will have sufficient manpower to effectively use RIA when it comes online. The RIA summer school is jointly organized by the 88-Inch Cyclotron, ATLAS, HRIBF, and NSCL, and will be an annual event, rotating among these laboratories. The summer school will be divided equally between lectures on RIA science in the mornings and "hands-on" experimental demonstrations in the afternoons. The topics of the lectures are given below.

The application deadline has passed, and we have had an outstanding response to our announcement of the summer school. Over 70 applications were received from students and postdocs representing 33 institutions in 11 countries. The enrollment was increased ~10% to accommodate the demand. We are very pleased with the level of interest and expect this enthusiasm to translate into a successful summer school. For further information, please visit our RIA Summer School web site at http://www.orau.gov/ria/.

Return to Contents

4. ORIC celebrates 40 years of service

ORIC attained a significant milestone on March 18, 2002, which marked the 40th anniversary of the first circulated beam. On March 17, 1962, the ORIC logbook indicated a cautious optimism that something important was about to happen: Good Operation. Looks very hopeful. Indeed, on March 18, 1962, the first ion beam was circulated in ORIC, approximately 3 years after groundbreaking for building 6000. Specifically the log records that a beam of 8-MeV protons, 65 uA reached extraction radius.

A celebration was held at ORNL to commemorate the occasion and nearly 100 current and former staff members attended. The celebration was dedicated to the memory of Dr. Robert S. (Bob) Livingston who, as Director of the ORNL Electronuclear Division, worked diligently to obtain funding and assure completion of ORIC. Dr. Livingston regrettably passed away just two weeks prior to the celebration. The program included remarks by former ORNL Director Dr. Alvin Weinberg, former Physics Division Director Dr. Jim Ball, and several of the literally hundreds of support personnel, crafts, operators, engineers, and physicists who made the day possible through 40 years of dedicated service to the physics community.

Return to Contents

RA1 - RIB Development

The uranium carbide targets used at HRIBF to produce the neutron-rich nuclei consist of a thin layer of UC2 deposited onto a rugged carbon matrix. This porous, low-density substrate is made of reticulated vitreous carbon (RVC) fibers and has the feature that the pores throughout the matrix are contiguous. The target disks are made by saturating the matrix with a solution of uranyl nitrate and then heating the sample up to 400 C to convert the uranyl nitrate to uranium oxide. This step is repeated several times to build up the desired amount of uranium in the target and then the sample is heated to 2000 C to convert the uranium oxide to uranium carbide.

The manufacturer of our original UC targets is no longer able to make these for us, so we initiated a collaboration with a group from the Metals and Ceramics Division at ORNL to manufacture our UC targets in-house. The initial batch of these targets has recently been tested at the UNISOR facility. The finished UC/RVC disks have diameters of 1.5 cm and are 0.2 cm thick, which are the same dimensions as the initial RVC substrates, so no shrinkage occurred during the heating cycles. The density of the newly delivered samples (0.8g/cm3) was lower than that of the original samples (1.2 g/cm3), so the number of disks required to make a RIB production target will increase to 13 compared to the 9 presently used.

Off-line tests involved heating the targets up to 2000 C in a target holder that was coupled to an electron-beam-plasma (EBP) ion source. If the conversion of uranium oxide to uranium carbide is incomplete, oxygen and carbon monoxide will be present at levels that can adversely affect the efficiency of the ion source. Fortunately low levels of O and CO ions were observed and the targets were relatively free of any volatile contaminants.

The yields of several radioactive ions were measured using a low intensity proton beam from the tandem to irradiate the targets. The extracted radioactive ion beam intensities were comparable (+/- 15%) to those previously measured. There were no immediately obvious trends in the yield differences based on radioactive half-life or chemical species, but the analysis is continuing. Based on these results, work on the remainder of the targets has begun. We plan a series of tests varying the uranium content and the thickness of the UC layer to determine the optimum target configuration for various species.

We have also made further tests of the technique for selecting specific beams by adding sulfur vapor to the system (via H2S gas through a variable-rate leak valve) and extracting the desired nucleus as a sulfide molecular ion. This technique has been previously reported for Sn beams. We now have shown that it also works for Ge beams. The yield of the radioactive GeS beam is about 1.8 times higher than the yield of the atomic Ge beam, while the SnS beam intensity is about equal to the Sn beam intensity. In both cases, the neighboring elements are suppressed by at least a factor of 1000 and, in most cases, the extracted beam intensity of these 'contaminant' beams is well below our detection threshold.

Return to Contents

RA2 - Accelerator Systems Status

ORIC Operations and Development
At the beginning of the reporting period, Dr. Merrit Mallory was again consulting with the HRIBF operations staff regarding improvements in beam energies and extraction efficiencies. He was successful in demonstrating that the proton energy could be increased from the nominal 42-MeV level to the 44-48-MeV range through a tedious and systematic tuning process. Development time was limited by the experiment schedule, but this process will be resumed in the near future due to the potential for increasing neutron-rich beam intensities at the higher energies.

ORIC was then utilized to deliver 2-3 uA of 40-MeV deuterons to the KENIS on the RIB injector platform for the production of 17F during the latter portion of February and early March. In mid-March, the ORIC coaxial magnetic extraction channel developed an internal water leak on the insert, and the operation cycle was interrupted for several weeks for diagnosis and repair of the highly activated channel. Following the repairs, ORIC operation resumed with the delivery of 44-MeV deuterons to the RIB injector for 17F production.

Tandem Operations and Development
The Tandem Accelerator has operated more than 870 hours since the last report with about 240 hours being 17F. The machine ran at terminal potentials of 4.10 to 24.4 MV and 124Sn, 124Te, 58Ni, 32S, 17O, 19F, 116Cd, and 1H were also provided. About 100 hours were used to condition the machine, which should allow operation up to 24.5 MV. The ticking problem, that had been going on for some time, appears to be going away as the tick frequency is lower and the voltagethreshold of onset is higher. The tank was opened one time during this period because the gas stripper valve failed open. Several shifts during this period were spent developing 102Ru and 116Cd beams from the stable source. 116Cd looks very promising but 102Ru still needs some work.

RIB Injector Operations and Development
During this reporting period, we delivered pure beams of Unfortunately, we have been unable to complete the Fluorine RIB campaign due to a series of problems with the two Kinetic Ejection Negative Ion Sources (KENIS) that were used to produce these beams. Electrical shorts on the cone and the grid, shown in Fig. RA2-1, prevented the formation of ions. Such shorts can develop when Cs vapor condenses on electrical insulators resulting in a conducting film which bypasses the insulators. This is often corrected by increasing the temperature of the affected area and driving the Cs off of the insulators. It is common to do this every few days. However, this time the shorts were more serious and could not be corrected. Each of the sources were able to produce beams for only about 7 days.

Fig. RA2-1 - A drawing to scale of the KENIS showing the positions of the grid and cone which had shorted.

We will perform autopsies on these ion sources to determine the causes of the failures. In addition, the KENIS from last fall will also be examined; it had a very successful run of several hundred hours and successfully completed its expected campaign. We will also reexamine our procedures for ion source assembly.

Return to Contents

RA3 - Experimental Equipment - Bragg Detector at the RMS

A new diagnostic station has been commissioned at the zero degree viewport of the RMS first dipole. This viewport directly views the CLARION target position, and in the present implementation, the diagnostic station is equipped with an axial ion chamber, or Bragg-Curve Spectrometer for the purpose of characterizing the isobaric content of RIBs. A photo of the station with the Bragg-Curve Spectrometer installed can be seen in fig. RA3-1.

Fig. RA3-1 - Photograph of the Bragg-Curve Spectrometer as installed behind D1 of the RMS.

Initial testing of the detector was performed with 3-MeV/A 124Sn and 124Te beams. Transmission to the detector and isobaric resolution were characterized in the absence and presence of a 1 mg/cm2 C target at the CLARION target position. Transmission to the detector with the present 1-inch diameter window was 100% without the target and 30% with the target. Isobaric resolution at 3 MeV/A was determined to be roughly one FWHM per Z at Z=50, and the performance with the target present was consistent with the energy loss of the beam in the target. A plot of E vs. dE for a mixed beam of 124Sn and 124Te with the target present can be seen in fig. RA3-2.

Fig. RA3-2 - A spectrum of the energy loss versus total energy of a beam "cocktail" comprised of Sn and Te isotopes with A=124.

The portion of the diagnostic station where the Bragg-Curve spectrometer is mounted can be easily removed to accommodate other detectors as dictated by experimental needs. A scale drawing of the station with the Bragg-Curve Spectrometer removed can be viewed in fig. RA3-3.

Fig. RA3-3 - Scale drawing of station.

Return to Contents

RA4 - User Group News - Transfer Workshop

The HRIBF Users group is sponsoring a workshop on transfer experiments with RIBs at the HRIBF. The goal of the workshop is to bring together users who are interested in utilizing transfer reactions with RIBs in inverse kinematics with the equipment at the HRIBF. The HRIBF is well equipped to address low-energy transfer reaction studies with three magnetic and/or electrostatic separators, a large collection of silicon strip detectors, as well as other charged particle and heavy ion detectors.

The workshop is chaired by Jolie Cizewski (Rutgers University, cizewski@physics.rutgers.edu) and Ray Kozub (Tennessee Technological University, rkozub@tntech.edu) and locally organized by Jeff Blackmon blackmon@mail.phy.ornl.gov and Felix Liang liang@mail.phy.ornl.gov.

More information, including the tentative program and registration form, may be found at http://www.phy.ornl.gov/workshops/transfer/.

Return to Contents

RA5 - Suggestions Welcome for New Beam Development

HRIBF welcomes suggestions for future radioactive beam development. Such suggestions may take the form of a Letter of Intent or an e-mail to the Liaison Officer at liaison@mail.phy.ornl.gov. In any case, a brief description of the physics to be addressed with the proposed beam should be included. Of course, any ideas on specific target material, production rates, and/or the chemistry involved are also welcome but not necessary. In many cases, we should have some idea of the scope of the problems involved.

Beam suggestions should be within the relevant facility parameters/capabilities listed below.

Return to Contents

Return to Contents

RA-6. HRIBF Experiments, February - April 2002

Schedules may be found by choosing this link.

Return to Contents

You may contact us at the addresses below.
Witek Nazarewicz Carl J. Gross Chang-Hong Yu
Deputy Director for Science Scientific Liaison Newsletter Editor
Mail Stop 6368 Mail Stop 6371 Mail Stop 6371
witek@mail.phy.ornl.gov cgross@mail.phy.ornl.gov chy@mail.phy.ornl.gov
+1-865-574-4580 +1-865-576-7698 +1-865-574-4493
Holifield Radioactive Ion Beam Facility
Oak Ridge National Laboratory
Oak Ridge, Tennessee 37831 USA
Telephone: +1-865-574-4113
Facsimile: +1-865-574-1268