HRIBF Newsletter, Edition 15, No. 2, Aug. 2007
1. HRIBF Update and Near-Term
As we enter the last quarter of FY2007, we can look back on nine months of excellent scientific output and solid facility operations. Our operations statistics are especially gratifying because of funding uncertainties that extended through half the fiscal year, and the continued fallout from the very austere FY2006 budget (see the February 2007 Newsletter), as well as several technical problems that had to be overcome. We are told a Continuing Resolution (CR) extending up to about four months is again likely in FY2008, but provided budgets close to the FY2008 President's Recommendation are eventually passed, the fiscal condition of HRIBF and the HRIBF research program will be better than it has been in several years.
The period covered by this Newsletter (January 1 to June 30, 2007) began with the facility in a scheduled shutdown for tandem maintenance, which continued through March. During the tandem maintenance period, ORIC beam was delivered to the HPTL in February for ISOL R&D. The shutdown was extended into March due to uncertainty arising from the extended Continuing Resolution for FY2007. Beginning in April, we initiated stable beam operation in support of the RIB program, and then after completing replacement of a failed ORIC PA tube (see Accelerator Systems article) began a neutron-rich RIB campaign which concentrated on central-collision (fusion and fission) experiments led by ORNL and Oregon State groups, along with a magnetic moment measurement led by the Rutgers group. During FY2007 up to June 30, we delivered 1595 hours of RIB for research (including 1317 ISOL hours and 278 in-flight RIB hours) out of 2337 total research hours. We plan to continue the current neutron-rich RIB campaign to the end of Fiscal 2007, though an interruption in the RIB program is possible in July and/or August due to a budgetary shortfall. Such an interruption could result in extension of the neutron-rich campaign past October 1. Following the current neutron-rich campaign, a radioactive fluorine campaign is planned which will continue to the next tandem maintenance period, now scheduled from January 2008 through early March. ISOL R&D on HPTL is planned during the maintenance period. In March 2008, we plan campaign of radioactive beryllium beams, followed in April by another neutron-rich RIB campaign.
Several other events of note occurred during the period covered by this report. The IRIS2 project remains on schedule and under budget and is proceeding very well. A visit by a team of scientists from GANIL in March culminated in the signing of a memorandum of understanding (MoU) between SPIRAL and HRIBF. On May 17, members of the research staff of the HRIBF made presentations to the 2007 DOE Review of the Low-Energy Research Program. On June 11 and 12, 2007, DOE Nuclear Physics held a Scientific and Technical (S&T) Review of the HRIBF facility. The HRIBF-SPIRAL MoU and the HRIBF S&T review are discussed briefly in subsequent articles. No report on the low energy program review is yet available.
2. Recent HRIBF Research - Magnetic Moment Measurements of Short-Lived Excited States in
a Radioactive Environment: 132Te
The magnetic moment of a nuclear state is a property that reflects directly the microscopic nature of the state and provides stringent tests of theoretical models. Magnetic moments of short-lived excited states have been measured for many years by a technique which couples Coulomb excitation with the transient field experienced by fast ions traversing ferromagnetic materials.
Recent developments of the experimental procedure involve measurements on beams rather than on targets of the material of interest. This particular approach lends itself well to the use of radioactive beams and opens a large variety of isotopes for investigation. The general details of the method are described in a recent review paper .
The first magnetic moment with a Coulomb-excited radioactive beam utilizing the transient field method (TF) has been measured at Berkeley. The g factor of the 2+1 state of 76Kr was determined . The radioactive beam was produced by the "recyclotron" method. This technique, in a two-step process, involves the production of the isotope of interest by the cyclotron, followed by extraction and re-injection of the isotope into the same cyclotron for acceleration.
However, most of the nuclei that are amenable for study will become available as primary radioactive beams at facilities such as HRIBF and NSCL today, and European, Japanese facilities as well as FRIB in the US in the future.
The first isotope studied at ORNL has been 132Te. The g factor of the 2+1 state has been determined by the Recoil-in-Vacuum (RIV) technique which yields the magnitude of g factor but not its sign. The results have been described in Ref. . The same state has been proposed for TF experiments in order to determine the requirements and limitations of this approach to the radioactive-beam environment.
Transient field experiments with radioactive beams necessitate changes to the conventional setup, where the beam typically is stopped in the target and forward-scattered light target nuclei are recorded in a particle detector placed downstream in the beam path. Special care has to be taken so that little radioactive material will be stopped in the target chamber or beam pipes nearby, where its decay radiation can be seen by the detectors. Fig. 2-1 shows the schematic setup for this experiment. The total target thickness was limited to less than 6 mg/cm2 to avoid excessive beam spread. The target was designed to let the beam and Coulomb-scattered projectiles exit from the target and mostly pass through a gap in the particle detector to be stopped down stream in a distant beam dump. A 9.15 mg/cm2 stopper foil of Cu in front of the particle detector let only the forward-scattered light target nuclei reach the detector.
Nonetheless, over time a fair amount of radioactivity accumulates in the target and stopper foil. The stopper foil could be exchanged in principle over the course of an experiment, if necessary several times, depending on the lifetimes of the radioactivities involved. In this experiment the "background" radiation in each Clover detector segment was kept below 1 kHz.
The 132Te beam contains about 10-15% 132Sb (T1/2 = 2.79 min). The Sb decays by β emission into 132Te and populates the 2+1 state of interest. This is a potential problem, since the radioactive decay is much stronger than the Coulomb excitation. But, since the Coulomb-excited projectiles exit from the target and decay in flight, their γ energies are Doppler shifted. The radioactive decay, on the other hand, originates from stopped nuclei and occurs only randomly. As Fig. 2-2 shows, both components are well separated in the gamma spectra and the random-subtraction yields clean coincidence spectra. The removal of the random and stopped components in the peak integration reduces the errors greatly.
The decay-in-flight of the projectiles recoiling into vacuum has a negative side effect. The angular distribution of the decaying &gamma-rays is attenuated due to the hyperfine interaction of the ions recoiling into vacuum. The attenuation leads to a reduced logarithmic slope of up to 30%, compared to that measured for 130Te with a Cu backed target.
Three experiments have been carried out on 132Te at ORNL. The initial run exposed the difficulties of working in a radioactive environment with a beam contaminated by isobars. The target of 10 mg/cm2 Gd proved to be too thick and the accumulated radioactivity near the particle detector and at the exit pipe of the target chamber quickly overwhelmed the γ detectors. A test run with a stable 130Te beam also revealed problems with heat dissipation of the target beam spot. A new, larger chamber was built and a thinner target was prepared. In a second attempt, radioactive 132Te beam was not available, so the new setup was tested with stable 130Te with a beam intensity (3X107 p/s) corresponding to that expected for the radioactive beam to assure thermal stability of the target under beam.
Finally, the radiaoactive beam experiment was carried out, albeit with a 132Te beam weaker than expected. Two different targets were used: 1.3 mg/c2 C on 4.9 mg/cm2 Gd backed by 0.8 mg/cm2 of Cu (added solely to improve thermal conductivity) and 1 mg/cm2 C on 4.4 mg/cm2 Fe. Both targets were cooled to 77 K with liquid nitrogen. The coincidence rate was at best 1 every 18 seconds per Clover detector for a total of about 700 counts per Clover and field direction. The data are still being analyzed and the preliminary result is in agreement with Ref. . An important part of this experiment is the determination of the sign of the g factor, which is as expected positive. The success of this experiment provides a proof of principle and is a guide for future measurements. In a radioactive environment, every isotope carries new challenges and a novel approach may have to be developed for each one.
 N. Benczer-Koller and G. Kumbartzki, J. Phys. G: Nucl. Part. Phys.
34, R321 (2007).
3. Recent HRIBF Research - Measurement
of the 134Te(d,p) Reaction for the Study of the Single-Particle
Structure of 135Te
The study of the single-particle structure of neutron-rich nuclei adjacent to shell closures yields important information for understanding the evolution of nuclear structure away from stability. Furthermore, the properties of neutron-rich nuclei near shell closures are significant to the understanding of neutron-capture processes that are responsible for the formation of the heavy elements. The low-level densities and low neutron-separation energies in such nuclei result in low neutron-capture rates, with significant components from direct capture into low lying states and isolated resonances. The properties of these single-particle states are crucial for determining direct capture rates. Transfer reactions are well established as a powerful spectroscopic tool for studying the single-particle structure of nuclei, and the (d,p) reaction preferentially populates the low-spin single-particle levels that are important for neutron capture.
The neutron-rich nucleus 135Te (Z=52, N=83) is one neutron beyond the N=82 closed shell, and two protons beyond the Z=50 shell closure; consequently, the properties of many low-lying states are predominantly of single-neutron structure. Its proximity to these shell closures makes it of particular importance to nuclear structure and astrophysics. Astrophysically, the structure of low-lying levels in 135Te is of significance to the isotopic abundances of xenon measured in pre-solar diamond grains, which indicate an overabundance of light (124,126Xe) and heavy (134,136Xe) isotopes compared to intermediate mass Xe isotopes, relative to the solar abundances. These anomalous isotopic ratios cannot be explained satisfactorily by addition of the p and r process xenon abundances, as observed in the solar system, as the relative excesses of 134Xe and 136Xe do not correspond to the isotopic ratios of the average r process .
Proposed explanations for this heavy isotope anomaly include the rapid separation of xenon from its precursors (iodine and tellurium) in supernova ejecta [1, 2], processes in which the rate of neutron capture is somewhere between the s and r processes, or a low-entropy r process . The relative abundances of 134Te and 136Te in such an astrophysical environment has a direct bearing on the final abundances of the β-decay daughters 134Xe and 136Xe. Therefore, knowledge of the properties of the low-lying neutron states with low ℓ in 135Te will add constraint to the range of conditions under which such isotopic ratios of xenon can be formed.
We have measured the 134Te(d,p)135Te reaction in inverse kinematics, utilizing a beam of 134Te incident at 643 MeV on a 100 μg/cm2 CD2 target (effective thickness due to rotation). The proton ejectiles were measured in an array of position-sensitive silicon detector telescopes, comprised of an early implementation of the Oak Ridge Rutgers University Barrel Array (ORRUBA) augmented by the Silicon Detector Array (SIDAR). The setup covered the angular ranges 55° to 125° with ORRUBA detectors, and 145° to 165° with SIDAR detectors. The ORRUBA detectors subtending angles forward of 90° were set up as charged particle telescopes, to enable the separation of proton ejectiles from elastically scattered deuterons and carbon nuclei from the target.
A preliminary plot of proton energy vs angle, for one strip of a single forward-angle ORRUBA telescope, is shown in Fig. 3-1, gated on protons from the ΔE-E measurement. Loci can be seen corresponding to transfer to the ground state and a number of excited states in 135Te, along with the locus from protons scattered elastically from the target.
Figure 3-2 shows a Q-value spectrum for the data shown in Figure 3-1. The presence of a strongly populated state near 1.8 MeV, along with the relative strength of the peak at about 1 MeV (relative to the ground and first-excited state peaks) is suggestive that the majority of the f5/2 strength lies at 1.8 MeV, rather than at about 1 MeV, as previously supposed . Further analysis will yield angular distributions, enabling the determination of spin assignments and the extraction of spectroscopic factors for the levels populated.
 Ulrich Ott, Astrophys. J.
463, 344 (1996).
4. What's New at HRIBF - Update on Injector for Radioactive Ion Species 2 (IRIS2)
The IRIS2 Project remains on schedule and budget. The major effort in the facility modifications portion of the project during the reporting period was installation of the Target Room HVAC system (Fig.4-1), which was successfully completed in April. Support structure and the localized shield around the target ion source has been fully installed (Figs. 4-2 and 4-3). Long lead-time technical equipment components are beginning to arrive. National Electrostatics Corporation (NEC) delivered the injector beamline platform in June (Fig. 4-2), and the instrumentation platform (Fig. 4-4) was delivered in August. Field mapping of the first stage mass separator magnets (Fig. 4-5) was nearing completion at the end of June, and they are expected to arrive at ORNL by late August. Fabrication and testing of the four high voltage conduits was extremely successful, and they will also be installed in August. By the end of September, we expect to have both of the platform structures, the high voltage conduits, and the injector beamline dipole magnets installed.
5. FY2007 S&T Review
A Science and Technology (S&T) Review of the HRIBF and its scientific program was held on June 11 and 12, 2007. Since the scientific program at HRIBF had been examined as part of the extensive national review of the Low-Energy Nuclear Physics Program in May 2007, the S&T Review concentrated on HRIBF operations. We expect a formal report on the S&T Review (and the Low-Energy Program Review) within a few weeks. At present we have only a set of bullets presented at closeout at the end of the review. It is inappropriate to discuss this unofficial feedback in any detail, but both facility and Physics Division management clearly see the results of the review as very favorable, based on this closeout document.
We must respond in writing to all Recommendations produced by the Review Committee. The "closeout bullets" presented only two recommendations: one deals with providing details of our plans for improving ORIC, and a second is concerned with development of performance measures for projected RIB campaigns. These will be discussed further when the formal report is received.
6. Message from the New Chairman of the Users Executive Committee
The last time that I served as chair of the UEC was 1995-96 - before operation of the HRIBF started. It was a rather abstract experience. Now the science at the HRIBF has reached the stage where it's necessary to think about how the facility should evolve. This comes at a time when budget constraints have had a real impact on productivity throughout the community. I look forward to working with the HRIBF to address both the concerns of the present and plans for the future.
A number of HRIBF users have expressed concerns about the reliability of the run schedule at the HRIBF, which is shared by the facility management. This issue was examined by the UEC. We considered the current situation at Oak Ridge in comparison with our experience at other facilities, drafted a number of recommendations, and discussions were held with the HRIBF management. There is general agreement that budget compression has taken a toll, for example, it is not possible to maintain a 24/7 repair staff and only selected spare parts are available. Nonetheless, it was also agreed that steps could be taken within the current budget reality. One improvement that will be implemented shortly is the creation of a "RIB Specialist" position. This person will have the overall responsibility for the delivery of a beam to an experiment and will also serve as the facility contact person before and during an experiment. There have also been discussions related to operational efficiency. The general state of the budget will continue to be an impediment. However, we feel that the current situation will be improved in the near future.
As many of you know, there have been news rules and restrictions regarding computer usage. Robert Varner reports that the impact of the new DOE rules and the various laboratories' interpretations and adaptations are in a state of flux and their impact to the facilities, including HRIBF, is not yet clear. There is considerable uncertainty over how the research needs of the users can be accommodated, but it is clear that we will have to live with this new reality.
Finally, planning is underway for a workshop on physics with
the proposed electron driver. Stay tuned for details.
7. HRIBF-GANIL Memorandum of Understanding
On March 9, 2007, a group of scientists from GANIL visited ORNL to discuss possible collaborative and synergistic efforts related to radioactive isotope beam (RIB) production and research with RIBs. In addition to the discussions at HRIBF, various topics related to high-power linear accelerators and handling high-power beams were discussed with SNS staff. The team consisted of Sydney Gales, Director of GANIL; Marcel Jacquemet, Deputy Director and Project Leader for SPIRAL2; and Marek Lewitowicz, Scientific Coordinator of SPIRAL2.
The MoU signed at the end of the visit (see above photo) outlines a broad program of common interest in target development, ion source development, beam purification techniques, remote handling, research equipment, and possible synergies in development of SPIRAL2 and the as yet unfunded electron driver upgrade at HRIBF. Collaborative agreements such as this one can be of substantial value to HRIBF, providing us early access to developments elsewhere and helping us achieve faster implementation of our own innovative ideas.
8. eRIBs '07 to be Held in Newport News on Oct. 10, 2007
You are cordially invited to attend the International Workshop eRIBs07 "Electron Drivers for Radioactive Ion Beams" to be held on October 10th, 2007 at the Marriott Hotel , Newport News, VA, USA. This workshop explores the scientific and technological issues associated with the production of radioactive ion beams by photofission of actinides. The workshop is organized by users of the ORNL and TRIUMF RIB facilities. It will be held during the 2007 DNP Fall meeting of APS.
An agenda will be posted on the workshop website
closer to the meeting. We anticipate that the workshop
will begin around 9:00 am and conclude around 5:00 pm.
RA1. RIB Development
In the last few months, a number of on-line and off-line tests have been performed at the On-Line Test Facility (OLTF) and at the High Power Target Laboratory (HPTL). In addition, a new target and ion source (TIS) assembly station is near completion.
The current assembly station has the capability for assembling and testing TIS modules for the IRIS-1 (Injector for Radioactive Ion Species 1) production platform. At the new assembly station we will be able to fully assemble and test TIS assemblies for both RIB production platforms. The TIS module tests include alignment of the utility connections (cooling water, electrical, vacuum, thermocouples, pressure gauges, and gas feeds), check for vacuum and water leaks, alignment of the RIB production target, check of the electrical connections in the ion source, and a high temperature test of all systems. The high temperature test also helps to outgas the target and ion source before it is moved to the production platform.
At the OLTF, targets are tested using low-intensity beams from the tandem accelerator. Proton and deuteron beam intensity is limited to less than 50 nA with energies up 40 MeV. A uranium boride target was irradiated with 40-MeV protons to look for the release of zirconium isotopes as molecular boride ions. After several on-line tests and an irradiation and release measurement, it appears that zirconium has very low release efficiency from uranium carbide or uranium boride targets. Andreas Kronenberg (ORAU) has been the principal investigator for this series of tests. In collaboration with Will Talbert (TechSource), Jerry Nolen (ANL), and John Greene (ANL), we recently tested a very high density (10.5 g/cm3) uranium carbide target. Using 40 MeV protons to induce fission, we measured the beam yields for a large number of fission fragments. The preliminary analysis indicates lower release efficiencies than are observed for lower density targets. The details of this analysis will be presented later. Experiments can also be made using heavy-ion beams from the tandem. A recent experiment to investigate targets for the production of beams in the region of Ag-97 used a Ni-58 beam on a Cr target. Chris Goodin (Vanderbilt) is the principal investigator for these experiments.
At the HPTL, targets are tested using light-ion beams from the cyclotron. Using 54-MeV and 42-MeV proton beams with intensities up to 15 μA, we have measured the yields of Al-25 and Al-26m beams from various silicon carbide (SiC) targets. A series of tests using SiC fiber targets (15 micron diameter fibers) have shown limited release of short-lived aluminum isotopes and the targets suffer significant damage when the production beam current exceeds 7 μA. In collaboration with Alberto Andrighetto and coworkers from Legnaro, we recently tested a SiC target design that allows higher production beam currents. The SiC disks are about 1.2 mm thick and separated by about 4 mm to allow for faster heat dissipation via radiation to the walls of the target chamber. So far there is no indication of target damage up to 12 μA and the normalized yields for aluminum isotopes are comparable to those observed with the SiC fiber targets. The Legnaro group has developed a detailed model to simulate temperatures in the target and these simulations agree quite well with the observed target temperatures. The HPTL is presently off-line to allow for the installation of the IRIS-2 platform and high voltage conduits. We plan to resume the on-line development of SiC targets later this year.
RA2. Accelerator System Status
ORIC Operations and Development (B. A. Tatum)
RIB Injector Operations and Development (P.E. Mueller)
Tandem Operations and Development (M. Meigs)
RA3. User Group News
The HRIBF will hold its annual Users Group Meeting at this year's DNP meeting in Newport News, VA on Thursday evening, October 11. As has become tradition, this meeting will be held jointly with the ATLAS, NSCL, GAMMASPHERE/GRETINA, and RIA Users groups. More details will be provided in future emails.
The next Users Group Workshop will be on the physics to be done with beams produced by photofission. HRIBF hopes to submit a proposal to DOE at the end of the calendar year to build a 50 kW electron accelerator capable of inducing 1013 fissions per second. Current HRIBF neutron-rich beams are produced with 3x1011 fissions per second. In addition, the photofission process results in more neutron-rich isotopes such as 134Sn. Additional information including projected beam intensities are given on our electron driver website.
Dates for the workshop have yet to be determined.
RA4. 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 firstname.lastname@example.org. 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.
RA5. HRIBF Experiments, January through June 2007