HRIBF Newsletter, Edition 18, No. 2, August 2010
1. HRIBF Update and
The funding outlook for FY 2011 is much more uncertain than it appeared when the last edition of this newsletter was drafted. The outlook for FY 2012 appears even more uncertain. However, we have had a period of strongly enhanced funding since the extended FY 2009 continuing resolution ended. This has enabled us to begin our long-planned staffing enhancement (see my update article in the February 2010 Newsletter) and to enhance our spare parts and maintenance materials inventory after several years of operation with badly depleted inventories. This should substantially improve our ability to respond rapidly to equipment failures. In addition, the IRIS2 project is now complete and awaiting ORIC beam for commissioning. The enhanced facility efficiency due to the availability of a second RIB production station, plus the new beam production and purification capabilities that IRIS2 will enable us to deploy, will have a dramatic impact on HRIBF operations.
We have been unable to operate ORIC reliably since the nine-month enforced shutdown that followed the July 2008 Operational Emergency. Consequently, we have been unable to launch an extended neutron-rich RIB campaign. In February 2010, one of the ten ORIC trimming coils failed. Previous trimming coil failures occurred in 1986 and 1997. The most recent failure left us with only seven operational coils (see ORIC Operations and Development for details). We were unable to operate ORIC successfully in this configuration, so the decision was made to install the replacement set of trimming coils fabricated after the second coil failure in 1997. We expect this installation to be complete in early November, 2010.
While the work on ORIC was ongoing, we provided radioactive beams of four long-lived species (7Be, 10Be, 26Al and 82Sr) to user experiments. We have also carried out stable-beam runs, almost exclusively in support of the PAC-approved RIB experimental program. Only about 300 hours of stand-alone stable beam experiments were run.
Kate Jones and her collaborators are to be congratulated on a high profile paper on the Magic Nature of 132Sn published in Nature  during the period covered by this newsletter. Nature provided a commentary on this work written by Paul Cottle of Florida State University in the News and Views section of the same issue. More details about the public interests generated by this paper is summarized by Witek Nazarewicz in this issue of the Newsletter.
Another high-profile result was the discovery of a new superheavy element with atomic number Z=117 by a US-Russian collaboration that included a HRIBF scientist, Krzysztof Rykaczewski, as a key member . The experiment was made possible by a target prepared with 249Bk material made at the ORNL High Flux Isotopes Reactor, and separated by ORNL chemists. This result was featured on the cover of the April 9 issue of Physical Review Letters. More details about this experiment is described by Krzysztof in a separate article in this Newsletter.
On August 2-6, HRIBF hosted the Ninth Summer School on Exotic Beam Physics under the leadership of Michael Smith and Caroline Nesaraja. More details about this very successful Summer School is given by Michael in this Newsletter.
On May 17, 18 and 19 DOE held an Operations Review of HRIBF. We have not yet received a final report from this review; however, in the closeout session strong concern was expressed about the negative impact of ORIC reliability on the HRIBF science program. The preliminary indication at the closeout was that the sole recommendation of the review would be that HRIBF hold an additional review with a panel of cyclotron experts focusing exclusively on ORIC, and including a deeper investigation into ongoing ORIC maintenance as well as possible future reliability and performance improvements. After further consultation with DOE Office of Nuclear Physics, we have scheduled this intensive ORIC review in the last half of August. HRIBF staff looks forward to the opportunity to discuss issues related to ORIC in more detail, and to profit from outside expertise. The only regret is that this review will deal only with ORIC, and will not consider the many advantages of replacing ORIC with a commercial cyclotron (see the article on the HRIBF driver upgrade in the February 2009 Newsletter, and the report on the HRIBF User Workshop in the February 2010 Newsletter).
The near-term schedule at HRIBF will be constrained by the trimming coil assembly replacement, which is expected to extend through October. The schedule from now to October will be filled with experiments using long-lived radioactive beams along with stable beam runs in support for RIB experiments. We anticipate very few stand-alone stable beam experiments will be scheduled. A tandem tank opening is scheduled to begin in October and last for seven weeks. Before the tank opening is complete, we intend to have ORIC operational. When the tank opening is complete, we plan to initiate an extended neutron-rich RIB campaign that will run well into CY 2011.
1. K. Jones, et al., Nature 465, 454 (2010).
2. Recent HRIBF Research -
Inelastic 17F(p,p)17F Scattering at 3 MeV and the 14O(&alpha,p)17F Reaction Rate
The 14O(α,p)17F reaction is an important trigger reaction in X-ray burst nucleosynthesis, and despite several indirect , direct [2,3], and time-reversed [4,5] studies, significant uncertainties remain. In particular, a recent direct measurement  observed an unexpected peak in the thick-target excitation function at Ec.m. = 1.45 MeV. Since no known 18Ne resonances exist at that energy, their conclusion was that the peak arose from a known 4+ resonance at 1.95 MeV (~3.1 MeV in the time-reversed 17F+p frame) and populating the first excited state of 17F at 495 keV in the exit channel. This was somewhat surprising because previous studies had not found such a large branch populating the first excited state of 17F. In principle, such a resonance should also be observable in a measurement of the 17F(p,p')17F* excitation function at 3 MeV.
To confirm the existence of this resonance, the inelastic scattering reaction was studied at the HRIBF with a 17F beam bombarding a CH2 plastic foil. Several beam energies between 52-58 MeV were measured with 17F beams of intensities of ~4x105 17F/s and purities of17F/17O ~ 2/1. Protons were detected in the SIDAR Silicon Detector Array in coincidence with beam-like recoils detected at forward angles in an isobutane-filled ionization chamber. The ion chamber could distinguish 17F from 17O recoils and was thus used to unambiguously identify the reaction events of interest.
Data collected from SIDAR in coincidence with recoil 17F ions are shown in a Q-value plot in Fig. 2-1. The large peak at Q=0 arises from elastic scattering 17F(p,p)17F events whereas inelastic events should result in a peak at -495 keV. No evidence for inelastic scattering was observed at these energies in contradiction to data gated on 17O recoils (Fig. 2-2) which show clear evidence for inelastic scattering as a peak at Q=-871 keV. Upper limits  on the 17F(p,p')17F* cross section are shown in Fig. 2-3 in comparison with what would be expected if the Notani et al.  interpretation of their data were correct. Clearly more work is needed to understand the mysterious peak observed in their data.
3. Recent HRIBF Research - Measurement of 10Be(d,p)
11Be Cross-Section in Inverse Kinematics at Several Energies
Light neutron-rich nuclei present an excellent arena for studying the evolution of nuclear structure as they represent the nuclei with the most extreme neutron to proton ratios. 11Be, in particular, is commonly used as a benchmark for theoretical studies because it exemplifies the distinctive properties of neutron-rich nuclei such as level inversion, and weakly bound ground states, leading to halo structure. Two recent experiments have been performed at HRIBF to study low-lying states in 11Be via the neutron transfer reaction 10Be(d,p) in inverse kinematics at a range of energies between 60 MeV and 107 MeV. The cross sections measured in these experiments will be used to help characterize sources of uncertainty in cutting-edge reaction calculations.
Data analysis is nearly complete for the experiment at 107 MeV. The preliminary differential cross-sections for the two bound states and first resonance are shown in Fig. 3-1. Analysis of the data at 60, 75, and 90 MeV and reaction calculations are ongoing. Several new experimental tools were used for these experiments, including batch-mode 10Be RIB beams, the first full implementation of ORRUBA, the QQQ silicon detector array for recoil detection, the Dual MCP for beam counting with real-time efficiency measurement, and a new Fast Ionization Chamber.
3. Doubly-Magic Tin and HRIBF in the News
A recent paper published in Nature reporting the results of transfer studies at HRIBF on the doubly-magic 132Sn, the nucleus sometimes called a Cinderella of the periodic table, has attracted great interest and been reported in a wide circle of public media. The team, led by Kate Jones and involving physicists from the University of Tennessee, Rutgers University, ORNL, Tennessee Technological University, Michigan State University, Ohio University, Colorado School of Mines, and University of Surrey, demonstrated for the first time that 132Sn is probably the best closed-shell nucleus. NATURE provided a commentary in the News & Views section of the same issue, explaining in layman terms the significance of the discovery. Following the publications, ORNL, UT, Tennessee Tech, NSCL, and Rutgers issued press releases. The work was also reported in Science Daily, Physics World, and in Physics Today as a cover article (August 2010 issue). For more details about the experiment and related science, see YouTube Interview with Kate Jones. Congratulations to the team on the outstanding result!
5. Element 117 And Beyond
The synthesis and identification of new elements is a long standing challenge, with major experimental contributions made over the last 50 years by the US, Russia, Germany and Japan, see, e.g., [Due10, Hof00, Mor04, Oga04, Oga05,Oga07,Oga10, Sta09]. The questions like "What is the heaviest atomic nucleus which can be bound against immediate disintegration?" or "Where does the Periodic Table of Elements end?" are of large interest not only to physicists but to the whole scientific community and general public. Most theoretical calculations point to the existence of elements 119 and up, to at least 126, and this has motivated many scientists to continue the effort of searching for the next superheavy element.
This year a team including scientists from Joint Institute for Nuclear Research (JINR Dubna, Russian Federation), Oak Ridge National Laboratory, Lawrence Livermore National Laboratory, Vanderbilt University, Las Vegas University, and the Research Institute of Atomic Reactors (RIAR, Dimitrovgrad, RF) reported the discovery of new chemical element Z=117 [Oga10]. The synthesis of 293(117) and 294(117) isotopes and other 9 new isotopes of Superheavy Elements (SHE) was achieved through a close collaboration of Russian and US laboratories. Unique ORNL contributions to this discovery included 22.2 mg of very high purity 249Bk (T1/2=320 days) material with only 1.7 ng of 252Cf, and no other detectable impurities. The 249Bk material was produced in a two-year campaign at the High Flux Isotope Reactor and was purified during the three-month long chemical separations at the Radiochemical Engineering Development Center at ORNL.
Six arc-shaped targets, of total area 36.0 cm2 area and about 0.3 mg/cm2 thick, were made at RIAR. Target disk was rotated at 1700 rpm perpendicular to the beam direction during the irradiation with over 1 particle-microAmp of 48Ca beam. This high intensity beam was accelerated at the heavy-ion cyclotron U-400 at JINR. The experiments were performed by means of the Dubna Gas-Filled Recoil Separator (DGFRS) [Oganessian1998]. A total target exposure to 4.4 x 1019 48Ca ions was achieved during a 150-day bombardment.
Evaporation residues (ER) passing through the DGFRS were transmitted and registered by two Multi-Wire Proporional Chambers with a total detection efficiency of over 99%, and were implanted into a Si-detector array with 12 vertical position-sensitive strips (10 mm wide, and 40 mm long) surrounded by eight 4cm x 4cm side detectors. The preamaplifier outputs with two amplifications allowed to measure alpha-energies in the range of up to 12 MeV with a reasonable resolution as well as to detect high energy fission events. The energy resolution for implanted alpha-particles was 60 to 140 keV. The total kinetic energy released in the spontaneous fission (SF) of nuclei with Z>101 was determined from the measured energy release Etot + 23 MeV. The 23-MeV correction corresponds to the mean energy loss of fission fragments in the dead layer detector as determined from a 252No measurement. The position resolution of the strip detector in registering correlated decay chains of the ER followed by alpha chain and SF event, was below 1.2 mm. The 48Ca beam was switched off for several minutes after a recoil signal was detected with parameters of implantation energy expected for Z=117 ERs, followed by an alpha-like signal with an energy between 10.7 MeV and 11.4 MeV, in the same strip, within a 2.2-mm-wide position window.
Two 70-day long irradiations were performed, at 252-MeV and 247-MeV energies of 48Ca projectiles, corresponding to about 39-MeV and 35-MeV excitation of the compound nucleus 297(117). Five decay chains observed at higher beam energy, with three subsequent alpha emission and ending in fission with Total Kinetic Energy (TKE) of 218(5) MeV, were assigned as starting at 293(117) nucleus, see Fig. 5-1. A single event consisting of six alpha decays and again ending with fission of TKE=219(5) MeV was observed at lower beam energy. The observation of this single long decay chain has been very recently confirmed at JINR Dubna, by two events observed during the chemistry-focused experiment using stationary 249Bk targets of 0.5 mg/cm2 [Dmi10].
The properties of 11 new isotopes identified among the products of 249Bk+48Ca reaction [Oga10] indicate a strong rise of stability for heavier isotopes with Z=111 to 117, validating the concept of the long sought island of enhanced stability for superheavy nuclei, see Fig. 5-2.
Presently, most of nuclear models point to the N=184 spherical shell closure. However, the superheavy nuclei identified to date are still several neutrons away from N=184. The observed decay properties do not point clearly to the one of predicted proton shell closures at Z=114, Z=120 or Z=126. Therefore, more experimental data and more theoretical work on the production and properties of superheavy elements are needed before the coordinates of the Island of Stability can be determined with high confidence.
One of the methods to push towards new isotopes of the heaviest nuclei is to employ intense 50Ti beams as well as new targets like 251Cf and even 254Es, see Fig. 5-3. One should note that attempts to use heavier beams like 54Cr, 58Fe and 64Ni on 238U and heavier radioactive targets have not been successful, probably due to the drop of the cross section, see, e.g., [GSI120, Oga08,Oga09]. 50Ti projectiles represent the stable beam closest to 48Ca, with the same magic neutron number of N=28 and Z larger by 2 protons. While larger Z helps to create heavier elements, the 2-proton excess with respect to the doubly-magic 48Ca will likely make the cross section for production of new isotopes smaller. It implies that the reaction between 50Ti and actinide targets should be well mapped and understood before attempting a long experiment aimed at producing new element (e.g., 249Bk + 50Ti to create new element Z=119). The development of high intensity50Ti beams and the excitation function mapping can be pursued using the longer-lived targets between 238U and 248Cm. However, since the production rates are expected to be low, an upgrade of the detection system to higher sensitivity is necessary. We believe that it is possible to reach these nuclei using ORNL-made target materials and a set of improved detectors.
The first step to meet this challenge is to build a new detector system using novel techniques. ORNL main contributions to the research on superheavy nuclei are currently related to the world-unique production capabilities of actinide target materials and to the theoretical work on the structure of the heaviest nuclei, see, e.g., [Cwi05]. These ORNL contributions to the SHE studies are getting extended, by constructing new detectors and their signal processing system at the Holifield Radioactive Ion Beam Facility. This new detection scheme should be capable of identifying the radioactivities having sub-microsecond lifetimes and offer a better discrimination between the high energy alpha signals and partial recoil energy deposition. This will be achieved with a large area and high granularity Double-sided Silicon Strip Detector with matching alpha-escape detectors around, all connected to the digital signal processing units like 100 MHz Pixie-16 modules of Xray Instrumentation Associates (XIA) [Grz2007]. The new recoil detection chamber should be able to distinguish the recoils having atomic number around Z=118 from the Z=84 contaminants, by tracking their energy loss. The new detection scheme should allow us to largely purify the alpha decay spectra and would increase confidence in the obtained results, even the cases where a single event is observed. New detectors and data acquisition system proposed for the SHE research will profit from the experience gained at the at HRIBF [Lid06, Maz07, Dar10] as well as from the joint experiments performed with Optical Time Projection Chamber [Mie07] in collaboration with the Warsaw University team at the National Superconducting Cyclotron Laboratory (Lansing, MI).
6. The Exotic Beam Summer School (EBSS 2010) Was Held in Oak Ridge in August
HRIBF hosted the Ninth Summer School on Exotic Beam Physics on August 2-6, 2010. The aim of this annual school is to educate young researchers on the excitement and challenges of radioactive ion beam physics and associated theoretical approaches. Through these schools, the research community will be able to more fully exploit the opportunities created by the next generation exotic beam facilities, such as the Facility for Rare Isotope Beams (FRIB) in the U.S.
Lecturers and topics of their lectures for the summer school were:
A unique feature of this summer school series is the hands-on activities where students spend their afternoons in the lab of a radioactive beam facility, learning about the techniques and instrumentation needed to carry out experiments with unstable beams. This year, the nine hands-on activities were:
On one evening, attendees had a poster session where they presented their research projects. On another evening, they had a tour of the ORNL Jaguar Supercomputer [the fastest in the world at 1.75 Petaflops and 250,000 processors] and EVEREST Visualization Center, followed by a Question and Answer session with the Lecturers.
Directors for the school were Michael Smith [ORNL], Kim Lister [ANL], Augusto Macchiavelli [LBNL], Mark Stoyer [LLNL], and Remco Zegers [MSU]. Caroline Nesaraja [ORNL] played a pivotal role in the local organization efforts. This annual school, which rotates between the organizing laboratories, is specifically designed for graduate students, senior undergraduate students who are actively involved in research, and postdocs (within 2 years of the PhD degree).
Additional information, lecture notes, and photos can be found at
RA1. RIB Development
Seven on-line experiments were performed at the OLTF during the last six months. This beam time was used to test two different uranium carbide targets, to optimize ion source performance for radioactive beams of Sr, and to test neutron detectors for the VANDLE array. A novel uranium carbide material with a density of 5.2 g/cm3 was tested with mixed results on the release rate. Also, a target of uranium carbide disks produced for the SPES project at Legnaro was tested using a 40-MeV proton beam from the Tandem. At three target temperatures (1600 C°, 1800 C°, and 2000 C°), we measured yields of fission fragments and release rates for comparison with other uranium carbide target geometries. These targets had an average density of 4.25 g/cm3 and the release rates compared quite favorably with our standard HRIBF uranium carbide targets.
Substantial efforts at the three off-line ion source test facilities have resulted in a number of improvements and better understanding of ion sources that will impact RIB production at HRIBF in the near future. At ISTF1 work continues on the design of the gas-filled RFQ negative-ion cooler that will be used to purify radioactive beams of F, Cl, and Ni at IRIS2. The design and fabrication of the deceleration and reacceleration regions need to be finished before the system will be implemented at IRIS2. At ISTF2, a campaign has been under way to better understand the design and operation of the HRIBF hot plasma ion source, the EBPIS (Electron Beam Plasma Ion Source). The extraction efficiency of Xe has been used to diagnose the effects of various hardware designs and operational modes. Investigations include the gap distance between the anode and cathode, the percentage of open area in the anode grid, the anode potential, the size of the source extraction aperture, and the strength and direction of the magnetic field of the solenoids that surround the plasma region. So far, Xe efficiencies of about 33% have been achieved when the parameters listed above are optimized. At ISTF3 a project is underway to develop a He-jet transport system to move radioactive atoms from an actinide production target to a Bernas-Nier ion source. Since the recoiling fission fragments are stopped in He and transported to the ion source without touching any surfaces, this is a chemically-independent ISOL technique and will allow refractory fission fragments to be ionized and post-accelerated for experiments. A modular room has been built in Bldg. 6010 and the assembly of equipment in this room has begun. While waiting for the room to be completed, the ion source chamber was designed, fabricated, assembled and tested, and it is now ready for use. Also, the He-recirculation cart has been refurbished and is now fully functional.
The laser system for the laser ion source was originally purchased and tested at ISTF2 in 2009 with a Nd:YAG pump laser and three tunable Ti:Sapphire lasers in a single package. We have decided to separate the system into three independent and identical packages each consisting of a Nd:YAG pump laser and a Ti:Sapphire laser with the ability to change the optics to deliver a beam of the fundamental wavelength or the frequency doubled, tripled, or quadrupled beams. These units will be interchangeable and a spare unit will be purchased to reduce downtime due to laser failure. A new laser table has been installed in the IRIS2 laser room and most of the optical components of the IRIS2 laser system have been ordered.
A new long-lived radioactive ion beam was developed during this
period. With maintenance activities on ORIC underway, a number of
beams of long-lived radioactive isotopes have been accelerated using
the multi-sample Cs-sputter ion source on IRIS1. These beams have
included Be-7, Be-10, and Al-26, and now Sr-82 (half-life is 25.4
days) has been extracted and delivered to an experiment. We purchased
10 mCi of Sr-82 from the DOE Isotopes Program. This isotope is
shipped with the Sr-82 atoms dissolved in weak hydrochloric acid.
This liquid is transferred to a metal powder sample and pressed to
form a solid sputter target. This first attempt resulted in beams
with low intensity, with a peak of about 2000 ions per second on
target and an average of about 1200 ions per second over a seven-day
period. Improvements in the transfer of activity to the cathode and a
different metal powder should result in higher quality beams
(intensity and purity). These improvements will be tested later this
year and we are also considering a similar process to deliver beams of
Fe-59 (half-life is 44.5 days).
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. Experimental Equipment - Digital Pixies at HRIBF
After a succesfull series of experiments[1-9], the nearly decade-old digial-data acquisition sytem based on CAMAC DGF4C boards found its new-generation successor in the Pixie16 modules. These boards were developed and are manufactured by XIA LLC, a Silicon Valley high-tech company who also developed the DGF4C units. The main advantages of the Pixie16-based system are: higher digitization frequency (100MHz), higher channel density (16 ch per board) and implementation of the fast readout (PCI), which enables much higher data throughput than previously used CAMAC-based system. Pixie16 is much better suited to systems with high channel count.
The main task of intergration of the Pixie16 at HRIBF was unertaken by the University of Tennessee researchers and postdocs. The libraries provided by XIA LLC were used in order to build the readout codes. Pixie-generated data-stream is formatted in such a way that it is compatible with the HRIBF's acquisition system. The new analysis codes (including event builder) were written. The software evolved from the original DEC-Fortran code (used with the DGF4C units), to the new C- and C++ based codes with built-in possibility of using either SCAN or ROOT suites for histogramming. A control software was developed using command line and GUI. All Pixie16-based systems use PC computers running Linux operating system.
This new system was already used in several experiments serving a broad range of detectors, including semiconductor, gas and scinitillator detectors. Only parameters of the on-board codes need to be modified to suit particular detector type. The new system proved to be very stable and reliable . Recently Pixie16 was used during the EBSS 2010 Summer School to demonstrate the advantages of digital electronics . Presently at HRIBF the prototype-generation boards Pixie16 Rev A, and a newest type pixie16 Rev D are avialable. The latter uses a revamped internal architecture and a new, more efficient readout scheme.
One example of the experimental application of the Pixie16-based system is in decay-spectroscopy studies at LeRIBSS facility . A purely digital system was used to acquire gamma-ray data from clover detectors and beta decays from fast-plastic scintillator detectors. The main advantage of using the Pixie16 system was the time stamping allowing for flexible-time correlations between decays. Pixie16 was also used at the Recoil Mass Spectrometer (RMS)  in experiments searching for new proton and alpha emitters. These experiments took advantage of the board's capabilities for real-time operations, in particular, the complex-pulse shape-based triggering ability and large data throughput. There are also future plans to employ the Pixie16-based system in super-heavy element research .
Recently significant development was made in exploring the capabilities of using the Pixie16 for fast-timing applications with plastic scintillators of the VANDLE detector system. The timing resolution of 200 ps have been demonstrated using new algorithms [17,18]. Presently a new 500-MHz-based system called Pixie500 is being impleneted at HRIBF for specialized electronic fast-timing applications.
1. W. Królas, et al., Phys. Rev. C 65, 031303 (2002).
RA4. Users Group News
The HRIBF will not hold its annual Users Group Meeting this year. In conjunction with ATLAS and NSCL a joint users meeting will be held at one of the labs sometime in 2011. A survey of users was taken earlier this year and the response was overwelmingly positive to such a move. The motives behind the joint meeting are:
It is expected the meeting will take place over a two-day period and will rotate between the laboratories. Several users suggested other facilities or groups may also wish to hold meetings at this forum. It is unclear at this time if any of the suggested expansions will be implemented.
The HRIBF Users Executive Committee Election will be held soon to replace Art Champagne (UNC) and Alfredo Galindo-Uribarri (ORNL). They formed a committee together with Witek Nazarewicz and nominated the following slate of candidates who have agreed to serve if elected. At-large candidates have been solicited from the entire users group. The current ballot is:
For the seat of Art Champagne
For the seat of Alfredo Galindo-Uribarri
Ballots are to be sent sometime in October.
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 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.
RA6. HRIBF Experiments,
January through June 2010