HRIBF Newsletter, Edition 18, No. 2, August 2010

Feature Articles

1. HRIBF Update and Near-Term Schedule
2. Recent HRIBF Research - Inelastic ¹⁷F(p,p)¹⁷F Scattering at 3 MeV and the ¹⁴O(α,p)¹⁷F Reaction Rate
3. Recent HRIBF Research - Measurement of ¹⁰Be(d,p) ¹¹Be Cross-Section in Inverse Kinematics at Several Energies
4. Doubly-Magic Tin and HRIBF in the News
5. Element 117 And Beyond
6. The Exotic Beam Summer School (EBSS 2010) Was Held in Oak Ridge in August

Regular Articles

RA1.  RIB Development

RA2.  Accelerator Systems Status

RA3.  Experimental Equipment - Digital Pixies at HRIBF

RA4.  Users Group News

RA5.  Suggestions Welcome for New Beam Development

RA6.  HRIBF Experiments, January - June 2010

1. HRIBF Update and Near-Term Schedule
(J. R. Beene)

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 [1] 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 [2]. 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.

References:

1. K. Jones, et al., Nature 465, 454 (2010).
2. Yu. Ogannessian, et al., Phys Rev. Lett. 104, 142502 (2010).

2. Recent HRIBF Research - Inelastic ¹⁷F(p,p)¹⁷F Scattering at 3 MeV and the ¹⁴O(&alpha,p)¹⁷F Reaction Rate
[D. Bardayan (ORNL), spokesperson]

The ¹⁴O(α,p)¹⁷F reaction is an important trigger reaction in X-ray burst nucleosynthesis, and despite several indirect [1], direct [2,3], and time-reversed [4,5] studies, significant uncertainties remain. In particular, a recent direct measurement [2] observed an unexpected peak in the thick-target excitation function at E_c.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 ~4x10^{5 17}F/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.

Figure 2-1: In (a)-(g), Q-value spectra are shown for ¹⁷F+p scattering events at bombarding energies of 52-58 MeV, respectively.

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 [6] 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. [2] interpretation of their data were correct. Clearly more work is needed to understand the mysterious peak observed in their data.

Figure 2-2: The same as Fig. 2-1, but now gated on ¹⁷O recoils.

Figure 2-3: Upper limits on the ¹⁷F(p,p')¹⁷F cross section. The dashed curve shows the expected cross section if the interpretation from Ref. [2] was correct.

References:

[1] K. I. Hahn, et al., Phys. Rev. C54, 1999 (1996).
[2] M. Notani, et al., Nucl. Phys. A746, 113c (2004).
[3] S. Kubono, et al., Eur. Phys. J. A27, 327 (2006).
[4] J. C. Blackmon, et al., Nucl. Phys. A688, 142c (2001).
[5] B. Harss, et al., Phys. Rev. Lett. 82, 3964 (1999).
[6] D. W. Bardayan, et al., Phys. Rev. C81, 065802 (2010).

3. Recent HRIBF Research - Measurement of ¹⁰Be(d,p) ¹¹Be Cross-Section in Inverse Kinematics at Several Energies
[K.T. Schmitt & K. L. Jones (Univ. of Tennessee), spokespersons]

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.

Figure 3-1: Measured differential cross-section for the population of the first three states in ¹¹Be via ¹⁰Be(d,p) with a beam energy of 107 MeV.

3. Doubly-Magic Tin and HRIBF in the News
(W. Nazarewicz, HRIBF Scientific Director)

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!

Cover picture of the August 2010 issue of Physics Today, showing the Oak Ridge National Laboratory's 25-megavolt tandem electrostatic accelerator, seen here rising 30 meters to the ceiling of its domed pressure vessel. The tandem accelerator can produce the highest-energy beams among all the existing electrostatic accelerators in the world. In the experiment described on page 16 of the August 2010 issue of the magazine, it accelerated a beam of short-lived tin-132 ions to 630 MeV. The experimenters sought to test whether the ¹³²Sn nucleus, with magic numbers of both protons and neutrons, does indeed have the "doubly magic" properties predicted for it by the shell model of nuclear-structure theory.

5. Element 117 And Beyond
(K. P. Rykaczewski)

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 (T_1/2=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 10^{19 48}Ca 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].

Figure 5-1: The observed decay chains were interpreted as originating from the 294(117) isotopes and from 293(117). Average time and energy values obtained from five identified events are displayed for the 293(117) of the new element Z=117. The measured and predicted [Sob10] lifetimes and alpha-particle energies are shown in black and blue, respectively (figure adapted from [Oga10]).

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.

Figure 5-2: Alpha decay energy (plot a) and half-lives (plot b) of Superheavy nuclei with an odd atomic number are displayed as a function of neutron number for the isotopes of elements with Z=111 to 117. All the nuclides with N>165 have been produced in ⁴⁸Ca induced reactions. The isotopes identified in [Oga10] are shown as solid symbols. In the plot (b) the values of T_α^exp are given only for the decay chain originating from the 293(117) isotope.

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.

Figure 5-3: Potential future chart of superheavy nuclei. The green squares indicate previously produced isotopes. The blue background shows predicted stability of isotopes (the more intense color indicates the more stable nuclide). The colored squares with colored frames indicate nuclei which may be potentially created in the fusion reactions between actinide targets and beams of ⁴⁸Ca and ⁵⁰Ti followed by evaporation of 3 and 4 neutrons. The solid colors indicate targets of ²⁴⁹Cf (pink), ²⁴⁹Bk (yellow), ²⁵¹Cf (orange) and ²⁵⁴Es (grey). The frames correspond to the use of ⁴⁸Ca (black) and ⁵⁰Ti (red) beams. There are other new isotopes of Z = 118 elements to be potentially produced with ⁵⁰Ti beam not indicated in this figure, e.g., 295(118) produced in the 3n evaporation channel of the ²⁴⁸Cm + ⁵⁰Ti reaction. Note that reactions between ²⁵⁴Es target nuclei and ⁵⁰Ti projectiles would theoretically produce the new element Z=121, predicted to start a new row of elements, after lanthanides and actinides.

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).

References:

[Cwi05] S. Cwiok, et al., Nature 433, 705 (2005).
[Dar10] I. G. Darby, et al., Phys. Rev. Lett. (2010), in press.
[Dmi10] S.N. Dmitriev, in contr. to the 45th Zakopane School of Physics "Extremes of the Nuclear Landscape", Zakopane, Poland, September 2010.
[Due10] Ch.E. Dullmann, et al., Phys. Rev. Lett. 104, 252701 (2010).
[Grz07] R. K. Grzywacz, et al., Nucl. Instrum. Methods. Phys. Res. B 261, 1103 (2007).
[Hof00] S. Hofmann and G.Munzenberg, Rev. Mod. Phys. 72, 733 (2000).
[Lid06] S.N. Liddick, et al., Phys. Rev. Lett. 97, 082501 (2006).
[Maz07] C. Mazzocchi, et al., Phys. Rev. Lett. 98, 212501 (2007).
[Mie07] K. Miernik, et al., Phys. Rev. Lett. 99, 192501 (2007).
[Mor07] K. Morita, et al., J. Phys. Soc. Jpn. 76, 045001 (2007).
[Oga98] Yu. Ts. Oganessian, et al., in Proc. of the Fourth International Conference on Dynamical Aspects of Nuclear Fission, Casta-Papiernicka, Slovak Republic 1998, World Scientific, Singapore, p. 334, (2000).
[Oga04] Yu.Ts. Oganessian, et al., Phys. Rev. C 69, 021601(R) (2004).
[Oga05] Yu.Ts. Oganessian et al., Phys. Rev. C 72, 034611 (2005).
[Oga07] Yu. Ts. Oganessian, Jour. Phys. G, 34, R165 (2007) and earlier references therein.
[Oga10] Yu. Ts. Oganessian, et al., Phys. Rev. Lett. 104, 142502 (2010).
[Sob10] A. Sobiczewski, Acta Phys. Pol. B41, 157 (2010).
[Sta09] L. Stavsetra, et al., Phys. Rev. Lett. 103, 132502 (2009).

6. The Exotic Beam Summer School (EBSS 2010) Was Held in Oak Ridge in August
[M. S. Smith (ORNL), Summer School Organizer]

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:

Carl Brune (Ohio Univ.):	Nuclear Astrophysics
Partha Chowdhury (Univ. Massachusetts Lowell):	Nuclear Structure Experiments
Martin Freer (Univ. Birmingham):	Nuclear Reactions
Alejandro Garcia (Univ. Washington):	Weak Interactions
Ken Gregorich (Lawerence Berkely National Lab):	Superheavies
Robert Gryzwacz (Univ. Tennessee):	Digital Signal Processing
Ritu Kanungo (St. Mary's Univ.):	Nuclear Reactions
Suzanne Lapi (Washington Univ. St. Louis):	Medical Physics
Kim Lister (Argonne National Lab):	FRIB Overview
Dave Morrissey (Michigan State Univ.):	RIB Production
David Radford (Oak Ridge National Lab):	Gamma Ray Tracking
Bob Wiringa (Argonne National Lab):	Nuclear Structure Theory
Vladimir Zelevinsky (Michigan State Univ.):	Nuclear Structure Theory

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:

ISOL Radioactive Ion Beam Production Using Tandem Beams;

Beam Emittance Characterization;

Digital Gamma-Ray Spectroscopy;

Position Sensitivity in Gamma-Ray Detectors;

Silicon-Strip Detector and GRETINA Digitizer;

Neutron Detection with Digital Electronics;

Digital Tricks in Nuclear Spectroscopy;

Neutron Attenuation & Scattering Measurements Using VANDLE;

Nucleosynthesis Calculations with the Computational Infrastructure for Nuclear Astrophysics.

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 the EBSS 2010 website.

Regular Articles

RA1. RIB Development
(D. W. Stracener)

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)

This has been a very difficult year for ORIC with limited operation. Following some difficulties with the extraction and rf systems, operation with proton beam resumed in February. However, this was short-lived. In late February, trimming coil eight (T8) completely failed due to an un-repairable internal water leak.

ORIC was designed with a set of ten concentric water-cooled trimming coils that nominally operate with currents up to 800 Adc. All ten copper coils are contained in a single stainless steel enclosure (see Figure 1). Matching coil assemblies are mounted on each of the two main magnet pole faces. This is not the first trimming coil failure. In fact, there have been several. Trimming coil T10 failed in 1986, 1/3 of T8 failed in 1988 (the coils in each assembly have either 2 or 3 coolant loops), and T9 failed in 1997 prompting us to fabricate a spare set of coils. With each of the prior failures, we were able to determine an alternate tuning configuration that allowed operation to continue. So we prepared for the possibility of another failure, not knowing when it might occur or whether it would prevent ORIC from delivering beam.

When T8 failed this year, there were also concurrent failures of the deflector septum and rf system. After drying the coil assemblies and repairing the extraction and rf systems we attempted to extract beam without T8, which we did, but not with high enough beam energy or intensity. Thus it became necessary to install the spare coils. Replacement of the trimming coils requires about four months, and we are concurrently implementing several other upgrades and performing considerable preventive maintenance. Note that in replacing the trimming coils, all ten coils are replaced at the same time. Thus ORIC will be restored to its original coil configuration, which should lead to improvements in beam extraction efficiency.

The trimming coil replacement project is scheduled for completion in early November. Once ORIC is again operational, beam will first be delivered to IRIS2 to complete commissioning activities with radioactive ion beam.

Figure RA2-1: New ORIC Trimming Coils During Fabrication

RIB Injector Operations and Development (P.E. Mueller)

During the period from 1 January 2010 to 30 June 2010, the 25 MV Tandem Electrostatic Accelerator delivered beams of

53 kpps [23.05 MV 14+/26+ terminal foil / high energy foil stripped] 520-MeV 95% ¹³²Sn,

73 kpps [23.57 MV 15+/26+ terminal foil / high energy foil stripped] 540-MeV 95% ¹³²Sn,

85 kpps [23.28 MV 15+/27+ terminal foil / high energy foil stripped] 550-MeV 95% ¹³²Sn,

90 kpps [23.7 MV 15+/27+ terminal foil / high energy foil stripped] 560-MeV 95% ¹³²Sn,

53 kpps [23.94 MV 15+/28+ terminal foil / high energy foil stripped] 580-MeV 95% ¹³²Sn, and

23 kpps [24.08 MV 15+/29+ terminal foil / high energy foil stripped] 600-MeV 95% ¹³²Sn to the time-of-flight endstation in Beam Line 23, and

800 kpps [20.93 MV 5+ terminal gas & foil stripped] 117-MeV ^26gAl to Beam Line 41.

The high purity tin beams were produced via proton induced fission of ²³⁸U by bombarding a pressed powder uranium carbide target coupled to an Electron Beam Plasma (positive) Ion Source (EBPIS) with 9-10 uA of 50-MeV ¹H and passing a positive tin sulfide beam through the recirculating cesium jet charge exchange cell and selecting the negative tin beam resulting from molecular breakup. The ^26gAl beam was produced with the multisample cesium sputter negative ion source using pressed powder copper targets loaded with ^26gAl₂O₃.

Tandem Operations and Development (M. Meigs)

The Tandem Accelerator was operated for more than 2100 hours since the last report.   The machine ran at terminal potentials of 2.85 to 24.34 MV.  IRIS1 provided 164 hours of the radioactive beams ²⁶Al and ¹³²Sn.  While the UC source was still on IRIS1, two shifts of ²³⁸U were provided.  In addition, the SNICS was used to provide the stable beams ¹H, ¹⁶O, ¹⁹F, ²⁷Al, ³²S, ⁵⁴Fe, ⁵⁸Ni, ^74,76Ge, ⁷⁶Se, ¹²⁴Sn and ^122,130Te.  About 88 hours were spent conditioning to assure operation above 24 MV.  No tank openings were required during this period.

RA3. Experimental Equipment - Digital Pixies at HRIBF

(R. Grzywacz [Univ. of Tennessee], spokesperson)

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[10]. 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 [11]. 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 [12]. A purely digital system was used to acquire gamma-ray data from clover detectors and beta decays from fast-plastic scintillator detectors[13]. 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) [14] 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 [15].

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[16]. 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.

References:

1. W. Królas, et al., Phys. Rev. C 65, 031303 (2002).
2. M. Karny, et al., Phys. Rev. Lett. 90, 012502 (2003).
3. R. K. Grzywacz, et al., Nucl. Instrum, Methods Phys. Res. B 204, 649, (2003).
4. S. N. Liddick, et al., Phys. Rev. Lett., 97, 082501 (2006).
5. R. K. Grzywacz, et al.,Nucl. Instrum, Methods Phys. Res. B 261, 1103 (2007).
6. C. Mazzocchi, et al., Phys. Rev. Lett., 98, 212501 (2007).
7. M. Karny, et al., Phys. Let. B 664, 52 (2008).
8. J. A. Winger, et al., Phys. Rev. Lett. 102, 142502 (2009).
9. I. Darby, et al., Phys. Rev. Lett. (2010).
10. H. Hubbard-Nelson,M. Momayezi, W.K. Warburton, Nucl. Instrum, Methods Phys. Res. A422, 41 (1999).
11. M. Smith, this HRIBF Newsletter.
12. LeRIBSS Website.
13. S. Padgett, et al., submitted to Phys. Rev. C.
14. C. Gross, et al., Nucl. Instrum, Methods Phys. Res. A 450,12 (2000).
15. K. Rykaczewski, this HRIBF Newsletter.
16. http://vandle.oit.utk.edu/vandlewiki.
17. M. Madurga, et al., CAARI 2010, to be published.
18. R. Grzywacz, http://fribusers.org/4_GATHERINGS/2_SCHOOLS/2010/lectures.html.

RA4. Users Group News
(C. J. Gross, HRIBF User Liaison)

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:

• more time to each facility than is available at the DNP
• foster interaction between groups that work at the different laboratories
• provide opportunities to form collaborations
• minimize travel expense for users who work at more than one laboratory

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

• Uwe Greife (Colorado School of Mines)
• Kate Grzywacz-Jones (University of Tennessee)

For the seat of Alfredo Galindo-Uribarri

• Elizabeth Padilla-Rodal (National Autonomous University of Mexico)
• Noemie Benczer-Koller (Rutgers University)

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 grosscj@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.

• The tandem accelerates negative ions only.
• Positive ions may be charge-exchanged or used directly off the platform.
• ORIC presently produces up to 52 MeV of ¹H (12 uA); 49-MeV ²H (12 uA); 120-MeV ³He (not yet attempted, costly); 100-MeV ⁴He (3 uA). Higher currents may be possible.
• Typical reactions required to produce more than 10⁶ ions per second are n, 2n, pn, and alpha-n fusion-evaporation reaction channels and beam-induced fission products. More exotic reactions are possible if extremely low beam currents are all that is needed.
• Species release is strongly related to the chemistry between the target material and the beam species. It is best when the properties are different and the target is refractory. Thin, robust targets (fibrous, liquid metals, a few grams per square centimeter) must be able to withstand 1500 degrees Celsius or more.
• Minimum half-life is seconds unless chemistry is very favorable.
• Very long-lived species (T_1/2 > 1 h) are probably best done in batchmode, i.e., radioactive species are produced with ORIC beams and then transported to the ion source where beams are produced via sputtering. Sputter rates of the species and target substrates are important.
• Isobaric separation is possible for light beams (fully stripped ions), while isobaric enhancement may be possible for heavy beams.
• Beware of long-lived daughters or contaminant reaction channels.

RA6. HRIBF Experiments, January through June 2010
(M. R. Lay)