HRIBF NEWS


Edition 12, No. 2 August 2004 Price: FREE

Feature Articles Regular Articles

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

Feature contributors: J. R. Beene, C. Baktash, C. J. Gross, J. Gomez del Campo, D. C. Radford, W. Nazarewicz, G. R. Young, C.-H. Yu
Regular contributors: J. C. Blackmon, C. J. Gross, M. R. Lay, M. J. Meigs, A. J. Mendez, P. E. Mueller, D. W. Stracener, B. A. Tatum



1. HRIBF Update and Near-Term Schedule

In the February issue of the HRIBF News, this article began with the phrase "These are exciting times at the HRIBF..." The excitement has not abated. The HPTL project is proceeding well, and we have just completed a successful and rewarding radioactive beam campaign which has seen us deliver about 1200 RIB hours to experiments in the last five months.  We now have delivered in excess of 1500 hours of RIB to experiments during FY04, part an overall operation total in excess of 4000 hours.  We have therefore achieved our beam delivery goals for this year.  We have also stretched our resources (human and financial) very thin.  We will stop the current neutron-rich campaign after completion of the current 134Sn experiment. A five-week shutdown for tandem maintenance and improvement is scheduled to begin near the end of August.  The main activity during this period will be installation of a new power supply for the terminal bending magnet.  This is a difficult and challenging task, but the old supply has become essentially impossible to maintain.

We will begin next fiscal year with a short 7Be campaign followed by an extended neutron-rich campaign which will last approximately three months.  It will be interrupted by a period of approximately one month during which ORIC will be unavailable due to work on the HPTL.

Our excellent radioactive beam delivery performance during this fiscal year, and especially during the last four months, is a tribute to the hard work and dedication of the HRIBF staff.  This has been achieved under difficult circumstances, as the HPTL project has demanded much attention from several key staff members.  In addition, we have had to accommodate a series of ORNL or DOE audit and fact-finding exercises during this period.  The most significant of these was a major environment safety and health management inspection of ORNL by staff of DOE  Office of Independent Oversight and Performance Assessment (OA), Office of ES&H Evaluations (OA-40). Although the Physics Division is a rather small part of ORNL, we found ourselves closely scrutinized. In fact, one of the OA-40 auditors was assigned to our division full-time throughout the audit.  Much of his attention was devoted to HRIBF.  It is generally agreed that we came through this exercise very well.  The Physics Division R&D was given the highest available rating of “Effective Performance.”  Out of more than 20 findings presented in the draft final report of the OA-40 team, only one dealt directly with the Physics Division.  Since the report is still in draft form and a laboratory Corrective Action Plan to address the finding has not yet been completed, it is too early to discuss further on how the response to the report might impact HRIBF operations.  In any event, it is clear that the performance of HRIBF and other Physics Division Staff in all the exercises during this difficult period has been exemplary.

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2. Recent HRIBF Research - The Astrophysically Important 2H(84Se,p)85Se Experiment* (Jeff Thomas, Spokesperson)

The low-lying states in nuclei near the closed shells are key benchmarks for nuclear structure models.  The experimentally determined energies, spins, and parities of these typically single-particle-like states constrain the effective interactions used in the models.  The available data for neutron-rich nuclei away from stability are limited, but both theoretical arguments [1] and experimental evidence [2] suggest modifications of nuclear shell structure in weakly-bound, neutron-rich systems.  The low-lying single-particle structure of neutron-rich nuclei near the closed shells may also influence how abundances formed in stellar explosions by the rapid neutron capture process (r-process) are modified following freeze out from nuclear statistical equilibrium.  Since the Q value for these captures is small and the level density of the compound nucleus is low, direct capture to bound states typically dominates the neutron capture process.

  
Figure 2-1: Strip number vs. proton energy (chan.).  The protons are in coincidence with the Se.    Figure 2-2: Preliminary results: Counts vs. Q-value (chan.) spectrum. 

Our recent measurement of the 2H(84Se,p)85Se reaction complements a similar study of 2H(82Ge,p)83Ge that was performed last year at the HRIBF [3]. These measurements investigate the single-neutron excitations around the N=50 closed shell that are important for the production of A~80 nuclei in the r-process.  Observations have shown substantial variations in the abundances of these nuclei in metal-poor halo stars.

A 200 ug/cm2-thick deuterated polyethylene target (CD2) was bombarded with a mass 84 mixed beam (93% Br, 7% Se) at 4.5 MeV/nucleon.  A segmented, gas-filled ionization counter (IC) at 0° was used to identify the atomic number of the beam and beam-like recoils through traditional energy loss techniques.  Protons were detected with the silicon detector array (SIDAR) [5] in a "lampshade" geometry and an annular strip detector, subtending laboratory angular ranges of 105°-150° (38°-12° in the c.m.) and 160°-170° (8°-4° in the c.m.), respectively. Protons detected in SIDAR in coincidence with the Se in the IC reveal the states of 85Se populated in the reaction (Fig.2-1).

A preliminary analysis of the 85Se Q-value spectrum shows four strong groups populated in the reaction (Fig. 2-2). Preliminary angular distributions for the ground state and first-excited state have been extracted and are displayed in Fig. 2-3.  Distorted wave Born approximation calculations are also shown as the solid line preliminary fits to the data.  The preliminary assignments, based on the fits of the calculations, are an l = 2 (presumably 5/2+) ground state, and an l = 0, 1/2+ first-excited state.


Figure 2-3: Preliminary angular distributions for the ground state (black) and first-excited state (red) of 85Se.  The solid curves are DWBA fits to the data.

Analysis is continuing to extract the other angular distributions and spectroscopic factors for the low-lying states, and working with theorists to understand the dramatic decrease in the s1/2-d5/2 energy spacing in the neutron-rich N=51 isotones.

* This work is the result of a collaboration between Rutgers Universtiy, ORNL, Tennessee Tech. Univ., Colorado School of Mines, Univ. of Tennessee, Univ. of North Carolina, and ORAU.

[1] J. Dobaczewski, et al., Phys. Rev. C 53, 2809 (1996).
[2] J. P. Schiffer, et al., Phys. Rev. Lett. 92, 162501 (2004).
[4] J. S. Thomas, et al., http://www.phy.ornl.gov/hribf/usersgroup/news/jul-03/jul-03.html
[5] D. W. Bardayan, et al., Phys. Rev. C 63, 065802 (2001), and references therein.

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3. Recent HRIBF Research - Recent Nuclear Structure n-Rich RIB Experiments

  • First Coulomb Excitation Experiment Using Odd-A Neutron-Rich Radioactive Beams* (Chang-Hong Yu, Spokesperson)
  • In the first experiment to Coulomb excite an odd-A radioactive beam, a 400-MeV A=129 radioactive beam provided by HRIBF was bombarded on to 50Ti targets with thicknesses of 1 and 1.5 mg/cm2. The CLARION Ge detector array was used to detect the gamma rays, the Hyball CsI detector array (absorbers removed) was used to detect scattered beams and recoiling target ions, and a Bragg detector was used to monitor the beam composition. Doppler corrected (event-by-event) gamma rays that are correlated with the 50Ti ions are shown in Figure 3-1. Almost all peaks in this spectrum can be identified in 129Te or 129Sb, which are the main components of the mixed beam. Gamma-gamma coincidence data also indicated the presence of new levels in 129Te that were not previously established. Preliminary data analysis yielded a B(E2, 7/2+ --> 11/2+) in 129Sb that is about ten times smaller than the B(E2, 0+ --> 2+) values in its even-even neighbors. Data are also being analyzed to extract B(E2)'s of about 10 other transitions (both stretched and unstretched E2 transitions) in 129Sb and 129Te. The results from the present experiment, as well as the planned future experiments to Coulex odd-A and odd-odd neutron-rich RIBs, could provide fresh insight to our understanding of nuclear structure near 132Sn, especially concerning the coupling of single-particles to corresponding even-even cores.

    Figure 3-1: Spectrum of gamma rays in coincidence with the 50Ti target recoils from the Coulomb excitation of A=129 neutron-rich RIBs. The inset shows the preliminary B(E2, 7/2+ --> 11/2+) value deduced for the 1128-keV transition in129Sb compared to the B(E2, 0+ --> 2+)'s [2] in its even-even neighbors.

    * This work is the result of a collaboration between ORNL, The Univ. of Tennessee, and ORAU.

    [1] D. C. Radford, et al., Phys. Rev. Lett. 88, 222501 (2002)
    [2] S. Raman, et al., Atomic Data and Nuclear Data Tables, 78, 1 (2001)

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  • Multi-Step Coulomb Excitation of 136Te* (Cyrus Baktash, Spokesperson)

    An earlier Coulomb excitation study of the neutron-rich isotopes of Te revealed an unexpectedly small B(E2)=0.103(15) e2b2 for the first 2+ state in 136Te [Ref. 1]. This result has generated a great deal of theoretical interest. While QRPA [Ref. 2], QMCD [Ref. 3], and shell model calculations with realistic interactions [Ref. 4] reproduce the energy spectra well, their predicted B(E2) values for the first 2+ and higher excited states vary greatly. The differences in these predictions primarily reflect the different degrees of contributions of neutrons to the wave function of this state. These results are quite surprising, given the relatively simple structure of this nucleus, and the previous successes of shell model calculations in describing properties of long chains of Sn isotopes across the N=50-82 shell. It is, therefore, of great interest to obtain experimental data on other quadrupole excitations in 136Te in order to constrain the choice of input parameters for microscopic shell model calculations. To this end, Coulomb excitation studies of 136Te in inverse kinematics using targets of 50Ti and 90Zr at beam energies of 410 and 470 MeV have been completed. The average intensity of the 136Te beam was ~105 pps. For calibration purposes, 126Te was also Coulomb excited using the same experimental setup and targets of 12C and 50Ti. A very preliminary analysis of these data indicates that Coulomb excitation of the second 2+ has a negligibly small cross section, while the first 4+ was excited with a larger cross section than predictions, based on theoretical strengths of the B(E2; 2->4). It is anticipated that the quadrupole moment of the first 2+ state can be measured by analyzing the gamma ray yield as a function of scattering angle.

    * This work is the result of the collaboration between ORNL, Florida State Univ., Univ. of Tennessee, and Vanderbilt Univ.

    [1] D. C. Radford et al., Phys. Rev. Lett. 88, 222501 (2002).
    [2] Terasaki, J. Engel, W. Nazarewicz, M. Stoitsov, Phys. Rev. C.
    [3] T. Otsuka (private communication).
    [4] A. Covello (private communication).

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  • Search for i13/2 State in 135Te by Neutron Transfer* (David Radford, Spokesperson)

    The goal of this experiment was to identify the single-neutron i13/2 state in 135Te by neutron transfer in inverse kinematics. The reaction 13C(134Te,12C)135Te was used, at energies just above the Coulomb barrier (about 4.2 MeV per nucleon). Light ions from the reaction were detected in the new bare HyBall, in coincidence with gamma rays in CLARION. Beams of up to 3 x 106 134Te ions per second were obtained.

    The state of interest was found through its expected decay to the 11/2- state at 1180 keV. Gamma-gamma-12C coincidence events were analyzed for coincidences with the 1180->0 transition, which yielded a definite peak (~ 20 counts) at 929 keV, as shown in the figure below. By then examining the angular correlations of the more numerous gamma-12C events, this 929 keV transition is found to be consistent with a stretched-dipole transition deexciting a strongly aligned (high spin) state. We therefore assign a new level at 2109 keV as the i13/2 state in 135Te. This energy fits well with expectations based on the systematics of N = 83 isotones where the level has been previously assigned (139Ba and heavier).
    [Tranfer spectra]

    Figure 3-2: Gamma-C coincidence data (bottom) and gamma-gamma-C coincidence data (top) from the 13C(134Te,12C)135Te reaction, showing the identification of the single-neutron i13/2 state.

    We also ran for several days using the similar 9Be(134Te,8Be)135Te reaction. The resulting data are very clean, but no evidence for the population of the the i13/2 state was observed in this reaction. To the level of the statistics, this is also consistent with expectations, since the momentum-matching for the 13C reaction is much more favorable for the required high-L neutron transfer.

    * This work is the result of the collaboration between ORNL, Univ. of Tennessee, and Vandibilt Univ.

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  • Study of Neutron-Rich Sb Nuclei via Proton Pickup Reactions* (Jorge Gomez del Campo, Spokesperson)

    An experiment to search for the missing s1/2 single-proton state in 131Sb nuclei has been performed using a 550-MeV 130Sn beam impinging on a 2 mg/cm2 7Li target. In addition, a decay experiment of a beam of 131Sn (beta decay to the excited states of 131Sb) was also performed. Measurements were done with the HYBALL (to detect charged particles) and CLARION (for detection of gamma rays) detector arrays.

    Using the 130Sn beam, the states in 131Sb are populated via a transfer of a proton from the target to the beam. Previous studies with 124Sn beams have indicated that the reaction is more like an incomplete fusion reaction (rather than a direct transfer of a proton ) by which the 7Li target breaks up into a triton and an alpha particle, transferring the triton to the beam and further decaying by 2 neutrons. Therefore, the p transfer is studied by the coincidence of alpha particles and gammas . A preliminary spectra of the gammas in coincidence with the alphas is shown in Figure 3-3. The analysis of these data is in progress.

    Figure 3-3: Gamma-ray spectrum gated on alpha particles from HYBALL. Most of the peaks shown in the figure belong to decay of states in 131Sb.

    * This work is the result of a collaboration between ORNL, The Univ. of Tennessee, and ORAU.

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    4. PAC-10 Results

    PAC-10 met May 6-7, 2004, in Oak Ridge and considered 18 proposals and letters of intent which requested 183 shifts of radioactive beams (RIBs), 52 shifts of low intensity stable beams (SIB for RIBs), and 26 shifts of stable beams (SIBs). Of these, a total of 115 RIB shifts, 29 SIB for RIB shifts, and 17 SIB shifts were approved. Approved experiments, which are posted on our website, requested RIBs of 7Be, 78,85,86,87Ge, 130,132Sn, 135Te, and 135I. The total number of accepted proposals from outside HRIBF was 8 out of a total of 13 with a ninth proposal granted discretionary time.

    5. HRIBF Workshop on In-beam Gamma-ray Spectroscopy April 5-7, 2004

    The Holifield Radioactive Ion Beam Facility (HRIBF) at Oak Ridge National Laboratory (ORNL) hosted a workshop on in-beam gamma-ray spectroscopy this spring, April 5-7, 2004. The purpose of this workshop was to bring together those interested in doing in-beam gamma-ray experiments at the HRIBF, to acquaint them with HRIBF capabilities for such experiments, and to address the facility's need to map out its gamma-ray detector requirements for the rest of the decade.

    HRIBF has an extensive arsenal of in-beam gamma-ray detectors, used for example in RIB Coulomb excitation and inverse-kinematics single-nucleon transfer experiments. The primary germanium detector array is CLARION, which consists of 11 segmented Clover detectors and has an efficiency of 2.2% at 1.33 MeV. In addition, experiments have been carried out using the ORNL-MSU-TAMU Barium Fluoride array, and we anticipate in the future making use of the rejuvenated Spin Spectrometer.

    A cost-effective upgrade of CLARION by the addition of another clover together with up to twelve GAMMASPHERE-style two-way segmented coaxial detectors was presented at the workshop by Cyrus Baktash. This idea was generally very well received by the participants. Also discussed was the possibility of an ORNL proposal to host GAMMASPHERE at the HRIBF in 2006 or 2007, since the possible coupling of RIBs and GAMMASPHERE (9% efficiency at 1.33 MeV) could provide opportunities to do significantly more detailed studies on nuclei further from stability. Paul Fallon, as chair of the GAMMASPHERE Users Group, presented informal input from the users of GAMMASPHERE concerning this possibility.

    The Physics Division is currently developing a funding proposal to the DOE that outlines various options for upgrating the gamma-detector array at the HRIBF.

    The program of the workshop is available for downloading. Please contact David Radford if you have questions and comments about issues discussed at the workshop.

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    6. RIA Summer School Update

    The third annual RIA Summer School on Exotic Beam Physics took place at Argonne National Laboratory from August 8-14, 2004, at the ATLAS Facility on the campus of Argonne National Laboratory, Argonne, Illinois. The aim of the summer school is to nurture future RIA scientists so that the community will have sufficient manpower to effectively use RIA when it comes online. The RIA Summer School is jointly organized by LBNL's 88-Inch Cyclotron, ANL's ATLAS, ORNL's HRIBF, and MSU's NSCL, and is an annual event, rotating among these laboratories.

    The total number of accepted students is 52 (42 graduate students and 10 postdocs), including 43 experimentalists and 9 theorists. Please see the summer school web site for more details about the school and its program.

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    7. ENAM'04 will be held September 12-16, 2004, at Callaway Gardens in Georgia

    The Fourth International Conference on Exotic Nuclei and Atomic Masses, ENAM'04, will be held at Callaway Gardens, September 12-16, 2004. The conference received more than 250 contributed abstracts and the program will consist of invited, oral, and poster presentations. Attendance is expected to be comparable to previous meetings held in Arles, France, Shanty Creek, MI, and Hämeenlinna, Finland. The conference originated by merging two other well-established conferences in 1995: Nuclei Far From Stability and the Atomic Masses and Fundamental Constants.

    The conference is organized by the Physics Division at Oak Ridge National Laboratory with Witek Nazarewicz and Carl Gross as co-chairs. The website is located at http://www.phy.ornl.gov/enam04/. The conference site is a well-known resort in the pine forests of Georgia about an hour southwest of Atlanta. The meeting will be held at the new Southern Pine Conference Center, and lodging will be in neighboring two-bedroom cottages. The Gardens encompass 14,000 acres with nature and bicycle trails, golf and tennis, butterfly pavilion, and much more.

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    8. SESAPS 2004 to Take Place in Oak Ridge November 11-13, 2004

    The 71st Annual Meeting of the Southeastern Section of the American Physical Society (SESAPS) will take place Thursday, November 11, through Saturday, November 13, 2004, in Oak Ridge, Tennessee. ORNL will serve as the official meeting host.

    The goal of SESAPS is the advancement and diffusion of the knowledge of physics within the Southeastern region of the United States, including the states of Alabama, Florida, Georgia, Kentucky, Louisiana, Mississippi, North Carolina, South Carolina, Tennessee, and Virginia. These states are home to many universities and colleges, as well as several national laboratories. Each November SESAPS brings together physicists from the region for a meeting hosted by either a university or a national laboratory.

    More information about SESAPS 2004 can be found at its web site.

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    9. Physics Staff Members Receive Prestigious Awards

  • Jeff Blackmon received the PECASE award in May

    Jeffery C. Blackmon of the Physics Division at Oak Ridge National Laboratory is one of five Department of Energy recipients of the latest Presidential Early Career Awards for Scientists and Engineers (PECASE), presented May 4, 2004, in a White House ceremony. Jeff is cited for his pioneering work performed at HRIBF toward understanding stellar explosions. The Presidential Early Career Awards program was established in 1996 to encourage and recognize the work of the nation's young scientists and engineers. In those years, ORNL researchers have received 11 of the awards, and Physics Division researchers have accounted for five PECASE awards.

    As a member of the Physics Division's Astrophysics group, Jeff has been involved in the development and operation of several devices at HRIBF, including the SIDAR array, the DRS spectrometer and the new windowless gas target. Jeff is also one of the key players in the group's experiments using the HRIBF's unique beams of radioactive fluorine isotopes and uranium fission fragments, which provided clues to reactions in cosmic novae and supernovae that are believed to be the origins of the synthesis of many of the elements. One of Jeff's most recent efforts has been a collaborative one to study transfer reactions using the neutron-rich beams now available from the fission target ion source.

  • Jim Beene named an ORNL Corporate Fellow
  • James R. Beene has recently been named a Corporate Research Fellow in ORNL's Physics Division. He is an experimental nuclear physicist, has served as Director of the Holifield Radioactive Ion Beam Facility since 1995, and also serves as Group Leader for low energy experimental nuclear physics. His research interests include the structure and reactions of nuclei far from stability and collective modes of nuclear response, the impact of these nuclear properties on astrophysical phenomena, and the physics of neutrinos and their interaction with nuclei. His citation notes his pioneering and seminal work in nuclear structure physics, notably concerning population and decay of giant resonances, and for directing the HRIBF to become a forefront facility for nuclear science, producing radioactive ion beams for nuclear physics research using the isotope-separator-online (ISOL) technique. Jim has led many of the developments at HRIBF, in particular the recent utilization of a uranium target to produce neutron-rich RIBs, making HRIBF unique in its ability to produce post-accelerated beams of both proton-rich and neutron-rich nuclei.

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    RA1 - RIB Development

    Recent target tests at the On-Line Test Facility (OLTF) include production of aluminum isotopes from a SiC target, chlorine isotopes from a CeS target, and n-rich fission fragments from a uranium carbide target. No large improvements have been made in the yields of 25Al, 26mAl, or 34Cl, and the projected accelerated beam intensities are still in the range of 104 ions/sec on target. We need to make some high power tests of our SiC and CeS targets to determine the maximum proton, or deuteron, beam intensity that can be used.

    Two uranium carbide (UC) targets from Argonne have been tested, and the yields are promising. The targets were manufactured from powders and pressed to densities that were higher than the average density of our standard targets where UC is deposited onto a low-density carbon matrix. The density of the standard UC target is about 1.2 g/cm3, while the densities of the pressed-powder targets that were tested were 2.5 g/cm3 and 6 g/cm3. The measured yields for the pressed powder targets averaged about 1.7 times higher than the measured yields from the lower density target. In addition, the targets are simpler and less expensive. We plan to make a high power test of these pressed-powder targets on the RIB Injector to see if the useful lifetimes (about 2 months) are comparable.

    Photofission of uranium can produce, in some cases, a much higher yield of interesting radioactive isotopes than can be produced by proton-induced fission. In past newsletters, there was mention of a project to verify this and to determine if the fission fragments could be efficiently transported in a He-jet to an ion source and formed into a beam. The initial tests were made with a Cf-252 source but the equipment has now been moved to the ORELA facility, which can deliver 150-MeV electron beams. On-line tests are planned for later this year.

    To make way for the new High Power Target Laboratory (HPTL), the Ion Source Test Facility II (ISTF-2) had to be relocated from Bldg. 6000 to a lab space in Bldg. 6010. It has been commissioned and is now fully functional in its new home. Upgrades that were implemented at this time include a new horizontal mass-analysis magnet with higher resolving power, a new RF power supply, a new water chiller/recirculator to provide cooling water to the magnet and the ion source, and an extension of the beam line after the magnet for increased flexibility.

    Performance tests of a flat-B field ECR ion source have resumed at the new ISTF-2 facility. Results from these recent tests will be included in the next newsletter. Some parameters that were investigated include the use of a biased disk in the plasma, the plasma potential, and a polarizer for the incoming RF signal. These tests will resume and be completed later this year after a test of a laser ion source. The laser ion source test is planned for September of this year in collaboration with Klaus Wendt and his group from Mainz. Klaus will provide three Ti:sapphire lasers for the test and ORNL is providing the pump laser and an ion source.

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    RA2 - Accelerator Systems Status

    ORIC Operations and Development

    ORIC has routinely provided 54-MeV proton beams to the target/ion source for neutron-rich RIB production throughout the period.  Generally, ORIC has continued to run well, but some unscheduled maintenance was required for repair of several aspects of the RF system.  Notable items requiring replacement or repair were the drive line insulator, a variable plate capacitor, and the power amplifier tube screen power supply.  Development activities have been minimized so that staff could focus on both routine accelerator operations and High Power Target Laboratory design efforts.  However, design studies continue for a possible future axial injection system for ORIC.

    Tandem Operations and Development

    The Tandem Accelerator was operational, to provide beam, for approximately 2389 hours since the last report.   The machine ran at terminal potentials of 3.26 to 23.76 MV and the stable beams 1,2H, 12C, 24Mg, 28Si, 35,37Cl, 54Fe, 58Ni, 70,76Ge, 76,82Se, 124Sn, and 122,126,130Te were provided.  Radioactive beams of 84Se, 118,120,122,124Ag, 129Sb, 130,131,132Sn, and 134,136Te accounted for 1142 hours. Three tank openings were necessary during this period; twice due to failures of the terminal bending magnet power supply and once because of a broken shorting rod string.  Both failures of the power supply were due to clogged heat sinks and included failures of output transistors which are obsolete and could not be replaced. The new power supply mentioned in the last report has been received but is not completely ready to be installed, but should be by the time we have a scheduled tank opening in late September.

    Approximately 228 hours of conditioning was done, primarily to keep the voltages up. The pair of units, 19/20, that had displayed deconditioning, seems to have gotten better with time. There have been a few other problems which may have been caused by broken resistors. At this time, there is one tube section (~1% of the machine) that is shorted due to an apparent resistor problem. Two other resistor problems were fixed during the last tank opening. The machine can probably run at 24 MV with no more conditioning, if everything stays as is.

    The ten-year internal inspection for the third and final storage tank was finished in June. This inspection, required by the Accelerator Safety Envelope, takes a significant effort to complete. The next inspection cycle will begin in eight years.

    Beam lines to the RMS and the OLTF have been removed for the HPTL construction project.

    RIB Injector Operations and Development

    During this reporting period, we delivered beams of

    These heavy neutron-rich beams were produced via proton induced fission of 238U by bombarding a uranium carbide coating on a reticulated vitreous carbon fiber target coupled to an Electron Beam Plasma (positive) Ion Source (EBPIS) with 6-7 uA of 54-MeV 1H.

    The pure tin beams were produced by passing a positive tin sulfide beam through the recirculating cesium jet charge exchange cell and selecting the negative tin beam resulting from molecular breakup. We replaced the charge exchange cell when suddenly, after years of reliable operation, frequent addition of cesium was required to maintain charge exchange efficiency. This may be related to the extended period when the cell was removed from the beam line during the negative 7Be campaign last year.

    Beams of several thousand particles per second (pps) of 340-MeV single foil stripped 14+ were measured in an ion chamber in Beam Line 23 to assay their silver, indium, and tin content.

    A vacuum accident caused the anode power supply to draw an arc, resulting in the failure of a target/ion source. A 300 mA PLC interlock was subsequently implemented for the anode power supply.

    A spurious actuation of the vacuum interlock for the isolation valve just upstream of the target/ion source resulted in 400 W of beam power being deposited in the valve disc, requiring repair of the valve. The vacuum interlock was subsequently removed.

    The original tape system at the 12_1 position in Beam Line 12 has been removed.

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    RA3 - Experimental Equipment - The New Windowless Gas Target System

    A windowless gas target system has been developed and commissioned at the HRIBF.  One of the primary uses of the gas target is to be the measurement of hydrogen fusion reactions important for nuclear astrophysics in inverse kinematics using radioactive ion beams from the HRIBF. The gas target is currently installed at the target position of the Daresbury Recoil Separator (DRS). The target consists of a central disk-shaped chamber with 4 differential pumping stages on the downstream and upstream sides. A photograph of the target is shown in Figure 1. A series of apertures, through which the beam and reaction products pass, separates each stage. The most restrictive apertures are the entrance and exit apertures of the central disk. As the target is currently configured, the entrance and exit apertures have diameters of 3 mm and 6.5 mm, respectively, and are separated by a distance of 14 cm. Good transmission (more than 75%) of the beam through the gas target apertures is typically achieved due to the good emittance of the HRIBF tandem beam.



    Figure RA3-1:  Photograph of the windowless gas target.  The beam enters from the left of the figure.  A constant uniform pressure is maintained throughout the 14-cm wide central disk.  Silicon detectors with double collimators are mounted at various angles around the central chamber to monitor elastic scattering of the beam with the gas.

    The pressure profile of the target has been characterized with hydrogen and helium gases using various vacuum instruments and by scanning a collimated NaI detector to measure the yield of gamma rays from the 1H(19F,alpha-gamma) reaction.  The pressure in the central disk is uniform.  Large roots vacuum pumps on the first pumping stages closest to the central chamber reduce the pressure to a few percent of the pressure in the central disk. The other stages are pumped by turbo-molecular pumps, and the pressure is reduced by about a factor of about 106 in a distance of less than 0.5 m.

    The total areal density of the target was determined by measuring the energy loss of charged particles with known stopping powers. At a central pressure of 6 Torr, the areal density of hydrogen was found to be 7.6x1018 atoms/cm2, corresponding to an effective length of 19.7 +/- 0.6 cm. Measurements at a central pressure of 4 Torr and 5 Torr of hydrogen determined an areal density consistent with the same effective length. Measurements with helium gas determined the effective length of the target to be 19.6 (0.5) cm, corresponding to 3.8x1018 atoms/cm2 at 6 Torr.

    The exit apertures of the gas target as currently configured accept a recoil cone of 0.1 msr, much less than the angular acceptance of the (DRS).  Larger exit apertures may be used to increase the acceptance, but the resulting higher pressure DRS velocity filters can limit the voltages on the electrostatic plates.  The 1H(12C,13N) capture reaction near the energy region of the well-known broad Ecm=1.7-MeV resonance was measured as a  test of the performance of the gas target with the DRS. A particle identification spectrum from the ionization chamber at the focal plane of the DRS is shown in Figure 2. These data were collected using a 180 particle-nA beam of 12C beam at 20.23 MeV on the gas target operated at a central pressure of 5 Torr of hydrogen. A 13N count rate of 70 Hz was observed at the focal plane, with the 12C beam being suppressed by a factor of approximately 4x10-11.

    12C+1H fusion

    Figure RA3-2:  Particle identification spectrum from the ionization chamber at the focal plane of the DRS with a 12C beam incident on hydrogen gas at an energy of 20.2 MeV.


    Other gas species can be run in the gas target, and the exit apertures may be somewhat increased for greater acceptance or decreased to allow greater target pressures. A pressure of up to 6 Torr of hydrogen can be operated in the gas target under the current configuration while still maintaining pressures in the first velocity filter of the DRS of less than 1x10-5 Torr. More effective pumping is achieved with heavier gases, allowing higher operating pressures. Pressures up to 10 Torr are possible with helium and significantly higher pressures with other heavier gases, e.g. nitrogen or neon.

    The gas target is currently operational and available for experiments. Measurements of the 1H(7Be,8B) and 1H(17F,18Ne) reactions are planned in the near term.  The gas target is also being used to measure stopping powers for various beam species in hydrogen and helium gas. A recirculating system is also currently being developed that will allow measurements to be performed with a small inventory of target gas.

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    RA4 - Users Group News - Election and Annual Meeting

    The Users Executive Committee election is scheduled for October of this year. Under the new charter, a nominating committee has been formed by the outgoing members, Ed Zganjar and Paul Mantica, along with Witek Nazarewicz. The slate of candidates are:

    Seat vacated by Ed Zganjar:
    Robert Grzywacz (University of Tennessee)
    Jeff Winger (Mississippi State University)
    Seat vacated by Paul Mantica:
    Jolie Cizewski (Rutgers University)
    Walt Loveland (Oregon State University)
    According to the new charter, the user group at-large may also nominate additional candidates. Ten members should submit letters of support to
    Carl Gross on or before September 20. Please identify whose seat the candidate should compete.

    See the new charter for details and requirements. The new members will join Uwe Greife (Colorado School of Mines) and David Radford (ORNL) on the committee and will serve four years.

    The HRIBF Users Group will hold its annual meeting at the Fall Meeting of the DNP in Chicago on Thursday, October 28. The format will be similar to last year's meeting with brief presentations from HRIBF, ATLAS, NSCL, GAMMASPHERE-GRETINA, and RIA Users Groups followed by a social hour. Come join us and learn what has been happening at our facility!

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    RA6 - HRIBF Experiments, January - June 2004

    Date Exp. No. Spokesperson Title of Experiment
    1/1-4 Shutdown
    1/5-7 RIB-014 Stracener/ORNL Target ion source development
    1/7-9 RIB-013 Blackmon /ORNL DRS commissioning
    1/9 RIB-037 Meigs,Juras/ORNL Tandem development
    1/10-11 Shutdown
    1/12 RIB-000 Gross/ORNL
    1/13 RIB-117 Thomas/Rutgers University Single-particle states in neutron-rich nuclei near the N=50 closed shell: 83Ge
    1/13-14 RIB-000 Gross/ORNL
    1/14-15 RIB-037 Meigs,Juras/ORNL Tandem development
    1/15-16 RIB-122 Winger/Mississippi State University Structure of neutron-rich nuclei near 78Ni studied via beta-decay of post-accelerated Cu beams - Static quadrupole moment of the first 2+ level in 126Sn via the reorientation effect
    1/17-19 Shutdown
    1/19 RIB-122 Winger/Mississippi State University Structure of neutron-rich nuclei near 78Ni studied via beta-decay of post-accelerated Cu beams - Static quadrupole moment of the first 2+ level in 126Sn via the reorientation effect
    1/20-21 RIB-117 Thomas/Rutgers University Single-particle states in neutron-rich nuclei near the N=50 closed shell: 83Ge
    1/21-23 RIB-014 Stracener/ORNL Target ion source development
    1/23-25 Shutdown
    1/26-27 RIB-014 Stracener/ORNL Target ion source development
    1/27-29 RIB-013 Blackmon/ORNL DRS commissioning
    1/29-30 RIB-068 Galindo-Uribarri/ORNL HYBALL charged particle detector system response
    1/30 RIB-105 Hausladen/ORNL Proof of principle for mass measurements using the tandem accelerator
    1/31-2/1 Shutdown
    2/2-6 RIB-060 Galindo-Uribarri/ORNL Opportunities for accelerator mass spectrometry at HRIBF
    2/7-8 Shutdown
    2/9 RIB-013 Blackmon/ORNL DRS commissioning
    2/10 RIB-117 Thomas/Rutgers University Single-particle states in neutron-rich nuclei near the N=50 closed shell: 83Ge
    2/10-13 RIB-039 Mueller/ORNL High voltage injector development
    2/14-15 Shutdown
    2/16 RIB-117 Thomas/Rutgers University Single-particle states in neutron-rich nuclei near the N=50 closed shell: 83Ge
    2/16-18 RIB-039 Mueller/ORNL High voltage injector development
    2/18-3/2 RIB-117 Thomas/Rutgers University Single-particle states in neutron-rich nuclei near the N=50 closed shell: 83Ge
    3/2-4 RIB-122 Winger/Mississippi State University Structure of neutron-rich nuclei near 78Ni studied via beta-decay of postaccelerated Cu beams - Static quadrupole moment of the first 2+ level in 126Sn via the reorientation effect
    3/4-5 RIB-107 Krolas/University of Tennessee; Piechaczek/Louisiana State University Evolution of the (proton h11/2 neutron h11/2) and (proton h11/2 neutron s1/2) configurations in deformed N=77 isotones near the proton drip line: Decay studies of metastable states in 142Tb, and 144Ho
    3/6-7 Shutdown
    3/8-9 RIB-107 Krolas/University of Tennessee; Piechaczek/Louisiana State University Evolution of the (proton h11/2 neutron h11/2) and (proton h11/2 neutron s1/2) configurations in deformed N=77 isotones near the proton drip line: Decay studies of metastable states in 142Tb, and 144Ho
    3/9-10 RIB-035 Stracener/ORNL Target ion source development
    3/11 RIB-013 Blackmon/ORNL DRS commissioning
    3/11-12 RIB-068 Galindo-Uribarri/ORNL HYBALL charged particle detector system response
    3/13-14 Shutdown
    3/15 RIB-068 Galindo-Uribarri/ORNL HYBALL charged particle detector system response
    3/16 RIB-037 Meigs,Juras/ORNL Tandem development
    3/17 RIB-068 Galindo-Uribarri/ORNL HYBALL charged particle detector system response
    3/17-19 RIB-124 Yu/ORNL A pilot study of B(E2) values in odd-A nuclei via Coulomb excitation using neutron-rich radioactive beams
    3/20-21 Shutdown
    3/22-25 RIB-124 Yu/ORNL A pilot study of B(E2) values in odd-A nuclei via Coulomb excitation using neutron-rich radioactive beams
    3/25-28 RIB-116 Radford/ORNL Identification of the neutron i13/2 state in 135Te by the 13C(134Te,12C)135Te neutron-transfer reaction
    3/29-4/4 Shutdown
    4/5 RIB-037 Meigs,Juras/ORNL Tandem development
    4/5-7 RIB-116 Radford/ORNL Identification of the neutron i13/2 state in 135Te by the 13C(134Te,12C)135Te neutron-transfer reaction
    4/8 RIB-037 Meigs,Juras/ORNL Tandem development
    4/9-13 Shutdown
    4/13-14 RIB-127 Stone/University of Tennessee Request for exploratory test time relating to recoil in vacuum g-factor studies at HRIBF, preparatory to full proposal
    4/14-16 RIB-013 Blackmon/ORNL DRS commissioning
    4/16 RIB-035 Stracener/ORNL Target ion source development
    4/17-18 Shutdown
    4/19-20 RIB-127 Stone/University of Tennessee Request for exploratory test time relating to recoil in vacuum g-factor studies at HRIBF, preparatory to full proposal
    4/20-21 RIB-035 Stracener/ORNL Target ion source development
    4/21-24 RIB-084 Gomez del Campo/ORNL Study of neutron-rich Sb nuclei via proton pickup reactions
    4/25-26 RIB-037 Meigs,Juras/ORNL Tandem development
    4/26-27 RIB-084 Gomez del Campo/ORNL Study of neutron-rich Sb nuclei via proton pickup reactions
    4/27-5/3 Shutdown
    5/3 RIB-037 Meigs,Juras/ORNL Tandem development
    5/4-7 RIB-084 Gomez del Campo/ORNL Study of neutron-rich Sb nuclei via proton pickup reactions
    5/7 RIB-039 Mueller/ORNL High voltage injector development
    5/8-9 Shutdown
    5/10 RIB-035 Stracener/ORNL Target ion source development
    5/10 RIB-037 Meigs,Juras/ORNL Tandem development
    5/11-13 RIB-085 Loveland/Oregon State University Study of fusion enhancement/hinderance with massive neutron-rich projectiles
    5/14 RIB-037 Meigs,Juras/ORNL Tandem development
    5/14 RIB-085 Loveland/Oregon State University Study of fusion enhancement/hinderance with massive neutron-rich projectiles
    5/15 RIB-037 Meigs,Juras/ORNL Tandem development
    5/15-17 RIB-085 Loveland/Oregon State University Study of fusion enhancement/hinderance with massive neutron-rich projectiles
    5/17 RIB-037 Meigs,Juras/ORNL Tandem development
    5/18-20 RIB-085 Loveland/Oregon State University Study of fusion enhancement/hinderance with massive neutron-rich projectiles
    5/20-25 RIB-084 Gomez del Campo/ORNL Study of neutron-rich Sb nuclei via proton pickup reactions
    5/26-28 RIB-116 Radford/ORNL Identification of the neutron i13/2 state in 135Te by the 13C(134Te,12C)135Te neutron-transfer reaction
    5/29-31 Shutdown
    6/1-9 RIB-114 Baktash/ORNL Coulomb excitation of the low-lying states in 136Te
    6/9-10 RIB-140 Padilla-Rodal/UNAM Request for extension of RIB077: COULEX of neutron-rich Ge isotopes near the N=50 shell closure
    6/10-11 RIB-013 Blackmon/ORNL DRS commissioning
    6/11-15 RIB-107 Krolas/University of Tennessee; Piechaczek/Louisiana State University Evolution of the (proton h11/2 neutron h11/2) and (proton h11/2 neutron s1/2) configurations in deformed N=77 isotones near the proton drip line: Decay studies of metastable states in 142Tb, and 144Ho
    6/15-17 RIB-013 Blackmon/ORNL DRS commissioning
    6/17-23 Shutdown
    6/23-24 RIB-037 Meigs,Juras/ORNL Tandem development
    6/25 RIB-121 Shapira/ORNL Subbarrier fusion of 134Sn with 64Ni
    6/28-29 RIB-121 Shapira/ORNL Subbarrier fusion of 134Sn with 64Ni
    6/29-30 RIB-013 Blackmon/ORNL DRS commissioning
    6/26-27 Shutdown

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    You may contact us at the addresses below.

    Chang-Hong Yu Carl J. Gross Witek Nazarewicz
    Newsletter Editor Scientific Liaison Scientific Director
    Mail Stop 6371 Mail Stop 6371 Mail Stop 6368
    chy@mail.phy.ornl.gov cgross@mail.phy.ornl.gov witek@mail.phy.ornl.gov
    +1-865-574-4493 +1-865-576-7698 +1-865-574-4580

    Holifield Radioactive Ion Beam Facility
    Oak Ridge National Laboratory
    Oak Ridge, Tennessee 37831 USA
    Telephone: +1-865-574-4113
    Facsimile: +1-865-574-1268