| HRIBF NEWS |
|
| HRIBF NEWS |
| |
||
| Edition 12, No. 1 | February 2004 | Price: FREE |
| |
||
Feature ArticlesRegular Articles
- 1. HRIBF Update and Near-Term Schedule
- 2. Call for Proposals; PAC-10 Meeting Scheduled for May 6-7, 2004
- 3. Recent HRIBF Research - First Identification of Excited States in 61Ga and 62Ge
- 4. Recent HRIBF Research - Test of Calculations with Single-Particle Density Dependent Pairing in 132Te
- 5. Results from PAC-9
- 6. Dan Bardayan Received Prestigious UT-Battelle Award
- 7. High-Power Target Laboratory Update
- 8. HRIBF Workshop on In-Beam Gamma-Ray Spectroscopy to be held
on April 5-7, 2004, at ORNL- 9. ENAM'04 will be held September 12-16, 2004, at Callaway Gardens in Georgia
- 10. Decay Spectroscopy Workshop Held at HRIBF in August 2003
Editors: C.-H. Yu, C. J. Gross, and W. Nazarewicz
Feature contributors: J. R. Beene,
C. J. Gross, D. C. Radford,
D. Rudolph, M. S. Smith, B. A. Tatum,
K. Rykaczewski, V. Zamfir
Regular contributors: C. J. Gross, P. A. Hausladen
M. R. Lay, M. J. Meigs, A. J. Mendez, P. E. Mueller,
D. W. Stracener, B. A. Tatum, C.-H. Yu
These are exciting times at the HRIBF. The High Power Target Laboratory (HPTL) project, the first major upgrade to the HRIBF since it was commissioned in 1997, is now officially underway, and we are in the process of developing the proposal for an additional upgrade project. Nevertheless, the past few months have not been uniformly successful. The next neutron-rich beams campaign, which is now underway, was scheduled to begin in late fall, but was delayed. It is critical that we make up for some of this lost time by running the facility efficiently and effectively over the next several months, in parallel with the HPTL-related demolition and construction. This will be a challenge, but it is a challenge we must meet.
The current neutron-rich campaign will run until April, and may be extended if target/ion source performance looks favorable, since work on the HPTL will make delivery of beam to the RMS and the On-Line Test Facility (OLTF) impossible for several months beginning in late May. At this point, we plan a campaign of radioactive fluorine experiments followed by a 7Be campaign during which we hope to complete a 7Be(p, gamma) measurement. Recent developments on 7Be have been very promising, including a proton elastic scattering measurement with a 7Be beam in October and November. We believe we can increase the 7Be beam intensity by more than an order of magnitude over what was produced in the October test run. These campaigns, with a modest sprinkling of associated stable-beam runs, should take us through the end of the fiscal year (October 2004).
It is worth mentioning that some of the positive developments mentioned in the last newsletter are proving their value. The improvement in ORIC extraction that offered the prospect of an increase in proton beam energy from 42 MeV to "above 50 MeV" is now being routinely utilized to produce 54-MeV proton beams. The really substantial list of upgrades and improvements made to the tandem during the extended maintenance period last summer are having major impacts on operations and will lead to substantial reduction in our maintenance load. The new recirculating gas stripper is a good example. It improves our normal gas stripping and offers us a new mode of operation for dealing with molecular beams; molecular breakup in dilute gas followed by foil stripping, which dramatically reduces degradation of beam quality and intensity due to effects of Coulomb explosion. The new gas stripper also eliminates the titanium sublimation pumps associated with the gas stripper it replaces -- eliminating a minimum of two to three weeks of maintenance every year and eliminating the danger of an accident spreading titanium dust through the accelerator tubes (it has happened before). The old foil stripper shared a common vacuum enclosure with a stripper foil magazine, adding significant outgas-time overhead to routine foil replacement. This situation was also corrected.
HRIBF is pleased to issue a Call for Proposals for consideration by the next Program Advisory Committee (PAC-10) meeting. This Call is for any experiment using all available radioactive and stable beams. The deadline for the receipt of the proposals is April 4, 2004. Proposals are only accepted electronically and may be submitted by email attachment to liaison@mail.phy.ornl.gov.
Specific instructions and information on HRIBF equipment may be found on our website and in past newsletters. Alternatively, you may contact the liaison officer at the address above. The Program Advisory Committee will meet on May 6-7, 2004, in Oak Ridge. Spokespersons should be informed of the decisions by June 1, 2004.
| J. Äystö | University of Jyväskylä |
| C. Baktash | Oak Ridge National Laboratory |
| J. C. Hardy (chair) | Texas A & M University |
| R. V. F. Janssens | Argonne National Laboratory |
| W. Loveland | Oregon State University |
| A. C. Shotter | TRIUMF |
| M. Wiescher | University of Notre Dame |
Isospin T is a good quantum number under the fundamental assumption of charge symmetry and charge independence of the strong force. Isospin breaking effects can be studied in pairs of mirror nuclei, in which the number of protons and neutrons are interchanged. They lead to shifts of typically 10-100 keV between the excitation energies of a mirror pair, which have proven to be precise and challenging probes of nuclear structure.
In an attempt to extend the experimental knowledge of mirror nuclei into the mass A=60 region, the stable-beam reaction 40Ca + 24Mg at 104 MeV was used in August 2003 to identify excited states in the hitherto unknown isotopes 61Ga (Z=31,N=30) and 62Ge (Z=32,N=30). The experimental setup comprised CLARION, the RMS, and the Ionization Chamber, and the collaboration saw students and physicists from Lund University, Sweden, Keele University, UK, and the ORNL Physics Division.
The analysis of the data set has been performed by two Master students at Lund University, namely Lise-Lotte Andersson and Emma Johansson. Despite problems with the ion source during the second week of the experiment, they were able to clearly identify for the first time gamma-ray transitions in 61Ga [1] and to provide tentative evidence for the 4+ -> 2+ -> 0+ cascade in 62Ge [2]. Elaborate Doppler correction routines using both the segmentation of the CLARION detectors and the total energy deposited in the ion chamber were developed along with novel approaches to obtain optimal Z-resolution by trying and comparing several combinations of the three energy-loss signals of the ion chamber, so-called energy-loss functions [1,2].
Figure 3-1 shows the comparison of two mass A=61 gated spectra, which are sliced at different regimes of an energy-loss function and with peaks originating from 61Cu already subtracted. The blue spectrum contains transitions from 61Zn only, while the red spectrum should comprise transitions from both 61Zn and 61Ga. Clearly, there is a peak at 271 keV in the red spectrum, which is absent in the blue spectrum. Another weak and tentative transition may be identified at 220 keV.
Fig. 3-1: Normalized gamma-ray spectra comprising transitions from 61Zn and 61Ga (red) and only 61Zn (blue).
The 271-keV transition in 61Ga is believed to be the "mirror" transition to the 124-keV 5/2- -> 3/2- ground state transition in 61Zn. The rather large energy difference of 150 keV is most likely due to Coulomb monopole effects such as radial or electromagnetic spin orbit contributions. The former may play a significant role as 61Ga is bound by only some 200 keV [3]. The latter arise from proton vs. neutron excitations from the j=l+1/2 p3/2 orbital into the j=l-1/2 f5/2 orbital, similar to recent findings in the A=35 mirror system [4]. A similarly large energy difference is (tentatively) seen for the 4+ state in the 62Ge-62Zn pair. In both cases large-scale shell-model calculations seem to support this preliminary interpretation.
[1] L.-L. Andersson, Master thesis, Lund University,
LUNFD6/(NFFR-5022)1-47/(2004);
http://wwwnsg.nuclear.lu.se/projects.asp .
[2] E.K. Johansson, Master thesis, Lund University,
LUNFD6/(NFFR-5023)1-47/(2004);
http://wwwnsg.nuclear.lu.se/projects.asp .
[3] L. Weissman et al., Phys. Rev. C 65, 044321 (2002).
[4] J. Ekman et al., Phys. Rev. Lett., in press.
The recent Coulomb excitation studies with beam of exotic nuclei have mapped level energies and B(E2) values across the N = 82 shell closure disclosing anomalies in the structure of Te isotopes with N > 82 [1,2]. These experimental results have motivated new microscopic calculations in which the dependence of pairing on single particle level density is explicitly taken into account [3]. New data, obtained from beta decay of 132Sb radioactive beam at HRIBF, has led to a significantly revised gamma-decay scheme for 132Te. The changes to the level scheme include a number of new, likely 2+, states below 2.5 MeV, which allows a test of very recent quasiparticle random phase approximation calculations with a density-dependent pairing force, and the removal of a 3- state at 2280 keV which resolves an incompatibility with the shell model and leads to a simple interpretation of the low lying negative-parity states.
The nucleus 132Te was populated in beta decay and studied through gamma-ray coincidence spectroscopy. A radioactive ion beam of about 107 particles/s of 132Sb at 396 MeV was embedded in a thick 14.3 mg/cm2 Al + 1.0 mg/cm2 C foil target. The 132Sb nuclei decay via two beta channels, with half-lives of 2.8 minutes and 4.2 minutes from the 4+ ground state and 8- excited state, respectively, to 132Te. The subsequent gamma rays were detected with the CLARION array [4] consisting of eleven clover Ge detectors with a total photopeak efficiency of 2.3% for a 1.33-MeV gamma ray (at 22 cm from the target). The experiment was run for two days and a total of ~2.5x107 gamma-gamma events were collected.
Fig. 4-1 Low-lying levels in 132Te populated in 132Sb beta decay and their depopulating gamma-ray transitions with energies in keV (uncertainties +- 0.2 keV) and, in parentheses, their relative intensities. New levels and gamma rays identified in the present work are marked in red.
The new gamma-gamma coincidence data has led to a significantly revised gamma-decay scheme for 132Te. The new work has confirmed 20 of the transitions in the previous scheme and given 19 other transitions new placements. There are also 198 new transitions that have been added. Due to the higher statistics, the lowest resolvable intensity was reduced by more than an order of magnitude, down to about 0.02% per decay from the previous intensity of about 0.5%. Figure 4-1 shows the level scheme of 132Te up to 2400 keV as deduced in this work. Newly placed gamma rays and those with revised placements are marked in red. It is seen that, in fact, they comprise half of the transitions connecting these levels.
Five new levels were found below 2500 keV (marked in red in Fig. 4-1). Based on the fact that they decay only to the 0+ ground state and the 2+ first excited state, tentative 2+ assignments have been made to four of these new levels, namely those at 1665 keV, 1788 keV, 2249 keV, and 2364 keV. The 2+ states identified in this work have significant consequences for our understanding of this region. Recently, an anomaly was found in B(E2; 0+1 --> 2+1) values and 2+1 energies in the 132Sn region [5]. Normally, these two quantities vary inversely to each other. However, if one compares 132Te80 and 136Te84, both the 2+1 energy and the B(E2) value are lower in 136Te. In an effort to understand this anomaly, Terasaki et al. [3], noted that the density of neutron single particle levels below and above N = 82 were quite different, being denser below N = 82. This would give a reduced neutron pairing gap above N = 82 and, in turn, a lower 2+1 energy. Since the 2+1 state could then also be more neutron-dominated, a lower B(E2) value than for 132Te would also result.
Fig. 4-2: Comparison of experimental and theoretical [3] 2+ states in 132Te.
However, the interpretation in Ref. [3] was developed a posteriori to account for an already observed violation of the Grodzins rule [E(2+1) x B(E2; 0+1 --> 2+1) ~ constant]. It is therefore very valuable to have an independent test of these calculations. The present set of 2+ energies for 132Te does precisely that. A comparison of these energies with those predicted in the calculations of Ref. [3] is shown in Fig. 4-2. Clearly, the agreement is quite good. The correct number of low lying 2+ levels is predicted and, except for one state, at approximately the observed energies.
Another interesting feature of the level scheme concerns the lowest negative-parity states. Previous literature evaluations give the set 7- (1925 keV), 5- (2053 keV) and 3- (2280 keV). Negative parity levels available at low energies must involve the 1h11/1 orbit (both for neutrons and protons) and, for neutrons, either the 2d3/2 or 3s1/2 orbits, or, for protons, the 2d5/2 and 1g7/2 orbits are relevant. The previously existing level scheme presented a significant puzzle. Only the proton excitations include a 3- level. For the |1h11/22d5/2J> configuration, it should be lower than the 7- and 5- levels, not higher. Therefore, no simple configuration gives the previous assigned level sequence.
However, the placement of the 3- state was based primarily on three transitions from this level to the 4+1, 2+1 and 0+1 states. The new coincidence data show that all three gamma rays have the reported intensities but, in fact, must be placed elsewhere. The new placements mean there are no transitions supporting a 3- level at 2280 keV, and so it can be removed from the level scheme. With the removal of the 3- level the low lying negative-parity states can now be naturally explained as having a large amplitude for the neutron configuration |n1h11/22d3/2 for which the sequence, in order of increasing energy, should be 7-, 5-, and nearly degenerate 6- and 4- levels. The 6- level at 2422 keV is likely part of this multiplet. These assignments thus fit the new level scheme quite well.
To recapitulate, in this study, a large number of changes were made to the existing 132Te level scheme. Many new transitions were placed, and new levels proposed. A number of previous placements were found to be inconsistent with the high quality coincidence data presented here. Several previously proposed levels were shown not to exist. Some of these changes also appear in the unpublished work of Ref. [6].
This work is the result of a collaboration between Yale University and ORNL.
[1] D. C. Radford et al., Phys. Rev. Lett. 88
222501 (2002).
[2] C. J. Barton et al., Phys. Lett. B 551, 269
(2003).
[3] J. Terasaki, J. Engel, W. Nazarewicz, and M.
Stoitsov, Phys. Rev. C 66, 054313 (2002) and private
communication.
[4.] C. J. Gross et al., Nucl. Instrum. Methods Phys.
Res. A 450, 12 (2000).
[5.] D. C. Radford et al., Eur. Phys. J. A15,
171 (2002).
[6] R. A. Meyer and E.A. Henry, unpublished, private communication.
At the 2003 UT-Battelle Awards Night held last November, ORNL Physics Division's Dan Bardayan was honored with the Early Career Award for Scientific Accomplishment. This award recognizes an ORNL staff member who, in the early years of his career, has made significant scientific, engineering or technical contributions in support of UT-Battelle's programmatic goals. UT-Battelle is the contractor who manages ORNL for the U.S. Department of Energy. Dan won the award "For innovative precision spectroscopy measurements that clarify the production of elements and radioisotopes in exploding stars." For his Yale University Ph.D. thesis under Prof. Peter Parker, Dan led the 1999 HRIBF measurement of the 17F(p, p)17F reaction, the first measurement with a reaccelerated radioactive ion beam in North America. In 2000, Dan was awarded the American Physical Society Dissertation Award in Nuclear Physics for this work. He has since gone on to lead additional HRIBF experiments, including precision measurements of the 18F(p,p)18F and 18F(p,alpha)15O reactions. Dan also serves as the mentor for the Silicon Detector Array (SIDAR), which was recently used in a measurement of the 82Ge(d,p)83Ge reaction by a Rutgers/ORNL/Tennessee Tech/Colorado School of Mines/Univ. of Tennessee collaboration. This was the first transfer reaction measurement on a nucleus in the astrophysical r-process path. Congratulations to Dan on these accomplishments and on winning this prestigious UT-Battelle award!
The High Power Target Laboratory (HPTL) project is now well underway. As mentioned in the previous edition, this is the first phase of a planned series of upgrades to the HRIBF. The HPTL will provide a location, decoupled from the production environment, where new and existing target geometries can be tested with high power light ion beams provided by ORIC. This important addition to the facility will yielding crucial information on RIB target design and provide a location for development of new beam species of interest to the user community.
A Project Management Plan was completed in July and the first phase of funding ($1.2M) became available for use in August. An additional $1.8M in FY04 funds are now available as well. There are two primary components to the HPTL: Facility Modifications, which entails reconfiguration of existing areas within building 6000, and Technical Equipment, which includes a new ORIC beamline, high voltage platform structure, target station, and RIB analysis system.
Initial efforts have focused on the completion of the Facility Modifications design. The A&E subcontractor has recently completed this effort and the project is certified for construction. A construction contract is expected to be awarded by the end of March. Key elements of the facility design include a target room with 7-foot thick poured concrete shielding walls, and a lightly-shielded instrumentation room. Provisions have been made to develop a RIB analysis system in available space above the target room. Rooms C-112, C-113, and the west end of the South Annex have been emptied in preparation for demolition. The "Purple and Orange" modular office buildings in the high bay have also been demolished.
Work is also progressing well regarding the Technical Equipment. An order was placed with NEC in December to fabricate the high voltage platform structure. Projected delivery is September 24. The ORIC beamline optics design has been completed and a system of four dipoles will be ordered by the end of March. Vacuum system design is also in progress.
HRIBF is planning a workshop on in-beam gamma-ray spectroscopy, to be held at ORNL on April 5-7, 2004. The purpose of this workshop is 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. In particular, we are currently exploring the possibility of siting GAMMASPHERE at the HRIBF, or of upgrading the efficiency of the CLARION array of segmented clover detectors. In this regard, we are actively seeking input from the users of the HRIBF and of GAMMASPHERE concerning the exciting physics that these options would offer, as well as any potential adverse effect on other research programs due to a reduced emphasis on stable-beam operation of GAMMASPHERE.
The recent production and acceleration of intense, neutron-rich radioactive ion beams (RIBs) at the HRIBF has yielded interesting results in the Coulomb excitation of these beams. Single-nucleon transfer reactions from light ions have been shown to populate excited states using RIBs. HRIBF has an extensive arsenal of in-beam gamma-ray detectors that are used for these experiments. The central germanium detector array is CLARION, which consists of 11 segmented Clover detectors. In addition, experiments have been carried out using the ORNL-MSU-TAMU Barium Fluoride Array, and we anticipate in the near future to reinstate the Spin Spectrometer, a 4pi array of 70 NaI detectors. An upgrade of CLARION by the addition of up to six Clovers, or of one clover and eleven large coaxial detectors, is under active consideration. The possible coupling of RIBs and GAMMASPHERE (9% efficiency at 1.33 MeV) would greatly enhance our high-resolution gamma-gamma detection capabilities and should provide opportunities to do much more detailed studies of nuclei far from stability.
At this time, we envision a mixture of overview talks, contributed talks, and working sessions that will target specific issues or types of experiments. There will be no registration fee for the workshop, and lunches will be provided. Details, such as registration, location, and lodging, can be found at http://www.phy.ornl.gov/workshops/gamma/.
The Fourth International Conference on Exotic Nuclei and Atomic Masses, ENAM'04, will be held at Callaway Gardens, September 12-16, 2004. The conference is held every three years with 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.
A call for contributed abstracts is expected to be issued shortly with a submission deadline of mid-April. We wish to encourage students and postdocs to apply as we have made some successful efforts to acquire funding support targeted to young researchers.
The conference is organized by 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 Pines 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.
On 18th-19th August 2003, the Executive Committee of the HRIBF Users Group hosted a workshop on nuclear decay studies at the HRIBF. Participants of the workshop include 55 scientists from 20 laboratories and universities representing six countries (Finland, Germany, Japan, Switzerland, UK and US). A total of twenty-five talks were presented, followed by much discussion.
The purpose of the workshop was to discuss current decay spectroscopy results and capabilities of the HRIBF and to develop new ideas and new collaborations for future programs. In particular, the recent developments of pure RIB beams produced from the fission of uranium carbide at the HRIBF triggered the talks about the measurements utilizing these neutron rich RIBs.
The HRIBF results, potential and detection systems were presented in the talks of C.J.Gross (including a prototype of a compact gas cell for the Z-identification and range discrimination), P. Hausladen (mass measurements of exotic Cu and Ge isotopes), R. Grzywacz (a setup for picoseconds level lifetimes measurements), K. Carter and J. Batchelder (beta decay studies of neutron rich nuclei), K. Rykaczewski (proton radioactivity studies) and A. Piechaczek (decay studies of N=Z nuclei 80Zr and 84Mo).
The capabilities of competing facilities presented in the talks reporting the results:
The concepts of new detectors and experimental methods for the HRIBF were discussed in the talks of C. J. Gross and W. Krolas (a RIB version of an isomerscope for neutron rich isomers produced in deep inelastic collisions), A. Woehr (efficient beta-delayed neutron counter), P. Regan (a setup for spectroscopy of products from RIB DIC collisions), K. Rykaczewski (proton emission studied at ENGE spectrometer with radioactive 56Ni beam) and by K. Carter and J. Batchelder (a development of Multi-pass Time-of-Flight device).
New theoretical developments and interpretations profiting from the results obtained with decay spectroscopy methods were presented in the talks of H. Grawe (nuclear structure near 100Sn and 78Ni), J. Wood (shape coexistence in neutron-rich nuclei), W. B. Walters (structure of nuclei near 78Ni and 132Sn) and W. Nazarewicz (proton emission from deformed nuclei). The astrophysical applications of decay data were included in a talk by M. Wiescher on cosmic signatures for nuclear decay.
An on-line test of a different type of uranium carbide (UC) target was made and the release rates were comparable (in some, cases, slightly better) to those measured with the standard HRIBF UC targets. The RIB yields were measured for both proton-induced and deuteron-induced fission with 40 MeV protons and deuterons at the On-Line Test Facility (OLTF, formerly UNISOR). The new UC samples were manufactured at Argonne National Laboratory by John Greene and Jerry Nolen from uranium oxide powder mixed with graphite powder. The mixture of powders was pressed into a pellet and then heated up to 1900° C for several hours to convert the uranium oxide to uranium carbide. We will soon measure release rates from a pressed-pellet target made directly from uranium carbide powder. The standard HRIBF UC targets are made by depositing a uranyl nitrate solution onto the fibers of an open-porosity, low-density, rugged graphite matrix. At about 300° C, the uranyl nitrate is converted to uranium oxide. This step is repeated until the desired amount of uranium is deposited onto the matrix and then the oxide is converted to carbide at high temperature (1900° C) for a few hours. Additional carbon atoms must be supplied to the system before heating in the last step to prevent the destruction of the carbon matrix. This process is quite expensive and requires unique capabilities to handle radioactive samples in a wet chemistry environment. The advantages of the pressed powder targets are the reduced cost and the ability to easily vary the density. The analysis of the yield data is not yet complete, but we plan to update the radioactive ion beam yield information on the HRIBF webpage as soon as possible.
On the proton-rich side, we are presently developing targets for 25,26Al and 33,34Cl beams. We have reported yields for some of these isotopes in previous HRIBF Newsletters, but the yields are low and some target development is needed. To this end, we have tested a new SiC target that was manufactured using a "paint" technique. In this technique, a small-diameter (about 1 micron) powder is suspended in an organic binder, forming a paint solution. This paint is vacuum-infiltrated into a low-density support matrix and the target is heated to 800° C to drive off the binder, leaving behind the target material on the fibers of the matrix. The support matrix is reticulated vitreous carbon, the same matrix used for the uranium carbide targets mentioned above. As previously reported, the measured yields of aluminum ions increased when SF6 was added to the target through a gas feed line. The yields of 25Al were slightly higher (about 3 x 104 pps) than was measured from previously tested targets (fibers and powders). However, the production rate in the target was lower by about a factor of five, so the efficiency for these targets is about an order of magnitude higher, but still low at 0.1%. The atomic ratio of Si atoms to C atoms in this target was about 1:9, which is significantly lower than the 1:1 ratio present in the previously tested targets. We are working to increase the Si concentration by starting with a lower density carbon matrix and depositing a thicker layer of SiC onto the fibers. We will increase the thickness of the SiC layer from about 2 microns up to around 6 microns.
ORIC was in scheduled shutdown during much of the period. However, significant development work was conducted to better understand the operation of the internal ion source and improve cathode lifetimes, especially for helium beams.
In its present configuration, the ORIC internal PIG source provides proton beams with high reliability and adequate output for present use and also for anticipated HPTL requirements. However, source lifetime and reliability are marginal at best when running alpha beams. The problem is the sputtering of the cathodes by back bombardment with He+ ions. There are two possibilities for reducing the sputtering: reducing the number of He+ ions in the source plasma or reducing the cathode voltage that accelerates the ions back into the cathodes. Experiments with cathode geometry to reduce the voltage were carried out in October and November 2003.
Because the cathodes are not directly heated, the back-bombarding ions are required to provide the heating necessary to raise the cathode temperature to the thermionic emission range. The power to do this (i.e. V*I from the arc power supply) depends on the thermal mass of the cathodes. By reducing the amount of cathode material that must be brought to emission temperature (~2200 K) for a given arc current, the required voltage should be lower.
The standard cathodes are 0.25" diameter pure tantalum rods press fit into tantalum holders. A number of modifications to this geometry were tested: (i) turning down a section of the cathode to reduce the thermal conductivity, (ii) increasing the length slightly (~ 0.1") to reduce the cathode-to-plasma chamber aperture gap and increase the length of the neck, and (iii) adding a heat shield to reduce the radiative losses from the hot end of the cathode.
Although more tests are needed to replicate the results and to measure actual lifetime increases, the experiments were very encouraging. These early results suggest the lengthening of the cathodes provides more stable running at low arc currents and easier starting. More significantly, the addition of the neck in the cathodes appears to lower the voltage by as much as 25%-40% at typical operating currents (0.5 A). This may translate to lifetime increases in the 20%-30% range. Experiments with the heat shield were inconclusive.
In addition to the experiments with the geometry, experiments with the gas flow rates were also performed. Although only qualitative results can be reported at this time, the tests showed the presence of a voltage minimum as the gas flow was varied across the operating range. Interestingly, this minimum did not correspond to the setting that maximized beam output from the source. What the test showed was that the output generally increased as the gas flow rate was lowered, up to the point where the plasma became unstable and that this point was well below the voltage minimum. The implication is that the operators have likely been tuning the gas lower than would be optimum for source lifetime when running alpha beams.
Future tests to study the other method of reducing sputtering, i.e. by reducing the amount of He in the plasma, are pending. These tests will involve operating the source with a mixture of H2 and He gases -- sustaining the plasma with H2 and admitting only enough He to obtain desired alpha beam current from the cyclotron.
The Tandem Accelerator was operational, to provide beam, for approximately 1767 hours since the last report. The machine ran at terminal potentials of 3.22 to 22.68 MV and the stable beams 1,2H, 7Li, 12C, 19F, 24Mg, 28Si, 40Ca, 58Ni, 76,82Se, 76Ge, and 124Sn were provided. 7Be was the only radioactive beam delivered during this period. The tank was still open at the beginning of this period to finish the major tank opening that was described in the last newsletter. Four more openings were necessary during this period; first to eliminate leaks in the tubes that had not been resolved before closing the tank from the major tank opening, once due to a broken Georator pulley shaft and twice due to failures of the terminal bending magnet power supply. Both failures of the power supply included failures of output transistors that are obsolete and extremely difficult and expensive to obtain. Therefore, a new power supply is currently being purchased and should be installed some time this year.
A large amount of conditioning, approximately 500 hours, was required to get back to the voltages needed for neutron-rich RIBs after the extended period the acceleration tubes were up to air during the major tank opening. Voltages up to approximately 24 MV should be available to the experimental program with one caveat: A pair of units, 19/20, has displayed deconditioning but has conditioned back readily within a shift. There are several theories as to the cause of this deconditioning and the units will be thoroughly examined the next time we open the tank.
There have been two firsts for tandem accelerator operation: the first operation of the new recirculating gas stripper and the first operation of the RF-exchange (Alphatross) source for the experimental program. The new gas stripper works well but it would do better with additional pumping in the terminal so plans are to install another 120 l/sec ion pump above the stripper. The Alphatross provided protons on target and negative helium beam was analyzed during this first operation. New leak valves for the stable injector will be installed this year to allow more reliable operation for both ion sources.
During this reporting period, we delivered beams of
These 7Be beams were produced with a negative ion sputter source with 9.8 mCi 7BeO in Nb powder in a Cu holder. Permanent magnets were installed around the beam line downstream of the first quadrupole multiplet to sweep away electrons. The recirculating cesium jet charge exchange cell was removed to maximize transmission.
The PC at platform potential was moved to the control room where it communicates with the PLC remaining at platform potential via fiber optic ethernet. This PC had been failing almost daily due to high voltage transients. A major overhaul of both motor-generators was performed that included replacement of all four coupling bearings and elements, replacement of both flange and pillow block bearings, and alignment of both motors. The manually operated rotating object slits for the first stage mass separator were replaced with a remotely actuated slit system that allows insertion of 2 mm X 1 cm, 5/8 inch hole, and 1 mm X 1 cm apertures. Both object and image slits are now remotely actuated.
An array of liquid-scintillation neutron detectors has been commissioned at the RMS target position. The primary purpose of this array is to function as a reaction-channel selection tool, which is very useful for experiments aimed at studying proton-rich nuclei.
Fig. RA3-1: Photograph of the neutron-detector array mounted at the RMS target position.
The array was designed to cover the available solid angle (from 0 to 40 degrees in the lab) as efficiently as possible with a single detector design while fitting in the limited space between the CLARION target position and the RMS. The resulting geometry consists of 19 tapered hexagonal detectors, each of which contains about 1.4 liters of liquid scintillator. A photograph of the detectors mounted in the CLARION support structure is shown in Fig. RA3-1. The efficiency of the array is reaction dependent, but estimated to be between 11% and 18%. The array can be operated in conjunction with CLARION, HYBALL, and the RMS, but space constraints preclude its use with the silicon forward array.
Fig. RA3-2: Two-D spectrum recorded by one of the neutron detectors demonstrating the n-gamma separation quality of the neutron detectors. The x-axis is the crossover time from the PSD circuitry and the y-axis is the energy. Data acquisition trigger condition was "n-gamma or gamma-gamma".
Since the scintillator material is sensitive to both neutrons and gamma-rays, these events are distinguished by both pulse shape discrimination (PSD) and time of flight. The pulse shape discrimination circuitry is conveniently packaged in five 4-channel programmable CAMAC modules developed in conjunction with RIS Corp which give excellent n-gamma separation down to 100 keV recoil electron energies. In the absence of beam bunching, time of flight must be measured against gamma-ray time.
The array was recently characterized in an in-beam test of channel selection using the 28Si + 58Ni reaction. In this test, the one-neutron detection efficiency was measured to be 11%. A 2D spectrum demonstrating the quality of n-gamma separation by PSD is shown in Fig. RA3-2. In the same test, time of flight (TOF) spectra were also obtained to assess their usefulness in aiding the n-gamma separation. Figure RA3-3 shows a sample 2D spectrum of PSD crossover time versus TOF for one of the neutron detectors. In this test, the TOF of the neutron detector signal was measured against the time of one selected Clover crystal.
Fig. RA3-3: The PSD crossover time (x-axis) versus TOF (y-axis) of one of the neutron detectors obtained from an in-beam test.
With the new year, the new charter accepted by the membership last year has gone into effect. The membership of the executive committee has been reduced from six to four members, and they are listed below. The committee had its first meeting of 2004 on February 5 and chose Ed Zganjar as this year's chair. Ed will represent the Users at the upcoming PAC and Science Policy Committee meetings. The committee is sponsoring a workshop on In-Beam Gamma-Ray Spectroscopy which is also covered elsewhere in this newsletter.
| Uwe Griefe | Colorado School of Mines | 2004-2006 |
| Paul Mantica | Michigan State University | 2002-2004 |
| David Radford | Oak Ridge National Laboratory | 2003-2006 |
| Ed Zganjar, chair | Lousiana State University | 2002-2004 |
The facility would like to thank last year's committee for their successful efforts to revamp the organization of the group. We wish to especially thank Paul Mantica who served as chair and also to Jeff Blackmon who gave up his position on the committee to ensure a smooth transition to the new charter. We also wish to thank Ani Aprahamian and Demetrios Sarantites for their three years of service which ended on December 31, 2003.
HRIBF welcomes suggestions for future radioactive beam development. Such suggestions may take the form of a Letter of Intent or an e-mail to the Liaison Officer at liaison@mail.phy.ornl.gov. In any case, a brief description of the physics to be addressed with the proposed beam should be included. Of course, any ideas on specific target material, production rates, and/or the chemistry involved are also welcome but not necessary. In many cases, we should have some idea of the scope of the problems involved.
Beam suggestions should be within the relevant facility parameters/capabilities listed below.
| Date | Exp. No. | Energy (MeV) |
Beam | Spokesperson |
| 7/21-25 | RIB-089 | 83 | 28Si | Grzywacz/ORNL |
| 8/4-7 | RIB-089 | 90 | 28Si | Grzywacz/ORNL |
| 8/7 | RIB-105 | 110 | 76Ge/76Se | Hausladen/ORNL |
| 8/8 | RIB-040 | 200 | 58Ni | Shapira/ORNL |
| 8/11 | RIB-013 | 17 | 19F | Blackmon/ORNL |
| 8/12-15 | RIB-013 | 17, 27 | 19F | Blackmon/ORNL |
| 8/18-19 | RIB-040 | 120 | 28Si | Shapira/ORNL |
| 8/20-22 | RIB-096 | 104 | 40Ca | Rudolph/Lund University |
| 8/26-29 | RIB-096 | 104 | 40Ca | Rudolph/Lund University |
| 9/2-3 | RIB-105 | 150 | 76Se/76Ge | Hausladen/ORNL |
| 9/3-4 | RIB-039 | 12 | 7Li/7Be | Mueller/ORNL |
| 9/4/-5 | RIB-118 | 50 | 12C | Greife/Colorado School of Mines |
| 9/8-12 | RIB-039 | 8 | 7Li, 7Be | Mueller/ORNL |
| 10/13-16 | RIB-054 | 32,37,39 | 1H | Bardayan/ORNL |
| 10/16-17 | RIB-014 | 35 | 1H | Stracener/ORNL |
| 10/20 | RIB-109 | 8 | 7Li | Greife/Colorado School of Mines |
| 10/24 | RIB-054 | 8 | 7Li | Bardayan/ORNL |
| 10/27 | RIB-109 | 8 | 7Li | Greife/Colorado School of Mines |
| 10/28-29 | RIB-000 | 292 | 58Ni | Gross/ORNL |
| 10/30-11/3 | RIB-109 | 4-27 | 7Be | Greife/Colorado School of Mines |
| 11/3-4 | RIB-000 | 292 | 58Ni | Gross/ORNL |
| 11/4-8 | RIB-103 | 292 | 58Ni | Batchelder/ORISE |
| 11/10-12 | RIB-036 | 40 | 1,2H | Stracener/ORNL |
| 11/13-14 | RIB-013 | 20 | 24Mg | Blackmon/ORNL |
| 11/17-21 | RIB-119 | 570 | 124Sn | Kozub/Tennessee Tech University, Jones/Rutgers University |
| 11/24 | RIB-036 | 40 | 1H | Stracener/ORNL |
| 11/25-26 | RIB-000 | 235,230 | 58Ni | Gross/ORNL |
| 12/1-3 | RIB-036 | 40 | 1H | Stracener/ORNL |
| 12/3-5 | RIB-013 | 20 | 12C | Blackmon/ORNL |
| 12/8-16 | RIB-081 | 235-340 | 58Ni, 28Si | Grzywacz/ORNL |
| 12/15-16 | RIB-013 | 20 | 12C | Blackmon/ORNL |
| 12/17-19 | RIB-117 | 450 | 82Se | Thomas/Rutgers University |
| 12/19 | RIB-013 | 16 | 19F | Blackmon/ORNL |
| 12/22-23 | RIB-013 | 16-17 | 19F | Blackmon/ORNL |
| 12/30-31 | RIB-014 | 40 | 1H | Stracener/ORNL |
| In addition, machine development and/or conditioning occurred on the following dates: July 17-18, 25; August 8, 12, 19, 25-26, 29; September 12, 15-19, 22-26, 29-30; October 9-10, 17, 20, 21, 27, 28; November 24; December 16, 29-30. We were shut down most weekends. | ||||
| |
||
| 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 | ||