|Edition 9, No. 5||Fall Quarter 2001||Price: FREE|
- 1. HRIBF Update and Near-term Schedule
- 2. Recent HRIBF Research - Measurement of 17F+p Inelastic Scattering
- 3. Recent HRIBF Research - First Observation of the Proton Drip-Line Nucleus 140Dy: Identification of a 7-us K-Isomer Populating the Ground-State Band
- 4. Recent Facility Review of HRIBF by NSAC
- 5. Reminder of the Latest Call for Proposals and Results from PAC-6e
Editors: C.-H. Yu, W. Nazarewicz, and C. J. Gross
Feature contributors: R. L. Auble, F.E. Bertrand, J.C. Blackmon,
C.J. Gross, W. Krolas,
Regular contributors: M. R. Lay, M. J. Meigs, P. E. Mueller, D.C. Radford, D. W. Stracener, B. A. Tatum
The hafnia target/Kinetic Ejection Negative Ion Source (KENIS) for the production of 17,18F began operation July 31 and continued through October 14. During this time, two 17F experiments (RIB-064, RIB-086) and two 18F experiments (RIB-020, RIB-066) were completed, along with several stable-ion beam experiments. Following completion of the 17,18F campaign, stable ion beams were provided for nuclear structure experiments and RIB target development.
The tandem was shut down on October 29 for scheduled maintenance and operation for the research program is presently scheduled to resume on December 10. When operation resumes, the tandem will provide stable-ion beams until late January. If the KENIS is still operational after the shutdown, there will be a short 17F beam campaign in late January or February. Following this, the KENIS will be replaced by the uranium carbide target in preparation for the production of pure-Sn RIBs.
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The hot-CNO cycle of reactions is the primary source of energy generation in nova explosions and in some phases of X-ray bursts. The small rate of the 14O(a,p)17F reaction restricts energy generation in the hot-CNO cycle and leads to the accumulation of a significant abundance of 14O. The cross section for the inverse reaction 17F(p,a)14O was measured at the HRIBF in 2000 [Bla01], and the 14O(a,p)17Fgs reaction rate was inferred by detailed balance. However, the contribution of the 14O(a,p1)17F* branch to the 495-keV (1/2+) first-excited state in 17F was not constrained by this measurement.
We have studied 17F+p inelastic scattering,
keV) reaction, in order to determine the 14O(a,p1)17F*
reaction rate. An isobarically-mixed beam of 17O
and 17F from the HRIBF bombarded a 59 mg/cm2
polypropylene target. Protons scattered from the target were detected
in the SIDAR silicon detector array arranged to subtend angles
= 16°-48°. Recoiling heavy ions were detected in coincidence,
and their Z identified, using a gas ionization counter at forward angles.
The experimental technique is the same as was used in a previous measurement
of 18F+p elastic scattering [Bar00].
Cross sections were measured at 11 bombarding energies ranging from 37.5
MeV to 45.0 MeV, corresponding to the most important energy range for the
reaction. Protons from the 1H(17F,p1)17F*(495
keV) reaction were clearly observed in some runs and were easily distinguished
from the much stronger 1H(17F,p0)17Fgs
elastic scattering by the energy of the detected protons in the SIDAR.
Sample SIDAR energy spectra are shown in Fig. 2-1.
Fig. 2-1. Energy spectra are shown from the SIDAR silicon detector array in coincidence with a recoiling 17F for 2 of the 11 measured bombarding energies. The particle energy has been converted into the Q-value for the reaction so that the peaks at both bombarding energies line up.In a preliminary analysis, the total cross section for the 1H(17F,p1)17F*(495 keV) reaction was determined by integrating the counts in the coincident inelastic scattering peak assuming the distribution to be isotropic in the center-of-mass frame. A single strong resonance is clearly visible in the excitation function. The cross section was fit using a 3-parameter, single-level Breit-Wigner functional form. The preliminary best-fit parameter values are as follows: Er= 2212 keV, G = 25.3 keV and wgpp' = 1.4 keV. This implies a state in 18Ne at Ex = 6134 keV with proton-partial widths of about 0.65G and 0.35G for decay to the ground state and first-excited state in 17F. This simple analysis does not distinguish which partial width is greater, but clearly the 14O(a,p1)17F* branch to this state will make a significant contribution to the reaction rate. At energies where a peak from the 1H(17F,p1)17F*(495 keV) reaction was not evident (Ecm = 2.083, 2.369, 2.403, 2.431, and 2.504 MeV), we are able to set an upper limit on the total cross section of about 2 mb. This implies a small branch for the previously observed states at Ex = 6.29 and 6.35 MeV in 18Ne [Hah96] for decay to the 495-keV first-excited state in 17F. A multi-level R-matrix analysis of the combined inelastic, elastic, and 17F(p,a)14O differential cross-section data is currently in progress. We expect to be able to determine the 14O(a,p)17F reaction rate to an accuracy of about 30%.
C. Blackmon, et al., Nucl. Phys. A688, 142c (2001).
[Bar00] D. W. Bardayan, et al., Phys. Rev. C 62, 042802(R) (2000).
[Hah96] K. I. Hahn, et al., Phys. Rev. C 54, 1999 (1996).
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A new 7-us isomer in the proton drip-line nucleus 140Dy was selected from the products of 54Fe (315 MeV) + 92Mo reaction by a recoil mass separator and studied with recoil-delayed gamma-gamma coincidences. Separated recoils, with A/Q = 140/27, passed through a thin-foil microchannel plate (MCP) detector placed at the RMS final focus. The MCP provided recoil time and position signals. The ions were stopped in a thin aluminium catcher surrounded by the Clover Array for Recoil Decay Spectroscopy (CARDS). The total photopeak efficiency of the setup consisting of four Eurisys segmented Clovers and one Ortec gamma-X detector was as high as 18% for 80-keV, 14% for 200-keV and about 4% for 1.33-MeV gamma rays. The preamplifier signals from the gamma-ray detectors and MCP were processed by XIA Digital Gamma Finder (DGF4C) modules. The pulses were digitized on-board to determine the amplitude and were real-time stamped with a 40 MHZ clock. As there were no restrictions on a recoil-delayed gamma correlation window we were able in the off-line analysis to substract the gamma background, both short-lived like the 0.3-us isomer in 140mEu and known long-lived isomers in beta decays of the A=140 isobars.
We have identified a new cascade of gamma transitions at 202, 364, 476, 550 and 574 keV with a half-life of 7.0(5) us correlated with the implantation of the selected A=140 recoils. The gamma spectrum gated on five labeled transitions is shown in Fig. 3-1a. A complete analysis of triple gamma coincidence data revealed Dy Kalpha and Kbeta X-rays at 45.8 and 52 keV, see Fig. 3-1b. The spectrum of time differences between the MCP signals and five gamma transitions is presented in Fig. 3-1c. It represents the decay curve of the isomer with a fitted half-life of 7.0(5) us.
Figure 3-1. (a) Gamma lines from the decay of the I = 8- isomer in 140Dy. (b) Low energy part of the double-gated spectrum reveals Dy Kalpha and Kbeta X-rays. (c) Fit to the decay pattern produced by gating on five transitions yields a half-life of 7.0(5) us.
The sequence of the gamma lines in the cascade cannot be determined by coincidence analysis. However, the intensities and energies of the transitions resemble those expected for a rotational band in a deformed nucleus like 140Dy fed by the isomeric level. Also, a comparison to the decay patterns of I = 8- K-isomers in less exotic N=74 even isotones of 134Nd, 136Sm, 138Gd, shows striking similarities, see Figure 3-2. It leads us to the interpretation of the isomeric level at 2166 keV as an I = 8- n9/2-  x n7/2+  K-isomer, which decays to the ground-state band in 140Dy via the 8+ -> 6+ -> 4+ -> 2+ -> 0+ sequence, see Figure 3-2.
Figure 3-2. Level scheme of 140Dy established in this work and the systematics of the decay properties of I = 8- K-isomers in even Z > 58, N=74 isotones. Corresponding hindrance per degree of K-forbiden factor f values derived from displayed decay schemes are shown for comparison (see text).
Considering the 574- and 550-keV transitions as the candidates for the E1 isomeric transition from the I = 8- K=8 to the I = 8+ K=0 level we find Weisskopf hindrance factors FW of 5.3(4) x 109 and 4.7(3) x 109, respectively. These are very close to the values 4.7(8) x 109, 5.9(4) x 109, and 8.4(6) x 109, reported for 8- K-isomers in neighboring N=74 isotones of 134Nd, 136Sm, and 138Gd. Corresponding hindrance per degree of K-forbiden factor, f, amounts to 24.5(3) and 24.1(3) for the 574- and 550-keV transitions, respectively. We cannot unambiguosly determine the ordering of the 574- and 550-keV transitions based on these small differences. However, the proposed level scheme (see Figure 3-2) seems to be most likely based on energy level systematics.
The ground state of 140Dy is populated in the decay of the deformed proton emitters 141gs,mHo [1,2,3]. The structure of the p-emitting states as well as the energies of the ground-state band in 140Dy, in particular the energy of the first 2+ level, govern the probability of the so-far unreported fine structure in proton emission from 141gs,mHo. >From the properties of the ground-state band in 140Dy one can learn about its deformation, i.e. the shape of the potential tunneled by the emitted protons.
According to the Grodzins formula [4,5], the observed value E(2+) = 202 keV gives a deformation parameter beta2 of 0.244 for 140Dy. The experimental level schemes of 140Dy (this work) and 141Ho  provide a reliable experimental input for the predictions of proton emission rates. Following the model presented in [6,7], we calculated the fine structure branching ratio Ip(2+ ) for the proton emission from the 7/2-  ground state and 1/2+  isomer in 141Ho. With the beta2 fixed at 0.244, the branching ratio is close to 2% for the 7/2- ground state assuming a hexadecapole deformation beta4 around -0.05. The Ip(2+) value of about 2% is above the reported experimental upper limit of 1% . The present RMS-based setup for proton radioactivity studies at the HRIBF  is sensitive down to Ip(2+) of about 0.5% for the 4-ms activity of 141gsHo. The search for the fine structure in the proton emission from 141gsHo is within experimental reach; this will be reported in the next newsletter.
 K. Rykaczewski et al., Phys. Rev. C60, 011301 (1999)
 D. Seweryniak et al., Phys. Rev. Lett. 86, 1458 (2001)
 C. N. Davids et al., Phys. Rev. Lett. 80, 1849 (1998)
 L. Grodzins, Phys. Lett. 2, 88 (1962)
 F. S. Stephens et al., Phys. Rev. Lett. 29, 438 (1972)
 A. T. Kruppa et al., Phys. Rev. Lett. 84, 4549 (2000)
 B. Barmore et al., Phys. Rev. C62, 054315 (2000)
 K. Rykaczewski, Eur. Phys. J. A, in press
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On July 17, 2001, a letter was sent from James Decker, Acting Director of the Office of Science at the USDOE, and Robert Eisenstein, Assistant Director for Mathematical and Physical Sciences at the NSF, to James Symons, Chair of NSAC, requesting that NSAC "...review and evaluate current and future scientific capabilities in the area of nuclear structure and astrophysics supported by the DOE Low Energy Nuclear Physics subprogram and make recommendations of priorities consistent with projected resources and the scientific opportunities identified in the new long-range plan." The NSAC appointed a committee consisting of Brad Filippone (chair), Juha Aysto, Larry Cardman, Vijay Pandharipande, Mark Riley, Gene Sprouse, Vic Viola, and Michael Wiescher to carry out the review. The review was to focus on facilities but was to also consider the research carried out under the DOE Low Energy Program. Laboratories reviewed were ATLAS (ANL), 88-Inch (LBNL), HRIBF (ORNL), Texas A&M, TUNL, University of Washington, and Yale.
The committee visited ORNL on September 10 and 11. We provided a series of talks and an in-depth tour of the HRIBF to the committee. A brief version of the agenda for the visit is shown below. The tour involved many of the ORNL staff as well as outside users. In addition, a lunch was held on the first day during which the committee spoke with 15 of our Users. Thanks to all of the members of our staff and our User community who helped with the review. We believe this was the best prepared review we have held in many years, and it would not have been so without the exceptional effort of all of you.
The review provided an opportunity for us to present the excellent science, the latest information on the beam development at HRIBF, and our plans for the future to the committee and to DOE. The progress at HRIBF has been truly outstanding over the past two years, and every indication points to continued success in the future. The report of the review committee will be presented to the NSAC at a meeting to be held November 29 and 30 in Washington, D.C. We will provide a summary of the report to all Users after it has been approved by NSAC.
|Low Energy Program||Fred Bertrand|
|Science Overview||Witek Nazarewicz|
|Facility Overview||Jim Beene|
|Facility Tour||Division Staff|
|User Program||Witek Nazarewicz|
|Low Energy Research Overview||Cyrus Baktash|
|Physics with Neutron-Rich RIB Beams||David Radford|
|Astrophysics Program||Michael Smith|
|Research Around the Proton-Drip Line||Kris Rykaczewski|
|Reactions with RIB Beams||Alfredo Galindo-Uribarri|
|Theory Connection||David Dean|
|Target/Ion Source Development||Gerald Alton|
|Beam Development||Dan Stracener|
|Research Apparatus||Cyrus Baktash|
|HRIBF Future Directions||Alan Tatum/Jim Beene|
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A Call for Proposals (PAC-7) was issued in early November. This Call is for any experiment using all available radioactive and stable beams. The deadline for the receipt of the proposals is December 13, 2001. Proposals are only accepted electronically and may be submitted by email attachment to firstname.lastname@example.org. Specific instructions may be found on our website. The PAC will meet in Oak Ridge on January 18-19, 2002.
The Program Advisory Committee held a telephone conference call on October 29, 2001, to discuss the five proposals received from our limited Call for Proposals using pure neutron-rich Sn beams. All proposals received some allocation of time although only two were allocated immediate RIB time. As expected, all the proposals emphasized experiments with 132Sn. Titles and spokespersons of accepted experiments are available.
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Aluminum RIB Development
Recent tests of the release of 25Al and 26Al from a SiC powder (1 micrometer diameter) target coupled to an EBP ion source were disappointing. The yields were much lower than expected from the previous tests with the SiC fibers (15 micormeter diameter) and, in fact, were about the same from both of these targets. The shorter diffusion path in the powder target should have resulted in higher yields. While the powder target did change slightly in color during the measurement, an inspection afterwards revealed that the powder did not sinter so the diffusion path was still very short. Another difference in the two measurements was that the target holder was constructed of graphite for the fiber target test and was tantalum and molybdenum for the powder target test. The relatively low operating temperature of this target (1650°C) may have resulted in long holdup times on the Ta and Mo surfaces so we plan to try the powder target again in a graphite target holder.
As we have seen in other systems, the addition of a "carrier" can have a significant effect on the intensity of the extracted beam. In this case, SF6 was added to the system to supply a vapor of fluorine atoms that combined with the radioactive aluminum atoms to form the relatively inert AlF molecule. From previous experience with radioactive fluorine beams, we know that AlF effuses rapidly through the ion source and has a high ionization efficiency in the EBP ion source. The 25Al and 25AlF beam currents increased linearly with the extracted 19F beam current as the leak rate of SF6 into the ion source was gradually increased. The intensities of these extracted beam currents increased by a factor of ~25 with the ratio of Al to AlF remaining nearly constant, and close to one.
Other RIB Development
The batch-mode source to be used for the production of 56Ni beams has been installed on the UNISOR separator, a low-intensity online target and ion source test facility. This involved some minor modifications to the front end of the separator but these were easily accomplished and the batch-mode source was installed, pumped down, and initial operation checks have been done. A new power supply is needed to provide the wheel potential (up to 5 kV). This has been ordered so we plan to continue tests with this target/ion source in the next couple of months.
Design and off-line testing of a positive ion source using surface ionization from a hot tantalum surface has been started. This ion source has very high efficiency for elements from Group IA, and we are particularly interested in using it to provide beams of neutron-rich isotopes of rubidium that are produced via proton-induced fission in our uranium carbide targets. Following the current accelerator maintenance period, we plan to perform online tests with this ion source.
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ORIC provided up to 2.6 uA of 44.5-MeV 2H for the 17F campaign and up to 3 uA of 85-MeV 4He for the 18F campaign, each of which is detailed in a companion article. ORIC experienced both air and water leaks on the ion source assembly during the period which required several days to correct. However, these leaks were located and repaired during a thorough overhaul of the entire source assembly. Aside from this problem, ORIC operation was quite successful.
Cathode lifetimes were shorter than desired when producing 4He. The presence of the heavier ions causes the tantalum cathodes to sputter much more rapidly, and the sputtered material is deposited on the chimney end aperture plates. Lifetimes were limited to less than 60 hours, as compared to almost 800 hours for proton and deuteron production. Development work and source testing is underway to establish a modified helium source which will not only last longer, but provide somewhat higher intensities. A longer term possibility would be the installation of an axial injection external source.
At the conclusion of the scheduled runs, a leak in the RF shorting plane and a water pump motor starter were repaired. Additionally, installation of a new TV switching system and improved instrumentation for existing utilities has begun.
The Tandem Accelerator has provided more than 1160 hours of beam on target since the last report with about fifty percent of the time for radioactive beams. The machine ran at terminal potentials of 3.2 to 22.52 MV and 58Ni, 54Fe, 32S, 18O, and 1H were provided from the stable injector while 17F,and 18F were provided from the RIB injector. The tank was opened on the last day of this period for a long awaited maintenance period. Work will be done to try to isolate the ticking problem arising from units 21/22 during this opening.
In the first week of September we received 38,090 pounds of SF6, which should allow enough pressure in the accelerator tank to reach higher terminal voltages. This amount was purchased at this time because a very good price was offered.
During this reporting period, we delivered the following radioactive fluorine beams to the astrophysics endstation in front of the Daresbury Recoil Separator (DRS) and the nuclear reaction physics endstation at the ENGE spectrometer:
|17F||1.0x106||58.8%||40.1||5+||foil||DRS||2H||2.0 uA||44.5 MeV|
|17F||0.1x106||42.6||5+/9+||foil||DRS||2H||2.0 uA||44.5 MeV|
|17F||0.12x106||43.7||5+/9+||foil||DRS||2H||2.6 uA||44.5 MeV|
|17F||1x106||33.3%||38.5||5+||foil||DRS||2H||2.7 uA||44.5 MeV|
|17F||0.9x106||33.3%||38.9||5+||foil||DRS||2H||2.7 uA||44.5 MeV|
|17F||1.3x106||38.5%||41.5||5+||foil||DRS||2H||2.7 uA||44.5 MeV|
|17F||8.5x106||122||7+/9+||foil||ENGE||2H||2.6 uA||44.5 MeV|
|18F||0.02x106||25||2+/9+||gas/foil||ENGE||4He||1.0 puA||85 MeV|
|18F||0.42x106||20%||6.6||1+||gas||DRS||4He||1.5 puA||85 MeV|
|18F||0.15x106||20%||7.5||1+||gas||DRS||4He||1.5 puA||85 MeV|
Shown are the charge state(s) of the beam and type(s) of stripping, the isobaric purity level for beams that were not fully stripped in a foil after the tandem electrostatic accelerator, and the ORIC beam that bombarded the fibrous HfO2 target/kinetic ejection negative ion source.
The target/ion source performed well through the end of the fluorine campaign and will remain on the high voltage platform to test the new control system. There is a possibility that this target/ion source will be used again to provide a few days of 17F for some remaining PAC approved experiments in January 2002 prior to our 132Sn campaign with a uranium carbide target/electron beam plasma ion source in February 2002.
We have begun the long awaited upgrade of the control system software for the high voltage platforms from VISTA to EPICS. We have been plagued by a bug in VISTA that improperly handles memory allocation in the VAX ELN single board computer and frequently causes a loss of communication with the high voltage platforms.
We are also taking advantage of this control system changeover to improve software documentation, make the control system windows more operator friendly, and incorporate serial control of the turbopumps. When the work on the high voltage platform control system is complete, similar work will begin on the control system for the injection line to the tandem electrostatic accelerator.
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The CLARION array comprises eleven segmented clover Ge detectors, together with ten smaller HPGe detectors. All are equipped with BGO anti-Compton shields. The total efficiency at 1.33 MeV is about 2.5%. The clover detectors are equipped with dedicated electronics modules which use ECL-bus readout. More information may be found at http://www.phy.ornl.gov/hribf/research/equipment/clarion/.
Recently five of the clover detectors were temporarily relocated to the focal plane of the RMS, where they were used for spectroscopic studies of isomeric decays. The XIA Digital Gamma Finder modules were used instead of the standard analog electronics for these detectors at the focal plane, with excellent results.
Software for improved Doppler correction based on gamma-particle coincidences with the HyBall array is being developed for the neutron-rich RIB Coulomb excitation program, and should be available soon. For information on this, and on CLARION detector efficiencies and angles, etc., please contact David Radford at email@example.com
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The ballots for the annual election of the Users Executive Committee have been counted. We welcome Paul Mantica and Ed Zganjar to the committee. Their terms will begin in January and will last three years. We would like to thank I-Yang Lee and Bill Walters for their service on the committee.
|Ani Aprahamian (chair)||Notre Dame University|
|Paul Mantica||Michigan State University|
|Peter Parker||Yale University|
|Kris Rykaczewski||Oak Ridge National Laboratory|
|Demetrios Sarantites||Washington University|
|Ed Zganjar||Louisiana State University|
This year, the HRIBF provided a brief overview of our facility at the annual DNP Meeting in Hawaii. It was given by David Radford who described the facility and emphasized the availability of neutron-rich radioactive ion beams including pure Sn beams.
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HRIBF welcomes suggestions for future radioactive beam development. Such suggestions may take the form of a Letter of Intent or an e-mail to the Liaison Officer at firstname.lastname@example.org In any case, a brief description of the physics to be addressed with the proposed beam should be included. Of course, any ideas on specific target material, production rates, and/or the chemistry involved are also welcome but not necessary. In many cases, we should have some idea of the scope of the problems involved.
Beam suggestions should be within the relevant facility parameters/capabilities listed below.
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|RIB-000||Commissioning of the RMS||Rykaczewski/ORNL||9/13
|RIB-014||RIB development on UNISOR||Stracener/ORNL||10/15-10/16(08:00)|
|RIB-020||Determination of rates of 18F(p,gamma)19Ne & 18F(p,alpha)15O reactions in nova & X-ray burst explosions||Parker/Yale||10/10-10/14|
|RIB-033||A search for fine structure in a proton decay in deformed nuclei 141Ho and 131Eu||Rykaczewski/ORNL||10/16-10/19
|RIB-039||18F beam development||Mueller/ORNL||10/01-10/02(06:30)|
|RIB-044||Hot CNO breakout: proposal to measure single nucleon transfers to 18F||Kozub/TN Tech||8/16(08:00)-8/17|
|RIB-064||1H(17F,p')17F inelastic scattering cross section||Blackmon/ORNL||8/1- 8/4(12:00)
|RIB-066||HRIBF proposal to search for missing states in 19Ne||Bardayan/ORNL||10/02(06:30)-10/05|
|RIB-070||Study of the proton-drip line nucleus 69Br and the implications for the astrophysical rp process||Batchelder/ORISE||8/27-8/31|
|RIB-080||Isomers in 140Dy||Krolas/Vanderbilt||9/17-9/19
|RIB-086||Breakup of 17F at 120 MeV||Liang/ORNL||8/9-8/11
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|Witek Nazarewicz||Carl J. Gross||Chang-Hong Yu|
|Deputy Director for Science||Scientific Liaison||Newsletter Editor|
|Mail Stop 6368||Mail Stop 6371||Mail Stop 6371|
|Holifield Radioactive Ion Beam Facility|
|Oak Ridge National Laboratory|
|Oak Ridge, Tennessee 37831 USA|