HRIBF NEWS


Edition 7, No. 1 Winter Quarter 1999 Price: FREE

Contents:

Feature Articles Regular Articles

Editor: Carl J. Gross

Feature contributors: D. Bardayan, A. Piechaczek, B. Sherrill, J. D. Garrett
Regular contributors: F. Ervin, J. D. Garrett, C. J. Gross, M. J. Meigs, D. W. Stracener, B. A. Tatum, R. F. Welton


1. HRIBF PAC-4 Meets

The HRIBF Program Advisory Committee (PAC) met at the Holifield Radioactive Ion Beam Facility (HRIBF) in Oak Ridge on February 12, 1999, to consider the next round of proposals for experimental studies at the HRIBF. Two hundred seventy-six shifts of radioactive ion beams and 250 shifts of stable beams were requested in the nineteen proposals considered. In general, the scientific content of the proposed experimental studies was judged to be excellent, and the proposals were well written. Obviously, it was not possible to grant all requested experimental time at the HRIBF, which is expected to run a radioactive ion beam research program of about 2000 hours this fiscal year. A sizeable backlog of experiments also awaits the development of more intense beams and specific experimental setups. One hundred two shifts of radioactive ion beams were assigned to six of the ten radioactive ion proposals. In addition, 133 shifts of stable beams were also assigned to these proposals. Five proposals were accepted that requested only stable beams. The accepted proposals were prioritized to help establish the order in which beam time is granted.

A summary of the proposals accepted by PAC4 is given on the HRIBF web site at http://www.phy.ornl.gov/hribf/users/pac4_app.html. The composition of the HRIBF PAC is given at http://www.phy.ornl.gov/hribf/users/pac.html. Brad Sherrill, the chair of the HRIBF User’s Executive Committee, represented the Users at the PAC Meeting.

2. Recent HRIBF Research - A Study of the 3+ State in 18Ne via 17F(p,p)17F and Its Importance to Astrophysics

The 17F(p,gamma)18Ne reaction is important in the Hot CNO cycles which occur in stellar environments such as novae, supernovae, and X-ray bursts. Its reaction rate is uncertain, however, because of an expected 3+ state in 18Ne that has never been conclusively observed. This missing 3+ state would provide a strong l=0 resonance and, depending on its excitation energy, could dominate the 17F(p,gamma)18Ne rate. Predictions of its excitation energy [1,2,3] vary by 300 keV, corresponding to a factor of 1000 variation in the resonant component of the reaction rate.

Previous measurements of the 16O(3He,n)18Ne and 20Ne(p,t)18Ne reactions [2,4,5,6] did not find conclusive evidence for the existence of the 3+ state. These measurements were hindered by using reactions that suppress the population of states with unnatural spin and parity. We have therefore measured the 1H(17F,p)17F excitation function with a radioactive 17F beam at the HRIBF. This reaction is extremely sensitive to 2+ and 3+ states in 18Ne, and our measurement provided the first unambigious evidence for the existence of the missing 3+ state.

A 17F beam was used to bombard a 48 ug/cm2 polypropylene CH2 target, and the scattered protons were detected in an annular array of single-sided silicon strip detectors located 10.5 cm downstream from the target. This SIlicon Detector ARray (SIDAR) is comprised of 128 segments with 16 radial (from 5 to 13 cm) and 8 azimuthal divisions, similar to the LEDA array used at Louvain-la-Neuve.[7] The SIDAR covered lab angles between 25 and 51 degrees. Recoiling 17F ions were detected in coincidence with the protons in an isobutane-filled ionization counter. Proton yields were measured at 12 beam energies between 10 and 12 MeV. The yield at each energy was determined by summing the coincident proton yields in all strips of the SIDAR and normalizing to the incident beam current. The coincident proton yields were used (instead of singles) to eliminate background from betas and scattered beam ions. The resonance was clearly visible, and a fit to the data yielded a resonance energy of 599.4 +/- 2.4 keV (excitation energy of 4523.3 +/- 2.9 keV) and a total width of 18 +/- 2 keV. We conclude that we have observed the long-sought 3+ state in 18Ne for the following two reasons. First, our measurement is sensitive to 3+ and 2+ states only, and there are no 2+ states in 18O in this energy region whose analogs have not been identified in 18Ne. Second, the observed width is consistent with the expected width of the 3+ state, but is five times larger than that expected for a 2+ state.

Using these new resonance parameters, we have calculated the contribution to the 17F(p,gamma)18Ne rate from the 3+ state. This contribution, now on sound experimental footing, is a factor of 2 larger at T=0.5 GK than the most commonly used estimate from Garcia et al.[2] It is different, however, by orders of magnitude from the predictions of Wiescher et al.[1] and Sherr et al.[3]. While discovery of the 3+ state resolves the greatest uncertainty in the 17F(p,gamma)18Ne rate, the total rate is still somewhat uncertain because the direct capture cross section and partial gamma width of the 3+ state have not been measured. These problems could be solved with a direct measurement of the 17F(p,gamma)18Ne resonance strength.

References

[1] Wiescher et al., Astrophys. J. 326, 384 (1988).
[2] Garcia et al., Phys. Rev. C 43, 2012 (1991).
[3] Sherr et al., Phys. Rev. C 58, 3292 (1998).
[4] Nero et al., Phys. Rev. C 24, 1864 (1981).
[5] Hahn et al., Phys. Rev. C 54, 1999 (1996).
[6] Park et al., preprint, submitted to Phys. Rev. C.
[7] Coszach et al., Phys. Lett. B353, 184 (1995).

3. Recent HRIBF Research - Moving Tape Collector Experiments Along the N=Z Line

A series of experiments using the recoil mass spectrometer (RMS) and the Louisiana State University moving tape collector (MTC) has been completed by a collaboration between researchers in the UNIRIB consortium, HRIBF, Notre Dame University, and Clark University. The measurements, centered along the N=Z line, include

Although varied between the experiments, the detection setup consisted of up to four Clover germanium detectors (equipped with anti-Compton shields) large-area plastic scintillators and silicon detectors for beta detection, a LN-cooled internal pair/internal conversion spectrometer, as well as LEPS and LOAX photon spectrometers. Depending on the half-life of the investigated species, the MTC was either operated in "collect mode," transporting activity from a collection point to the detection station, or in "take-away" mode, removing long-lived contaminating and daughter activities from a collection point within the detection station.

The half-life of the rp-process waiting point nucleus 80Zr, produced in the reaction 24Mg(58Ni,2n), was measured for the first time using delayed-gamma tagging. The mass A=80 activity was transported by the MTC to a detection station, where delayed coincidences between beta particles and a known beta-delayed 84 keV gamma ray [1] in 80Y were registered. Since the gamma-ray emitting level has a half-life of 4.7 us, the delayed beta-gamma coincidence spectrum is dominated by the 84-keV gamma rays, which permit a unique tagging of 80Zr beta-decay events among the huge gamma-ray background from the pn and 2p reaction channels. In this way, the 80Zr half-life was determined to be 4.1(5) s. The result leads to the conclusion that the rp-process is not terminated at mass A=80 for stellar processes with lifetimes comparable to or longer than this halflife. For example, the time scale of X-ray bursters which has been associated with a hot rp-process is of the order of 20 seconds.

The beta-decay propetrties of a 2.2-s isomer, discovered earlier in a mass-separator study at ISOLDE [2], were investigated. The present results assign the isomer unambiguously to 70Br and determine its spin and parity as 5+. Based on deformed shell model calculations, it can be concluded that 70Br is a spherical or nearly spherical nucleus. As such it marks the "northeastern" limit of the region of sphericity induced by the doubly-magic shell closure at N=Z=28.

By conversion-electron and gamma-ray spectroscopy, the K-shell conversion coefficient of the 4.7-s isomeric transition [3] in 80Y was determined to be 0.47(15) and the transition was unequivocally characterized as M3. This result corroborates the spin and parity assignments for the isomer and the 80Y ground state as 1- and 4-, respectively, which were in doubt for a long time since no beta-decay feeding to negative-parity states in 80Sr was observed.

"Isomeric spectroscopy" was performed on the self-conjugate nucleus 66As. Two microsecond isomers [4], which are copiously produced in fusion-evaporation reactions, allowed a detailed spectroscopy of low-spin levels, which presently cannot be investigated in such detail by any other method. Among the preliminary results is the excitation energy of the first T=0, 1+ level in 66As at 394.2 keV, which extends the E(1+) systematics for the first time beyond 54Co [5].

[1] P. Regan et al., Acta Phys. Pol. 28, (1997) 43; R. Grzywacz et al., Phys. Rev. C 55, (1997) 1126.
[2] B. Vosicki et al., Nucl. Instr. Meth 186, (1981) 307.
[3] J. Doering et al., Phys. Rev C 57, (1998) 1159.
[4] R. Grzywacz et al., Phys. Lett B 429 (1998) 247.
[5] I. Reusen et al., Cont. 2nd Int. Conf. Exotic Nucl. & At. Masses, 23 - 27 June 1998, Bellaire, Michigan, U.S.A.

4. From the Chairman of the HRIBF Users Executive Committee

The Holifield Radioactive Ion Beam Facility (HRIBF) Users Group Executive Committee advises HRIBF management about issues of concern to the user community. The group is composed of six scientists, elected to a three-year term by a plurality of the members of the users group.

The Users Executive committee for 1999 is:

Noemie Koller, nkoller@physics.rutgers.edu
I-Yang Lee (Vice-chair for 1999), iylee@lbl.gov
Brad Sherrill (Chair for 1999), sherrill@nscl.msu.edu
Michael Smith, msmith@mail.phy.ornl.gov
Bill Walters, ww3@umail.umd.edu
Michael Wiescher, wiescher.1@nd.edu
If any of the HRIBF users have any concerns or needs, they should feel free to contact any of the Executive committee members or Carl Gross, the HRIBF Liaison Officer, at cgross@mail.phy.ornl.gov.

5. NSAC ISOL Task Force Visits ORNL

On December 15, 1998, the NSAC Isotope Separator On-Line (ISOL) Task Force visited ORNL. This Task Force, chaired by Hermann Grunder, has been charged to recommend a technical option for constructing an ISOL Facility in the U.S. During the day-long visit various members of the ORNL staff and collaborators from LBNL, LLNL, and Giessen University presented our plans for constructing the Next-generation ISOL (NISOL) Facility at the Spallation Neutron Source (SNS) site. The Task Force also toured the HRIBF (the only ISOL facility in the U.S.) during their Oak Ridge visit. This Task Force is expected to present its preliminary findings to NSAC in the spring and to deliver a final report in the autumn of 1999.

Plans for picking off up to ten percent of the 1-GeV proton beam after the SNS linac and using it as the basis of an ISOL facility are available from the ORNL Physics Division web site, http://www.phy.ornl.gov/nisol/.

6. Presidential Early Career Award Given to Tony Mezzacappa

Anthony "Tony" Mezzacappa of ORNL's Physics Division received the Department of Energy's Young Scientist Award and the Presidential Early Career Award for Scientists and Engineers at ceremonies in Washington on February 10, 1999. These awards are given each year to a few outstanding scientists and engineers at the beginning of their research careers. For example, this year the Presidential Early Career Award was given to about 60 young researchers from all scientific and engineering disciplines. These awards are intended to recognize individuals who show exceptional potential for leadership at the frontiers of scientific knowledge during the 21st century. The Presidential Early Career Award is the highest honor bestowed by the U.S. government on outstanding scientists at the onset of their careers. Tony received the awards in recognition of his research on the collapse of a supernova's core.

Tony follows Michael Smith and David Dean as the third recipient of a Presidential Early Career Award from the ORNL Physics Division in the three years that these awards have existed.

RA1 - RIB Development

In the last three months of operation at UNISOR we have extracted negative fluorine ions directly from an ion source using three different target materials, tested a new concept of liquid germanium target, and installed two new turbo-pumps which should improve the reliability of the vacuum system. In the next few weeks we plan to do a series of off-line tests, followed by on-line experiments, on the new liquid germanium target system which is described below.

Fluorine RIB Development

The ion source for the production of negative fluorine ions has now been tested with fibrous targets of aluminum oxide, zirconium oxide, and hafnium oxide. The 17F atoms were produced in the oxide targets using a (d,n) reaction on 16O. The beam was 20 nA of 30 MeV deuterons.

The measured yield of 17F- ions from the aluminum oxide target was 9x10**5 ions/s normalized to 1 uA of deuteron beam. This represents a target-ion source efficiency of 0.02% which is comparable to the previously reported efficiency for a similar source using a fibrous aluminum oxide target (HRIBF News 3/27/98).

Observed yields of 17F- from the hafnium oxide and zirconium oxide targets were an order of magnitude better. This is partially due to higher target temperatures achievable in these materials, but the yields were higher even at the same temperatures. Having observed AlF+ as the dominant fluorine carrier molecule in the EBP ion source for both aluminum and hafnium oxide targets, we inserted a small amount of aluminum oxide fibers along with the refractory targets to provide a source of aluminum vapor.

The 17F- yield from the fibrous zirconium oxide target was 1x10**7 ions/s/uA of deuterons, and the target ion source efficiency was 0.24% at a target temperature of 1700 C. At higher target temperatures the yield remained constant up to 1830 C, which was the highest temperature measured. The target was at the operating temperature for 40 hours and no measurable decrease in yield was observed. At the end of the experiment, we found the target still intact and only slightly reduced in volume (about 5%).

The fibrous hafnium oxide target also performed well up to 1830 C at which point the yields decreased dramatically due to significant out-gassing of the target material which decreased the overall efficiency of the source. The measured yields increased exponentially with target temperature up to 8x10**6 ions/s/uA of 2H at 1830 C which corresponds to a target ion source efficiency of 0.22%. Since we did not see any saturation of the yields, we infer that the yields would be even higher at higher temperatures if the target material is fully out-gassed. This target was run for about 20 hours and when removed was intact and reduced in volume by less than 10%.

Arsenic RIB Development

Radioactive arsenic ions have been extracted both at UNISOR and from the RIB platform using sources with liquid germanium targets. The targets used thus far have been pools of liquid Ge which are 4 mm thick and 9 mm in diameter. We have been working to reduce the thickness of the Ge layer through which the As atoms must diffuse thus reducing the hold-up times in the target. We are currently testing a design which takes advantage of the wetting properties of Ge on molybdenum. A mesh of Mo wire is coated with a relatively uniform layer of Ge which is about 0.1 mm thick. This target performed well in off-line tests at temperatures up to 1250 C for 20 hours. The long-term durability and highest operating temperatures of this target are the focus of the next off-line tests. Also, we will soon measure the release efficiency of this target for radioactive arsenic atoms.

RA2 - Accelerator Systems Status

ORIC Operations and Development

In December, the Oak Ridge Isochronous Cyclotron (ORIC) successfully provided a 44.5 MeV deuteron beam to the RIB injector target ion source for the production of 17F. Maximum beam intensity on target was 8 microamps. Since that run, ORIC has been shut down for numerous machine upgrades. Two new trim coil power supplies are being installed for T7 and T8. Following a successful run cycle, two more new units will be installed to replace T5 and T6. Conversion of the harmonic coil power supplies from the Modcomp computer system to Vista is underway. Only a few additional controls will need to be converted to fully eliminate the Modcomp. All trim coil and harmonic coil hoses are being replaced and lengthened to minimize low resistance problems which occurred during recent runs. Flow switch wiring for these coolant circuits has been entirely replaced and interfaced to a PLC for improved interlocking and problem diagnosis. The "temporary" lower-channel power leads are being replaced by permanent water-cooled bus, and other maintenance tasks such as re-brushing the main field generator have been completed.

Tandem Operations and Development

The tandem accelerator was opened for scheduled maintenance on December 21, 1998. During this maintenance period the low-energy quadrupole at D2 was removed and repaired. The quadrupole was tested on the bench while under vacuum, and no signs of the previous problems were seen. During the tank closing checks, it was determined that the old NEC gas stripper leak valve had failed. A replacement model, which is advertised to have better control of the leak rate, was ordered from NEC and installed. Also, during this shutdown, a new high-voltage stable injector power supply was installed. Tests show that this power supply has less ripple than the old supply and should improve stability of the beam entering the tandem accelerator. As soon as any leaks are fixed, the tank will be closed and conditioning will proceed.

RIB Injector Operations and Development

In November, we demonstrated that a fibrous target of HfO2 (1.6 g/cm2) could be used in conjunction with an Al vapor oven and our positive plasma ion source to produce ~3x10**8 p/s of Al17F+. This represented an increase in the 17F intensity by a factor of ~600 above the previous maximum yields measured with Al2O3 fiber target material. Shortly after this our first 17F physics experiment began where the excitation function of 17F(p,p)17F in the 10 MeV range was studied at the astrophysics experimental station. After approximately one week of operation the Al oven failed and the experiment ended. Details of this experiment can be found in the second article of this newsletter. Having shown this technique to be a viable means of producing 17F, we have designed and constructed a robust transport vapor oven system to allow the addition of a variety of elemental transport species to the target ion source. This system should have an essentially unlimited operational lifetime compared with the lifetime of the target ion source and operate over the temperature range of 200-1500 C. When used with Al metal, a boron nitride reservoir is employed and temperatures of ~700 C are required.

In December this target system was employed to provide a 17F beam to the astrophysics experimental station. The ionization efficiency of the source did not achieve the values of the first experimental run. In fact, the ionization efficiency as measured by feeding a known flow-rate of Xe into the ion source was only about half the earlier value. Consequently, this reduced the 17F yield by approximately a factor of two from the previous run, delivering only ~15,000 p/s to the experimental end station. The low ionization efficiencies are most likely the result of variations in the preparation of the ion source either in the out-gassing or assembly phase. The run was terminated when the charge exchange cell failed due to an internal heater short. During both experimental runs a gradual decrease in the 17F yield with time was observed indicating some target damage may be occurring. Inspection of the target material revealed significant damage to the target interior which likely occurred toward the end of the run when ORIC production beam intensities greater than 8 uA were applied to the target.

To correct this problem and increase the total overall yield of 17F, a series of high-power target configurations have been designed and are currently being constructed. The designs are based on the concept of redistributing the production beam intensity over larger transverse and longitudinal target dimensions. At operating temperatures above ~2000 C radiative cooling is the dominate heat transfer process and geometrical target configurations, which allow maximum viewing of cool surfaces, will radiate most efficiently. By wiggling or defocusing the beam and adjusting the size of the beam collimator we hope to increase the beam spot size on target to ~23 mm, effectively reducing the beam power density by a factor of ~4. Either by inclining the target material with respect to the beam or by adding gaps within the material, a much larger effective radiating surface can be achieved allowing more heat to flow to the cooler reservoir walls. The combined effect of increasing both the beam size and transverse target dimension and longitudinal target dimensions will increase the effective radiating surface area of the target by a factor of 20-50 over the present target configuration. This should allow stable target operation with increased total beam intensities. A third approach under consideration is to add a W conductive heat sink to the central axis of the target. In parallel to these target developments, the capabilities of the RIB injector to withstand higher total beam intensities have been enhanced by relocating log amplifiers for the faraday cups and beam profile monitors to a shielded area.

In addition to the implementation of these high-power target systems, we believe further total yield gains can be made by replacing the conventional positive plasma ion source with a highly efficient direct negative ion source. This source has been previously developed at our on-line and off-line test facilities specifically for the ionization of metal fluorides such as AlF (see article RA1 in this newsletter). In January, the retrofit of this negative ion source to the RIB injector began, and tests should commence within about one week. Many hardware and software changes to the injector were required to accommodate the new source which includes a Cs vapor reservoir and transport system, a repel grid feed-through, and an SF6 feed system.

With regard to long-term operation of the RIB injector, perhaps the most significant development to have occurred during this period has involved the target ion source vacuum enclosure. Since the RIB target ion source requires remote handling after irradiation, a module containing the source must be manipulated with a robotic arm, remotely transported to a storage area and placed in a storage vessel. The mechanical design of this structure should balance reliability and cost since these enclosures are, in essence, "disposable." Toward this end the first prototype of an all-Al enclosure has been tested and used during the HfO2 bombardment in December. Prior to this a comparatively expensive, stainless steel assembly has been employed which was adequate for initial development work but far too costly to fabricate the multiple copies required in routine operation. A team of engineers has settled on the final design for additional prototypes of this structure, and the project is currently in the bidding process. More information on RIB injector and beam development can be found at our web site.

RA3 - Experimental Equipment - Recoil Mass Spectrometer

The Recoil Mass Spectrometer (RMS) has been the workhorse of nuclear structure research the past year. With the refinements made to the Double-sided Silicon Strip Detector (DSSD), LSU's Moving Tape Collector (MTC), Clover Ge Array (CLARION), and Ionization Chamber (IC), a complete experimental program to study nuclear structure has been established. To date, the following experimental techniques have been used:

Highlights from our recent experiments include the discovery of several new proton emitters in the A=145 region (see last newsletter), beta spectroscopy in self-conjugate nuclei in the A=70 region (see this newsletter), isomer spectroscopy (see this newsletter), and identification of gamma-ray transitions in 151Lu and 79Y.

Many changes and additions have been made to the RMS. The new support structure for CLARION (CLover Array for Radioactive ION beams) was installed in August. The new target chamber, with its provisions for a charge reset foil and a large forward fantail to remove scattered RIBs from the focus of CLARION, has been installed earlier this year. The electronics for CLARION is nearing completion. The vacuum chambers near the achromatic focus has been rearranged to more easily allow experiments at this location. A support structure for Clover Ge at the final focal plane has been used with the MTC, and we are working on a more permanent structure. A new chamber for the DSSD has been developed to allow the installation of additional silicon detectors and thin windows for use with a large-area X-ray detector. Collimator slits have been added to the focal plane chamber.

RA4 - HRIBF Users

I-Yang Lee was unanimously elected vice-chairperson for 1999. He will become chairperson on January 1, 2000, and serve for 1 year. The HRIBF web site is in transition to our new format, and any bookmarks you may have stored in your browser may no longer be valid. We are updating the information and are expanding the site. Some links may not work at times, and we ask for your patience and help. Should you find missing articles or have other difficulties, please contact the person listed at the bottom of each page. The HRIBF Calendar is now up on the web site. The schedule for the rest of this quarter and the following quarter may be found at http://www.phy.ornl.gov/hribf/users/01-03-1999.html and http://www.phy.ornl.gov/hribf/users/04-06-1999.html, respectively.

RA5 - Experiments, Spokespersons, and Dates Run During the Past Quarter

Experiment

Spokesperson/Institution

Dates

RIB-018 - Proposal to Measure the p(17F,17F)p Reaction at the Holifield Radioactive Ion Beam Facility

Bardayan/Yale University

11/2-5/98
12/7-9/98
12/11-16/98

RIB-031 - States in 18Ne Populated by Resonance Scattering of 17F on 1H Using Thick Targets

Galindo-Uribarri/ORNL

11/5-6/98

RIB-036 - Identification of the Beta Decay of 66Se

Piechaczek/Louisiana State University

11/9-13/98

RIB-014 - Target-Ion-Source Development - Arsenic and Fluorine

Stracener/ORNL

11/16-17/98
11/25/98
12/1/98

RIB-012 - As + Ti Sub-barrier Fusion

Liang/ORNL

11/18/98
12/1-4/98

RIB-000 - Recoil Mass Spectrometer Commissioning

Gross/ORISE, ORNL

11/19-20/98
11/23-24/98
11/30/98

Unscheduled Maintenance

12/10/98

Scheduled Maintenance

12/17/98-
1/29/99

For this quarter's schedule go to http://www.phy.ornl.gov/hribf/users/01-03-1999.html.





Additional copies of the newsletter and more information about HRIBF can be found on the World Wide Web at www.phy.ornl.gov. You may contact us at the addresses below.

Jerry D. Garrett Carl J. Gross
Scientific Director Scientific Liaison
Mail Stop 6368 Mail Stop 6371
garrett@mail.phy.ornl.gov cgross@mail.phy.ornl.gov
+1-423-576-5489 +1-423-576-7698

Holifield Radioactive Ion Beam Facility
Oak Ridge National Laboratory
Oak Ridge, Tennessee 37831 USA
Telephone: +1-423-574-4113
Facsimile: +1-423-574-1268