HRIBF Newsletter, Edition 13, No. 1, Feb. 2005

   


Feature Articles

Regular Articles



1. HRIBF Update and Near-Term Schedule
(J. R. Beene, M. J. Meigs, R. C. Juras)

The performance of HRIBF during fiscal 2004 (ending October 1) was truly outstanding.  We were able to provide a record number of 1670 hours of RIB to experiments, an increase of 35% over FY2003 (which was itself a record year with an increase of 20% over 2002).  This result is particularly remarkable given the fact that we are in the midst of the construction of the new High Power Target Laboratory (HPTL).  It will not be possible to continue this rapid increase in RIB hours delivered for another year.  We have been aware for some time that greater interference between HRIBF operations and the HPTL project was inevitable in fiscal 2005.  We were also aware that the replacement of the tandem terminal magnet power supply carried out at the end of FY04 was bound to impact operating hours during the first quarter of FY2005.  We did not anticipate a third issue that will impact our beam schedule.  This was the failure of several coils in the downstream element of the second stage mass analyzer (Isobar Separator).  This is among the newest magnets in the facility and it must operate for us to inject beams from the RIB injector to the tandem.  New coils have been ordered (a full set, since there is apparently a design flaw) and will be delivered in late April 2005. We expect the Isobar Separator to be fully restored to operation by the end of May. (See the following subsetions 1.1 for detailed status of the RIB isobar separator magnet system.) For the time being, we are limited to injecting beams of A less than or equal to 40 from the RIB injector.

As a result of these developments, the neutron-rich campaign planned for early spring has been delayed until summer.  We expect the time between now and May to be occupied by a continuation of the present 7Be campaign, followed by a nine-week 17,18F campaign, along with short stable-beam runs and a brief tank opening for tandem maintenance.  The remainder of the year will be occupied by a ~4-month neutron-rich RIB campaign.

This has been - and will continue to be - a particularly trying time for HRIBF operations and research staff.  We have many irons in the fire and are stretched extremely thin.

We had two major DOE reviews of HRIBF related activities in 2004.  An annual review of the HPTL project, including a preliminary review of the new IRIS2 project, was carried out in September, and a Scientific and Technical Review of HRIBF operations and low-energy nuclear physics research was held in November.  More detailed discussions on the status of HPTL and IRIS2 can be found in a subsequent article. We do not yet have a written report on the S&T review, but the oral closeout was extremely favorable.  Our staff and users are to be commended for the excellent work they have done and continue to do.


1.1 RIB isobar separator magnet system update

The RIB isobar separator magnet system on beam line 12 experienced a partial coil failure in August 2004 while providing 134Sn for RIB-121. Beam instabilities were a direct result of a water leak on the upper coil of the second separator magnet. The run was stopped to try to address the magnet problem. There was an attempt to remove water from the coil and to dry it by passing air through the middle pancake that was determined to be leaking. This pancake was then shorted and 318 amps were applied to the magnet in an attempt to further dry it. At this time, the top pancake in the bottom coil failed and subsequently yet another pancake in the bottom coil failed. The magnet had been specified to run at 360 amps, but evidently there was a flaw in the design or manufacture which caused it to fail at even lower currents. Prior to this point, the magnet had not been run at more than half the capacity for any length of time. Therefore, at this time, three of the six pancakes in the second magnet are shorted which limits transport through the system to masses less than 40. The system is usable for the Be and F campaigns, but not for neutron-rich beams.

Replacement coils are under fabrication and should be shipped at the end of March. Since they will be transported by sea, it will probably be the end of April before they arrive at ORNL. Installation of the new coils will be challenging and will probably require about a month from disassembly to subsequent operation for the experimental program. Specifications and testing for these coils is more stringent than for the original to try to prevent the same type of failure.

1.2 Tandem accelerator terminal magnet update

A new power supply was purchased and installed to replace the original power supply for the tandem accelerator terminal magnet (BM T-1). The original power supply was manufactured in 1976 by Alpha Scientific Inc. and was showing signs of its age. Power supply failures resulted in several tank openings in the previous two years, and replacement parts for the power supply were becoming difficult to obtain and very expensive. For several months before it was removed, the power supply was operating without a full complement of transistors in the 57-transistor regulator bank because we had not been able to obtain suitable replacements. In addition to the problem of obtaining replacement transistors, water passages in heat sinks for the transistors banks, in transformers and chokes, and in the water-cooled shunt were becoming corroded or plugged.

New power supply installation was challenging because the only access to the terminal is by means of the central service platform (CSP) and then access to the second level of the terminal, where the power supply is located, is by means of a ladder through a narrow opening. The original power supply was installed before the terminal was completely assembled. The old power supply was cut into pieces for removal and the new power supply, purchased from Alpha Scientific, was custom designed to be separable into pieces suitable for transport in the CSP and up the ladder.

When the new power supply went into operation, it was initially susceptible to accelerator high voltage transients. Working with the manufacturer, the susceptibilities were identified and corrected. Each repair and modification resulted in significant lost time because the power supply is located in the terminal of the accelerator and requires a tank opening to access. Of nine unscheduled tank openings during the first quarter of FY05, seven were caused by the power supply. The first six failures were on the power-supply regulator amplifier circuit board. These were solved by circuit modifications that would better protect components on the circuit board, and modifications of the power supply enclosure and cabling aimed at reducing any transient energy leading to the circuit board. The seventh failure was caused by failure of an input fuse. All of the input fuses will be replaced by a slower-acting fuse type. Until December 2, every tandem accelerator spark over 20 MV resulted in damage to the regulator circuit board. Since December 2, there have been more than ten sparks over 20 MV with no regulator circuit board damage. The manufacturer will provide a regulator amplifier circuit board that incorporates all of our modifications and will also provide slower-acting fuses and an appropriate fuse block. The circuit board will be packaged in a separate, shielded enclosure. The new parts will be installed during the next scheduled maintenance period.


2. Recent HRIBF Research - Measurement of Evaporation Residues at Sub-Barrier Energies
(D. Shapira, Spokesperson)

Experiment RIB121 was approved last year in order to study excitation function for fusion of 134Sn with 64Ni. Since the mass A=166 ions extracted from the ion source (sulfides of Sn and Te) contained approximately equal amounts of 134Sn and 134Te sulfides, it was realized that to obtain the 134Sn + 64Ni data two experiments will have to be run. Evaporation residues from the mixture of both beams were measured together with a count of the number of 132Sn and 134Te particles that hit the target (and also any other contaminants). After that the tune of the source was changed from extracting mass 166 to extracting mass 134, the latter contained more then 96% of 134Te. The evaporation-residue cross section for 134Sn + 64Ni was measured with the same target and energy. Since the ratio of Sn to Te is known in both experiments, the two measurements yielded evaporation-residue cross sections for both 134Sn + 64Ni and 134Te + 64Ni. The system used in these measurements is shown in Fig. 1. Events recorded in the ionization chamber that show the separated evaporation residue data appear in Fig. 2, and Fig. 3 shows the 2-d map of A=134 beam particles delivered in the experiment. A full excitation function for 134Te + 64Ni evaporation residue data is shown in Fig. 4. The runs for 134Sn + 64Ni data were cut short because of a breakdown of the isobar separator magnet and will be continued this summer.


3. Recent HRIBF Research - First RIB g-factor Measurement Using Recoil-in-Vacuum Technique
(N. J. Stone, Spokesperson)

The advent of RIBs presents experimenters with new challenges as well as new opportunities. It has long been recognized that the g-factor of a nuclear ground or excited state gives valuable evidence as to its single particle and/or collective makeup. The usual method of ~ps excited state g-factor study has, in recent years, been the transient field (TF) method. In TF beam nuclei are excited by Coulomb excitation and, as they decay by a highly anisotropic gamma emission, the angular distribution of this emission is caused to rotate by their passing through a ferromagnetic metal layer in which they are subject to Larmor precession. The observed rotation yields both magnitude and sign of the g-factor. However, the rotation angles are small, requiring high statistical accuracy in the data, and, to prevent any other perturbation of the distribution, after emerging from the ferromagnetic layer, the excited nuclei, plus the rest of the beam, are stopped in a third region of the target. With RIBs TF has problems; RIBs are orders of magnitude weaker than SIBs, so high statistical accuracy requires long runs, and stopping a RIB in the target produces large unwanted activity which can be particularly troublesome in the not uncommon case that beam impurities decay through the excited state under study.

A team from HRIBF, combined with others from Oxford, Tennessee, ANU, Brno, Maryland, Rutgers and Yale, have recently completed a successful g-factor measurement on the first 2+ state of the RIB isotope 132Te [1]. The experiment took full advantage of the combination of the detector systems CLARION and Hyball. Thin C targets, without backing, were used so that, following Coulomb excitation, both the excited Te isotopes and the recoiling C nuclei left the target. The C nuclei were detected in Hyball, which has a series of segmented rings subtending different ranges of angle q with respect to the beam axis. The specific segment recording the event determines its reaction plane. Gamma decay of the Te 2+1 excited state at 973 keV was detected in one of the CLARION detectors, determining both the value of qg and the angle of emission f relative to the reaction plane. The un-reacted RIB activity has long lifetime and leaves the target region, so causing no unwanted background.

As the excited Te ions leave the target, their nuclear spins I are aligned perpendicular to the beam following the Coulomb excitation process. For 396-MeV beam energy incident on an 0.83 mg/cm2 C target the Te ions emerge with charge state range from ~ 32+ - 36+. For each charge state there is a range of electronic states having different total angular momentum J. I and J then precess about their mutual resultant F and this precession leads to de-alignment of the nuclear spins, producing, in turn, attenuation of the anisotropy of the angular distribution of their gamma decay. The attenuation is caused by Larmor precession over the lifetime of the excited state and is thus a measure of the g-factor of the excited state. This method of measuring g-factors, known as the Recoil-in-Vacuum method [RIV] was first developed in the 1970's, and is set to make a comeback with the advent of RIBs [2].

The attenuation was calibrated by short measurements on the first 2+ states in the SIB isotopes 122,126,130Te having known g-factors and lifetimes [Figs.3-1 and 3-2]. Fig.3-1 shows the unperturbed angular distribution from 130Te when it does not recoil into vacuum [the SIBs could be stopped in a second, Cu backed, target], compared with the attenuated distributions found for RIV 126,132Te using the thin C target, all having the same range of ionic charge state. Fig.3-2 shows the attenuation coefficients for the SIBs plotted as a function of the product of their excited state lifetimes and g-factors, gτ. The measured attenuations for 132Te, again with the same ionic states, are also shown in Fig.3-2. With the lifetime of 2.6(2) ps from the recent HRIBF measurement of the B(E2) of this level [3], these results give the g-factor as (+)0.35(5).

It is predicted that the g-factors of the 2+1 states of the Te isotopes should increase from their level of close to 0.3 at mid-shell to a sharp maximum of about 0.7 at the neutron shell closure, before falling steeply beyond the shell, possibly to negative values. The observed increase is rather less than recent calculations predict [4,5].

The combination of HRIBF detection facilities CLARION plus Hyball has proved ideally suited for RIV experiments, opening the prospect of g-factor study with RIBs of existing strength. The team aim to continue this work over the coming year, planning to measure on 134,136Te by the RIV method. We also will explore the practicability of TF measurements.


[1]N. J. Stone et al., submitted to Phys.Rev.Letters
[2]G. Goldring, in Heavy Ion Collisions Vol 3, ed R. Bock (North Holland, Amsterdam, 1982) p. 484.
[3]D. C. Radford et al., Phys.Rev.Letters 88, 222501 (2002).
[4]J. Terasaki et al., Phys.Rev. C66, 054313 (2002).
[5]B. A. Brown et al., arXiv:nucl-th/0411099v1 24 Nov 2004.



3. Laser Ion Source Tests Performed at HRIBF
(Y. Liu, Spokesperson)

Laser ion sources (LIS) based on resonant photo-ionization have already proven to be of great value at existing ISOL radioactive ion beam facilities for generating useful intensities of isobarically pure radioactive ion beams [1-5]. In these ion sources, ions of a selected isotope are produced via stepwise atomic resonant excitations by two or three laser beams followed by ionization in the last transition. Because each element has its own unique atomic energy levels, the resonant photoionization process can provide elemental selectivity of nearly 100%. A hot-cavity laser ion source with three tunable Ti:Sapphire lasers has been set up at the off-line Ion Source Test Facility of HRIBF, and initial laser ionization experiments have been successfully performed, in collaboration with the Atomic Physics Group of the ORNL Physics Division and the research group led by Dr. Wendt of the University of Mainz. The hot-cavity ion source was modified from a Ta tubular surface ionization source. It has the same basic structure as the standard high temperature RIB ion sources employed for on-line operation at the HRIBF. A schematic view of the ion source is shown in Fig.4-1. The Ti:Sapphire lasers have been developed and provided by the Mainz group [6]. They are pumped by a commercial frequency-doubled Nd:YAG laser operating at 10 kHz repetition rate with a maximum of 60 W average power at 532 nm. The Ti:Sapphire lasers are tunable over the range of 720 to 925 nm. With 2nd and 3rd harmonic generation capabilities, the laser system can provide tunable laser radiation in 360-463 nm (frequency doubling) and 240-308 nm (frequency tripling) spectral region. Fig.4-2 shows a picture of the Ti:Sapphire laser system in operation at HRIBF.

In the initial laser ionization experiments, three-step resonant ionization of Sn, Ni, and Ge has been observed. The excitation schemes used for the three elements are shown in Fig.4-3. A known three-step excitation scheme for Sn, which leads to an autoionization state in the last step [5], was used in this study. No previous laser ion source work on Ge has been reported. Resonant laser ionization of Ge in a hot-cavity LIS was demonstrated for the first time in this experiment. Ge atoms were excited from the 4p2 3P1 ground state level to the 4p5s 1P1o excited state (lambda1 = 253.39 nm), and then to the 4p5p 1S0 state (lambda2 = 909.845 nm). For the last transition, a careful search for autoionization was conducted by scanning the third laser wavelength over the tuning range of the Ti:Sapphire laser. Both Rydberg states and autoionization states were successfully observed in Ge. The best ionization yield was obtained using the autoionization state at 63818.262 cm-1. An extensive search was also conducted to look for autoionization states in Ni. Although no autoionization states were found, a variety of Rydberg states were detected. The ionization of Ni was then accomplished by three-photon resonant excitation to a Rydberg state. Displayed in Fig. 4-4 are measured mass spectra showing the Ge and Ni ions observed when the laser beams were turned on and the surface ionized Ga ions when the lasers were off. The temperature of the hot-cavity was about 1700-2000° C. No surface ionized Ge and Ni ions were observed when the laser beams were turned off.

Overall ionization efficiencies of 22%, 2.7%, and 3.3% were measured for Sn, Ni, and Ge, respectively, using calibrated samples containing ~1017 atoms. For Sn, the efficiency obtained in this study (22%) is better than the reported off-line and on-line performance at ISOLDE using either dye or Ti:Sapphire lasers [7]. Our results represent the first LIS work reported for Ge and the first efficiency measurement reported for Ni using Ti:Sapphire lasers. The ionization process was not saturated for either Ge or Ni in this work. Thus, improvements for both elements are expected using more efficient excitation schemes. The Rydberg and autoionization states identified in Ge and Ni will be of value in future work. Analysis of these atomic spectroscopic results is in progress.


[1.] V .N. Fedoseyev, G. Huber, U. Koester, J. Lettry, V.I. Mishin, H. Ravn, V. Sebastian, Hyperfine Interact. 127 (2000) 409.
[2.] P. Van Duppen, Nucl. Instrum. Methods Phys. Res. B126 (1997) 66-72.
[3.] Yu. Kudryavtsev, et al., Nucl. Phys. A701 (2002) 465c-469c.
[4.] M. Koizumi, et al., Nucl. Instrum. Methods Phys. Res. B204 (2003) 359.
[5.] U. Koester, V.N. Fedoseyev, V.I. Mishin, Spectrochim. Acta Part B58 (2003) 1047.
[6.] Christian Rauth, et al., Nucl. Instrum. Methods Phys. Res. B215 (2004) 268-277.
[7.] K. Wendt, et al., International Conference on Laser Probing, Oct. 16-23, 2004, Argonne, IL, USA.



5. HPTL and IRIS2 Status Update
(B. A. Tatum)

The High Power Target Laboratory (HPTL) upgrade of HRIBF is proceeding well, although there have been several challenges. Facility modifications were scheduled to be completed by the end of December, but that effort will now extend a few weeks into 2005. NEC has delivered the new high voltage platform system, and installation will begin in late January after the Target Room is completed. ORIC beam line magnets are also scheduled to ship from Sigma Phi in early January. Procurement issues caused the magnet contract to be awarded later than anticipated, but the delay is not expected to affect the overall project completion schedule. The 90-degree low-energy RIB analysis magnet design is being finalized, and the schedule calls for delivery by the end of March. Overall, the project is still on budget and schedule, but the months ahead will be extremely busy for our staff as we focus on installation, testing, and commissioning of the beamlines, platform system, and target station. The DOE Office of Nuclear Physics (DOE-ONP) conducted an Annual Progress Review of the HPTL project in late September and the report was quite positive, concluding that "the completed project will provide unique capabilities to the national nuclear physics program."

A proposal was also submitted to DOE-ONP in September for an additional upgrade project known as IRIS2, or second Injector for Radioactive Ion Species. This approximately $4.7M,two-and-a-half-year upgrade project would result in a second RIB production station, thus providing improved facility reliability through much needed redundancy in our RIB generation and preparation systems. IRIS2 would be co-located with HPTL and include a first stage RIB analysis system located on a greatly expanded high voltage platform system, a beam transport line from the HPTL to the existing second stage isobar separator, and a laser lab relevant to laser ion sources and beam purification. The HPTL Review Panel was also tasked with evaluating the IRIS2 proposal for technical merit and significance. Feedback from that review was also very positive, and HRIBF has been directed to prepare a Project Management Plan by May 1 in preparation for a potential baseline review.




6. Recent Scientific and Technical Review of HRIBF by DOE
(G. R. Young)

The DOE Office of Nuclear Physics held a science and technology review of the HRIBF at ORNL on November 22-23, 2004. This review asked an outside panel of experts to advise the Office regarding all aspects of the HRIBF program. Jim Beene presented an overview of HRIBF and its science progam, and Alan Tatum described the recent operational experience with HRIBF and the now-underway project to add a new High-Power Target Lab to HRIBF to improve our ability to develop and test new ion sources at full power. Alan also presented a plan for a second RIB injector platform and for a future driver accelerator which would produce RIBs via electrofission. Dan Stracener reviewed the ion source development program, including the new results on laser-purification of beams. Sandra Kennedy reviewed operational experience for the Physics Division and for its users and gave a perspective on recent safety initiatives. Witek Nazarewicz, Carl Gross, and Ed Zganjar gave an overview of the user program, UEC membership, and recent running and PAC statistics. The scientific program was presented in two sets of talks, with Michael Smith and Raph Hix reviewing astrophysics experiment and theory, and Dave Radford and Krysztof Rykaczewski covering nuclear structure experimental work prior to Witek Nazarewicz's review of structure theory. Cyrus Baktash drew these results together and presented the scientific and technical case for improvements to the HRIBF experimental apparatus, with an emphasis on upgrades to the gamma-ray detection capability, an improved gas-jet target, and a modernized analysis computing farm.

These reviews have become a regular means for the Office of Nuclear Physics to gather recent information about its users facilities and plan for their future use and improvement. They are a key forum to present matters which will enhance the facility's scientific output and strengthen the user program.




7. Physics Staff Members Became APS, IOP Fellows

Three members of the Physics Division have been elected fellows of the American Physical Society (APS), and another two were elected fellows of the Institute of Physics (IOP) in London, UK.

The three APS fellows are David J. Dean, Anthony Mezzacappa and Predrag S. Krstic. Election to fellowship in the APS is limited to no more than one half of one percent of the annual APS membership and is in recognition of outstanding contributions to physics.

Glenn Young and Witold Nazarewicz have been elected fellows of the Institute of Physics.

The APS recognized Dean for his important contributions to understanding of quantum many-body systems and for applications of computational quantum mechanics to the structure of atomic nuclei.

Mezzacappa was cited by the APS for his pioneering work toward identifying the explosion mechanism of core collapse supernovae and his leadership in the development of U.S. computational science.

The society recognized Krstic for his important and diverse contributions to atomic theory, in particular, to the theory of non-adiabatic heavy-particle collisions and of relativistic effects in ultra-strong laser-atom interaction.

The Institute of Physics elected Young, who directs the Physics Division at ORNL, for his experimental research that focuses on states of nuclear matter at very high energy density.

Nazarewicz, whose nuclear theory research focuses on the description of nuclei far from stability, is scientific director of the Holifield Radioactive Ion Beam Facility as well as a professor at the University of Tennessee in Knoxville.




8. PAC-11 Results and PAC-12
(C. J. Gross)

PAC-11 met January 13-14, 2005, in Oak Ridge and considered 22 proposals and letters of intent which requested 206 shifts of radioactive beams (RIBs), 106 shifts of low intensity stable beams (SIB for RIBs), and 63 shifts of stable beams (SIBs). Of these, a total of 105 RIB shifts, 71 SIB for RIB shifts, and 24 (+9 additional shifts depending on results) SIB shifts were approved.

Approved experiments requested RIBs of 7Be, 18F, 56Ni, 84Se, 132Sn, and 132,134Te. The total number of accepted proposals from outside HRIBF was 10 out of a total of 13. Uwe Greife, this year's chair of the Users Executive Committee (see RA4) represented the Users at the meeting. We wish to thank Juha Äystö for his service on the PAC. He will be replaced on PAC-12 by Peter Butler of the University of Liverpool. We anticipate PAC-12 to occur in November of this year with proposals due in October.




9.The Fourth International Conference on Exotic Nuclei and Atomic Masses (ENAM) was held on September 12-16, 2004.
(C. J. Gross)

ENAM is a triennial international conference series devoted to the physics of nuclei far from stability and radioactive ion beams (RIB). RIB science is extremely broad and diverse. It spans the gamut from nuclear structure to astrophysics, tests of fundamental laws of nature, and myriad applications. ENAM'04 can trace its origins to the 1950s and 1960s with the Atomic Mass and Fundamental Constants (AMCO) and the Nuclei Far From Stability (NFFS) series of conferences. The Fourth Conference was held at Callaway Gardens in Pine Mountain, Georgia; it was organized by the ORNL Physics Division.

The science program covered the topics of atomic masses, nuclear structure of exotic nuclei, reactions with radioactive ion beams, clusters and nuclear driplines, nuclear astrophysics, heavy elements, nuclear fission, and production of radioactive ion beams and applications. The program consisted of over 33 hours and 45 minutes of oral presentations and more than 160 poster presentations over two sessions. There were no parallel sessions, and ample time was provided for collaborative discussions as well as relaxation in the gardens.

The conference received more than 280 abstracts (including invited presentations) from which 130 oral presentations were selected; 43 of which were 5-minute oral advertisements (oral-posters) for their posters. The talks were broken down into:

36 invited talks of 25 or 30 minutes
48 contributed talks of 15 or 20 minutes
1 contributed talk of 25 minutes
43 oral-posters of 5 minutes
2 welcome/summary

As a service to our community, oral presentations from the conference have been posted on the web. The abstracts of the poster contributions are also available. The proceedings will be published in The European Physical Journal A direct, and will be available on-line at no cost soon.

The conference received an initial registration of more than 350 possible participants. Of these, 280 attended representing 23 countries on 4 continents. The United States had the largest contingent with 45% of the participants. The participants came from the following countries: Belgium (6), Bulgaria (1), Brazil (2), Canada (11), Croatia (2), Denmark (2), Finland (12), France (14), Germany (23), India (2), Israel (2), Italy (7), Japan (28), Mexico (3), Netherlands (2), Poland (8), Russia (5), Spain (3), Sweden (2), Switzerland (8), Ukraine (2), United Kingdom (9), United States (126). The complete list of participants can be downloaded from the ENAM website.




10. RIA Summer School on Exotic Beam Physics Will be held at Lawrence Berkeley National Laboratory
(W. Nazarewicz)

The fourth annual RIA Summer School on Exotic Beam Physics will be held at Lawrence Berkeley National Laboratory from July 31-August 6, 2005. Lectures will take place on the Berkeley Lab site, with hands-on activities at the 88-Inch Cyclotron. The aim of the summer school is to nurture future RIA scientists so that the community will have sufficient manpower to effectively use RIA when it comes online. The RIA summer school is supported by the DOE, NSF and the following laboratories: ANL, LBNL, LLNL, ORNL and NSCL/MSU. It is an annual event, rotating among the 88-Inch Cyclotron, ATLAS, HRIBF, and NSCL.

The summer school has three main components. The mornings will be devoted to lectures covering broad topics in both the experimental and theoretical physics of nuclei far from stability. This year's lecturers have been set and are listed below:

Robert Grzywacz (Tennessee) - Nuclear Structure
Guy Savard (ANL) - Fundamental Symmetries
Heinrich Schatz (MSU) - Nuclear Astrophysics
Jim Vary (Iowa) - Nuclear Theory
Sherry Yennello (TAMU) - Nuclear Reactions

Other, shorter lectures will focus on specific areas of R&D leading towards RIA and the instrumentation and techniques which are being developed for use at RIA. These talks will complement the hands-on activities taking place in the afternoons. The afternoons will provide opportunities for "hands-on" experience with experimental equipment and techniques useful in RIA research.

For more information, please visit the 2005 RIA Summer School website.




11. Summer School and Workshop on Radioactive Ion Beam Production Targets and Ion Sources to be held in May 2005
(H. K. Carter)

The first summer school and workshop in the U.S. on "Radioactive Ion Beam Production - Targets and Ion Sources." will take place May 23-25, 2005, followed by a workshop on the same topic on May 26-27, 2005. Both will be held at the Holifield Radioactive Ion Beam Facility (HRIBF) at Oak Ridge National Laboratory in Oak Ridge, Tennessee.

The purpose of the Summer School is to introduce young researchers in various disciplines to the research opportunities and challenges in this important and growing field. Following the school a workshop will be held for experts in the field to focus on a few current "problem beams". The summer school students will be encouraged to participate in the workshop.

The format of the school will be a half day of lectures from experts in the field followed by a half day of hands-on experiments using the HRIBF facilities. The workshop following the school will focus on the production of new beams that are presently not available, as well as improving existing beams in intensity and purity. The workshop will consist of presentations followed by discussions that will take advantage of the various viewpoints of the cross-discipline audience.

Advanced undergraduates through early post-doctoral researchers are strongly encouraged to apply. The workshop is intended for all researchers interested in developing radioactive ion beams.

Students accepted for the school will be provided with partial travel support and local expenses for the period of May 23-28.

To learn more about these programs and how to apply, please visit the Summer School website.

Applications for the Summer School must be received by March 1, 2005.




RA1. RIB Development
(D.W. Stracener)

We have recently completed two separate tests using laser beams to improve the quality of our radioactive ion beams. The first is an initial test of an ion source where the ions are produced using three independently-tuned Ti:Sapphire lasers to selectively ionize the element of interest. This project is described in another article in this Newsletter. In the other development project a laser is used to purify a 56Ni beam by eliminating a large fraction of the 56Co contamination from the low energy beam that is extracted from the ion source.

Accelerated beams of 56Ni have a wide range of interest in the nuclear structure and astrophysics communities. Some time ago, we developed a target and ion source to deliver 56Ni beams. A nickel pellet is irradiated with 42-MeV protons and the 56Ni is produced via the (p,p2n) reaction on 58Ni. In addition, 56Co is produced via the (p,2pn) reaction and the cross-section for the 56Co production is about an order of magnitude larger than for the 56Ni production. This irradiated sample is then rotated into a Cs-sputter ion source and negative-ion beams are produced. The sputter efficiencies of nickel and cobalt are similar, so the composition of the A=56 beam is about 90% cobalt and 10% nickel.

We have demonstrated an efficient way to purify the Ni beam by selectively neutralizing the negative Co ions using a CW Nd:YAG laser interacting with the beam in a gas-filled RF quadrupole (RFQ) ion guide. The photons are absorbed and the additional electrons are excited into the continuum. The technique of laser photo-detachment was demonstrated in the Co/Ni system over a decade ago by Berkovits et al. (NIM A281 (1989) 663 and NIM B52(1990) 378).

The electron affinities are 0.661 eV for Co and 1.156 eV for Ni and the fundamental wavelength of a Nd:YAG laser is 1064 nm (1.165 eV). This photon energy is high enough to efficiently neutralize negative Co ions and, with much lower efficiency, negative Ni ions will also be neutralized. The beam energy from the ion source is 5 keV and the energy is reduced to <50 eV before entering the He-filled RFQ ion guide, which is 40 cm long. In the RFQ ion guide, ion energy is further reduced by collisions with the buffer gas molecules. This results in a laser-ion interaction time on the order of 1 ms.

In a test with stable isotopes (see Fig.RA1-1) 95% of the Co ions were neutralized and 10% of the Ni ions were neutralized. For this test the laser beam power was 5 W with about 50% transmission through the RFQ. The figure shows the relative beam intensities of nickel and cobalt ions with the laser beam interacting with the ion beam (Laser On) and with the laser beam physically blocked (Laser Off).

Several improvements must be realized before the technique can be implemented on the RIB Injector as a means to deliver purified beams of 56Ni. The transmission of the ion beam through the RFQ ion guide must be improved. A new RFQ ion guide system must be designed for beams with energies of about 20 keV. Higher laser power is needed to further suppress the negative Co ions in the beam. While it will certainly be possible to implement this technique in a second, more spacious radioactive beam injector, we are trying to devise a way to use this purification technique on the present RIB Injector.




RA2. Accelerator System Status

ORIC Operations and Development (B. A. Tatum)

ORIC provided a 52-MeV proton beam for the neutron-rich RIB campaign during most of July and August.  The machine ran quite well with only a few minor power supply instabilities.  ORIC was shut down for the remainder of the year during the tandem tank opening, stable beam runs, and the Beryllium RIB campaign with the batch-mode ion source.

There were a few key maintenance and upgrade activities that took place during the shutdown.  First, a new ion source service platform was installed that makes source maintenance much easier.  This platform is considerably larger than its predecessor, has better access, and is designed to reduce the spread of possible radiological contamination.  Second, the ORIC control system was converted from Vista to the EPICS platform.  This conversion was the final step in unifying all of the HRIBF control systems under EPICS and is expected to result in more efficient and reliable operation.  Third, preparations were made for installation of the new ORIC beamline that will deliver beam to the High Power Target Laboratory.  Installation of the beamline will begin in February 2005. 

Tandem Operations and Development (M. J. Meigs)

The Tandem Accelerator was operational, to provide beam, for approximately 1769 hours since the last report.   The machine ran at terminal potentials of 3.47 to 24.15 MV and the stable beams 1H, 7Li, 10B, 12C, 14N, 18O, 19F, 24Mg, 58Ni, 124Sn, and 130Te were provided.  Radioactive beams of 7Be, 132,134Sn, and 132Te accounted for 571 hours. Ten tank openings were necessary during this period with the first being a scheduled maintenance period.  During this scheduled opening, the new power supply for the terminal bending magnet and an additional ion pump were installed.  The additional ion pump shares the gas load of the stripper with ion pump T-2 and allows better accelerator tube vacuum while allowing stripping at a higher gas pressure.  Seven of the openings were for failures of the new supply after sparks.  A separate article in this newsletter gives details on the status of this power supply.   The other openings were for communication failures after sparks; once to D4 and once to the terminal.  About 85 hours were spent on conditioning.

During this period an experiment was done with 100-MeV 7Be.  This beam must be injected as the molecule 7Be16O and must be fully stripped in the terminal to achieve the desired energy.  This was accomplished by breaking up the molecule with the gas stripper and fully stripping the 7Be with the foil stripper.  Before the new gas stripper installation and relocation of the foil stripper above the gas stripper, this would not have been possible since the coulomb explosion from deep stripping with the foil stripper would have caused extremely low transmission down the high energy tube.  This was in fact, experimentally confirmed by the operators who could get no transmission of beam with either stripper alone but had good transmission when using both strippers. 

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

During this reporting period, we delivered beams of

  • 60 kpps 14+/26+ terminal foil / high energy foil stripped 530 MeV pure 132Sn to Beam Line 23,
  • 64 kpps 14+/26+ terminal foil / high energy foil stripped 545 MeV pure 132Sn to Beam Line 23,
  • 51 kpps 14+/27+ terminal foil / high energy foil stripped 560 MeV pure 132Sn to Beam Line 23,
  • 29 kpps 15+/28+ terminal foil / high energy foil stripped 590 MeV pure 132Sn to Beam Line 23,
  • 20 kpps 16+/29+ terminal foil / high energy foil stripped 620 MeV pure 132Sn to Beam Line 23,
  • 450 pps 14+/25+ terminal foil / high energy foil stripped 500 MeV 40% 134Sn to Beam Line 23,
  • 600 pps 14+/26+ terminal foil / high energy foil stripped 530 MeV 40% 134Sn to Beam Line 23,
  • 200 kpps 1+/ 4+ terminal gas / post foil stripped 12 MeV pure 7Be to the Daresbury Recoil Separator, and
  • 3.7 Mpps 4+ terminal gas / terminal foil stripped 100 MeV pure 7Be to the astrophysics endstation in Beam Line 21.

The tin beams were produced via proton-induced fission of 238U by bombarding a uranium carbide coating on a reticulated vitreous carbon fiber target coupled to an Electron Beam Plasma (positive) Ion Source (EBPIS) with 7.5-8 uA of 54-MeV 1H and passing the positive tin sulfide beam through the recirculating cesium jet charge exchange cell and selecting the negative tin beam resulting from molecular breakup.

The neutron-rich campaign ended when the second stage mass separator (SIGMA PHI) second dipole upper coil developed a short in the middle winding and a water leak. During subsequent testing, the second dipole lower coil developed a short in the upper and lower windings. This ten-year-old magnet system has always been run well within specifications for current and cooling water flow. The second dipole upper and lower coils will be replaced in Spring 2005.

Fortunately, the first dipole and the remaining windings in the second dipole allow us to bend 200-keV 1- 17F, 18F, and 7BeO beams.

The 7Be beams were produced with a cesium sputter negative ion source with 19.6 mCi 7BeO in Ag powder in a Cu holder. Permanent magnets were installed around the beam line upstream of the first quadrupole multiplet to sweep away electrons. The recirculating cesium jet charge exchange cell was removed to maximize transmission.

The 100-MeV 4+ 7Be beam was produced by gas stripping followed by foil stripping in the terminal. This has only been possible since the new recirculating gas stripper was installed in front of the foil stripper a little over a year ago. Gas or foil stripping alone only allowed the transmission of a 3+ beam. Presumably, the combination of the two allowed lower charge state 7Be from less violent breakup of 7BeO in the gas to be fully stripped to the 4+ charge state in the foil.




RA3. A New Experimental Technique for Decay Sepectroscopy: Ranging Out of Ions from An Accelerated Beam
(C.J. Gross)

Beta-decay studies on nuclei far from stability have traditionally been carried out at isotope separator facilities and at the focal plane of recoil and fragment separators. The isotope separator facilities extract and accelerate beams to a few tens of kilovolts, mass analyze the beam to one part in 1000, and rely on the purity of the resulting beam to study these nuclei. Recoil and fragment separators rely on the reaction kinematics to convey enough energy to spatially separate and/or electronically tag the ions prior to implantation and study of the decay properties. In both cases, very weak components of the beams can be swamped by contaminants. In order to improve upon the isotope separator technique we propose to accelerate the ion beams to approximately 3 MeV per nucleon and use a transmission ionization chamber to detect and enhance the purification of a beam of neutron-rich nuclei.

Fig. RA3-1 - A photograph of the transmission ionization chamber used in the present study. The chamber is approximately 8 cm long with 6-anodes and one common cathode. The windows are mylar and approximately 16 mm in diameter.

A picture of our small ionization chamber is shown in fig. RA3-1. The energy loss of energetic ions in gas is dependent upon the Z of the ion; at our energies high-Z ions lose more energy than low-Z ions. For neutron-rich RIBs, the more exotic species has a longer range in the gas. Thus, by operating the ion chamber at high enough pressures the high-Z components of the beam will stop in the gas or exit window while the lower-Z components are transmitted to the measuring station.

We have performed tests using a A=120 beam consisting of radioactive Ag and In components (ΔZ=2). In fig. RA3-2 a γ-ray spectrum showing the decay of the 0.3-second isomer in Ag and transitions following In β decay is shown at two gas pressures. A factor of 5 relative reduction in the amount of In has been observed. The overall transmission efficiency is better than 70%.

Fig. RA3-2 - A portion of spectra taken at two different gas pressures corresponding to full transmission of all ions and optimal transmission of 120Ag ions with maximum reduction of 120In ions. The spectra are normalized to the 1461-keV transition from room background.

Improvements to the setup are possible. There was no shielding of the single Ge detector used for this test. In addition, the support wires of the ionization chamber's windows will be changed to minimize bowing of the window. Initial experiments will occur this summer and will involve isotopes with Z~30.

Our collaboration consists of scientists from ORNL, University of Tennessee, Mississippi State, University, Louisiana State University, UNIRIB, Joint Institute for Heavy Ion Research, and Vanderbilt University.




RA4. HRIBF Users Group News
(C. J. Gross)

The HRIBF Users Group Executive Committee election was held in October and elected Robert Grzywacz (University of Tennessee) and Walt Loveland (Oregon State University). They took office this month and joined Uwe Greife (Colorado School of Mines) and David Radford (ORNL). We thank Ed Zganjar (Louisiana State University) and Paul Mantica (Michigan State University) for their service.

A conference call was held in December and Uwe Greife was elected chair for 2005. The other major item of business was the topic of this year's users workshop. We are planning on hosting a workshop on fusion-fission reactions in either late summer or fall of this year. Walt Loveland, Jim Kolata (Notre Dame), ORNL's Dan Shapira and Felix Liang comprise the organizing committee.




RA5. Suggestions Welcome for New Beam Development

HRIBF welcomes suggestions for future radioactive beam development. Such suggestions may take the form of a Letter of Intent or an e-mail to the Liaison Officer at 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.

  • The tandem accelerates negative ions only.
  • Positive ions may be charge-exchanged or used directly off the platform (E < 40 keV).
  • ORIC presently produces up to 52-MeV of 1H (12 uA); 49-MeV 2H (12 uA); 120-MeV 3He (not yet attempted, costly); 100-MeV 4He (3 uA). Higher currents may be possible.
  • Typical reactions required to produce more than 106 ions per second are n, 2n, pn, and alpha-n fusion-evaporation reaction channels and beam-induced fission products. More exotic reactions are possible if extremely low beam currents are all that is needed.
  • Species release is strongly related to the chemistry between the target material and the beam species. It is best when the properties are different and the target is refractory. Thin, robust targets (fibrous, liquid metals, a few grams per square centimeter) must be able to withstand 1500 degrees Celsius or more.
  • Minimum half-life is seconds unless chemistry is very favorable.
  • Very long-lived species (T1/2 > 1 h) are probably best done in batchmode, i.e., radioactive species are produced with ORIC beams and then transported to the ion source where beams are produced via sputtering. Sputter rates of the species and target substrates are important.
  • Isobaric separation is possible for light beams (fully stripped ions), while isobaric enhancement may be possible for heavy beams.
  • Beware of long-lived daughters or contaminant reaction channels.