HRIBF Newsletter, Edition 17, No. 1, Feb. 2009
Feature Articles
- HRIBF Update and Near-Term Schedule
- Recent HRIBF Research - Dynamic Polarization in the Two-Body Breakup of 17F
- Recent HRIBF Research - The 132Sn + 96Zr Reaction: A Study of Fusion Enhancement/Hindrance
- Recent HRIBF Research - Commissioning Experiments at the Low-Energy Radioactive Ion Beam Spectroscopy Station (LeRIBSS)
- Update of the IRIS2 Project
- Update on the New SNICS II (Source of Negative Ions by Cesium Sputtering) Ion Source
- More Thoughts on a Driver Upgrade for HRIBF
- HRIBF Strategic Plan
- PAC-15 and Call for Proposals
- The 3rd LACM-EFES-JUSTIPEN Workshop Held at ORNL
Regular Articles
1. HRIBF Update and Near-Term Schedule
(J. R. Beene)
The six months covered by this report has been a difficult time for everyone involved with HRIBF, as we strive to recover from the Operational Emergency declared at HRIBF in July 2008. The events which precipitated that declaration were discussed in the August 2008 Newsletter. I will recap and update this discussion very briefly in the following paragraphs. Before I do that, I will skip to the bottom line and provide some information which must be of greatest interest to our users. When and how will we resume full operation of HRIBF?
ORNL line management agreed to allow us to proceed with a phased restart of the facility. The first step in this process was the resumption of stable-beam experiments in mid-September 2008. The next steps, in sequence, are (1) resumption of ISOL target R&D at the On-Line Test Facility (OLTF, based on the UNISOR facility), (2) resumption of RIB experiments using beams of long-lived isotopes (e.g., 7,10Be, 26gAl) employing what we refer to as batch mode operation, (3) resumption of ISOL target R&D with ORIC beams at HPTL, (4) resumption of ISOL RIB experiments employing selected proton-rich species, (5) resumption of target R&D at OLTF with actinide targets, and finally (6) full HRIBF operation including neutron-rich RIB production with ORIC beam on uranium carbide targets. We have initiated, or have permission to proceed with steps 1 through 3. Permission to proceed with steps 4 and 5 are expected soon. The final step is more problematic. It now appears that the set of requirements we must meet before resumption of neutron-rich operation cannot be completed before late May, or perhaps not until the first week in June.
I now return to a brief recap of the July event. The declaration of an Operational Emergency at HRIBF was made by the ORNL Laboratory Shift Superintendant following the evacuation of Building 6000 early on the morning of July 28. The evacuation resulted from the observation of airborne contamination on the first floor of Building 6000 near the door of the RIB production vault (C111S). The maximum dose rate observed outside of a radiological area was 4 mrem/h. Following this event, ORNL management commissioned a Management Investigation Team to probe the event, determine causes, and provide input to potential corrective actions. In addition, a Recovery Team was constituted, consisting primarily of HRIBF staff with the mission of preparing HRIBF for resumption of operation. However, our initial effort was largely devoted to carrying out physical on-site investigation and providing information to the Investigation Team.
The Investigation Team released a 150-page report on November 20, 2008. The direct cause of the event was determined to be release of radioactive noble gas into the air in C111S from a leak in the HRIBF off-gas system. The leak resulted from a small corrosion-induced hole in an oil-fill plug on the exhaust side of a mechanical roughing pump which served as a backing pump for the turbomolecular pumps that provide high-vacuum on the target/ion-source system (TIS) and the RIB production beamline. The plug was made of stamped-carbon steel approximately 1-mm thick, with threads cut into the 1-mm thickness. The corrosion-induced leak was on the threaded region of the plug. A contributing cause was found to be the failure of the belt driving an exhaust fan in the ORIC-shielded area HVAC system. This resulted in a slight positive pressure inside C111S (the RIB production vault), which pushed airborne contamination through the gaps around the shield door into occupied areas. An interlock on the exhaust fan motor was intended to prevent operation of the supply fan if any exhaust fan was not operating. However, the interlock was based on fan motor operation, rather than fan motion or room pressure.
The most important fact about the event was that no individual received a measurable radiological dose as a result of this incident. All personnel who entered Building 6000 on July 28 or the three preceding days had their thermoluminescent dosimeters (TLD) checked immediately. A number of scientists involved in the experiment that was underway were equipped with electronic dosimeters in addition to TLDs. Six individuals most directly involved with the event (scientists and radiological control technicians) were sent for whole body counts. All whole-body scans and dosimeter readings produced negative results.
A set of thirty-two corrective actions related to the event were established based on the Management Investigation Team Report. We have agreed with ORNL management on a mapping between closing of the corrective actions and the beginning of the various phases of the facility restart plan. Requirements for all phases except full facility operation to produce neutron-rich beams with ORIC beam on uranium carbide targets should be met before the end of March. An absolute prerequisite for full neutron-rich RIB operation is the completion of the Triennial Review of the HRIBF by the ORNL Accelerator Safety Review Committee (ASRC), a recommendation for resumption of operations by the ASRC, and formal approval by ORNL line management. This review was scheduled, before the July event, for the spring of 2009; its scope and significance are now enhanced. Unfortunately the earliest possible date for the start of this review is now mid April. Allowing time for the ASRC to draft its report, and for HRIBF staff to respond to any findings, we arrive at the estimate of full restart of neutron-rich beam operation quoted earlier.
Not all the news is bad. We are doing research with radioactive beams
again. The IRIS2 project
has proceeded extremely well over the past
year, in spite of a somewhat stretched-out funding profile. We are
well into FY2009 but are, yet again, operating under a Continuing
Resolution (CR), and are still uncertain what our actual budget for
the year will be. However, there is real optimism that funding for
science will see substantial increases over the next several years,
and that the budget process may become a bit less capricious, at least
for a few years.
2. Recent HRIBF Research - Dynamic
Polarization in the Two-Body Breakup of 17F
(J.F. Liang, spokesperson)
Coulomb dissociation is a useful technique for studying radiative capture reactions in nuclear astrophysics when direct measurements are difficult or impossible, such as those involving short-lived radioactive nuclei [1]. It is important to understand the breakup processes in order to correctly extract relevant information. For loosely bound proton-rich nuclei, first order perturbation theory calculations are not reliable and the inclusion of higher order effects is required. Of particular importance is the dynamical polarization effect where the valence proton is displaced behind the core nucleus and shielded from the target. This is similar to the tail of a comet pointing away from the sun when it flies close to the sun. This effect is proportional to the cube of the target charge, Z3T. It manifests in reducing the breakup probability as compared to first order perturbation theory and the reduction in breakup probability is expected to be smaller for a target of lower Z [2].
We have measured the breakup of 17F by bombarding 58Ni and 208Pb targets. The objective is to compare the breakup of 17F into oxygen and proton with respect to first-order-perturbation theory for the two targets. The radioactive 17F was produced at the HRIBF by an ISOL method and accelerated to an energy of 10 MeV per nucleon. The reaction products were measured by a stack of three large-area Si-strip detectors. The front and middle detectors form an ΔE-E telescope for identifying the breakup oxygen. The breakup proton has sufficiently high energy to pass through the first two detectors and is detected in the third detector. This enables us to measure angular distributions of oxygen ions in singles, and oxygen and proton in coincidence.
Figure 2-1 presents the measured data (red circles) and first order perturbation theory predictions (blue curves). The angular distribution of the oxygen ions in coincidence with protons for the 17F on 58Ni reaction is reproduced by first-order perturbation theory with E1 and E2 excitations included. For the 208Pb target, the angular distribution of oxygen and proton in coincidence shows a larger reduction of breakup probability with respect to first-order-perturbation theory. Comparisons with dynamic calculations that take into account the Z3T correction to study the dynamical polarization in the breakup of 17F are underway.
Figure 2-1: Angular distributions of the oxygen ions in coincidence with protons from the breakup of 17F by bombarding 58Ni (left panel) and 208Pb (right panel) at the energy of 10 MeV per nucleon. The experimental data are closed circles and the solid curves are predictions by first-order perturbation theory including E1 and E2 excitations.
[1] G. Baur, C. A. Bertulani, H. Rebel, Nucl. Phys. A458, 188 (1986).
[2] H. Esbensen, G. F. Bertsch, Nucl. Phys. A706, 383 (2002).
3. Recent HRIBF Research - The
132Sn + 96Zr Reaction: A Study of Fusion
Enhancement/Hindrance
(W. Loveland, spokesperson)
In fusion reactions induced by neutron-rich radioactive nuclei, one expects to observe a lowering of the fusion barrier, relative to that observed in reactions induced by stable nuclei with smaller neutron to proton ratios. This is simply a geometrical effect due to the greater size of the n-rich projectile. In addition, in the synthesis of heavy nuclei with n-rich projectiles, one expects a higher survival probability of the completely fused system due to its lower fissility and the lower excitation energies.
In the systems investigated to date, one does not generally expect significant fusion hindrance because ZpZt < 1600. For ZpZt > 1600, it is believed that fusion hindrance effects become prominent. Fusion hindrance generally takes the form of an extra energy that must be supplied to the fusing system to drive it from the contact point inside the fission saddle point. This energy is loosely referred to as the "extra-push" energy although the formal definition of this energy is that it is the "extra-extra push energy". Evaporation residue (ER) cross section measurements with massive projectiles (A ~ 100) have clearly established the occurrence of fusion hindrance with an extra energy needed to cause fusion. This fusion hindrance was explained successfully by the extra push model developed by Swiatecki et al. [1].
Studies by Sahm et al. for the 90-96Zr + 124Sn reactions showed an unexpected result. As the fusing system became more neutron-rich (decreasing fissility), the fusion hindrance, as measured by the extra push energy, increased in contradiction to predictions of most theoretical models. (See Fig. 3-1) If this trend were confirmed, it would reduce the attractiveness of fusion using very n-rich projectiles as a pathway to the heaviest elements. We have carried out, over a period of several years, a set of measurements of the capture-fission excitation functions for the 124,132Sn + 96Zr system to try to define this effect. The results of these investigations have been published [2,3].
Figure 3-1: Extra push as determined from the mean fusion barrier height EB and the barrier height calculated with the Bass potential, VB, versus the effective entrance channel fissility. The experimental points are taken from Sahm, et al., with 124Sn+96,94,92,90Zr systems selected. The theoretical prediction by Bjornholm and Swiatecki is shown by a line.
Figure 3-2: Comparison of the capture fission excitation functions for the 132Sn+96Zr (solid squares and line) and 124Sn+96Zr reactions (open circles and dashed line). The lines are to guide the eye through the data points.
The measured capture-fission excitation functions for the two systems are shown in Fig.3-2. Clearly the interaction barrier is shifted to lower energies for the more n-rich system.
We have measured capture cross sections in this work. The direct measurement of fusion cross sections requires the separation of the quasi-fission and fusion-fission components of the capture cross sections, which is not feasible with the current generation of radioactive beam facilities. However, for many purposes, such as heavy element syntheses, we need to know the properties of the fusion cross sections in these collisions. What should we do?
We have chosen to extrapolate from the capture cross sections to the fusion cross sections, which, at least, involves cross sections of similar magnitude. We do this extrapolation using the dinuclear system (DNS) model. This model was successful in reproducing evaporation residue cross sections for 16O+204Pb and 124Sn+96Zr systems.
Figure 3-3: Excitation function for 132Sn+96Zr capture-fission reaction. The one dimensional barrier penetration model prediction is shown as a dashed line. The predicted capture, and fusion cross sections predicted by the DNS model are shown by solid, and dotted lines, respectively. See text for details.
Calculations by Giardina et al. using the DNS model for capture cross sections in the 132Sn+96Zr reaction are shown in Fig. 3-3 as a solid red line. The predictions of this model for the fusion cross sections for this reaction are shown as a dotted line. Since the DNS model calculations agree reasonably well with the observed capture cross sections for the 132Sn+96Zr and 124Sn+96Zr reactions [2] and the evaporation residue cross sections for the 124Sn+96Zr reaction, it is reasonable to speculate that the interaction and fusion barrier heights for these reactions are those predicted by the DNS model. For the 132Sn+96Zr reaction, the deduced (DNS) interaction barrier height is 192.3 MeV while the fusion barrier height is 201.8 MeV. Similarly, for the 124Sn+96Zr reaction [2], the deduced interaction barrier height is 204.4 MeV while the fusion barrier height is 208.9 MeV. The Bass barrier heights for these reactions are 213.8 and 216.3 MeV, respectively.
One immediately comes to several conclusions. The deduced fusion barrier heights from the DNS model for the reactions induced by neutron-rich radioactive beams (201.8 and 208.9 MeV) are substantially below the Bass barrier heights. We take the definition that "extra-push" energy is the difference between the interaction barrier height and the fusion barrier height. Both the 124Sn+96Zr and 132Sn+96Zr reactions show positive extra push energies (fusion hindrance) of 4.5 and 9.5 MeV, respectively, as deduced from the DNS model. (Refs. [2,4] would have estimated these energies as 10 and 7 MeV, respectively). Some but not all of the advantage of using neutron-rich radioactive beams is predicted by the DNS model to be lost due to fusion hindrance in the Sn + Zr system.
[1]W. J. Swiatecki, Physica Scripta 24, 113 (1981); Nucl. Phys. A
376, 275 (1982). 4. Recent HRIBF Research - Commissioning Experiments at the Low-Energy Radioactive Ion Beam Spectroscopy Station (LeRIBSS)
The first LeRIBSS experiments were performed in July 2008.
The 54-MeV ORIC proton beam impinging on the ~7 grams of the HRIBF 238UCx
target typically had an intensity of about 8 μA.
Fission products are released as positive ions from the
HRIBF ion source. These ions are usually passed through
the cesium-loaded charge exchange cell to create the negative ions
needed for post-acceleration in the HRIBF Tandem.
However, some elements, like zinc and cadmium, do not create
negative ions. In the first phase of LeRIBSS experiments,
zinc-free copper beams were obtained using the charge exchange cell.
Later phases of the experiment used positive ions in order to avoid
losses due to the charge exchange process (typically ~ 5% efficiency).
Figure 4-1: The activity collected on tape of the MTC was viewed by two plastic
beta energy-loss detectors and four clover gamma
detectors during first LeRIBSS experiment.
Figure 4-2: The LeRIBSS digital data acqusition system based on Pixie-16 modules of
XIA (lower crate) and the remotely controlled Wiener MPOD-ISEG High Voltage
supply for clover and beta detectors (upper crate).
Two LSU plastic beta detectors and four clover gamma-detectors around the
thin beam pipe surrounded the collection point, see Fig. 4-1.
All detector signals were digitally processed using XIA Pixie-16 modules [1].
This new data acquisition system was developed at the
Digital Pulse Processing Laboratory of University of Tennessee
Knoxville (UTK) by Robert Grzywacz and his collaborators
Sean Liddick and Iain Darby, see, e.g., [2]. The high-voltage
for all detectors was remotely controlled using a Wiener MPOD minicrate
with ISEG High Voltage cards, see Fig. 4-2.
Figure 4-3: The beta-gated gamma spectrum collected during 3 hours
measurement with pure 75Cu negative ion beam.
The decays of copper isotopes in the vicinity of doubly magic
78Ni were studied following the proposals
of Jeff Winger and Sergey Ilyushkin of Mississippi State University.
Practically pure beams of 75Cu and 77Cu
were obtained, at the maximum rates of about 3000 pps and 130 pps,
respectively.
An example of the low-energy beta-gated gamma spectrum collected
during the 3 hour
experiment on 75Cu decay is given in Fig. 4-3.
The respective time patterns of the 191-keV, 419-keV and 722-keV
gamma transitions following 75Cu decay are given in Fig. 4-4.
The beam was pulsed with 5-seconds on and 7-seconds off in order to measure
the activity grow-in/decay-out profile.
Figure 4-4: The grow-in/decay-out pattern observed for several gamma transitions
following beta decay of 75Cu.
The total measurement time was about 80 minutes.
The second part of the LeRIBSS experiment was performed without the use
of the charge exchange cell.
The calibration of the RIB-injector magnet and initial beam tuning
were done using positive ions of stable Krypton and Xenon isotopes.
The decays of neutron-rich 79Zn,
80Zn and 81Zn isotopes were
studied following the HRIBF proposals by Sean Liddick and Steven Padgett
of UTK. The resolving power of the HRIBF injector magnet was high
enough to separate the neighboring isobars of Z=31, 81Ga and Z=30,
81Zn.
The ratio of mass difference between 81Ga and 81Zn to the mass of
81Zn is ΔM:M ~ 1:6400. The positive ions of 81Ga were produced
at the rate of about 106 pps, while the rate of 81Zn ions was of
the order of 10 pps.
Figure 4-5: The section of beta-gated gamma spectrum measured for 81Zn decay.
The 216-keV and 711-keV gamma transitions can only result from
the decay of 81Ga produced as a daughter activity of 81Zn.
The 81Ga ions, initially produced at a rate about five orders of magnitude
higher than that of 81Zn, were practically removed from the separated
positive-ion beam.
Despite the large difference in composition of the A=81 beam,
practically pure 81Zn samples, at about half its maximum rate, were
studied at LeRIBSS after fine tuning
of the HRIBF high-resolution beam-injector magnet, see Fig. 4-5.
In the preliminary analysis, the half-life of 81Zn was
measured to be 315(18) ms, see Fig. 4-6. The gamma transitions
feeding the levels up to ~2-MeV excitation energy in the N=50
81Ga daughter nucleus were
established using beta-gamma-gamma coincidence information.
Figure 4-6: The grow-in/decay-out pattern observed for the strongest
gamma transition at 351 keV following 81Zn beta decay
The total measurement time was about 5 hours.
5. Update on Injector for Radioactive Ion Species 2 (IRIS2)
The IRIS2 Project continues to progress well and remains scheduled for
completion by September 30, 2009. Suspension of HRIBF operations
following the July 2008 Operational Emergency (see HRIBF
Updates in this and the
previous
newsletters)
has had a positive impact on the project, allowing us to devote craft
resources to installation activities without interruption by higher
priority operation issues. Thus substantial progress has been made,
particularly in the areas of injector and transport beamline assembly,
routing of utilities, and electrical/control systems.
Vacuum systems for both the injector and transport beam lines have
been successfully installed, leak checked, and pumped down to better
than the specified high vacuum level of 5.0X10-6 Torr. This includes
the transport beamline 35 degree and 90 degree electrostatic
deflectors which have been fully assembled, successfully bench tested
in terms of vacuum and high voltage specifications, and installed on
the transport beamline.
Injector beamline electrical/control system installation and
functional testing are nearing completion. Installation of transport
beamline electrical/control system components and functional testing
are underway. Remaining storage boxes for activated target ion
sources and other aspects of the handling system will be procured as
soon as the FY09 funds become available. Procurement of the modular
laser room and associated laser safety systems is in
progress. Drilling of the 8-inch diameter hole for the laser and utility
port plug through the 9.5-foot-thick concrete shielding wall has been
completed.
An Operational Readiness Review was held in early December 2008 so
that any concerns identified could be addressed prior to beginning
commissioning with beam. Draft revisions to Authorization Basis
documents, procedures, and training were presented to the committee
for discussion and review. There were no surprises and we are
awaiting the Review report. The stable ion beam
portion of commissioning is expected to begin in February.
Commissioning with RIB produced by an ORIC proton beam on a uranium
carbide target will probably not begin earlier than May 1, since
commencement of this activity requires permission of ORNL management
granted as part of the Operational Emergency recovery process. The
project remains on schedule and budget with ample contingency still
available.
6. Update on
the New SNICS II (Source of Negative Ions by Cesium Sputtering) Ion Source
The new SNICS II (Source of Negative Ions by Cesium Sputtering) ion
source, purchased from National Electrostatics Corporation (NEC), was
installed on the stable injector August 12, 2008. Experience is being
gained by the operators to learn the SNICS personality. It does
typically produce an abundance of beam when compared to the previous
source. More difficult beams, which require gas feeds, will be
possible when the gas cathode option is installed. A specific date
for this installation has not been set.
Since its installation, the new SNICS II source has produced the following
beams: H, B, C, Ti, Ni, Ga, Ge, As, Br, Ag, In, Sn, Sb, and Te.
7. More Thoughts on a Driver Upgrade for HRIBF
Several articles in this Newsletter over the past few years have
discussed our Integrated Strategic Plan
for development of the HRIBF.
The HPTL and IRIS2 projects, along with
development of new experimental tools, are all part of this plan. In
the February 2007 Newsletter,
a brief discussion of the next step in
this plan, a driver accelerator upgrade, was discussed. That article
described a particular driver proposal involving a high-power electron
beam accelerator to enable production of neutron-rich species by
photo-fission (see the "HRIBF Initiatives" web
page), which we refer to as
the electron driver upgrade (EDU). Implementation of the EDU would
lead to truly remarkable increases in the yield of very neutron-rich
species. One important feature of any potential driver upgrade is the
use of a commercial accelerator system which would allow us to focus
the efforts of our small facility operations staff on ISOL technology,
beam purification, post-accelerated beam quality, and reliability.
In the summer of 2008, we became aware of another commercial
accelerator which could be very interesting as a driver at HRIBF: the
C70 cyclotron, built by IBA in Belgium. This is a multi-species
cyclotron capable of delivering up to 750 μA of protons at energies up
to 70 MeV, a minimum of 50 μA of deuterons at energies up to 35 MeV,
along with an alpha-particle beam at 70 MeV (fixed) and 35 μA. The
proton- and deuteron beams are accelerated as negative ions, and
extracted by stripping. In this negative-ion mode, the C70 also has a
dual port extraction capability.
A White Paper
exploring how the C70 might be employed to enhance the
capability and reliability of the HRIBF was completed and submitted to
the DOE Office of Nuclear Physics in early November 2008. This
document is available on the "HRIBF
Initiatives" web page. Some of
the points raised in that document are outlined in the next paragraph.
Please read the
White
Paper for details. For convenience, the C70
cyclotron-based option is referred to in this article as the HDU
(Hadron Driver Upgrade).
Even though the commercial electron accelerator employed in the EDU
concept is less expensive than the C70, the HDU total project
cost (TPC) is substantially less than the EDU TPC. This is
because the HDU makes effective use of all the basic
infrastructure of the existing HRIBF, including the IRIS1 and
IRIS2 production stations, with only minor modification, while
the EDU cost is dominated by substantial new civil
construction. The C70 would replace essentially all the
current applications of ORIC. It would improve our
proton-rich beam production as well as the neutron-rich
production, while the EDU is strictly a neutron-rich driver
and would depend on the 50-year-old ORIC for proton-rich
production.
The completed HDU would be less expensive to operate than the current
HRIBF. The size of staff required to operate the upgraded facility is
estimated to be the same as that required for the present facility.
The electric power usage of the C70 is one-fifth that of ORIC (400 kW
vs. 2000 kW). Maintenance of the C70 would be contracted to IBA,
reducing our need to purchase craft effort from ORNL.
The dual port extraction capability of the C70 offers two significant
additional capabilities to HRIBF. We now purchase long-lived isotopes
for batch-mode operation. The C70 would offer a cost-effective way to
produce batch-mode isotopes in-house at HRIBF without interfering with
normal ISOL operations. In addition, the ability to extract two beams
simultaneously could enable us to produce isotopes of interest to
medical (or other) research activities, again without interfering with
ISOL operations. This capability may be becoming more important as
the responsibility for the isotopes program within DOE has been
transferred to the Office of Nuclear Physics in FY2009.
Finally, we consider the performance of the HDU-based facility for
production of very neutron-rich species. This is the strength of the
EDU and represents a critical piece of the science program at HRIBF
over the next few years. The most obvious and straightforward way to
make use of the HDU is simply to extend our present method of
producing fission fragment beams with the 238U(p, fission)
reaction but
utilize the higher proton-beam currents and energy (70 MeV vs. 50 MeV)
available from the C70. The gains that can be made this way are
probably modest. We believe that the maximum 70-MeV proton current
that a conventional direct proton-induced fission target system can
sustain is not much more than 50 μA, a small fraction of the 750 μA
available from the C70. The total gain in neutron-rich beam yields
would probably be about a factor of 5 across the board compared to
current facility performance. This enhanced direct proton-induced
fission production could be implemented at HRIBF with essentially no
development effort, but the factor of five gain would be two to three
orders of magnitude less than the gains made by the baseline
expectations of the EDU. However, with additional development effort,
more complex production schemes can be employed which offer much
greater performance and might eventually make use of essentially the
full beam-current capability of the C70. A very promising production
method would be 238U(n, fission) using secondary neutrons produced by
high-current proton or deuteron beams. Because of the higher proton
energy available, and the very high proton currents available,
production of intense secondary beams of neutrons by proton
bombardment of thick targets of nuclei containing weakly-bound
neutrons is much more favorable than the use of deuteron breakup as a
neutron source. We believe an HDU baseline on the order of 50% to 75%
of the EDU baseline could be achieved with essentially the same
modest-scale fission targets planned for the EDU. The distribution of
the yield of neutron-rich fission fragments from this secondary neutron
fission is very similar to the production from photofission.
Calculated photofission beam yields can be used directly to estimate
RIB yields from secondary-neutron induced fission. For further
discussion of the potential performance of a C70-based facility,
please see the White Paper.
8. HRIBF Strategic Plan
The HRIBF strategic plan has been
placed on our website. This document was originally created at the
request DOE as part of our annual Science and Technology Review.
The document undergoes periodic modifications as a result of the
scientific progress in our field as well as the future outlook for the
facility. The plan outlines the research HRIBF is currently
involved with and where we think that research will take us.
It represents what our users tell us about their use of the
facility. The wide dissemination of the plan is meant to be
inclusive and to allow collaborators to come together to
work on specific research topics or experimental devices. It is
also intended to inform users of potential upgrades to the facility
and the expected resources and timelines. The plan has been shared
and discussed with the
HRIBF Users Group Executive Committee
as well as the
HRIBF Science Policy Committee. Feel free
to send and comments or suggestions about the plan to our
Scientific Director,
Witek Nazarewicz.
9. PAC-15 and Call for
Proposals
The Program Advisory Committee meeting (PAC-15) has been scheduled for
July 22-23, 2009. Two new members have been added to this committee.
Birger Back from Argonne National Laboratory and Ian Thompson from
Lawrence Livermore National Laboratory have replaced Robert Janssens and
Walt Loveland. We are grateful for the service that Robert and Walt
have given to the facility and look forward to working with Birger and Ian.
10. The 3rd LACM-EFES-JUSTIPEN Workshop Held at ORNL
The third LACM-EFES-JUSTIPEN Workshop was held on February 23-25,
2009 (with Feb. 26 and 27 devoted to more individual
collaborations) at the Joint Institute for Heavy Ion
Research of Oak Ridge National Laboratory. The meeting is a merger of
two workshops: (i) the US-Japan theory meeting under the auspices of
the Japan-US Theory Institute for Physics with Exoctic Nuclei
(JUSTIPEN) and (ii) the annual NNSA-JIHIR meeting on the nuclear large
amplitude collective motion (LACM) with an emphasis on fission.
The meeting, jointly organized by the JUSTIPEN Governing Board, the
UT/ORNL theory group, the Todai-RIKEN Joint International Program
for Nuclear Physics (TORIJIN), and the JSPS Core-to-Core program
"International Research Network for Exotic Femto Systems (EFES)",
brought together theorists and experimentalists with interests in the
physics of radioactive nuclei, LACM, and theoretical approaches
related to the SciDAC-2 UNEDF project. As in the preceding Joint
JUSTIPEN-LACM Meetings, one emphasis of the meeting was on topics
related to future collaborations between US and Japanese groups. The
meeting was attended by almost 90 researchers including about 20
Japanese visitors, and it consisted of about 50 talks. Some of the
highlights of the workshop were Arima's special lecture on "New
facilities for exploration of exotic nuclei", the dedication of new
office space in the JIHIR as the U.S. site of JUSTIPEN, and a tour of
the computational facilities Jaguar and EVEREST.
Please see the workshop web page for more details.
RA1. RIB Development
A novel technique using lasers is being developed at HRIBF for
efficient suppression of isobaric contaminants in negative ion beams.
Progress on this project has been described in earlier HRIBF
Newsletters but the data below are from a redesigned system that will
be implemented at HRIBF soon after the commissioning of the new RIB
Injector (IRIS2). Lasers shined on the negative ion beam can be tuned
in frequency so that their light is absorbed by contaminant negative
ions only, which then eject their extra electron and their negative
charge. They can then easily be separated from the negatively charged
ions of interest. In a recent experiment, the feasibility of near
100% suppression of isobar contaminants in negative ion beams was
demonstrated. In this work, negative ions of 58Ni and
59Co were
neutralized using a pulsed Nd:YAG laser beam at 1064 nm. A gas-filled
radio-frequency quadrupole ion guide was employed to slow down the
negative ions and dramatically increase the interaction time of the
ions with the laser. This substantially increased the efficiency of
removing the extra electron. More than 99.99% of the contaminant
59Co- ions were removed with less than 3W of laser power, while less
than a 20% loss of the 58Ni-
ions was observed under identical
conditions. This technique will be used at HRIBF to purify the
radioactive ion beams of 56Ni, 17,18F and
33,34Cl isotopes. A
U.S. patent on this technique has been granted (U.S. Patent 7,335,878,
Feb. 2008, Beene, Liu, and Havener).
RA2. Accelerator System Status
ORIC Operations and Development (B. A. Tatum)
ORIC provided a nominal 54-MeV, 10μA proton beam to IRIS1 for the
production of RIBS used for commissioning and initial experiments with
the new Low Energy Radioactive Ion Beam Spectroscopy Station (LeRIBSS)
until an Operational Emergency was declared at HRIBF on July 28
(see HRIBF
Updates in this and the
previous
newsletters).
RIB production and ORIC operations were
suspended by ORNL management in order to conduct a four month
investigation into the event. The remainder of the second half of 2008
was consumed by the development of a corrective action plan. ORIC
operations were resumed in early February
2009.
During the shutdown, some equipment improvements were made to ORIC
systems. A new 20-inch cryopump was installed on the rf resonator
enclosure, replacing a failed unit and providing supplemental pumping
to the three main diffusion pumps. The associated gate valve was also
extensively refurbished. Four new power amplifiers have been received
in addition to a new 4648A power amplifier tube, providing much needed
backup capability for the ORIC rf system.
A program was initiated to replace the 50-year-old ORIC substation
circuit breakers with new units with the goal of improving reliability
of operation and reducing maintenance. The new breakers can be easily
racked out and locked out, and also include remote power monitoring
capability. Concurrently, preventive maintenance was performed on
motor control centers and some deteriorating cabling and power
distribution panels were replaced.
RIB Injector Operations and Development (P.E. Mueller)
Prior to the unexpected shutdown of the facility at the end of July
2008, the radioactive ion beam injector (IRIS 1) delivered beams of
to the Low Energy Radioactive Ion Beam Spectroscopy Station (LERIBSS).
These beams were produced via proton induced fission of 238U by
bombarding a pressed powder uranium carbide target coupled to an
Electron Beam Plasma (positive) Ion Source (EBPIS) with 10 - 12 uA of
50-MeV 1H from the Oak Ridge Isochronous Cyclotron (ORIC).
Tandem Operations and Development (M. Meigs)
The Tandem Accelerator was operated for more than 1900 hours since the
last report. The machine ran at terminal potentials of 7.58 to 23.57
MV and the stable beams 1H, 12C, 58Ni,
71Ga, 73,76Ge, 75As, 79Br,
107Ag, 115In, 120,124Sn, 121Sb,
and 124Te were provided. This was the
first time that Indium has been accelerated at our facility. Due to
the 'operational emergency" reported in the last newsletter, no
radioactive beams were accelerated during this period. And as a
consequence of the same event, the tandem accelerator was put into
standby for over three weeks, after which, a tank opening was done and
then conditioning was allowed. About 125 hours were spent
conditioning, which allowed terminal voltages up to 24 MV. Four tank
openings were completed during this period; the first for regular
maintenance, one for a broken shorting rod, and two to repair CAMAC
power supplies which caused a lack of communication to D4. The new
SNICS source purchased
from NEC was installed during the first tank
opening.
RA3. Experimental Equipment -- The Low-Energy Radioactive Ion Beam Spectroscopy Station (LeRIBSS)
The Low-energy Radioactive Ion Beam Spectroscopy Station (LeRIBSS)
was designed to study the decays of exotic neutron-rich nuclei
produced in the proton-induced fission of 238U at the HRIBF.
The physics motivations of these studies are related to
the origins of nuclear structure evolution,
to the properties of very neutron-rich nuclei affecting nucleosynthesis within
the rapid neutron capture process, and to the decay properties
of fission products relevant to the operation of power reactors and nuclear
fuel processing.
Figure RA3-1: The main components of LeRIBSS setup:
The beam steering, focussing and diagnostic chamber at the left side is
separated with a remotely operated valve from the detection chamber
surrounded by the detectors supported by the CARDS ring.
The setup
is located downstream from the high-resolution magnet and under the HRIBF
tandem. It is suspended from the ceiling below which the tandem's
energy-analyzing magnet rotates to the various high-energy beamlines.
LeRIBSS has two separate vacuum sections, the ion optics and diagnostic
chamber and the detection section, see Fig. RA3-1.
The first chamber houses the beam X-Y steerer, focussing quadrupole
and beam diagnostic equipment which includes a high-sensitivity Faraday cup
and a fluorescent beam viewer made out of Alumina plate.
There is an additional fluorescent beam viewer in the second vacuum section,
only 2 inches upstream from the radioactivity
collection spot on the moving tape collector. These viewers die rapidly
under the impact of such low-energy ions. We hope to avoid requiring their
use once the system is better understood.
The beam intensity diagnostics are complemented
by a Microchannel Plate detector at the exit of the high-resolution
injector magnet, before the LeRIBSS setup.
Figure RA3-2: The Moving Tape Collector (MTC) at LeRIBSS was built by Ed Zganjar
at Lousiana State University. The half-inch-wide tape can move collected activity
within ~200 ms by about 20 inches.
The radioactive samples collected at the moving tape collector (MTC)
in the 2-inch-diameter aluminium beam pipe (0.9 mm thick)
can be viewed by detectors supported in a CARDS ring, see Fig. RA3-2.
The MTC and CARDS ring were designed and built by Ed Zganjar at
Louisiana State University (LSU).
An old-fashioned, half-inch-wide computer tape is used for collecting
the source for a preselected time and transporting
(within ~200 ms) behind the 2-inch-thick lead wall separating
CARDS detectors from the main MTC chamber.
The beam-on/beam-off capability is provided by using a surplus magnetic steerer
from the Tandem, allowing us to measure the grow-in and
decay-out of the radioactivity samples deposited
in front of the detector setup. The steerer is implemented upstream
from the the high-resolution magnet and will be replaced by an electrostatic
steerer.
The field of the HRIBF RIB injector can be set to allow either
positive or negative ions to be separated and transmitted to LeRIBSS.
We used positive ions of noble gases, e.g., krypton and xenon,
to calibrate and verify the ion optics before the first experiment
with radioactive ions.
The LeRIBSS Faraday cup and two fluorescent viewers helped to adjust
the beam positon during the first experiments.
Additionally, we have used a thin window, 10 mm diameter Silicon
detector to observe 200-keV ion signals,
about 2 inches upstream from the tape collector. For future experiments,
this Si detector will be replaced by an small MCP detector
currently under construction (Dan Shapira and Steven Padgett).
We are planning to use a very thin (~microgram/cm2) carbon foil
in this MCP setup, to allow the transmission of the 200-keV ions
to the collection spot without a substantial intensity loss.
The LeRIBSS collaboration would like to express their gratitude to the
following people for their important contributions to the design and
construction of this apparatus: Charles Reed (ORAU), Tony Mendez
(ORNL), Ed Zganjar (LSU), Jim Johnson (JIHR/UT), Ray Juras (ORNL) and
Daryl Dowling (ORNL).
RA4. Users Group News
The HRIBF Users Group met on October 24, 2008, in Oakland, CA at the
Fall Meeting of the DNP. More than 110 people attended the meeting
which was held jointly with the ATLAS, NSCL, GAMMASPHERE/GRETINA,
and RIA Users groups and sponsored by HRIBF, ATLAS, and NSCL.
Our portion was hosted by UEC chair Walt Loveland.
Carl Gross gave the facility update including an account of the
failures which resulted in our RIB-delivery shutdown.
Krzysztof Rykaczewski gave a report on the performance of the new
Low-energy Radioactive Ion Beam Spectroscopy Station (LeRIBSS)
and a summary of its capabilities and detector equipment. LeRIBSS
is also the subject of
recent research
and the
experimental equipment
reports in this newsletter.
The Users Group held its biennial election for the exceutive committee
this fall and selected Jeff Winger from Mississippi State and Ray Kozub
from Tennessee Tech to replace Walt Loveland and Robert Grzywacz.
The new members took office January 1 and will serve for 4 years.
We would like to thank Walt and Robert for the work they have done in
representing the users and their interests for the past four years.
Jeff and Ray join Art Champagne (North Carolina) and Alfredo Galindo-Uribarri (ORNL).
The planned users workshop on science applications and cross-disciplinary
fields at HRIBF has been postponed and will be rescheduled for later this
calendar year. Users should contact Alfredo Galindo-Uribarri for more
information. There are tentative plans to hold a workshop in early summer
on neutron detection techniques. Contact Jeff Winger for more information.
Their email addresses are on the
UEC web page.
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
grosscj@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.
RA6. HRIBF Experiments,
July through December 2008
[2]A. M. Vinodkumar, W. Loveland, P.H. Sprunger, D. Peterson, J. F. Liang, D. Shapira, R. L. Varner, C. J. Gross and J. J. Kolata,
Phy. Rev. C 74, 064612 (2006).
[3]A.M. Vinodkumar, et al., Phys. Rev. C 78, 054608 (2008).
[4]S. Bjoernholm and W. J. Swiatecki,
Nucl. Phys. A 391, 471 (1982).
(K. Rykaczewski for the LeRIBSS collaboration)
References
(B. A. Tatum, spokesperson)
(M. Meigs, spokesperson)
(J. R. Beene)
(C. J. Gross, HRIBF User Liaison)
(C. J. Gross, HRIBF User Liaison)
(T. Papenbrock)
(Y. Liu, C. Havener and D. Stracener)
Figure RA1: The fraction of 59Co and 58Ni
negative ions transmitted
through the RF quadrupole ion guide as a function of laser power. The
ion guide was operated at about 6 Pa He buffer gas and 2.76 MHz RF
frequency.
(K. Rykaczewski for the LeRIBSS collaboration)
(C. J. Gross, HRIBF User Liaison)
(M. R. Lay)
Date
Exp. No.
Spokesperson
Title of Experiment
7/2
RIB-037
Meigs,Juras/ORNL
Tandem development
7/2-6
Shutdown
7/7-11
RIB-159
Galindo-Uribarri/ORNL
Development of a poliarized solid target for radioactive ion beams
7/11-14
RIB-035
Stracener/ORNL
Target ion source development (actinide targets)
7/14-20
RIB-178
Cizewski/Rutgers University
73,74As(d,pgamma) reactions in inverse kinematics as a surrogate for neutron capture for
stewardship science
7/21-23
RIB-159
Galindo-Uribarri/ORNL
Development of a poliarized solid target for radioactive ion beams
7/24-25
RIB-035
Stracener/ORNL
Target ion source development (actinide targets)
7/25-9/12
Shutdown
9/12
RIB-037
Meigs,Juras/ORNL
Tandem development
9/13-14
Shutdown
9/14
RIB-037
Meigs,Juras/ORNL
Tandem development
9/15-19
RIB-082
Gross/ORNL
A time-of-flight system for measuring fusion-evaporation cross-sections using radioactive ion
beams
9/19-21
Shutdown
9/21-24
RIB-082
Gross/ORNL
A time-of-flight system for measuring fusion-evaporation cross-sections using
radioactive ion beams
9/24-26
RIB-000
Gross/ORNL
RMS development
9/27-10/2
RIB-082
Gross/ORNL
A time-of-flight system for measuring fusion-evaporation cross-sections using radioactive ion
beams
10/2-7
Shutdown
10/7-8
RIB-037
Meigs,Juras/ORNL
Tandem development
10/8-10
RIB-014
Stracener/ORNL
Target ion source development (As & F)
10/11-12
Shutdown
10/13-15
RIB-014
Stracener/ORNL
Target ion source development (As & F)
10/16-17
RIB-000
Gross/ORNL
RMS development
10/18-19
Shutdown
10/20-23
RIB-035
Stracener/ORNL
Target ion source development (actinide targets)
10/23-24
RIB-037
Meigs,Juras/ORNL
Tandem development
10/25-27
Shutdown
10/27-31
RIB-037
Meigs,Juras/ORNL
Tandem development
11/1-2
Shutdown
11/3-4
RIB-014
Stracener/ORNL
Target ion source development (As & F)
11/5-7
RIB-178
Cizewski/Rutgers University
73,74As(d,pgamma) reactions in inverse kinematics as a surrogate for neutron capture for
stewardship science
11/8-9
Shutdown
11/10-14
RIB-143
Kolata/University of Notre Dame
Fusion of 132,124Sn with 48,40Ca
11/15-16
Shutdown
11/17
RIB-014
Stracener/ORNL
Target ion source development (As & F)
11/17
RIB-037
Meigs,Juras/ORNL
Tandem development
11/18
RIB-014
Stracener/ORNL
Target ion source development (As & F)
11/18
RIB-037
Meigs,Juras/ORNL
Tandem development
11/19
RIB-014
Stracener/ORNL
Target ion source development (As & F)
11/19-21
RIB-152
Krolas/Institute of Nuclear Physics, Krakow
Structure of neutron-rich Cu and Zn isotopes produced in deep-inelastic transfer reactions with
radioactive ion beams
11/22-23
Shutdown
11/24-26
RIB-014
Stracener/ORNL
Target ion source development (As & F)
11/26-30
Shutdown
12/1-13
RIB-101
Page/University of Liverpool
Search for new alpha emitters above 100Sn
12/13-14
Shutdown
12/15-19
RIB-188
Bardayan/ORNL
Astrophysics stable beam experiments
12/20-31
Shutdown