HRIBF Newsletter, Edition 18, No. 1, February 2010
1. HRIBF Update and Near-Term Schedule
In many respects, the prospects for HRIBF look extremely favorable. There are certainly good reasons for optimism. The IRIS2/HPTL upgrade project is essentially complete (see IRIS2 update in this newsletter). Improved operational efficiency due to the availability of two RIB production stations, as well as the impact of new beam production and purification capabilities, should soon be evident. The near-term budget outlook for the facility is more favorable than it has been for many years. Finally, we are free of the direct impact of the July 2008 Operational Emergency which crippled facility operation for more than a year.
In spite of a Continuing Resolution that extended deep into fiscal year (FY) 2009, the final 2009 budget for the facility was very favorable. This upward trend was continued in the FY 2010 appropriation, and, at a reduced rate, in the FY 2011 President's budget. The 2009 budget for the HRIBF was the first to exceed the inflation adjusted FY 2005 allocation since the large cuts imposed in FY 2006. These favorable budgets have enabled us to replenish badly depleted materials, supplies, and maintenance budgets. Even more important we have finally been able to proceed with the hire of two new accelerator operators, Justin Leach and Andrew Barclay. These two hires are the first elements of an HRIBF staff enhancement plan that was developed more than five years ago, but has been delayed due to funding. Once Justin and Andrew have completed necessary training, we will finally be in a position to operate the facility in a seven-day twenty-four hour mode for extended periods.
One of the indirect legacies of the extended shutdown period during the recovery from the July 2008 Operational Emergency has been a series of ORIC failures as we attempt a restart of routine operations. Chronic unreliability of this fifty year old cyclotron is the most important issue we face. Continuous operation should certainly help these chronic problems, but by far the best solution would be replacement of ORIC with a commercial accelerator. In November 2009, more than 150 scientists attended a two-day HRIBF users workshop devoted to discussions related to upgrade of the facility by addition of a commercial cyclotron. A Summary of the workshop is available in this Newsletter. Addition of an accelerator with capabilities similar to those of the IBA C70 cyclotron would dramatically improve efficiency, reliability and scientific reach of HRIBF. Annual operating costs could be reduced by well over $1M per year. We could take full advantage of the capabilities of IRIS2, and we should easily achieve 4000 hours of RIB delivered to experiments. Most important, we could take advantage of higher driver accelerator beam currents coupled with the developments in ISOL technology we have made in recent years to deliver neutron-rich beams with baseline intensities in excess of one thousand times greater than are now available.
ORNL has made an investment worth approximately $5M in the HRIBF utilities infrastructure by replacing the fifty year old cooling tower and many of our electrical substation circuit breakers. These improvements will have a direct impact on HRIBF reliability.
Experiments with neutron-rich beams are a centerpiece of our science
program. The Operational Emergency ended our last successful
neutron-rich campaign in July 2008, and the associated facility
shutdown has exacerbated the accumulation of a substantial backlog of
important neutron-rich beam experiments. Consequently, our highest
priority is to initiate an extended (up to six month long)
neutron-rich campaign as soon as we are able to do so.
2. Recent HRIBF Research - Deviations from U(5) Symmetry in 116Cd
The cadmium isotopes near their mid-neutron shell, i.e., N=66, exhibit one of the well-known examples of shape coexistence . In addition to the spherical quadrupole vibrational levels (hereafter referred to as normal phonon states), which are described within the Interacting Boson Model (IBM) by the U(5) limit, they possess intruding proton particle-hole configurations that give rise to additional states in the low-lying spectrum of levels .
These intruding excitations may mix with the normal states, perturbing their properties. However, once this mixing is accounted for , the underlying normal phonon states have been claimed to be very close to the U(5) limit, and, in fact, are often cited as the best examples of U(5) nuclei  and are often used in textbooks to illustrate quadrupole vibrational spectra in nuclear systems [5,6]. These nuclei span the "bottom of the parabola" that is characteristic  of intruding particle-hole configurations that underlie shape coexistence. The low-lying levels of 112,114Cd have been explained  as mixtures of vibrational and intruder configurations. Experimental studies of 110Cd [7,8,9], 112Cd [10,11,12,13], 114Cd  and116Cd [15,16] have supported this description. An intruder band structure has also been well established [7,15,17] in 110-116Cd, with enhanced intraband B(E2) values observed.
In the cases of 110,112,114Cd, it is the lowest excited 0+ states that have large B(E2) values for decay to the one-phonon 2+ states (and also the 2+ intruder states decay to them with enhanced B(E2) values), whereas the second excited 0+ states have much smaller B(E2) values for their decay [10,12,14]. This pattern can be explained well within the strong mixing approach as outlined in , and shown in [17,18], although the weak mixing approach can also explain the B(E2) pattern in 114Cd . In 116Cd, it is the second excited 0+ state, rather than the first, which has the enhanced B(E2) value for decay to the one-phonon 2+ level , and it is also this level that is fed by the strong B(E2) value from the 2+intruder level , a pattern that remains unexplained [18,20].
In order to better understand the behavior of 116Cd and all of the Cd isotopes, we reinvestigated the decay of levels in 116Cd populated via the beta-decay of 116Ag. Silver-116 was produced via the proton-induced fission of 238U at the Holifield Radioactive Ion Beam Facility (HRIBF). The proton induced fission products were then mass-separated by the OLTF and deposited on a moving tape collector (MTC). The collected samples were subsequently moved to the counting position located at the center of the CARDS array (Clover Array for Radioactive Decay Spectroscopy), which consisted of three segmented-clover Ge detectors, plastic scintillators, and a high-resolution (FWHM 1.5 keV at 44 keV) Si conversion-electron spectrometer . The BESCA detector had an efficiency for conversion electrons of ~2%, while the clovers had a summed efficiency of ~5% for 344-keV 152Eu gamma rays.
The results of our experimental observations confirm the existence and placement in the decay scheme of the five levels (1869.8, 1916.0, 1928.6, 1951.4, and 2026.7 keV) assigned in  to be the complete three-phonon state quintuplet. However, while the experimental B(E2) values for decay of the 3+, 4+ and 6+ levels (see Figs. 2-1 and 2-2) compare well with the calculations, the decay of the 0+ and 2+ levels is not consistent with this picture.
The experimental B(E2) values for the decay of the 2+4 state are completely different than those predicted by the IBM2 for a vibrational phonon state. As mentioned above, the decay pattern is different with the IBM2 predicting decays to the 2+2, 4+1, 0+2, 0+3, and 2+3 states. Of these states, only the decay to the 0+2 state is observed, along with decays to the 2+1 and 0+1 states. For the one transition that is observed (2+4 → 0+2), the experimental B(E2) value is 61 W.u. compared to the calculated value of 4.2 W.u. For comparison, the experimental and calculated B(E2) for decay of this level are shown in Fig. 2-3. The decay pattern of this level is not consistent with a three-phonon interpretation and the 1951.4-keV, 2+ level is more consistent with an isolated weakly-deformed band structure in view of the strength of the connecting 668.8-keV transition.
For the case of the 0+4 state in 116Cd, the experimental decay of this state is completely different than what is predicted by the IBM-2 calculations. The experimental relative B(E2)'s versus the predicted values are shown in Fig. 2-4 (the lifetime of the 0+4 state has not been measured, so the experimental B(E2) is unknown). From the IBM-2 calculations one would expect that the state should decay strongly to the 2+2 state (B(E2) = 43.5 W.u.) and very weakly to the 2+1 state. In fact the opposite is true, as we only observe a single transition de-exciting this state to the 513.5-keV 2+ state and an upper limit for the relative B(E2) to the 2+2 of 14%.
The deviations in the experimental B(E2) values from IBM-2 calculations that are observed in 116Cd for the phonon-states cannot be explained through considered mixings with the intruder excitations or mixed-symmetry states. Along with the inability to explain the decays of the 0+2 and 0+3 levels, the present results for the proposed three-phonon levels show that the description of 116Cd as a vibrational nucleus with well understood mixing between normal and intruder states is inadequate.
A careful analysis of the other known even-even Cd isotopes (110-120) reveals that the discrepancy in the decay of the three-phonon 0+ and 2+ states with the U(5) description is a consistent feature in all these nuclei. Fig. 2-5 shows the decays of the 0+ and 2+ members of the three-phonon quintuplet for110Cd ,112Cd [24,25], 114Cd ,116Cd [this work], 118Cd , and 120Cd . In the U(5) description of the normal states, the 2+ three-phonon state would be expected to strongly decay to all three of the two-phonon states (0+, 2+, 4+). The decay of this state in 112,114,116Cd is to the 2+ two-phonon and 2+ one-phonon states, while in 110,118,120Cd, this state is only known to decay to the 2+ one-phonon state. (In Ref. , the 1915.8-keV state was labeled as a three-phonon state decaying to the 2+ two-phonon state, while Ref.  labels this state as an intruder state based on energy systematics). The three-phonon 0+ states in 110,112,114,116,118Cd decay only to either intruder states or the 2+ one-phonon state.
In all the neutron-rich even-even Cd nuclei from 110-120, none of the observed 0+ and 2+ states previously assigned as three-phonon states decay in a manner consistent with a three-phonon state. This discrepancy is unaccounted for to date in calculations that incorporate mixing between normal and intruder states. Further, the consistent decay pattern across the Cd isotopes, regardless of level spacings that would cause significant differences in energy denominators for mixing amplitudes, is suggestive that the deviations from the expected harmonic vibrator or U(5) selection rules are not due to mixing.
To continue this investigation, we have recently taken data with high statistics (1.4X108 events in the 2+1 → 0+1 gamma transition) on the beta-decay of 120Ag. This data is currently being analyzed.
3. Update on Injector for Radioactive Ion Species 2 (IRIS2)
The IRIS2 Project completion date has been delayed until March 31, 2010, to allow additional time for documentation updates and new review requirements. In turn, this has delayed our ability to complete commissioning with radioactive beam.
From a technical perspective, the project continues to progress very well with most of the equipment installation and testing complete. Additional high voltage tests have been performed on the platform systems to identify and mitigate the effect of any remaining corona points. Handrails have been installed on both of the platform structures. The modular laser room has been installed, and laser safety system installation is nearing completion. Testing of the automated sequences of the remote handling system bridge crane has also been successfully completed, and we are now finalizing related procedures and lift plans.
As previously reported, both the injector and transport beamlines are
fully assembled, tested and operational. In addition to the stable
beam commissioning activities reported in the last newsletter, which
focused on the setup of the 1st-stage mass separator, subsequent
stable beam tuning exercises have been completed. A 2 μA, 40Ar+ beam
was accelerated off the platform to 200 keV and tuned to LERIBSS. The
focus of the exercise was to maximize transmission rather than mass
resolution, and it was highly successful: transmission from the object
point of the 1st-stage separator to the 1st cup on beamline 12 was
essentially 100%, to within the accuracy of the Faraday cup
calibrations. To state it another way, there were essentially no
losses in transport through the entire IRIS2 system, comprising both
the injector and transport beamlines. Total transmission from IRIS2
to the LERIBSS Faraday cup was approximately 85%, 1.7 μA out of 2 μA,
with virtually all of the loss coming between the image point of the
isobar magnet (FC_12_3) and LERIBSS. This will likely improve with
experience and time.
4. Summary of the HRIBF Workshop, Upgrade for the FRIB Era
The HRIBF Users Executive Committee called a user workshop meeting on Friday and Saturday, November 13-14, 2009, at the Pollard Conference Center on the campus of Oak Ridge Associated Universities (ORAU) in Oak Ridge, Tennessee. The purpose of the workshop was twofold: (i) to solicit user input and support for a proposed new production driver, a 70-MeV variable-energy, light-ion cyclotron to replace the ORIC; (ii) to solicit user input to update the HRIBF strategic plan. The result of the meeting was a user-driven White Paper that contains a strong science case for a modern, reliable ISOL facility at ORNL. A description of the proposed upgrade is also available. The discussion in the White Paper and throughout the workshop assumed that the upgrade is based on a commercial cyclotron with specifications equivalent to or exceeding those of the recently developed IBA C70. The C70 specifications can be found here.
By any measure, the workshop was a resounding success. In all, there were 151 participants representing 44 institutions from 10 countries, and many suggestions and proposals for exciting work at HRIBF were made by this group. This high level of participation is clear evidence that user interest in a cyclotron upgrade for the HRIBF is both widespread and intense.
On the first day of the workshop, introductory talks covered the following topics:
This was followed on Friday afternoon and Saturday morning by parallel, breakout discussion sessions in working groups on nuclear structure (in-beam and decay), reactions, astrophysics, applications, and ISOL technology. During the breakout sessions, there were lively discussions on the unique capabilities the HRIBF would have in the future FRIB era to perform complementary research in support of FRIB science. Most sessions included a series of presentations illustrating the type of research the users would like to do with the new beams and higher intensities that would be available with the new cyclotron driver. The ancillary equipment needs were discussed in detail. Theoretical perspectives were offered on the specific type of data needed to impact the development of models for nuclear structure, reactions, and astrophysics. A brief outline of the presentation and discussion topics is given here:
- Nuclear structure by in-beam spectroscopy
The majority of the proposed research centered on neutron-rich beams from proton-induced fission. In addition, proton-rich beams important for astrophysics and unique 56Ni beams with low 56Co contamination were highlighted. Major research thrusts discussed included studies of the following:
Separate sessions were devoted to laser and electromagnetic techniques for producing and enhancing radioactive beams and to various applications of radioactive beams at the HRIBF, especially in the context of new, unique opportunities associated with the proposed upgrade. Examples of the latter included isotope research and development, fission-fragment data of interest to nuclear reactor operations, stockpile stewardship, medical wear studies, and accelerator mass spectrometry.
Working group summaries were presented later on Saturday morning, and the workshop closed early Saturday afternoon. The conveners of the working groups were responsible for writing drafts of their respective sections of the White Paper. They were recruited from the user community and included both non-HRIBF scientists and local personnel.
Apart from soliciting user input and support for the upgrade proposal, the rich information collected during the workshop will be used to update the HRIBF strategic plan. As usual, the facility management will do this, in close consultation with the HRIBF Users Executive Committee, HRIBF Program Advisory Committee (PAC), and HRIBF Scientific Policy Committee (SPC). We are pleased that 13 of the 18 members of these HRIBF advisory bodies attended the meeting and were actively involved in discussions and the actual production of the White Paper.
In conclusion, it is very clear that a very strong, compelling science case has been made in each of the main research areas by the HRIBF Users Group for a cyclotron driver upgrade. The White Paper which details those cases was submitted to DOE on January 22, 2010.
The HRIBF Users Executive Committee would like to thank all those who
made the workshop and the White Paper possible, including the ORNL
central administration and Physics Division staff, ORAU, and the many
conveners from various institutions who contributed to this process.
5. The HRIBF Cyclotron Replacement
The HRIBF is proposing to replace the nearly 50-year-old ORIC with a commercial 70-MeV cyclotron that can replace most of the abilities of ORIC as well as bring new features beneficial to RIB production. The new cyclotron provides higher and easily adjustable proton energy. Coupled with higher beam intensity, the new cyclotron would increase the yield of neutron-rich fission fragment beams by at least a factor of 8 and potentially much more through the use of a two-step target. The variable energy deuteron beam allows us to maintain our current (d,n) reactions at low-energy for proton-rich beam production, e.g., 17F. The 70-MeV alpha beam also adequately covers ORIC's beam energy and intensity capabilities while, at the same time, eliminating problems associated with running alphas on ORIC:
In addition, a possible dual port extraction capability will allow us to simultaneously extract two beams of deuterons or protons at different energies. One beam can be used for normal RIB production while the other can be used for isotope production such as 7Be which we could use in our batch-mode ion source.
Very large beam intensities are possible with a commercial cyclotron. This ability will allow us to develop two-step targets where neutrons generated on a Be or Ta target could be used to induce fission on a surrounding uranium target. This "cold fission" process will produce more neutron-rich radioactive species than our current proton-induced fission process resulting in orders of magnitude improvement of many of our beams.
A more in-depth discussion of the commercial cyclotron possibilities may be found in two white papers:
The next step will be to develop a proposal based on the above white papers which we hope to submit to DOE this summer.
RA1. RIB Development
The last few months of 2009 were a busy period at the On-Line Test Facility (OLTF). Efforts included twelve on-line tests to i) measure yields from a variety of uranium carbide targets, ii) measure yields from an ion source that is specifically designed to provide Sr beams with greater purity, and iii) provide 120Ag to the CARDS array that was installed on a beamline after the OLTF separator. Specifics of the ion source projects were detailed in the last HRIBF Newsletter. Uranium carbide targets that were recently tested include UC targets produced at ORNL and several higher-density targets that were produced using a hot-press technique. The ORNL-produced UC targets are made by mixing graphite with a uranyl nitrate solution, which is then heated to make uranium dicarbide. This material is ground to a very fine powder (less then 3 micron diameter) and mixed with synthetic graphite powder before a pellet is formed using a cold-press technique. The resultant UC targets are quite uniform with an average density of 2.2 g/cm3. While these targets have been used for RIB production at the HRIBF since 2006, this is the first extensive survey of fission fragment yields. The other UC targets that were tested had densities up to 5.2 g/cm3 and are being studied in collaboration with Will Talbert. The release from these targets has been shown to be reasonably fast and the yields are comparable to yields from other UC target geometries that have been tested at the OLTF. In addition, the OLTF provided beams of 3He and 4He at 40 keV, which were implanted in thin aluminum foils. These successful tests were made in collaboration with Ray Kozub and his student, Jonathan Wheeler, from Tennessee Tech. The 3He targets, to be irradiated with radioactive beams, will be used to study (3He,d) reaction cross sections that are important in the rp process of nucleosynthesis.
During this period, we also completed another series of off-line tests
with the HRIBF laser ion source in collaboration with Klaus Wendt and
Tina Gottwald from Mainz University, along with Jens Lassen and
Ruohong Li from TRIUMF. This year's campaign planned to investigate
ionization schemes for three elements (Sb, Te, Sr) that had not
previously been ionized at HRIBF and to make an efficiency measurement
for Ga. Laser ionization of Sr was successful and several Rydberg
states in Sr were observed. To the best of our knowledge, this is the
first time resonant ionization schemes for Sr were demonstrated in a
LIS with Ti:Sapphire lasers. On the other hand, we did not
successfully produce laser-ionized beams of Sb and Te. We have
several possible explanations (low laser power, misalignment,
inaccurate ionization schemes), which will be investigated in the
coming months. The measured efficiency for Ga was only 10%, which is
quite low when compared to the 60% efficiency that was previously
measured at Mainz and we are also investigating the cause for this
discrepancy. These tests were carried out using the complete HRIBF
laser system which includes a Nd:YAG pump laser and three tunable
RA2. Accelerator System Status
ORIC Operations and Development (B. A. Tatum)
RIB Injector Operations and Development (P.E. Mueller)
Tandem Operations and Development (M. Meigs)
RA3. Users Group News
The Users Executive Committee met via telephone on January 21, 2010,
and elected Jeff Winger to chair this year's committee. The meeting
consisted of report on the facility and discussed the final report
of the HRIBF Users Workshop: HRIBF, Upgrade for the FRIB Era held
November 13-14, 2009, in support of a proposed 70-MeV cyclotron for HRIBF.
The report has been
posted on the website and is discussed in the
workshop summary in this newsletter.
RA4. 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 firstname.lastname@example.org. In any case, a brief description of the physics to be addressed with the proposed beam should be included. Of course, any ideas on specific target material, production rates, and/or the chemistry involved are also welcome but not necessary. In many cases, we should have some idea of the scope of the problems involved.
Beam suggestions should be within the relevant facility parameters/capabilities listed below.
RA5. HRIBF Experiments,
July through December 2009