HRIBF Newsletter, Edition 16, No. 2, Aug. 2008
1. HRIBF Update and Near-Term Schedule
There is much to celebrate concerning recent accomplishments at HRIBF. Researchers have produced exciting new scientific results, and the facility has produced important beams at record intensity. There is also ample cause for concern. Once again we face funding uncertainty in fiscal 2009, and we face uncertainty associated with recovery from an "operational emergency" which was declared on the morning of July 28, 2008. While this event did not occur during the period normally covered by this newsletter (the six months from January 1 to June 30), it is so important to the near-term future of the facility that it is impossible to ignore it here. I will therefore include a brief summary of the event and what we know of its impact on operations at the end of this article.
We are coming off a record year in FY2007 in terms of hours of radioactive ion beam delivered to experiments. Furthermore, the facility achieved a major milestone as the astrophysics group completed a direct measurement of the 17F (p,γ) 18Ne capture reaction, using the DRS to detect the 18Ne. This achievement adds another important result to the study of reactions that create and destroy 18F in stellar explosions. The experiment was made possible by an improvement of a factor of 50 in the intensity of the 17F beam over our previous facility record. Another important milestone was achieved by the decay spectroscopy group who capped a truly remarkable year of scientific results with the completion and first use of the new Low Energy Radioactive Ion Beam Spectroscopy Station (LeRIBSS). We have also made excellent progress toward the completion of the IRIS2 project, though delays in funding have resulted in postponement of the completion date (see IRIS2 article).
In the six month period ending June 30, HRIBF provided 1667 hours of beam on target for research, including 795 hours of ISOL RIB (including 131 hours of low-energy RIB), 180 hours of "in flight RIBs" (stable beam for experiments using the RMS to select and propagate beams of radioactive species for implantation decay studies at the focal plane of the RMS), and 294 hours of stable beam in direct support of RIB measurements (setup tests, calibrations, etc.). A total of 399 hours were devoted to stand-alone stable-beam experiments.
In the January Newsletter, I discussed the expected impact of the FY2008 continuing resolution (CR), which included up to two months budgetary shutdown of the facility. Such a shutdown was planned for August 4, 2008. The budget situation for FY2009 is again troubling. A likely scenario is a six month CR (i.e. operation at FY2008 budget levels), followed by six months at the levels of whatever FY2009 budget is enacted. We have also been instructed to develop plans for a twelve month CR. Either case would require substantial reductions in facility operations.
Now I turn to a discussion of the event that has introduced great uncertainty into our near term schedule. At approximately 8 AM on Monday, July 28, during operations with approximately 12 μA of 50-MeV protons on a uranium carbide target, delivering neutron-rich 81Zn beam to the new LeRIBSS facility, a radiological control technician (RCT) reported higher than normal radiation levels just outside the shield door to the IRIS1 vault (the room in which RIBs are produced at HRIBF). The measured dose rate equivalent was 4 mrem/hr. The presence of radiological contamination on the floor just outside the shield door was subsequently noted, as was the possible presence of airborne radioactivity. These observations were reported to facility management. Accelerators were shut down immediately and the building evacuated. The event was subsequently declared a laboratory operational emergency. Parts of the building were cleaned for reentry to collect belongings on Monday afternoon. The entire building was cleared for reoccupation on Tuesday morning after a detailed radiological survey found no contamination outside the shielded vaults. No decontamination was required.
A "recovery team" was constituted by the laboratory consisting of HRIBF staff members supplemented by a few other ORNL staff members with relevant expertise. ORNL management also commissioned an investigative team who are charged with determining causes of the event and another team to develop corrective action. The function of the recovery team is to carry out a systematic collection of information under direction of the investigative team, and subsequently, to implement corrective actions and prepare for resumption of facility operations. The investigative team will report their findings in early October. Though the recovery team is convinced that the physical basis of the event is now understood, it would be inappropriate to discuss causes in any detail prior to the investigation team report. It is appropriate, however, to catalog a few facts about the event.
The first and most important fact is that no individual received any detectable radiological dose as a result of the event. Six individuals were sent for whole body counts on the morning of the event; all were negative. All seventy-one individuals who had been inside the HRIBF building (Building 6000) during the day of the evacuation and three days prior to it had their thermo luminescent dosimeters (TLDs) collected and read the morning of the event. Readings were all consistent with zero doses. All subsequent analysis is consistent with a release that consisted entirely of noble gasses (Xe and Kr isotopes). One of the several samples collected after the event contained some 131I (all others exhibited only Xe and Kr daughters). If we accept this single iodine sample, the suppression of iodine compared to its total inventory at the time of the event compared to that of Xe isotopes is on the order of a factor of 107. Four high-volume charcoal filter air samples were taken soon after the event to search for 131I. The largest air concentration measured was on the order of 10-12 μCi/ml (i.e., background). At this time, only the RIB production vault (C111S) remains classified as a contamination area (as it was before this event). The only measurable contamination present that is attributable to this event is low levels of 137Cs (from 137Xe) and 140Ba and 140La (from 140Xe). As noted earlier, it is inappropriate to discuss causes in any detail at this point. We believe we have identified two unrelated failures, one associated with the HVAC system and the other with the roughing system exhaust which accounts for both the escape of noble gasses into the IRIS1 vault and their migration outside the vault.
It is very difficult to guess when we will be able to resume radioactive beam operation. The barriers to operation are not technical. The plan to begin operation on batch mode beams may help us start a little sooner, but we may well be down through November, or perhaps even longer. We have recently been given permission to resume operation of the HRIBF tandem for stable beams. When we are given permission to resume radioactive beam operations, we will begin with a two-month campaign of "batch mode" operations with beams of 10Be and 26Al. This will be followed by an extended period of operation with neutron-rich radioactive beams that will probably fill the remainder of our FY2009 operating time.
2. Recent HRIBF Research -
Direct Measurement of 17F(p,γ)18Ne at HRIBF
The rate of the 17F(p,γ)18Ne reaction is of significant astrophysical importance in such explosive astrophysical events as novae and X-ray bursts. The decay of radioactive 17F is thought to help drive the expansion of the envelope ejected in the nova outburst, and the 17F(p, γ)18Ne reaction affects the production of 18F, a target of γ-ray astronomy, and influences the 17O/18O ratio produced in these explosions. It is also an important link in the alpha-p reaction chain during the ignition phase of X- ray bursts. The 17F(p,γ)18Ne reaction rate was thought to be dominated by the contribution from an unnatural parity 3+ state that was predicted to exist from analog nuclei but was missing in decades of spectroscopic studies of 18Ne. In 1999, this "missing" 3+ state was found at 599.8 keV in the center of mass via a measurement of the 17F(p,p)17F reaction at HRIBF, which was the first measurement with a reaccelerated beam in North America . However, the strength of this resonance for (p,γ) was still unknown, resulting in more than an order of magnitude uncertainty in the capture reaction rate.
To address this, the 17F(p,γ)18Ne reaction has now been measured directly, to roughly 40% statistics. The HRIBF measurement used a mixed beam of 35 - 70% radioactive 17F and the remainder stable 17O at beam intensities of typically more than 5X106 pps of 17F ions. This was a factor of ~50 increase in low-energy 17F beam from previous measurements. The radioactive beam bombarded a differentially-pumped windowless hydrogen-gas target, and the Daresbury Recoil Separator was used to separate 18Ne recoils from unreacted primary beam. The spectra below show the excellent separation of recoils in the gas-filled ionization chamber at the focal plane. Resonant proton capture cross sections, gamma widths, and strengths for two resonances at 1178 keV and 599 keV in the center of mass have been determined, as well as an upper limit on the direct capture cross section at 800 keV in the center of mass. The strength of the 599-keV level was found to be more than a factor of two larger than a recent estimate based on shell model calculations of the gamma width of the analog level in 18O , resulting in an increase in the total 17F(p,γ)18Ne reaction rate. A set of nova simulations have been run with this new rate, and preliminary results indicate that over 400 times more 18F and 17O can be made in the hot, inner zones of one nova model when compared to simulations run with the most widely used 17F(p,γ)18Ne reaction rate. This work was the Ph.D. thesis for Kelly Chipps from the Colorado School of Mines. Future work may include a measurement of the direct capture cross section to three states just below the proton threshold in 18Ne.
 D.W. Bardayan et al., Phys. Rev. Lett. 83 (1999) 45.
3. Recent HRIBF Research - Measurement of Nuclear Structure Properties
As part of a research program devoted to the measurement of nuclear structure properties of neutron-rich nuclei around the mass A~80 region, during the months of September and October 2007 our collaboration performed three experiments using Radioactive Ion Beams (RIBs) produced at the Holifield Radioactive Ion Beam Facility (HRIBF):
Our aim has been to obtain a comprehensive picture of the shell structure in this region through the study of a series of properties: E (2+), B (E2), g-factors and electric quadrupole moment Q. The beams, instrumentation and techniques developed at HRIBF specifically for this purpose have allowed us to systematically study the behavior of these observables along isotopic and isotonic chains using both stable and radioactive nuclei under almost identical experimental conditions.
We have done pioneering studies of the evolution of collectivity in the neutron-rich radioactive isotopes 78,80,82Ge using the B(E2) as an indicator of the nuclear structure . For this work we also did a systematic measurement of the stable germanium and selenium isotopes. Recently a collaboration using REX-ISOLDE at CERN reported on the first observation of the 2+ state in 80Zn by Coulomb excitation . This together with our measurements on other N=50 nuclei gives a more complete picture of the nuclear structure for this isotones.
High quality radioactive ion beams were delivered to the RMS experimental station for the three new experiments. In Fig. 3-1 we show a scheme of the experimental setup. Around the target position we used the combination of the CLARION array, consisting of 11 HP-Ge clover detectors formed by four crystals, each segmented longitudinally in half, and the new particle array BAREBALL, with 54 CsI(Tl) detectors with minimum absorbers arranged in five rings. The de-excitation gamma-rays emitted by the Coulomb-excited projectile were detected in coincidence with the light target recoil nuclei. The beam composition was continuously monitored using a Bragg curve detector placed at zero degrees with respect to the incoming beam.
The aim of the first experiment was to measure the B(E2) values of the N=50 84Se. The conditions were very favorable in terms of beam purity and intensity. We bombarded a natural Al target [~1.5 mg/cm2 ] with an A=84 RIB at a beam energy of 193.2 MeV (2.3A MeV). The measured average beam intensity was 4X104pps, and the beam composition was 45% 84Se, 54% 84Br, and 1% 84Rb.
84Se is the N=50 isotone that lies right in the middle between Z= 28 and Z= 40 and therefore is expected to have maximum sensitivity to constrain the shell model effective interaction. In Fig. 3-3 we show a particle-gamma coincidence spectrum Doppler corrected event-by-event using both the clover segmentation and the recoil information obtained with BAREBALL. New transitions in the odd-odd 84Br isobar and a very clean peak for the 2+1 to 0+g transition in 84Se are observed.
The aim of the second experiment was to measure the electrical quadrupole moment of 78Ge. Using a sulfur purification technique , a pure beam of 78Ge was produced with typical intensity of 2X106 pps. We bombarded two targets of 12C and 24Mg alternately to minimize systematic effects such as beam intensity, beam composition and target thickness variations.
In the last experiment we intended to measure the gyromagnetic ratio in the mass A~80 region (78Ge and 80Ge) using the recoil-in-vacuum technique. Fig. 3-4 shows the angular correlation for 78Ge and 76Ge. The beam conditions for the data of 80Ge were less favorable as the overall source intensity had started to decline. The analysis of these data is in progress to determine if more data will be required for 80Ge.
Data sets using stable beams under nearly equal conditions to the RIB experiments were obtained for reference, normalization or calibration. Tests have been performed with the Bragg Detector to measure target thickness homogeneity and energy loss variations due to intense beam bombardment.
 E. Padilla-Rodal et al. Phys. Rev. Lett. 94 (2005) 122501.
4. Update of the IRIS2 Project
The Annual Progress Review for the IRIS2 project was held in April. All technical and management aspects of the project were favorably reviewed with only one recommendation: to submit change requests discussed at the review. Funding delays in two consecutive years have necessitated the request for changes to the funding profile, and delay of major project milestones including the project completion date. In the original project baseline, all project funds ($4,735K) were to have been received early in fiscal year 2008. Presently, $635K of project funds are now delayed until FY 2009 and thus the completion date has been moved from February 28, 2009 to September 30, 2009.
Technical progress on the IRIS2 Project has been good during the reporting period. All of the major beamline components for both the injector and transport beamlines have been received with the exception of the vacuum pumps which are due to arrive in late July. Assembly of injector beamline vacuum components is approximately 70% complete. Preparations have been made for tying in the transport beamline to existing beamline 12 (beamline from IRIS1 through the isobar separator). These activities have included relocation of piping, electrical circuits, and cable trays. Electrodes and vacuum chambers for both of the transport beamline spherical electrostatic deflectors have been received. The 35-degree deflector has been assembled, aligned, and tested and is now ready for installation. Assembly of the 90-degree deflector has begun. Installation activities will continue to be the primary focus during the remainder of the calendar, with beamline commissioning expected to begin early in calendar year 2009.
RA1. RIB Development
Significant progress has been made in the last few months on a number of ion source and target development projects. The efforts of the ISOL development group are devoted to developing techniques to improve the quality of radioactive ion beams at the HRIBF. The following is a summary of capabilities and recent activities at the facilities that are available at the HRIBF for testing RIB production targets and ion sources.
Ion Source Test Facility I (ISTF-1)
Ion Source Test Facility II (ISTF-2)
On-Line Test Facility (OLTF)
High Power Target Laboratory (HPTL)
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. Experimental Equipment - New Dual MCP Fast Timing Detector
A variation of the foil+MCP base timing detector has been developed to address the lower efficiencies for detecting ions with low specific energy loss (DE/DX) . The system features two micro-channel plate (MCP) detectors viewing the opposing surfaces of a single foil. This allows for monitoring the detection efficiency in-beam as well as extracting a fast timing signal with significantly increased efficiency.
In this detector secondary electrons are ejected as ions pass through a thin carbon or aluminized Mylar foil. Electric fields guide the electrons emitted from the foils toward the surface of a micro-channel plate detector assembly where fast multiplication of the electron current takes place. The amplified electron current is collected at a metal anode and used to provide a fast timing signal.
Efficiency is measured using the count rates from the individual anodes and the count rate of signals that are coincident in time. Since the probability that signals coincide is the product of the efficiencies of the individual detectors, the efficiency of one detector is the coincident count rate divided by the count rate of the other detector.
The first experiment to take advantage of this detector will be 10Be(d,p) as part of a long-lived RIB campaign scheduled for this fall at HRIBF. A position-sensitive Dual MCP will be constructed in the coming months.
RA4. User Group News
It is time to hold the biennial election for the Users Executive Committee. The nomination committee (Walt Loveland, Robert Grzywacz, and Witek Nazarewicz) has selected the slate of candidates below.
Additional nominations may be submitted from the users group at-large by collecting the support of 10 members and forwarding the name of the nominee to Carl Gross at email@example.com. Deadline for at-large nominations is October 10. More information may be found in the Users Group Charter or by contacting Carl Gross. The candidate receiving the most votes for a given seat will take office January 1 and serve for 4 years.
The HRIBF will hold its annual Users Group Meeting at this year's DNP meeting in Oakland, CA, on Friday evening, October 24. As in the past, this meeting will be held jointly with the ATLAS, NSCL, GAMMASPHERE/GRETINA, and RIA Users groups. The meeting is scheduled to start at 7 pm with HRIBF going first. Each group will have 20 minutes. We hope you will be able to join us in Oakland.
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 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.
RA6. HRIBF Experiments,
January through July 2008