HRIBF Newsletter, Edition 17, No. 2, Aug. 2009

Feature Articles

1. HRIBF Update and Near-Term Schedule
2. Recent HRIBF Research - Measurement of the ²⁶Al(d,p)²⁷Al Reaction for the Study of the Astrophysical²⁶ Al(p, γ)²⁷Si Rate
3. Recent HRIBF Research - Study of the Elastic And Inelastic ²⁶Al+p Reactions at HRIBF
4. Recent HRIBF Research - Resonant Scattering of ¹⁰Be on ⁴He
5. Recent HRIBF Research - Measurement of Levels in the Halo Nucleus ¹¹Be via ¹⁰Be(d,p) in Inverse Kinematics
6. Recent HRIBF Research - Use of ⁷Be Beam in Wear Studies (Proof-of-Principle Experiment)
7. Update of the IRIS2 Project
8. PAC-15 Results and PAC-16
9. Improvements to User Access And Possible Future Changes
10. HRIBF, Upgrade for the Era of FRIB - A Users Workshop Will be Held on November 13-14, 2009

Regular Articles

RA1.  RIB Development

RA2.  Accelerator Systems Status

RA3.  Experimental Equipment - The Oak Ridge Isomer Spectrometer and Separator (ORISS)

RA4.  Users Group News

RA5.  Suggestions Welcome for New Beam Development

RA6.  HRIBF Experiments, January-July, 2009

1. HRIBF Update and Near-Term Schedule
(J. R. Beene)

In the last two editions of the Newsletter, the HRIBF Update dealt primarily with the July 2008 Operational Emergency (OE) and its aftermath. If you are not familiar with the OE, I encourage you to have a look at the HRIBF Update in the February 2009 Newsletter, since I will make reference to events, committees and reviews in this article without providing full background information. I am pleased to report that this Update will have a more optimistic tone than the last two. There have been many positive developments in the December 2008 through June 2009 period. First and most important, we have completed some excellent science with long-lived radioactive ion beams (^7,10Be and ²⁶Al) operating in "batch mode". Other important accomplishments included completion of the required corrective actions which were developed based on the Management Investigation Team report, and completion of the Triennial Review of the HRIBF by the ORNL Accelerator Safety Review Committee. We have now completed the phased restart plan discussed in the February 2009 Newsletter and obtained permission for full operation of the HRIBF. In addition, excellent progress has been made on the IRIS2 project, including completion of the Accelerator Readiness Review (see the Update on IRIS2 in this newsletter), which will be fully operational before the end of calendar year (CY) 2009.

In spite of an extended Continuing Resolution to start FY2009, HRIBF is in better fiscal shape than it has been in many years. We actually may be able to make much-needed additions to the facility staff as part of a long-standing, but much delayed, facility staffing plan. The President's Request for fiscal year (FY) 2010 also looks very encouraging, though the congressional marks are less so.

Restart of an accelerator facility after an extended shutdown (almost a year in this case) is seldom easy. ORIC has always been vulnerable to the effects of such shutdowns, and this case has been no exception. We have experienced several intermittent failures during the initial weeks of operation. We are systematically working through these problems to ensure successful RIB campaigns. We have obtained $2.5M of American Recovery and Reinvestment Act (ARRA/stimulus) funds which will be applied to the refurbishment of ORIC systems over the next four years. Our current plan is to complete the ongoing fluorine campaign in the fourth quarter of FY2009, and move to an extended neutron-rich campaign which will continue well into CY2010. This campaign will be the first to take advantage of a completed and fully commissioned IRIS2.

This has been very difficult and frustrating year. The HRIBF operations staff deserves high praise for the way they maintained morale and for the work they accomplished. Of course we realize that it has also been a terribly frustrating time for our users as well. We are determined to recover quickly from the restart issues, and to provide a much improved level of facility operation with IRIS2 on line and with new investments in ORIC beginning to have an impact (see my article on HRIBF driver upgrade in the February Newsletter).

2. Recent HRIBF Research - Measurement of the ²⁶Al(d,p)²⁷Al Reaction for the Study of the Astrophysical ²⁶ Al(p, γ)²⁷Si Rate
[S. D. Pain (Univ. of the West of Scotland), spokesperson]

Astronomical gamma-ray mapping, by charting the distribution of specific isotopes, yields information which can constrain the rate of explosive nucleosynthesis events (such as novae and supernovae) within the galaxy. One of the landmarks in observational astronomy has been the detailed galactic mapping of the decay of ²⁶Al, achieved by the observation of the 1809-keV gamma rays emitted following its beta decay using the COMPTEL instrument aboard the Compton Gamma Ray Observatory (see Fig.2-1), and more recently the European Space Agency's INTEGRAL satellite. With a half-life of ~105 years, the distribution of ²⁶Al provides an insight into the galactic nucleosynthesis over a timescale of about the last million years. The mechanisms contributing to the formation and destruction of ²⁶Al are consequently of direct interest to the interpretation of these gamma-ray maps [2].

Figure 2-1: COMPTEL galactic map of the 1809-keV gamma-ray line from the decay of ²⁶Al [1].

In most astrophysical environments, ²⁶Al is destroyed via the ²⁶Al(p,γ)²⁷Si reaction. Understanding states near the proton threshold in ²⁷Si is crucial for constraining this reaction rate, and thus for elucidating the amount of ²⁶Al which survives to enrich the interstellar medium. The difficulty in measuring directly the strengths of these resonances, due to their small cross sections, means that indirect approaches are required. Furthermore, due to the difficulties inherent in measuring proton transfer reactions, an alternative is to measure mirror states in ²⁷Al to obtain information about the ²⁷Si structure.

The ²⁶Al(d,p)²⁷Al reaction has been measured in inverse kinematics at the HRIBF, in order to study states in ²⁷Al which are mirror to those in ²⁷Si. A batch-mode beam of ²⁶Al, of typically 5 million particles per second, impinged on a ~150 mg/cm² CD₂ target for ~5 days. Proton ejectiles were detected in the SIDAR and ORRUBA silicon detector arrays, subtending angles from ~90-165 degrees in the laboratory frame. These data represent the first measurement performed with a complete barrel of ORRUBA. Elastic scattering was monitored in ORRUBA detectors mounted at angles just forward of θ_lab = 90°. A forward array for recoil tagging, comprised of annular segmented silicon detectors of Micron QQQ2 design, covering angles from ~1.5 to 10 degrees, was used to detect ²⁷Al ions coincident with (d,p) protons.

Figure 2-2 shows an online spectrum from a single strip of SIDAR, for a subset of the data taken during the experiment. Transitions at these backward angles are strong candidates for ℓ = 0 transfer, pending a full analysis of angular distributions. These data are under analysis by, and will contribute to the PhD thesis of Stephanie Brien from the University of the West of Scotland.

Figure 2-2: Proton energy spectrum from the ²⁶Al(d,p)²⁷Al reaction, for a single SIDAR strip, for a subset of the data.

References:

[1] S. Pluschke et al., "The COMPTEL 1.809 MeV Survey" in Proceedings of the 4th INTEGRAL Workshop, A. Gimenez, V. Reglero, and C. Winkler, eds. (ESA Publications, Noordwijk, 2001).
[2] J. Jose, A. Coc and M. Hernanz, Astrophys. J. 520, 347 (1999).

3. Recent HRIBF Research - Study of the Elastic and Inelastic ²⁶Al+p Reactions at HRIBF
[D. Bardayan (ORNL), spokesperson]

The detection of characteristic gamma-rays from the decay of ²⁶Al (t_1/2=7.2x10⁵ yr) has provided clear evidence of continuing nucleosynthesis in the Milky Way Galaxy. Detailed maps of Galactic ²⁶Al have been made by satellite gamma-ray observatories such as COMPTEL [1], RHESSI [2], and INTEGRAL [3]. The observed ²⁶Al was concentrated along the Galactic plane, but the source is still not known. Possible sources include Wolf-Rayet stars, novae, and core-collapse supernovae. Peak temperatures in these environments vary from 0.03 to ~3 GK meaning that relevant reaction rates need to be known over a broad temperature range to estimate the contributions of these events to the Galactic ²⁶Al abundance.

Figure 3-1: The experimental configuration used in the ²⁶Al(p,p)²⁶Al measurement.

Of the relevant reactions, the ²⁶Al(p, γ)²⁷Si reaction is thought to provide the dominant destruction mechanism over a large temperature range. Levels in ²⁷Si above the proton threshold at 7.463 MeV provide possible resonances in the ²⁶Al(p, γ)²⁷Si reaction. While there have been many studies of this reaction [4-7], there are many levels in the relevant energy range (Ex=7.5-8.5 MeV) for which their spins and proton-widths are not known. Also important to know is whether the effective lifetime of ²⁶Al is changed in the high-temperature stellar environment as a result of inelastic excitations. While the 5⁺ ground state has nearly a million-year half life, the 0⁺ metastable state only lives 6.3 s. If there are ²⁷Si levels through which ²⁶Al could inelastically-scatter into the metastable state, the effective half life of ²⁶Al could be greatly reduced.

To address these uncertainties, we have measured the elastic and inelastic ²⁶Al+p scattering reactions at HRIBF. Pure ²⁶Al beams with intensities of 2x10^{6 26}Al/s were used to bombard 50 μg/cm² CH₂ targets (see Fig. 3-1). Scattered protons were detected in the angular range 18-41 degrees by the SIDAR Silicon Detector Array [8]. The recoil ²⁶Al ions were detected in coincidence with the protons in an isobutane-filled ionization counter. The unscattered primary beam was prevented from entering the ionization counter by a 1-cm-diameter disk that was inserted in front of the counter entrance window during each run. The size of the disk was chosen so that for the proton angles covered by the SIDAR, the corresponding recoil ²⁶Al ions were not blocked by the disk. The energy spectrum detected in SIDAR is shown in Fig. 3-2 in singles and coincidence demonstrating the cleanliness of the proton identification in the coincidence data. The proton yields were measured at 45 energies from E_c.m.=0.5-1.5 MeV over a period of 7 days in March 2009. The data are currently under analysis and will constitute the Ph.D. thesis of University of Tennessee graduate student Stephen Pittman.

Figure 3-2: The energy spectrum observed at 27 degrees for a 15-MeV ²⁶Al beam bombarding a 50 μg/cm² CH₂ target in singles and in coincidence with a recoil ²⁶Al ion.

References:

[1]R. Diehl et al., Astron. Astrophys. 298, 445 (1995).
[2]D. M. Smith et al., Astrophys. J. 589, L55 (2003).
[3]R. Diehl et al., Nature (London) 439, 45 (2006).
[4]L. Buchmann et al., Nucl. Phys. A 415, 93 (1984).
[5]R. B. Vogelaar et al., Phys. Rev. C 53, 1945 (1996).
[6]C. Ruiz et al., Phys. Rev. Lett. 96, 252501 (2006).
[7]G. Lotay et al., Phys. Rev. Lett. 102, 162502 (2009).
[8]http://www.phy.ornl.gov/hribf/equipment/sidar/

4. Recent HRIBF Research - Resonant Scattering of ¹⁰Be on ⁴He
[M. Freer (University of Birmingham, UK), spokesperson]

The structure of the beryllium isotopes is rather unusual, but highlights the spectrum of possible structural characteristics of light nuclei. The nucleus ⁸Be is well-known to posses a structure which can be described in terms of two alpha-particle clusters. From an experimental perspective the ground state rotational band may be described by a moment of inertia commensurate with two touching alpha-particles. Such a structure is also found in the Greens Function Monte Carlo calculations [1] in which no cluster structure is apriori assumed. The addition of neutrons to the ⁸Be nucleus to form isotopes such as ^9,10Be produces some rather interesting characteristics. Rather than destroying the symmetries responsible for the clustering in ⁸Be, the two alpha-particle cluster structure remains and the valence neutrons are exchanged between the two alpha-particle cores in a covalent, molecular, manner [2]. The orbitals of the neutrons have either σ or π character, just as electrons exchanged in atomic molecules. The nature of the bonds depends on the orientation of the p-orbitals which are occupied by the neutrons at each alpha-particle centre. When the orbitals are aligned perpendicular to the separation axis then π-bonding results, whereas if the alignment is parallel the bonding is of σ-character (see Fig. 4-1) [2].

Figure 4-1: The formation of the molecular orbitals from the linear combination of p-orbitals built around two alpha particles.

In the ground-state of ¹⁰Be the two neutrons, asymptotically, have π-behaviour. Such systems are nuclear dimers. The question then arises as to if it is possible to form trimers, i.e. three-alpha-particles and valance neutrons. From a theoretical perspective the answer appears to be yes, though the precise geometric arrangement of the alpha-particles is unclear (linear or triangular) [3,4]. The experimental situation is a little more unclear. Reference [5] summarises the latest situation. A great number of resonant states have been observed that exist above the alpha-decay threshold (12 MeV), but their spins are largely unknown and this prevents any systematics being determined.

The present study was of resonances in ¹⁴C populated in the ¹⁰Be+⁴He resonant scattering reaction. The ¹⁰Be beam used had an intensity of ~10⁷ pps and the contamination from ¹⁰B was ~1%. Nine beam energies ranging from 25 to 46 MeV were used to perform the measurements. The experimental arrangement is shown schematically in Fig. 4-2. The SIDA chamber was positioned in front of the Daresbury Recoil Separator and was sealed with respect to the beam line with a 5 μm thick havar window. Helium gas was held at a pressure of 900 mb.

Figure 4-2: The arrangement of the two LAMP configurations used to coincidently detect the 10Be and 4He products. The detectors were placed within the gas volume.

Within the gas volume were a number of silicon detectors. At zero degrees a three-element silicon telescope formed from three, 150-μm thick detectors was screened from the beam by mylar foils. These foils allowed the resonantly scattered alpha particles to pass through to the telescope behind. This provided a measurement of the zero-degree yield for the alpha-particles. In addition two LAMP arrays of YY1 type silicon detectors (Micron Semiconductors Ltd) were located within the helium gas volume. These were used to detect the ⁴He+¹⁰Be nuclei in coincidence as illustrated in Fig. 4-2. These detectors allowed the elastic and inelastic channels to be identified, the angular distributions of the reaction products to be calculated, and finally the location (distance from the window) of the inte raction to be calculated. The data from these latter detectors permit the angular distributions of the resonances observed in the zero-degree detector to be calculated and thus the resonance spins to be deduced. The zero-degree excitation function for the 25-MeV beam energy is shown in Fig. 4-3 and the energy angle systematics for the ⁴He particles in Fig. 4-4. The analysis of data is ongoing.

Figure 4-3: The ¹⁴C excitation energy spectrum populated in ⁴He+¹⁰Be resonant scattering for the beam energy of 25 MeV.

Figure 4-4: Energy versus angle systematics (measured with respect to the chamber window) for the two LAMP arrays for E_beam=25 MeV.

References:

[1] R. B. Wiringa, S. C. Pieper, J. Carlson, and V. R. Pandharipande, Phys. Rev. C 62, 014001 (2000).
[2] M. Freer, Rep. Prog. Phys. 70, 2149 (2007).
[3] N. Itagaki, et al., Phys. Rev. C 64, 014301 (2001).
[4] W. von Oertzen, et al. Euro. Phys. J A 21, 193 (2004).
[5] P. J. Haigh, et al., Phys.Rev. C 78, 014319 (2008).

5. Recent HRIBF Research - Measurement of Levels in the Halo Nucleus ¹¹Be via ¹⁰Be(d,p) in Inverse Kinematics
[K. Schmitt & K.L. Jones (Univ. of Tennessee), spokespersons]

Phenomena such as neutron halos and level inversion make the light neutron-rich nuclei fascinating test cases to study the evolution of nuclear structure away from stability. ¹¹Be is an archetypal example of both of these effects, and is one of the heaviest nuclei for which ab-initio theory is presently practical. Although ¹¹Be has been studied thoroughly via (d,p) in normal kinematics [1], β-decay[2], breakup[3,4], and (p,d)[5] experiments, there still remains a paucity of knowledge about the bound first-excited state, for which spectroscopic factors are disputed, and the resonances.

We have performed a ¹⁰Be(d,p)¹¹Be experiment in inverse kinematics using a new batch-mode ¹⁰Be beam at the energies of 60 and 107 MeV and deuterated polyethylene targets. The angular distribution of protons ejected in the population of bound states and low-lying resonances in ¹¹Be will be used to extract spectroscopic factors for those states. The Oak Ridge Rutgers University Barrel Array (ORRUBA) was used in conjunction with the Silicon Detector Array (SIDAR) for a large solid angle coverage and nearly continuous coverage in polar angle from 45° to 165° in the laboratory reference frame. The new QQQ array and Dual MCP detectors were placed at forward angles to detect recoils and to help discriminate between (d,p) and fusion-evaporation events. A schematic view of the silicon detector setup is shown in Fig. 5-1. The angular coverage is shown with expected angular distributions in Fig. 5-2.

Figure 5-1: Silicon detector setup. The ORRUBA and SIDAR arrays were used for detection of protons from (d,p) and elastically scattered deuterons.

Figure 5-2: Angular coverage with expected angular distribution. Coverage in polar angle was nearly continuous from 45 to 165 degrees in the lab frame, with only a small gap due to target frame shadowing.

Preliminary analysis of the proton data in SIDAR yields excellent statistics for the bound states and first resonance, as shown in an ungated spectrum in Fig.5-3. The first excited state at 320 keV is clearly separated from the ground state (two groups to the right of the figure). Other low-lying resonances have been observed in our preliminary analysis of the proton energy spectra from SIDAR.

On completion of calibrations both for SIDAR and ORRUBA, the full data analysis is expected to yield angular distributions with very low statistical uncertainties.

Figure 5-3: Energy distribution of protons at back angles. Protons from the population of the ground state and 0.320 MeV first-excited state are well resolved, and tremendous statistics were recorded for the wider first resonance.

Special thanks are due to the staff at HRIBF for a very successful run, especially Dan Stracener.

References:

[1] D.L. Auton, Nucl. Phys. A 157, 305-322 (1970).
[2] D.E. Alburger and D.H. Wilkinson, Phys. Rev. C 3, 1492 (1971).
[3] B. Zwieglinski, et al. Nucl. Phys. A 315, 124-132 (1979).
[4] V. Lima, et al. Nucl. Phys. A 795, 1-18 (2007).
[5] S. Fortier, et al. Phys. Lett. B 461, 22 (1999).

6. Recent HRIBF Research - Use of ⁷Be Beam in Wear Studies (Proof-of-Principle Experiment)
[U. Greife (Colorado School of Mines), spokesperson]

Currently in the United States, about 200,000 hip-joint replacement surgeries are performed each year. Worldwide, the number is nearly 1 million [DeG04, Feh00]. Unfortunately, these implants do not last forever, but seem to have useful lifetimes (limited by wear) between 10 and 20 years. The aim of wear studies on artificial hip joints is to extend the lifetime of the implants through development of more durable materials, as well as to make recommendations for the patients on lifestyle (activity) choices.

Substantive work has been going on in the past decade to improve the lifetime of metal-plastics joints. The wear of plastics is nearly exclusively measured by gravimetric methods. Due to the low wear, long test times are necessary to achieve reasonable accuracy. Also, fluid soak from the lubrication fluids can lead to high systematic errors in this method. We have performed a proof-of-principle experiment at the Holifield Radioactive Ion Beam Facility (HRIBF) to show the principal viability of a radiotracer method based on uniformly implanted ⁷Be for wear analysis.

The radioactive ⁷Be for the experiment was obtained from the ATOMKI cyclotron institute in Debrecen, Hungary, where it had been produced via the ⁷Li(p,n)⁷Be reaction. After chemical extraction from the ⁷Li matrix at HRIBF, the ⁷Be matrial was transferred to a sputter cathode for injection in the HRIBF tandem accelerator. A new ⁷Be implantation setup had been developed at the Colorado School of Mines and was installed at a free beam line at HRIBF (Fig. 6-1). The activity available resulted in ⁷Be-beam currents of 10⁵-10⁶ ions per second at the sample location.

Figure 6-1: 7Be implantation setup during beam time at HRIBF.

The 8-MeV energy of the beam was transformed into a broad energy distribution (measured with a silicon detector in the implantation position) using a wheel of 20 foils (increasing thickness from zero to 10 μm in 0.5 μm increments) and additional energy "smearing" foils. Based on the foil thicknesses an activity plateau (with depth) in the polyethylene from 0 to approximately 9μm was achieved on which wear studies were subsequently performed. Total ⁷Be implantation doses varied from 10⁹ to 10¹⁰ nuclei on the 7 pins used.

For artificial hip joints with metal-plastic couplings, a typical wear range lies between 0.1-1 mg/(cm²*1 million motion cycles). This corresponds to a depth wear of about 1.08-10.8μm per 1 million motion cycles. Motion simulators work at a speed of about 1 Hz, giving a time of about 2 weeks for one million cycles, well matched to the ⁷Be half-life. The wear studies were performed with a specifically designed motion simulator supplied and operated by Rush University Medical Center in Chicago. The Pin-on-disk (POD) design replicates the motion trajectories of artificial joints. For this experiment 2 types of plastic materials were available: One conventional high-density polyethylene, the other cross-linked high-density polyethylene. The latter material had been advertised as significantly superior to the previously used conventional materials. However, due to the lower wear and the problems of the gravimetric method with fluid soak a direct quantitative comparison had not been possible.

Figure 6-2: Pin-on-disk orthopedic wear testing system.

The wear studies were performed at Argonne National Laboratory and used a 20% Germanium detector setup for ⁷Be gamma detection. The plastic pins underwent a known number of wear cycles in the POD system (lubricated with bovine serum) before being cleaned and transferred for activity measurement. The results of the complete wear experiment (extending over 4 months) are depicted in Fig. 6-3. Shown in the figure is the fraction of activity worn off (natural decay corrected) as a function of wear cycles. Clearly visible is the different behavior of cross-linked (the rising group of 4 samples) and conventional material (the relatively flat group of 3 samples) with a preliminary result of a wear ratio of app. 13 (40.6%/10⁶ cycles conventional; 3.1%/10⁶ cycles cross-linked). Error analysis and further simulations of implantation depth are still ongoing.

Figure 6-3: Cumulative ⁷Be activity wear loss as a function of wear cycles.

This proof-of-principle experiment shows the usefulness and practical potential of the use of ⁷Be implantion as a radiotracer for wear studies. Further analysis and experiments have to look at improvements in the activity measurements (to reduce scatter and systematical error), the influence of radiation dose on mechanical properties (will provide upper limits on allowable ⁷Be implantation dose) and the possibilities of extending the method to natural materials.

The experiment was performed as a collaboration between the Colorado School of Mines (U. Greife, L. Erikson, N. Patel), Rush University Hospital (M. Wimmer, Y. Dwiwedi, M. Laurent), Oak Ridge National Laboratory [K. Chipps (Rutgers), D. Bardayan, J. Blackmon (LSU), C. Gross, D. Stracener, M. Smith, C. Nesaraja, R. Kozub (TTU)] and Argonne National Laboratory (E. Rehm, I. Ahmad)).

References:

[DeG04] J. DeGaspari, Mech. Eng. 12 (2004)
[FEH00] P. Fehsenfeld et al., Nachrichten Forschungszentrum Karlsruhe 32 1-2 (2000) 91

7. Update on Injector for Radioactive Ion Species 2 (IRIS2)
(B. A. Tatum, spokesperson)

The IRIS2 Project will be completed this Fall. Suspension of HRIBF operations following the July 2008 Operational Emergency had a positive impact on the project for several months relative to installation activities, but the resumption of operations and required Authorization Basis documentation updates have slowed down some of the commissioning activities. Nevertheless, substantial progress has been made.

Installation of most equipment is complete. The injector and transport beamlines are fully assembled, tested, and operational. This includes the transport beamline 35 degree and 90 degree electrostatic deflectors.

The modular laser room has been installed, and 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. Laser safety system procurement is underway, and a segmented polyethylene plug with three laser beam conduits and two utility conduits is in fabrication.

Iron storage boxes for storing activated target ion sources have been received, as well as the contamination control boxes. The activated target ion source storage area outside of the target room has been constructed. This is a concrete wall storage compartment that will accommodate up to six of the iron boxes. A below the hook lifting fixture for manipulating the boxes with the existing 12.5 ton bridge crane has been tested and approved for use. Testing and programming of the target room crane is in progress in preparation for commissioning. Pick-up and set-down points within the room are being finalized.

The Operational Readiness Review was not received as early as expected, but most findings have been addressed with the primary exception being the completion of revisions to the HRIBF Safety Assessment Document (SAD) and the Accelerator Safety Envelope (ASE). Although the revisions that are specific to IRIS2 are straightforward, other revisions that are associated with our July 2008 Operational Emergency are more complex. We are in the process of incorporating Accelerator Safety Review Committee (ASRC) comments on our draft revisions and expect these documents to be approved in early September. This delay has not impacted IRIS2 stable ion beam commissioning, but RIB commissioning cannot begin until the updated SAD and ASE are approved.

Stable ion beam commissioning has proceeded quite well. Beam has been transported from the target ion source through the injector and transport beamlines into beamline 12, the existing isobar separator beamline. The 1st-stage mass separator has been optimized for maximum mass resolution. Mass separation of ¹³¹Xe and ¹³²Xe has been demonstrated, with a measured resolving power of m/Δm = 1250 (error bars of +/-30), as shown in Figure 7-1, and we continue to optimize beam optics components and transmission.

Figure 7-1: Screen capture of the Beam Profile Monitor (BPM) display showing the horizontal profiles of the separated isotopic beams of Xe at the image position of the separator magnet system. Units are arbitrary.

8. PAC-15 Results and PAC-16
(C. J. Gross)

PAC-15 met July 22-23, 2009, in Oak Ridge and considered 26 proposals and letters of intent which requested 371 shifts of radioactive beams (RIBs), 12 shifts of in-flight RIBs, 77 shifts of low intensity stable beams (SIB for RIBs), and 65 shifts of stable beams (SIBs). Of these, a total of 212 RIB shifts (57 long-lived isotopes), 6 in-flight RIB shifts, 63 SIB for RIB shifts, and 12 SIB shifts were approved.

Proposals accepted by HRIBF requested RIBs of ^7,10Be, ^17,18F, ⁵⁹Fe, ⁵⁶Ni, ^73,74,77Cu, ^81,82Zn, ⁸³Ga, ⁸⁵As, ^130,132Sn, and ¹³²Te. The total number of accepted proposals was 17; 13 of which were from outside organizations.

Ray Kozub, chair of the Users Executive Committee, represented the Users at the meeting. Alan Shotter's tenure on the HRIBF PAC has come to an end with PAC-15 and the facility thanks him for his service. He will be replaced by Jens Dilling of TRIUMF.

We anticipate PAC-16 to occur in late spring 2010.

9. Improvements to User Access and Possible Future Changes
(C. J. Gross)

ORNL is attempting to improve user access to HRIBF and other ORNL User Facilities. Intermittent visitors, ie. most users, will begin receiving a "user entry pass" via email 7 days before arrival and completion of ORNL's Site Access Training (SAT). This pass will contain relevant information including the start date as well as contact information in case problems arise. A bar code will also be included which the guard at the portal will scan. It is anticipated that this will shorten the time spent at the portal.

The user entry pass is optional; it is not required for entry so don't panic if it is lost or damaged. Our hope is that time spent with the guards at the portal will be minimized. Remember that proper photo ID is still required at the checkpoint, and foreign nationals will still need to present their passport and visa at the Visitor Center.

There is also a draft DOE order 142.3 pertaining to Unclassified Foreign Visits and Assignments. We are working with National User Facility Organization and others to provide comments and suggestions for improving the order. If these efforts are successful, we anticipate more improvements regarding HRIBF access to users in the future.

10. HRIBF, Upgrade for the Era of FRIB - A Users Workshop Will be Held on November 13-14, 2009
(R. Kozub, Chairman, HRIBF Users Executive Committee)

The science of rare isotopes is blooming worldwide. With the FRIB on its way, the time has come for the HRIBF to better define itself in a new era. Now, as we have learned about the scope of the FRIB, we are in an excellent position to assess the unique scientific opportunities provided by the intense ISOL beams of the HRIBF for nuclear structure and reactions, nuclear astrophysics, and applications of nuclear science, and to thereby promote the complementary features the HRIBF has with the FRIB and other RIB facilities.

To this end, the HRIBF Users Executive Committee is calling a workshop, HRIBF, Upgrade for the Era of FRIB, on Friday and Saturday, November 13-14, 2009, at the Pollard Auditorium in Oak Ridge to inform the user community about a proposed new cyclotron driver to replace the ORIC (click here for a description). The primary goal of the workshop will be to develop a user-driven white paper that will contain a science case for a modern, reliable ISOL facility at ORNL. This workshop will also offer an opportunity to attract new users, especially in the context of the envisioned future GRETINA campaign at the HRIBF.

There will be introductory talks on Friday morning about the FRIB and various ISOL facilities. This will be followed on Friday afternoon and evening by parallel discussion sessions in working groups on nuclear structure (in-beam and decay), reactions, applications, astrophysics, and ISOL technology. The discussions should focus on the scientific strengths of HRIBF in the future FRIB era and on the research that can be done in support of FRIB science. The proposed HRIBF upgrade would result in new isotopic beams and an increase in RIB intensity, so new equipment necessary to take advantage of these improvements should also be covered in the discussions. The format of these break-out sessions is expected to be characterized primarily by detailed round table discussions built around the topics specified by the conveners, with few if any formal presentations. The conveners of these groups are being recruited from the user community, and they will be responsible for writing their respective sections of the white paper. In addition to the areas mentioned above, the final document will also include sections on the complementarity of FRIB-HRIBF science, FRIB-HRIBF instrumentation, the worldwide context of the proposed upgrade, education and outreach, and the relation to nuclear theory. Working groups will summarize their findings on Saturday morning.

More details about the HRIBF Users Workshop can be found at its website, which will be updated frequently in coming weeks and months. Meanwhile, if you have questions or suggestions concerning this workshop, please contact us. Your active participation is very important to the future of the HRIBF and its niche of ISOL/RIB physics in the U.S. We hope to see you there!

Regular Articles

RA1. RIB Development
(D. Stracener)

The RIB development group has been involved in several efforts at the On-Line Test Facility (OLTF) and at the High-Power Target Laboratory (HPTL) to develop new radioactive ion beams and to improve the quality of the beams that are currently available at the HRIBF.

A group led by Jon Batchelder (ORAU) has made several on-line tests to optimize a target and ion source for the production of ⁹⁷Ag beams. The search for an isomer in ⁹⁷Ag is a PAC-approved experiment. The ⁹⁷Ag nuclei will be produced in the reaction of a ⁵⁴Fe beam with an enriched ⁵⁰Cr target. These tests have included measurements of silver holdup times using different materials for the target holder and catcher foil, including molybdenum, tantalum, graphite, and niobium.

Another project, led by Ron Goans (a student from University of Tennessee), is seeking to understand the differences in yields of neutron-rich tin and germanium isotopes from uranium carbide targets that are produced using various techniques. Our data from the IRIS1 injector indicate that the RIB intensities of neutron-rich Ge and Sn isotopes are higher when UC/RVC targets are used than the intensities observed when we are using targets fabricated from UC powders. The UC/RVC targets are produced by coating a low-density, highly-porous graphite matrix with a layer of uranium carbide, resulting in a target density of about 1 g/cm³. We have not used these targets lately since the fabrication process is complex and it has been difficult to consistently produce high-quality targets. Over the last couple of years, we have used targets produced by pressing a mixture of UC powder with graphite powder. These targets, with densities of about 2.2 g/cm³, have performed quite well in terms of longevity and RIB yields with the exception of Ge and Sn isotopes. Beams of these two elements are purified by extracting positively charged sulfide molecules of these elements. Sulfur is added to the system via the introduction of hydrogen sulfide. Ron is investigating different methods of introducing sulfur into the system and looking at differences in the holdup times of the elemental ions and the molecular ions as a function of target material, target temperature, and sulfur concentration.

Another student, Cara Jost (Mainz), is continuing her investigation of a technique to purify neutron-rich strontium beams by selectively trapping the rubidium contaminants on surfaces in the transfer line between the UC target and the ion source. It is known that Group I elements can be trapped on a quartz surface at relatively high temperatures (up to 1200° C). Cara has measured holdup times of several elements (both stable and radioactive) at different temperatures for a variety of materials (quartz, sapphire, tantalum) and geometries, including "straight-thru" tubes and "blocked" tubes where the average number of interactions with the wall is much higher.

At the HPTL, we have recently completed an on-line measurement using a proton beam from ORIC to irradiate a niobium silicide target for the production of ²⁵Al and ²⁶Al. The normalized yields were lower than we had previously measured from a SiC fiber target. This is partly explained by the high density (4.2 g/cm³) and low porosity of the niobium silicide targets. We will produce another set of niobium silicide targets that are more suitable for the release of short-lived isotopes. Further investigations are needed to determine the best targets for these aluminum beams. To this end, we have purchased a couple square meters of silicon carbide cloth woven from fibers with a nominal diameter of 10 microns. These materials are certified to have very low oxygen content, which is important since silicon oxides are quite volatile at the HRIBF ion source operating temperatures.

RA2. Accelerator System Status

ORIC Operations and Development (B. A. Tatum)

Due to the July 2008 Operational Emergency, ORIC remained shut down for most of the January--to-July period as various corrective actions were implemented. As part of the phased restart of the facility, we received permission in late winter to deliver light ion beam to non-uranium targets at the High Power Target Laboratory (HPTL). A nominal 54-MeV proton beam was subsequently delivered to a niobium silicide target at the HPTL for the production of ²⁵Al and ²⁶Al beams. Details of this beam development activity are provided in this Newsletter by Dan Stracener on RIB Development.

Permission to resume full facility operations including proton beam on uranium carbide targets was granted in late June following the completion of all necessary corrective actions related to the Operational Emergency. As mentioned by Jim Beene in his HRIBF Update and Near-Term Schedule, starting ORIC after many months of shutdown is difficult. We have been systematically restoring the machine to an operational status and have had a few setbacks along the way. Examples of difficulties include mechanical drive system vacuum seals which become rigid when not exercised, and current power supplies that have not operated at high current levels for an extended period. Nevertheless, we are anticipating a resumption of routine operation for the proton- and neutron-rich experimental programs during the remainder of the summer and into the fall.

We have been awarded $2.5M in American Recovery and Reinvestment Act (ARRA/stimulus) funds to be utilized over the next four years. One of the major uses of these funds will be the replacement of the 1938-vintage ORIC main magnetic field motor-generator set which provides up to 5000Adc at 360V to the main magnet coils. The MG set will be replaced with a dc power supply. The present plan is to retain the MG set as a backup. Improvements will also be made to the ORIC extraction and rf systems, the 1960's vintage diffusion pumps will be replaced, high-current beam stops will be installed, HVAC stack monitors will be implemented, and the IRIS1 target handling system electronics will be upgraded. These improvements combined with base capital equipment and Accelerator Improvement Project funds will substantially modernize ORIC and lead to higher reliability.

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

During the period from January 1, 2009 to June 30, 2009, the 25 MV Tandem Electrostatic Accelerator delivered beams of

510 kpps [ 6.88 MV 2+ terminal gas stripped ] 18-MeV ^26gAl and

1.5 Mpps [ 9.9 MV 3+ terminal gas stripped ] 36-MeV ^26gAl to Beam Line 41,

16 Mpps [ 21.47 MV 5+ terminal gas stripped ] 117-MeV ^26gAl to the ORRUBArray in Beam Line 21,

630 kpps [ 6.12 MV 1+ terminal gas stripped ] 8-MeV ⁷ Be to the Beam Line 22 tribology deposition chamber,

70 kpps [ 10.04 MV 2+ / 4+ terminal gas & foil / post foil stripped ] 23-MeV 100% ⁷Be to Beam Line 41,

10 Mpps [ 24.65 MV 4+ / 4+ terminal gas & foil / post foil stripped ] 107-MeV 99% ¹⁰Be to the ORRUBArray in Beam Line 21, and

27 Mpps [ 12.25 MV 2+ / 4+ terminal gas & foil / post foil stripped ] 29-MeV 99% ¹⁰Be to Beam Line 41.

Other beams of ^26gAl from 13 to 40 MeV were also delivered to Beam Line 41.

All of these beams were produced with the multisample cesium sputter negative ion source using pressed powder copper targets loaded with ^26gAl₂O₃, ⁷BeO, or ¹⁰BeO as appropriate.

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 1.34 to 24.65 MV. The multi-sample ion source was used on IRIS1 to provide the radioactive beams of ^7,10Be and ²⁶Al. In addition, the SNICS was used to provide the stable beams ¹H, ⁷Li, ^17,18O, ²⁷Al, ⁵⁴Fe, ⁵⁸Ni, ⁷⁶Ge, ¹⁰⁷Ag, ¹¹⁵In, and ¹²⁰Sn. About 64 hours were spent conditioning, most of which was necessary after a spark at 24.65 MV caused deconditioning. Three tank openings were completed during this period; the first to repair the foil stripper which had stuck in place, the second to do regular maintenance and to repair the low intensity diagnostics in preparation for radioactive beams, and the last to repair the gas stripper. Quite a few hours during this period were spent on SNICS development, leading to a greater understanding of the operation of the source.

RA3. Experimental Equipment - The Oak Ridge Isomer Spectrometer and Separator (ORISS)
[A. Piechaczek & K. Carter (UNIRIB), spokespersons]

In 2003, the UNIRIB Consortium set out to develop a universal means for providing high-purity radioactive beams for nuclear decay spectroscopy. High intensity backgrounds of adjacent nuclei are the principal impediment in many decay studies of such exotic nuclei. Many techniques exist for purifying RIBS, including laser ionization, molecular sideband, ranging out and others. Magnetic isobar separators are also frequently used.

The method chosen by UNIRIB was an electrostatic longitudinally dispersive separator based on the multi-turn time-of-flight technique [Cas01, Ish04]. The UNIRIB goal was initially a mass resolution of 15,000 as a separator. The technique was universal, applying equally well to all elements, and the goal of 15,000, which was beyond anything achieved at that time, is sufficient to significantly reduce other members of the mass chain.

Since that time the UNIRIB Consortium has advanced three separate technologies which when combined into one instrument will far exceed the original goal of a mass resolution of 15,000. Our instrument, ORISS, will consist of three components:

A multi-turn time-of flight spectrometer with an experimentally demonstrated mass resolving power of 110,000 [Pie08],

An electrostatic gate of the Bradbury-Nielsen type [Bra36] with locally developed fast pulser electronics [Gri09] to physically separate individual isobars,

A radiofrequency quadrupole (RFQ) cooler/buncher for cooling and injection of ions into the time-of-flight spectrometer [Koz09,Shc09].

Using funding from a recent DOE grant, the assembly and remaining construction will be completed. Off-line tests of ORISS should be performed within one year. ORISS will be initially commissioned and used for experiments at the online isotope separator, UNISOR, in 2011. A picture of the existing time-of-flight spectrometer is shown in Fig. RA3-1. A draft of the layout for ORISS at UNISOR is shown in Fig. RA3-2.

Figure RA3-1: View of the main component of ORISS, the multi-turn time-of-flight mass spectrometer in its vacuum chamber.

Figure RA3-2: Conceptual layout of ORISS at the UNISOR on-line test facility, where commissioning and first experiments will take place.

Experimentally determined performance data from the RFQ, the time-of-flight spectrometer, and the Bradbury-Nielsen gate together with calculations from an ion optics code we developed [Shc05] allowed us to predict the performance of ORISS when completed. We expect to obtain:

Mass resolving power of 400,000 full-width at half-maximum (consequence of ion cooling) as a mass spectrometer.

Capability to produce pure beams of nuclei with a relative mass difference M/ΔM < 200,000 as a mass separator.

Transmission of 50% from DC input beam to separated beam.

ORISS will provide pure beams of any isobar thus satisfying the original goal. However, the significantly improved performance of 400,000 provides a "quantum leap" in capabilities for decay spectroscopy by adding the ability to produce pure beams of isomers with a relative mass difference M/ΔM < 200,000 (corresponding to an excitation energy of an isomeric state > 470 keV in a mass A = 100 nucleus). For isomeric states with relative mass differences M/ΔM > 200,000, ORISS will be able to produce enriched beams. ORISS will provide unparalleled capabilities for nuclear structure experiments such as:

Decay experiments with pure sources,

Separation of isomers and study of their decays, and

Fast and efficient search for isomers in spectrometer mode.

In order to demonstrate the power of ORISS, we show in Fig. RA3-3 a simulated time-of-flight spectrum with a mass resolving power full-width at half-maximum of ~ 400,000 for the mass A = 77 isobaric chain. The full width at half-maximum of the time-of-flight peaks is 14 ns. All isobars and the 772-keV isomer in ⁷⁷Zn are clearly resolved in the spectrum. Pure beams of these nuclides can be produced using the Bradbury Nielsen time gate which can be switched from "open" to "closed" within 10 - 15 ns. The gate will be opened for ~30 ns when the desired nuclide arrives at the gate, and this nuclide will be transmitted to a detector station located further down-beam. All other nuclides arrive at the gate when it is closed, and they will be laterally deflected into a beam dump area. The isomeric state in ⁷⁷Ge with an energy 160 keV would not be resolved in the time-of-flight spectrum but could be identified since the resulting sum peak of ^77mGe and ⁷⁷Ge (ground state) has a larger width (20 ns) than the ground-state peak alone (14 ns).

Figure RA3-3: Simulated time-of-flight spectrum for A = 77 isobars in ORISS after 300 laps, a time-of-flight of 16 ms. Energy differences [MeV] relative to ⁷⁷Se (ground state) are indicated. The mass resolving power amounts to 430,000 full-width at half-maximum.

References:

[Bra36] Absolute Values of the Electron Mobility in Hydrogen, N.E. Bradbury and R.A.Nielsen, Phys. Rev. 49 (1936) 388.
[Cas01] A multi-reflection time-of-flight mass spectrometer for in-situ measurement on acomet core, A. Casares, PhD thesis, Justus-Liebig-University Giessen (2001) unpublished.
[Gri09] Gate driver for a Bradbury Nielsen gate, B.O. Griffith, to be published, Nucl. Instr. Meth. B.
[Ish04] A time-of-flight mass spectrometer to resolve isobars, Y. Ishida et al., Nucl. Instr. and Meth. B 219-220 (2004) 468-472.
[Koz09] Study of Ion Cooling and Ejection from Two Stage Linear Quadrupole Ion Trap consisting of RFQ ion guides, V.I. Kozlovskiy et al., To be presented at the 18th International Mass Spectrometry Conference, Bremen, Germany, Aug. 30 - Sep. 04, 2009. See http://www.imsc-bremen-2009.de
[Pie08] Development of a high resolution isobar separator for study of exotic decays, A. Piechaczek et al.,, Nucl. Instr. and Meth. B 266 (2008) 4510-4514.
[Shc05] Non-Linear Beam Dynamics In High Resolution Multi-Pass Time Of Flight Mass separator*, V.A. Shchepunov, H. Wollnik, Proceedings of Particle Accelerator Conference, Knoxville, TN, May 2005, P. 4105.
[Shc09] Design of an RFQ Interface for the UNIRIB ORISS high resolution separator, V. A. Shchepunov and V. I. Kozlovskiy, to be published in Nuclear Instr. and Meth. B.

RA4. Users Group News
(C. J. Gross, HRIBF User Liaison)

The HRIBF will hold its annual Users Group Meeting in Oak Ridge this year. The meeting is planned for November 13-14, 2009; more details may be found in Feature article 10 of this newsletter. This expanded Users Group meeting is expected to become an annual event which may supplant the usual meetings carried out at the annual fall DNP meetings. The previous meetings, held jointly with ATLAS, NSCL, GAMMASPHERE/GRETINA, and RIA Users groups, will most likely be reformatted now that we are in the FRIB era. At this year's DNP, it is expected that there will be a short informational-type presentation. More details will be provided by email.

One targeted user workshop will be held November 12, 2009, immediately before the annual meeting. The workshop will be on science applications and cross-disciplinary fields at HRIBF. It is hoped that this workshop will be held at the same location (Pollard Auditorium on the ORAU campus) although there may be a conflict. Users should contact Alfredo Galindo-Uribarri for more information.

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.

• 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 (T_1/2 > 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.

RA6. HRIBF Experiments, January through July 2009
(M. R. Lay)