HRIBF Newsletter, Edition 16, No. 1, Feb. 2008

Feature Articles

1. HRIBF Update and Near-Term Schedule
2. Recent HRIBF Research - Fusion of Mass-134 Isobars (Sn, Sb and Te) with ⁶⁴Ni - First Results
3. Recent HRIBF Research - Coupled-Cluster Approach to Nuclear Structure
4. What's New at HRIBF - A New SNICS II (Source of Negative Ions by Cesium Sputtering) Ion Source to be Installed on the Stable Injector.
5. PAC-14 Results and PAC-15
6. JIHIR Dormitory Is Available for Users at No Cost
7. New Results from Laser Ion Source Development at HRIBF
8. Update on Injector for Radioactive Ion Species 2 (IRIS2)
9. HRIBF and the GRETINA Workshop held in October, 2007
10. eRIBs'07 Workshop held in October 2007
11. The 2nd LACM-EFES-JUSTIPEN Workshop Held in January 2008
12. Twenty-Fifth Year Celebration of the JIHIR and JIHIR Expansion

Regular Articles

RA1.  RIB Development

RA2.  Accelerator Systems Status

RA3.  Experimental Equipment - A Rotating Target at the RMS

RA4.  Users Group News

RA4.  Suggestions Welcome for New Beam Development

RA5.  HRIBF Experiments, July - December 2007

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

Fiscal year 2007 (October 2006 thru September 2007) was, in many respects, the most successful year for facility operation in the history of HRIBF. We logged 1952 hours of ISOL RIB on target for experiments out of 3648 total research hours. Out of the 1696 research hours that were not classified as ISOL RIB hours, 278 hours were "in-flight RIBS" (stable beam decay spectroscopy experiments using the RMS to select and propagate beams of radioactive species for implantation decay studies at the focal place of the RMS). Another 837 hours were stable beams provided in direct support of RIB experiments (setup tests, calibrations, etc.). Only 255 hours were devoted to stand-alone stable-beam experiments. These excellent results were achieved while keeping the IRIS2 project on schedule, and in spite of more than a month of facility shut down due to budgetary restraints and uncertainty. These are remarkable achievements for a staff under budgetary pressure, and stretched very thin. They reflect significant and ongoing improvements in facility reliability.

The period between the last newsletter and the end of the fiscal year was occupied with a very successful campaign of neutron-rich species that produced several important physics results. This was followed by a tandem tank opening in mid October, primarily to repair the gas stripper. Since that time we have been engaged in selected stable beam experiments and an extremely successful radioactive fluorine beam campaign. We plan to continue the fluorine campaign until March when we will begin a major (8 week) scheduled shutdown for maintenance. Immediately after the shutdown we plan a campaign of long-lived (batch-mode) beams (7,10Be, 26Al). If we were funded at the level of the Presidents request for FY2008, this would be followed in June by a neutron-rich campaign which would run into FY2009. Unfortunately we have recently learned that our FY2008 budget will be almost 5% below the President's request. This, coupled with increases in the cost of doing business at ORNL, will impact our ability to operate for the full year. Our initial analysis suggests a shutdown on the order of two months will be required.

The HRIBF upgrade program outlined in our Integrated Strategic Plan is now well under way and going very well. The IRIS2 is on track for completion in FY2009 (the completion date delayed by transfer of funding to later years in the last two DOE budgets). The effect of our upgrades is already being seen in improved facility performance, and we expect IRIS2 to have a further significant impact on facility efficiency and reliability. As the next step in our upgrade strategy, we will propose to add a new driver accelerator, a high power (>100kW) electron accelerator to produce neutron rich species by photo-fission in actinide targets. If such an accelerator and associated production facilities were added to HRIBF, it would increase the intensity of our most neutron-rich beams by several orders of magnitude and ensure our relevance as a RIB facility at least until a next generation U. S. rare isotope facility is fully operational. This proposal has been favorably reviewed by our Science Policy Committee and has been discussed at several workshops. The latest forum was the eRIB07 Workshop organized jointly by HRIBF and TRIUMF scientists at Newport News, Virginia in October 2007.

2. Recent HRIBF Research - Fusion of Mass-134 Isobars (Sn, Sb and Te) with ⁶⁴Ni - First Results
(D. Shapira, spokesperson)

In the Feb. 2005 newsletter we reported our first attempt to measure the cross section of evaporation-residue production near and below the barrier in ¹³⁴Sn+⁶⁴Ni. At that time we were able to present the cross section of evaporation-residue production in ¹³⁴Te+⁶⁴Ni. In this article we present the data of combined evaporation residue production resulting from collisions between a mixture of A=134 isobars and a ⁶⁴Ni target.

By repeating the same measurement with different mixtures of the three main isobars in the A=134 beam, we were able to extract cross sections for evaporation residue production from ¹³⁴Sn+⁶⁴Ni, ¹³⁴Sb+⁶⁴Ni and ¹³⁴Te+⁶⁴Ni collisions. The unfolding was made possible by our unique experimental setup (see Fig. 1 in the Feb. 2005 newsletter and Ref. [1]), in which we sample the beam continuously throughout the experiment during which we measure the production of evaporation residues.

Figure 2-1: The mixture of isobars in the A=134 beam as observed in two different runs.

Fig. 2-1 shows the isobar mixture from two separate runs. Since we measure near and below the barrier of the ¹³⁴Sn+⁶⁴Ni collision, presence of ¹³⁴Ba in the mixture does not contribute to evaporation residue production and our main concern is the contributions from the Te,Sb and Sn isobars. Fig.2-2 presents cross sections for evaporation residue production of the three main components in our A=134 isobar mixture. The presentation of these data is preliminary. Because of the paucity of ¹³⁴Sn+⁶⁴Ni and ¹³⁴Sb+⁶⁴Ni data we could not apply our customary correction of the energy scale that takes account of the steep decline in cross section in the energy interval sample by the beam in the thick target. The data are presented with the beam energy fixed at the half-way point inside the target which is probably too high. We plan to take more data points along the excitation function which will enable us to apply similar correction as outlined in our publication of the ¹³²Sn+⁶⁴Ni data [2].

Figure 2-2: Cross sections of evaporation-residue productions in ¹³⁴Te+⁶⁴Ni, ¹³⁴Sb+⁶⁴Ni and ¹³⁴Sn+⁶⁴Ni reactions. Vertical bars indicate the locations of the Coulomb barrier in each system.

[1] Shapira et al. Nucl. Instrum. Methods Phys. Res. A551, 330 (2005).
[2] F.J. Liang et al. Phys. Rev. C75, 054607 (2007).

3. Recent HRIBF Research - Coupled-Cluster Approach to Nuclear Structure
(G. Hagen, spokesperson)

One of the major aims in the nuclear structure and reaction community today, is to understand the nuclear properties from the basic interactions among protons and neutrons. This effort has been labeled the "ab-initio" approach to nuclear structure and reactions. The "ab-initio" approach aims for a theory that is capable of not only explaining experimental data, but also making predictions, and therefore providing guidance for future experimental setups. In the nuclear structure/reaction context, this approach involves treating the nucleus as a many-body quantum system. The quantum many-body problem is a difficult undertaking. Today there exist several theoretical methods capable of virtual exact solution of the nuclear Hamiltonian in the lightest region of the nuclear chart (A<=12); these include the Faddeev [Nog00], Hyperspherical Harmonics [Bar99], No-Core shell-model [Nav00] and Green's function Monte-Carlo [Piep01] approaches. Due to the combinatorial or exponential scaling, these methods are limited to the lightest region of the nuclear chart.

Scientists are exploring different methods to extend the "ab-initio" program to medium-mass nuclei. Coupled-cluster theory is a very promising candidate for this purpose. Recently Coupled-cluster theory has seen a renaissance in nuclear structure. Coupled-cluster theory originated in nuclear theory, and was pioneered by Coester and Kummel in the late 50's Ref.[Coe60]. In quantum chemistry there was a parallel development of Coupled-cluster theory, and today it defines the state-of-the-art many-body theory in the quantum chemistry community, and a recent review of Coupled-cluster theory can be found in Ref.[Bar07]. Coupled-cluster theory is an ideal compromise between computational cost on the one hand and accuracy on the other hand. It brings in correlation in a very economical way when compared to other "ab-initio" methods. It has a polynomial scaling with system size, favoring it over methods with exponential or combinatorial scaling. Coupled-cluster is also capable of systematic improvements and recovers the exact wave function in the full limit. Coupled-cluster theory maintains the very important feature of size-extensivity: the energy of the system scales correctly with number of particles in the system regardless of the order of approximation made. This is crucial property must be a component of any "ab-initio" nuclear structure effort that moves into heavier regions of the nuclear chart.

"Ab-initio" calculations of light nuclei, starting from Hamiltonians with two-body forces only have shown consistent failure to meet experimental mass values. These calculations have therefore revealed the need for three-nucleon-forces (3NF'S) in order to account for this systematic discrepancy. The existence of 3NF's is not surprising since nucleons are not elementary point particles. A theory starting from nucleon degrees of freedom is therefore an effective theory where internal degrees of freedom (quark and gluon) are integrated out. The relevant low-energy degrees of freedom are given by a cutoff or resolution scale at which properties of the system are resolved and probed. The higher the resolution scale the more details of the inner structure is revealed. The removal of degrees of freedom by a cutoff at a given energy scale, has to be compensated by additional many-body forces in order to recover the richness of the system where all degrees of freedom are taken into account. The hope is that two- and three-body forces will be sufficient to approximately renormalize the nuclear many-body problem in a range of energy cutoffs. The modern understanding is that there are no unique 3NF, all nucleon-nucleon forces have their associated cutoffs, and therefore have to be accompanied with their own 3NF. A frontier in nuclear structure concerns how one can consistently relate 3NF's to a given realistic nucleon-nucleon force. A systematic way of relating low-energy nuclear physics to QCD through Chiral Effective Field Theory (EFT), was recently developed. Chiral EFT starts from an effective Lagrangian consistent with the symmetries of QCD. The relevant low-energy degrees of freedom of Chiral EFT are the nucleons and pions, all other degrees of freedom are integrated out of the theory. Expanding the nuclear amplitude in powers of a typical nucleon momentum or pion mass over the chiral symmetry break down scale (~1 GeV) a perturbative series is obtained, where NN forces, 3NF's and forces of higher rank appear systematically at a given order. At each order in the theory there are a finite number of diagrams determined by one- and two-pion exchange terms and contact terms. This approach further accounts for the natural hierarchy of forces, i.e. NN > NNN > NNNN ... In Ref.[Hag01] we performed large scale coupled-cluster calculations of the ground state energies of ⁴He, ¹⁶O and ⁴⁰Ca using a Hamiltonian with a renormalized two-body force of the low-momentum type (V-lowk). Our results were reasonably well converged with respect to the basis size, and we estimated that the ⁴⁰Ca ground-state energy were converged within 1% of the exact result. The calculated ground-state energy of ¹⁶O ( -148.2 MeV) and ⁴⁰Ca (-502.9 MeV) were largely overbound when compared to the experimental mass values of -127.6 MeV and -342.1 MeV, respectively. This is not surprising since we did not include the corresponding 3NF's which should accompany the two-body interaction we used.

One of our major aims in the coupled-cluster project is to investigate the role of 3NF's in medium-mass nuclei and in isotopic chains with extreme isospin asymmetry. Recently we developed and implemented coupled-cluster theory for three-body Hamiltonians [Hag1] and performed a benchmark calculation of the binding energy of ⁴He using a renormalized two-body interaction accompanied with a 3NF at NNLO in the Chiral EFT expansion. Our results were in excellent agreement with the numerical exact Faddeev-Yakubovsky calculation starting with the same Hamiltonian. We further found that the 3NF could be very well approximated by a density dependent zero-, one- and two-body term, see Fig. 3-1. This finding is very promising, since we can account for the full 3NF using well developed tools and machinery for two-body Hamiltonians. It remains to be seen whether this finding also holds in heavier nuclei.

Figure 3-1: Relative contributions ΔE/E to the binding energy of ⁴He at the CCSD level. The different points denote the contributions from (1) low-momentum NN interactions, (2) the vacuum expectation value of the 3NF, (3) the normal-ordered one-body Hamiltonian due to the 3NF, (4) the normal-ordered two-body Hamiltonian due to the 3NF, and (5) the residual 3NFs. The dotted line estimates the corrections due to omitted three-particle--three-hole clusters.

Another frontier in the nuclear structure and reaction community today concerns the theoretical understanding of structure properties and reaction mechanisms of nuclei located far away from the valley of beta-stability. At the limits of matter (neutron/proton drip lines), exotic features, which are not seen in the well-bound and stable nuclei, start to emerge, such as extreme matter clusterizations, melting and reorganizing of shell structure, ground states embedded in the continuum, and extreme dilute and extended matter densities. Another peculiar feature which appear in some of these exotic nuclei, is that the one-neutron decay threshold is above the two-neutron decay threshold. Some of these nuclei, like ⁶He and the cardinal case of ¹¹Li, have been labeled as Borromean nuclei. It is a great theoretical challenge to account for the properties of nuclei at the drip lines. In standard shell model approaches, the nuclear wave function is expanded in a finite set of harmonic oscillator states. While this approach works well for well-bound and stable nuclei, it is obvious that this description is not the appropriate description when moving towards the drip lines, where the nuclei become loosely bound and even unbound in their ground states. The proximity of the scattering continuum in these systems, directly relates to the exotic properties observed in these nuclei. As the outermost nucleons approach the scattering thresholds, the tail of their wave functions extend far out in radial space and therefore accounts for the spatially dilute matter distributions or halo densities observed in some of these nuclei.

A very promising way to account for these properties is by expanding the wave function in a Berggren basis [Berg68]. The Berggren basis is a generalized single-particle basis where bound-, resonant,- and continuum states are treated on equal footing. A representation of the many-body wave function in such a basis allows for description of both halo-densities of loosely-bound nuclei and calculation of lifetimes and decay widths of unbound nuclear states. This approach has been applied with great success in shell model calcuations here at Oak Ridge in W. Nazarewich group, led by J. Rotureau [Rot06] and N. Michel [Mic02] . In Ref.[Hag3] we applied a Berggren basis within the Coupled-cluster framework for the first time, and calculated masses and lifetimes of the Helium chain (^3-10He). This was the first "ab-initio" calculation of lifetimes of a whole isotopic chain. The results are summarized in Fig.3-2. The black dotted line gives our calculated masses, while the red dotted line gives the experimental mass values. The inset gives our calculated widths of the helium isotopes compared with experimental values. This figure shows that our results are in semi-quantitative agreement with experiment. With this interaction, all helium isotopes lack binding compared to experiment. However, the even-odd mass pattern is reproduced fairly well. We see that ⁵He is unstable with respect to one-neutron emission, while ⁶He is stable towards one-neutron emission. However, ⁶He is not stable towards two-neutron emission. This is mainly due to the missing three-body forces and inclusion of full triples in our calculation. ⁸He is stable towards one-, two-, and three-neutron emissions but not stable against the emission of four neutrons to the continuum and ⁴He. We believe that the growing discrepancy between theory and experimental mass values as we move along the helium chain is due to the lack of 3NF's. But for larger systems, triples corrections should play a more prominent role as well. By combining both of these missing ingredients, we believe that our results should be closer to the experimental values.

Figure 3-2: CCSD results (black dotted line) and experimental values (red dotted line) for the ground state of the helium chain ^3-10He using a Gamow-HF basis and a low-momentum interaction generated from the N³LO interaction model.

In summary, we are now in a position where we can answer questions in nuclear structure and reactions, which could previously not be addressed. In the near future we are going to explore the role of 3NF's in medium size nuclei. We will look at saturation properties of modern realistic forces in medium mass nuclei. We will implement and derive the Equation-of-Motion CCSD method, so that we can study excited states and properties of closed-shell nuclei and their neighboring nuclei. A particularly interesting project is to perform an "ab-initio" Coupled-cluster calculation of halo nuclei, and for the first time give "ab-initio" predictions of the drip lines. We are also aiming at merging of structure and reaction theory within the Coupled-cluster framework.

[Nog00] Phys. Rev. Lett. 85, 944 (2000).
[Bar99] Phys. Rev. C 61, 054001 (2000).
[Piep01] Ann. Rev. Nucl. Part. Sci. 51, 53 (2001).
[Nav00] Phys. Rev. C 62, 054311 (2000).
[Coe60] Nucl. Phys. 17, 477 (1960).
[Bar07] Rev. Mod. Phys. 79, 291 (2007).
[Hag1] Phys. Rev. C 76 044305 (2007).
[Hag2] Phys. Rev. C 76 034302 (2007).
[Berg68] Nucl. Phys. A 109, 265 (1986).
[Rot06] Phys. Rev. Lett. 97, 110603 (2006).
[Mic02] Phys. Rev. Lett. 89, 042502 (2002).
[Hag3] Phys. Lett. B 656 169 (2007).

4. What's New at HRIBF - A New SNICS II (Source of Negative Ions by Cesium Sputtering) Ion Source to be Installed on the Stable Injector.
(M. Meigs)

A new SNICS II (Source of Negative Ions by Cesium Sputtering) ion source has been purchased from National Electrostatics Corporation (NEC) to be installed on the stable injector. The source has been delivered in February and will be installed in the next long tank opening. This source is the 90^th single-cathode source that NEC has sold and they have 62 multi-cathode sources in use around the world. It is expected that the large user community will be a good resource when having problems or trying to develop beams. It is also expected that this source will provide higher beam currents than our present one. After the source has been installed, it will be the permanent stable ion beam source. Though we expect the SNICS II to be an improvement, there will be a learning curve to determine the exact parameters to optimize each beam.

A list of beams which will be available at the time of installation is presented below (all materials are of natural abundance unless specifically noted):

^1,2H, Li, ¹⁰B, C, N, ^16,17,18O,F, Mg, Al, Si, S, Cl, Ca, Ti, Fe, Ni, Cu, Ga, Ge, As, Se, Br, Zr, Mo, Ag, Sn, Te, I, Pt, and Au.

Contact Carl Gross if the beam you need is not on our list.

5. PAC-14 Results and PAC-15
(C. J. Gross)

PAC-14 met January 10-11, 2008, in Oak Ridge and considered 19 proposals and letters of intent which requested 219 shifts of ISOL radioactive beams (RIBs), 36 shifts of in-flight RIBs, 57 shifts of low intensity stable beams (SIB for RIBs), and 0 shifts of stable beams (SIBs). Of these, a total of 156 ISOL RIB shifts (18 non-ORIC ISOL), 36 in-flight RIB shifts, 57 SIB for RIB shifts, and 0 SIB shifts were approved.

Approved experiments requested RIBs of ²⁶Al, ^75-77,79-82Cu, ^84-87Ga, ^86,88As, ⁸⁷Br, ⁹⁷Ag, ^126,134Sn, and ¹³⁴Te. The total number of accepted proposals was 17; 11 of which were from outside organizations. We received 6 requests for our new beamline LeRIBSS (Low energy Radioactive Ion Beam Spectroscopy Station) and all were accepted so that a backlog of experiments for this type of experiment will be available.

Art Champagne, chair of the Users Executive Committee, represented the Users at the meeting. We anticipate PAC-15 to occur in late 2008 with proposals due in October.

The facility would like to remind experimenters that we have an on-site dormitory for your use during experiments. There is no cost for the rooms which have been renovated in 2006 and provide convenient 24-hour access to your experiment. It is located in building 6007 which is only a few hundred feet from your experiment. The dormitory includes a full kitchen, TV lounge with some cable TV channels, private bedrooms, and shared bathroom facilities. Bed linens are provided as are cooking and eating utensils. Wireless access through our visitor network is also available. Pictures of the dormitory are available on our website. Other HRIBF and Physics Division guests may also use the dormitory if space is available. See our guidelines and rules for more information.

7. New Results from Laser Ion Source Development at HRIBF
(Y. Liu, spokesperson)

Resonant ionization laser ion sources (RILIS) promise significant improvement over conventional ion sources in generating pure radioactive ion beams (RIB). We have made new progress in developing a RILIS for HRIBF.

A Ti:Sapphire laser complete with 2nd and 3rd harmonic units has been received and installed at the off-line ion source test facility 2 (ISTF-2). This laser is the first of three Ti:Sapphire lasers that we will need for a RILIS. It is manufactured by Photonics Industries International, Inc. and its prototype was first tested at ISTF-2 in 2006. Pumped by 18 W of pulsed 532 nm laser at 10 kHz from a Nd:YAG pump laser, it can delivery about 2 W fundamental peak power near 800 nm and more than 300 mW and 100 mW peak power in frequency doubled and tripled outputs, respectively. It is also continuously tunable from 700 nm to 960 nm, with typical linewidths of 1-3 GHz. Fig.7-1 shows a photo of the Ti:Sapphire laser together with the Nd:YAG pump laser.

Figure 7-1: The new Ti:Sapphire laser and its pump laser. Also shown are two additional Ti:Sapphire lasers from Mainz University, which are also pumped by the Nd:YAG laser.

In a RILIS, a particular isotope can be selectively ionized by laser radiation via stepwise atomic resonant excitations followed by ionization in the last transition. In order to yield a useful RIB current, maximized ionization efficiency is required. The RILIS efficiency is often limited by insufficient laser power to saturate the last ionization step. It is thus important to find schemes that lead to ionizing an excited atom resonantly through an autoionization (AI) or Rydberg state, which are much more efficient than non-resonant transitions to the continuum. Therefore, an important focus is to develop the most efficient ionization schemes for a range of elements of interest.

Ionization schemes for four new elements - Co, Ho, Tb and Dy - have been investigated in recent experiments conducted at ISTF-2 in collaboration with the LARISSA group led by Klaus Wendt of the University of Mainz and, for the first time, the TRIUMF ISAC laser ion source group led by Jens Lassen. The experiments were performed with our hot-cavity laser ion source, our new Ti:Sapphire laser, and two additional Ti:Sapphire lasers with harmonic generation units from the University of Mainz, which are also shown in Fig.7-1. Since there were no known resonant ionization schemes for the four elements using Ti:Sapphire lasers, searches for high lying Rydberg and AI states in these elements were necessary. With the new Photonics Ti:Sapphire laser, we were able to study the atomic spectroscopy of each element over a wide wavelength range. The Photonics Ti:Sapphire laser's continuous tunability proved to be extremely useful for such studies. Consequently, many AI states were observed for each element, most of them for the first time. Three-photon resonant ionization through AI states was achieved for Co. For Ho, Tb, and Dy, two-photon and three-photon ionization schemes were studied. Although no Rydberg states that led to resonant ionization were observed, many AI states were found in all three actinides. Fig. 7-2 shows some of the resonant ionization schemes established for these elements. Analysis of the experimental atomic spectroscopic data is in progress.

Figure 7-2: Resonant ionization schemes for Co, Ho, Dy, and Tb.

The ionization efficiency for Co and Ho has been measured using liquid samples that contained a known amount of the neutral atoms. Using the three-photon resonant ionization schemes shown in Fig.7-2, the overall ionization efficiency was found to be more than 20% for Co and about 40% for Ho. The 40% efficiency for Ho is the highest RILIS efficiency ever reported for any element.

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

A remarkable amount of work has been completed on the IRIS2 Project since the last newsletter report in February 2007. The HVAC for the target room was installed in April and is designed to maintain the room at a temperature at 70° F and the relative humidity at 50%. Most of the long lead-time major technical equipment items have been fabricated, received, and installed including the high voltage platform structures, high voltage conduits, and injector beamline magnets.

National Electrostatics Corporation (NEC) delivered the injector beamline platform structure in late June and the instrumentation platform structure in late August. The injector beamline magnet system manufactured by Sigma Phi was subsequently received and installed on the beamline platform. This system consists of a 25 degree pre-separator magnet that diverts the beam from the existing low energy analysis beamline, and a first stage mass separator which is made up of two 60 degree sector magnets. Four high voltage conduits were successfully assembled, potted, and tested by the HRIBF staff. These conduits consist of an aluminum pipe surrounded by an 18.5" x 8.5" x 16" polyethylene extrusion. The gap between the pipe OD and the polyethylene ID was filled by an elaborate process of standing the conduits on end and pumping RTV through the gap from bottom to top. All of the conduits were tested to +/-200kV and installed in the 9.5' thick concrete shielding wall between the target and instrumentation rooms, surrounded and supported by polyethylene laminations. The instrumentation platform, including dual 50kW motor-generator sets, was successfully installed and tested. Re-installation of the HPTL instrument racks, cables, and utilities is underway. Fixed and movable localized shielding around the target ion source has been fabricated and installed as has the remote handling system crane extension. Design has been completed and fabrication is underway for the transport beamline spherical electrostatic deflectors.

Installation activities will continue to be the primary focus during the remainder of FY2008, but now that the major items are installed we are beginning to deal with a wide array of detailed and time consuming tasks that include alignment, beamline assembly, utilities installation, testing, and commissioning. Due to Federal budget constraints, the Project will receive only $1,400k rather than $1,835k in FY2008. This shortfall, combined with a $400k shortfall in FY2007 means that the project completion date will be moved from March 1, 2009 to approximately July 1, 2009. At this time, however, the IRIS2 Project is on schedule and on budget.

Figure 8-1: Photos taken in October 2007 showing IRIS2 work in progress.

9. HRIBF and the GRETINA Workshop held in October 2007
(C. -H. Yu and D. C. Radford)

A GRETINA/GRETA Physics Working Group workshop was held on October 14 and 15, 2007, at the University of Richmond immediately following the Fall DNP meeting in Newport News.

At the workshop, it was agreed that GRETINA will be assembled, tested, and commissioned at LBNL and then it will be rotated among the national laboratories ANL, ORNL and MSU. The workshop also discussed in detail the physics opportunities at each laboratory and unanimously agreed on GRETINA's first rotation cycle, which will be physics campaigns of approximately 6 months duration at each of the following labs and in the order of:

1. MSU
2. ORNL (contingent on IRIS-2 being fully functional)
3. ANL (contingent on CARIBU being fully functional)

In the letter that HRIBF submitted to the GRETINA Advisory Committee, it was emphasized that the combination of GRETINA with radioactive beams available at the HRIBF will allow the community to address critical questions at the frontier of nuclear structure and nuclear astrophysics in a unique and powerful way, and will significantly enhance the productivity of the HRIBF research program.

Details of the report from GRETINA/GRETA Workshop can be found at the GRETINA website.

10. eRIBs'07 Workshop held in October 2007
(A. Galindo-Uribarri)

The International Workshop eRIBs07 "Electron Drivers for Radioactive Ion Beams" was held on October 10th, 2007, at the Marriott Hotel, Newport News, VA. The purpose of the workshop was to explore the scientific and technological issues associated with the production of radioactive ion beams by photofission of actinides. The workshop was organized by users of the ORNL and TRIUMF RIB facilities and sponsored by the Joint Institute for Heavy Ion Research, ORNL and TRIUMF. The event was opened to all interested participants. The workshop was aimed to raise awareness of the impact that a facility at HRIBF based on photofission of actinides can bring to nuclear science and to discuss the associated technical challenges.

The workshop program consisted of four sessions, a lunch-hour poster session and a general discussion session at the end. The program, list of participants, list of posters and all the talks are available at the workshop's website. Overviews of the projects and on the physics opportunities in nuclear structure and nuclear astrophysics were given followed by presentations describing efforts being made in an international context. The technical challenges associated with the targets and sources required, as well as the advantages of these machines were discussed.

Holding the workshop during the 2007 DNP Fall meeting of the APS allowed participation from a broad spectrum of members of the nuclear physics community including the local JLAB community, particularly the accelerator physicists. The workshop was well attended, with about 50 participants. It was recognized that science with neutron-rich fission fragment beams is the keystone of our research program and will continue to be. An electron-beam based facility can produce intense beams in a cost-effective way. Such a facility would be competitive world-wide for neutron-rich beams until FRIB-scale facilities are available. There is ample motivation and enhanced physics opportunities that such a facility can bring.

11. The 2nd LACM-EFES-JUSTIPEN Workshop Held in January 2008
(T. Papenbrock)

The 2nd LACM-EFES-JUSTIPEN Workshop was held on January 23-25, 2008 at the Joint Institute for Heavy Ion Research (JIHIR). As its predecessor, this meeting was a merger of the US-Japan theory meeting under the auspices of the Japan-US Theory Institute for Physics with Exotic Nuclei (JUSTIPEN) and the Annual NNSA-JIHIR Meeting on the nuclear large amplitude collective motion (LACM). It was jointly organized by the UT/ORNL theory group and the JUSTIPEN Governing Board.

The workshop consisted of 29 scientific talks, and its three days were dedicated to (i) supercomputing, (ii) gamma-ray spectroscopy and LACM, and (iii) nuclear structure, nuclear reactions and nuclear astrophysics. The late afternoon of each day saw a discussion session with focus on supercomputing in low-energy nuclear physics, gamma-ray spectroscopy, and JUSTIPEN, respectively. This gave the participants the opportunity to further exchange their views and ideas about future research directions and collaborations. Two of the highlights were the ceremony in celebration of the 25th anniversary of the JIHIR, and the tour of the supercomputers at ORNL and the visualization room EVEREST.

The workshop was attended by 78 physicists, including 21 Japanese researchers and a number of scientists from Europe. This is a considerable increase in participation compared to the first Joint JUSTIPEN_LACM Meeting and testifies to the popularity of the workshop and the interests in its themes. The workshop was preceded by a one-day DFT_UNEDF Meeting, and a considerable number of researchers attended both events.

The workshop was partially supported by the NNSA under the Stewardship Science Academic Alliance program "Theoretical Description of the Fission Process", the Joint Institute for Heavy Ion Research, the SciDAC Program UNEDF, and by ORNL. The Todai-RIKEN Joint International Program for Nuclear Physics (TORIJIN) and the JSPS Core-to-Core program "International Research Network for Exotic Femto Systems (EFES)" by the Japan Society for the Promotion of Science (JSPS) supported the travel of Japanese physicists. The talks presented at the meeting can be found on the meeting's website.

12. Twenty-Fifth Year Celebration of the JIHIR and JIHIR Expansion
(C. R. Bingham)

This year marks the 25th anniversary of the Joint Institute for Heavy Ion Research. It also marks a time of significant growth of the JIHIR building to accommodate the nuclear theory program in support of HRIBF, and in particular to provide a home for theory visitors to Oak Ridge as part of the Japan-U.S. Theory Institute for Physics with Exotic Nuclei (JUSTIPEN). On January 23 a number of friends of the JIHIR met in the auditorium of the JIHIR to recount and celebrate some of the accomplishments of the institute and to announce the eminent construction of a new eight-office addition to the building. This celebration was billed as a ceremonial session of the JUSTIPEN workshop meeting at the JIHIR on January 23-25, thus making it possible for many of the Japanese participants to join in the celebration of the groundbreaking event.

The JIHIR supports heavy-ion research at the Holifield Radioactive Ion Beam Facility. The Institute houses an intellectual center in the field of heavy-ion research, offering workshops, support for meetings and conferences, and physical accommodations to long- and short-term visiting scientists conducting research related to the Holifield facility. JIHIR operates under an agreement between The University of Tennessee, Oak Ridge National Laboratory, and Vanderbilt University, fulfilling goals of mutual benefit to sponsors and participants. The Institute includes a dormitory facility for use of people conducting research at the Holifield Facility and other short-term visitors.

Two of the founders of the JIHIR, Professors Joseph Hamilton from Vanderbilt and Lee Riedinger from UT Knoxville, were on hand for the celebration to revive some memories of the establishment of the first joint institute at ORNL, the dream of full cooperation of our university and laboratory scientists in the pursuit of forefront nuclear research, and some of the obstacles encountered in building a university building on DOE land. They also discussed many of the accomplishments of the JIHIR in the first 25 years of work and some of the lasting effects of those endeavors. The JIHIR is composed of two parts: the dormitory building, owned by ORNL, was built first with $239K from ORNL, $100K from UT and $100K from Vanderbilt, and the office part owned by UT was built with $350K from the State of Tennessee and $44K from UNISOR/ORAU to provide office space for the UNISOR (UNIRIB) collaboration. Soon after the second building was completed in 1984 the Science Alliance (a state-funded center of excellence at UT) was established, with the JIHIR being one of its cornerstones. The operating budget of the JIHIR has profited greatly from the continuing support of the Science Alliance (~$160K/year) and funds from the ORNL Physics Division and Vanderbilt University over the intervening years.

In his short talk, Lee Riedinger listed five areas of significant accomplishments of the JIHIR.

1. Support of visitors to work at HRIBF and on theory

Full or partial support is provided to approximately 100 guests each year. Guests conduct research in nuclear structure theory as part of the experimental program at the HRIBF; participate in cooperative experimental research at the HRIBF; help plan the experimental program at the HRIBF and other radioactive ion-beam accelerator facilities around the world; and work with the HRIBF, university, and UNIRIB research teams to upgrade, design, and construct complementary instrumentation housed in the accelerator facility. Over 25 years the JIHIR has hosted around 1200 different people, and some of these have participated via multiple visits. This is the most important activity of the JIHIR.

2. Development of Nuclear Structure Theory

One of the early goals of the JIHIR was to establish a strong nuclear theory group in support of Holifield-type experiments, especially in the realm of nuclear structure. The visitors program of the Institute filled the bill in attracting some of the world's leading structure theorists to Oak Ridge for both short- and long-term visits. Important theories relating to nuclei at extreme values of angular momentum and exotic nuclei with a proton/neutron ratio very different than for stable nuclei were developed through the visitors program. Best of all, one of our early long-term visitors, Witek Nazarewicz, was persuaded to stay here, becoming the root of the impressive nuclear structure group presently working in permanent positions at UT/ORNL.

3. Organization of Workshops and Conferences

A key activity of the JIHIR is to bring together people to discuss the most important new directions in heavy-ion physics. Our own conference rooms have been used routinely for small and intermediate sized workshops and conferences, many leading to the development of new collaborations to pursue research at HRIBF. We have organized or helped to organize other conferences to highlight the exciting results in the broad field of nuclear science, and these sometimes must meet in other places with better conference facilities. Many of the conferences are billed as international conferences; and because of the widespread interest in nuclear structure, even the small workshops often involve participants from several countries. In the last 25 years, the JIHIR has organized and/or sponsored over 100 conferences and workshops, attended by about 4500 people.

4. Development of experiments at HRIBF

The JIHIR has played a significant role in the development of the HRIBF into the successful radioactive ion beam facility that it is today. In the decade starting in 1984, the State of Tennessee, UT, and Vanderbilt provided around $1.6M for the construction of key pieces of detection equipment at Holifield. During the last decade less capital funds were delegated to this process, but through the JIHIR visitor program, many of the world's best experimental physicists have been brought to HRIBF to guide and work on new developing experimental equipment.

5. Development of people resources

An important outcome of the JIHIR visitors program has been to attract and help develop important programs in nuclear physics and astrophysics. Witek Nazarewicz and Anthony Mezzacappa are two prime examples of leading scientists aided in their formative years at UT and ORNL, but no doubt several other individuals have been helped to become established in their areas of expertise as permanent members of the science community revolving around the Holifield facility.

The newest major accomplishment of the JIHIR has been to obtain funding to expand the second building to include eight additional offices to house theory visitors. This new wing of the building will become the home of JUSTIPEN, providing a place for longer-term Japanese visitors to work while in Oak Ridge. The plans for the new wing have been completed and a contract for the building has been negotiated. The building is funded by a state appropriation of $250K and contributions of $137.5K from UT/UT Knoxville, $75K from ORNL, and $37.5K from Vanderbilt University. At the ceremonial session, Drs. Sidney Coon from DOE, David Dean from ORNL, and Takaharu Otsuka from Tokyo University discussed the importance of the JUSTIPEN collaboration in the development of physics of exotic nuclei in support of experimental programs in Riken and Oak Ridge.

The ceremonial session was concluded with congratulatory comments from Dr. Jan Simek, Interim Chancellor of UT Knoxville, Dr. David Milhorn, Executive Vice President of UT, and Dr. James Roberto, Deputy Director for Science and Technology at ORNL. Following the session, a lunch was served in the atrium of the Joint Institute for Computational Science. Dr. Roberto addressed the luncheon affirming the role of the JIHIR in providing a model to form the four new ORNL/UT joint institutes and gave a status report on the most important new developments at ORNL.

A more complete description of the work of the JIHIR can be found at the website of the Joint Institute.

Figure 12-1: Participants in the Ceremonial Session: Lee Riedinger, UT Knoxville; Jim Roberto, Deputy Director of Science and Technology, ORNL; Sidney Coon, Program Manager for Theory, Office of Nuclear Physics, Department of Energ; Joe Hamilton, Vanderbilt; Jan Simek, Interim Chancellor, UT Knoxvill; David Millhorn, Executive Vice President, UT; Carrol Bingham, UT Knoxville; Takaharu Otsuka, University of Tokyo and managing director of JUSTIPEN; Witek Nazarewicz, UT Knoxville/ORNL; and David Dean, ORNL and associate director of JUSTIPEN.

RA1. RIB Development
(D. W. Stracener)

After a short experiment in July to test a silicon carbide target, the HPTL facility was shutdown to allow for the installation of the high voltage platforms and conduits for the IRIS-2 facility. This silicon carbide target consisted of solid disks (about 1 mm thick) separated by gaps of about 4 mm to allow for radiation to the walls of the target holder. The simulations show that the production beam power can be significantly increased by allowing for heat dissipation via radiation when compared to the maximum beam power allowed when the dissipation of heat from the center of the targets is solely dependent on conduction through the silicon carbide disk. Early in 2008 we will be able to continue our target development at the HPTL and our initial tests will include a measurement of the release of radioactive aluminum isotopes from a niobium silicide target. We will also continue the investigation of silicon carbide targets to determine the maximum beam power that can be used on various target geometries. Our tests have shown that the silicon fiber targets have a higher release rate for aluminum as a function of beam power, but the silicon carbide disks can withstand up to ten times more production beam current, resulting in higher extracted beam currents of radioactive aluminum for experiments.

The "new" uranium carbide targets that were described in a previous Newsletter have performed quite well over the last few months and they have enabled the HRIBF to complete a good number of experiments with neutron-rich beams. These targets also helped the HRIBF achieve a record year in terms of the number of hours of radioactive beams delivered to experiments. During 2007 the production beam currents were gradually increased to the point where we now routinely use between 12 and 15 microAmps of 54-MeV protons on these uranium carbide targets. At these production beam currents, the useful lifetime of an ion source with a uranium carbide target is about 30 days.

As mentioned in the previous Newsletter, we made some initial tests of the release of fission fragments from a high-density (10.5 g/cm³) uranium carbide disk. The yields were lower than yields from low-density (2 g/cm³) targets by up to three orders of magnitude in some cases. We have recently received some uranium carbide samples that were produced using a similar technique but the densities are much lower at about 6 g/cm³ and in the near future we plan to test these targets with low production beam currents at the OLTF. Development of these high-density targets is a result of a collaboration between Will Talbert (TechSource), Jerry Nolen (ANL), and John Greene (ANL).

Another area of research at the OLTF has focused on the development of higher quality strontium beams, which are typically contaminated with rubidium isotopes. This effort is being led by Cara Jost, a student from Mainz. She is investigating ways to purify the strontium beams by looking at the formation and ionization efficiency of strontium oxide and strontium fluoride molecules and measuring the efficiency of formation of negative ion beams of strontium from these molecular beams passing through a Cs-vapor charge exchange cell. Another idea under investigation is the use of a selective adsorption technique where a quartz tube is used along a section of the transfer line between the target and the ion source. At some temperatures quartz will preferentially trap atoms of alkali metals (rubidium) while releasing atoms of other elements. This ion source coupled to a uranium carbide target will be tested on-line early in 2008.

RA2. Accelerator System Status

ORIC Operations and Development (B. A. Tatum)

ORIC's reliable operation during the reporting period was instrumental to the completion of the most successful operations year in HRIBF history. From July through mid October, ORIC provided 54 MeV protons to the High Power Target Laboratory (HPTL) for development of aluminum beams from SiC targets, and to the Injector for Radioactive Ion Species (IRIS1) for production of neutron-rich beams for the RIB experimental program. ORIC was shut down for much of the first quarter of FY2008 for budgetary reasons, machine maintenance, and the transition to deuteron beams that will be used for proton-rich RIB production in the second quarter. ORIC has continued to experience some chronic problems with the rf trimmer servos, a few older power supplies, and small vacuum leaks. Once the FY2008 budget is known, we plan to allocate capital equipment and AIP funds to order replacements for the 20" cryopump used on the south end of ORIC, the harmonic coil power supplies, the lower channel inside coil power supply, and to make additional rf improvements including procurement of a spare rf power amplifier tube.

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

During the period of 1 July 2007 to 31 December 2007, the 25-MV Tandem Electrostatic Accelerator delivered radioactive-ion beams of

10 Mpps [23.3 MV 16+ terminal foil stripped] 396 MeV 80% ¹³²Te to the general purpose endstation in Beam Line 21,

40 kpps [22.7 MV 15+/27+ terminal foil / high energy foil stripped] 545 MeV 95% ¹³²Sn,

62 kpps [22.7 MV 15+/26+ terminal foil / high energy foil stripped] 530 MeV 95% ¹³²Sn, and

65 kpps [22.4 MV 14+/26+ terminal foil / high energy foil stripped] 515 MeV 95% ¹³²Sn to the time-of-flight endstation in Beam Line 23, and

40 kpps [16.2 MV 11+ terminal foil stripped] 193 MeV 42% ⁸⁴Se,

1 Mpps [15.0 MV 11+ terminal foil stripped] 179 MeV 99% ⁷⁸Ge,

200 kpps [15.4 MV 11+ terminal foil stripped] 184 MeV 98% ⁸⁰Ge, and

20 kpps [22.7 MV 17+ terminal foil stripped] 408 MeV 90% ¹³⁶Te, to the Recoil Mass Spectrometer in Beam Line 14.

These beams were produced via proton induced fission of ²³⁸U by bombarding a pressed powder uranium carbide target coupled to an Electron Beam Plasma (positive) Ion Source (EBPIS) with 7.5 - 18 uA of 50 MeV ¹H. Additionally, the high purity germanium and tin beams were produced by passing a positive germanium or tin sulfide beam through the recirculating cesium jet charge exchange cell and selecting the negative germanium or tin beam resulting from molecular breakup.

Tandem Operations and Development (M. Meigs)

The Tandem Accelerator was operated for about 2450 hours since the last report. The machine ran at terminal potentials of 3.10 to 23.4 MV and the stable beams ¹H, ⁷Li, ¹⁵N, ¹⁷O, ¹⁹F, ⁵⁴Fe, ⁵⁸Ni, ⁷⁶Ge, ^76,78,80,82Se, ⁹⁰Zr, ⁹²Mo, ¹⁰⁷Ag, ^118,124Sn, and ¹³⁰Te were accelerated. Radioactive beams of ^78,80Ge, ⁸⁴Se, ^126,132Sn, and ^132,136Te accounted for more than 730 hours of beam on target. Conditioning was done for only 26 hours to return the machine to operation after having the terminal up to atmosphere to change foils. Only one tank opening was required during this period for scheduled maintenance.

RA3. Experimental Equipment - A Rotating Target at the RMS
(C. J. Gross for the Decay Spectroscopy group)

Recent software improvements to our digital data acquisition system has enabled us to use unique trigger requirements [1] in order to search for fast sequential decay events at the recoil mass spectrometer (RMS). These techniques have led to our discovery of superallowed alpha decay [2] in the ¹⁰⁹Xe-¹⁰⁵Te-¹⁰¹Sn decay chain. This new trigger requirement allows us to suppress uninteresting events while pursuing short (< 1 μs) half-life radioactivities. The suppression is so large, that we could increase our implantation rate by a factor of 3-5 and still maintain our detection capability and data throughput. Thus, we have recently developed a rotating target for use at the RMS.

Figure RA3-1: (left) The rotating target mounted in the target chamber of the RMS. A U. S. penny is shown for scale. (right) A ⁵⁸Ni target that was irradiated with 30+ pnA of ⁵⁸Ni. The photographs may be viewed separately by clcking on them.

Our design is unusual in that we have opted for a system completely enclosed in the vacuum chamber thereby eliminating mechanical (or magnetic fluid) feedthrough devices. After several iterations of motors (stepper motor, DC motors with various gear ratios) we have settled on a 24 VDC motor [3] with a gear ratio of 3.7:1. Although motors rated for vacuum use (low-vapor pressure lubricant) have been used, we have also tested non-vacuum-rated motors and both appear to work in the vacuum for several days. We selected DC motors due to their simplicity to control (a bench-top power supply is all that is needed) and their lack of producing excess heat (the stepper motor required copper cooling fins to radiate heat in order to stay below 50° C). There does not appear to be a similar build up of heat with the DC motors.

In addition, we do not use a wheel with multiple targets but rather a single target of roughly 14 mm diameter. The target moves in a 7-mm-diameter circle allowing plenty of space for our approximately 2-mm-diameter beam to avoid striking the frame and scattering primary beam into our detectors. Our system consists of: