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Plans for an Advanced Exotic Beam facility in the U.S.
Nuclear Physics A 787 (2007) 84c–93c
Plans for an Advanced Exotic Beam facility in the U.S. J.A. Nolena a
Physics Division, Argonne National Laboratory, 9700 South Cass Av., Argonne, IL 60439, USA The Office of Science of the Department of Energy is currently considering options for an advanced radioactive beam facility in the U.S. A committee, the Rare Isotope Science Assessment Committee, has been formed under the auspices of the Board of Physics and Astronomy of the National Academies of Science and Engineering with the charge of assessing the importance of and prospects for future research in this field world-wide. There is interest in identifying opportunities for a facility in the U.S. that will complement the capabilities that will exist elsewhere. There is an on-going R&D program sponsored by the DOE to develop the technologies that are essential to both the production of and use of beams of intense exotic beams at next-generation facilities. This paper presents the status of planning and options for such a facility, as well as, progress in the R&D. This program is developing concepts and prototypes for high power superconducting heavy ion linacs for rare isotope production and high power targets, ion sources, fragment separators, gas catchers, and reacceleration technologies that support the delivery of high quality, intense beams of exotic isotopes for research. 1. INTRODUCTION TO A ”RIA-LITE” CONCEPT The study of the properties of and reactions induced by short-lived isotopes is a key to our understanding of fundamental questions in nuclear physics, nuclear astrophysics and fundamental interactions at low-energy. This has long been recognized by the international nuclear physics community and is reflected by the investments in that field that have been or are being made in North America, Europe, and Asia. The US nuclear physics community addressed the importance of such studies in its 2002 Long Range Plan [1] where a Rare Isotope Accelerator Facility (RIA) was identified as its highest priority for new construction. The powerful combination of technologies that comprised the RIA concept were subsequently described in several papers [2,3,4] and much of the R&D towards realizing the facility was described in the references included in those publications. However, the estimated total project cost of the proposed RIA facility was $1.1B escalated to the planned years of construction, and this led to consideration of alternatives for how best to pursue science with radioactive beams. A committee, the Rare Isotope Science Assessment Committee, was formed under the auspices of the Board of Physics and Astronomy of the National Academies of Science and Engineering with the charge of assessing the importance of and prospects for future research in this field world-wide. The report of this committee is scheduled to be published in late October 2006. 0375-9474/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.nuclphysa.2006.12.018
J.A. Nolen / Nuclear Physics A 787 (2007) 84c–93c
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Figure 1. A simplified schematic layout of AEBL, a RIA-lite facility at Argonne.
Recently, the Department of Energy and the National Science Foundation have also charged the Nuclear Science Advisory Committee of the United States to carry out a study of the scientific capabilities and the technological feasibility of an advanced radioactive beam facility that could be constructed for approximately one half of the cost of the baseline RIA concept. The charge involves consideration of ”a modified RIA that focuses on capabilities which would make it unique in the world and would complement the rare isotope capabilities elsewhere.” The present plan is for completion of this study in early 2007 so that the recommendations can be provided as input to the upcoming long-range plan for nuclear physics in the United States which is due to be completed by the end of 2007.
Table 1 Beam list for selected ions from the AEBL Driver Linac with the onestripper option. The output currents indicated correspond to 400kW beam power for all beams. ECR Strip Output Beam Z A Source Qinj Energy Qstrip Frac Iout Energy pmicroA MeV/A pmicroA MeV/A 1 1 880 1 95.2 704.1 568 2 3 390 2 70.7 312.4 427 1 2 728 1 57.1 582.0 344 8 18 101 6 41.8 8.0 0.90 72.9 305 18 40 47 8 27.6 18.0 0.90 33.7 297 36 86 24 14 23.0 35.0 0.90 17.5 266 54 136 17 18 18.9 50.7 0.90 12.4 237 92 238 6+6 33+34 16.5 79.0 0.83 8.4 200
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Contingent upon the recommendations of these two committees and the United States Long-Range Plan for Nuclear Physics the tentative plan for a ”RIA-lite” facility could proceed with project engineering and design beginning in fiscal year 2011 with construction beginning soon after that. This schedule requires continued R&D on the technical components of the facility, a site selection, and the completion of a Conceptual Design Report in the coming years, prior to 2011. The following sections of this paper will address a possible technological approach and the associated R&D for the ”RIA-lite” facility. The goal is to design an advanced facility that in conjunction with other modern facilities worldwide will enable great advancements in this science. The resulting design will retain as many of the features of the baseline RIA concept as possible within the budget constraints, such as: • Use of a variety of production techniques for short-lived isotopes including new approaches based on gas catcher technology that remove the main existing limitations of standard ISOL techniques. • A powerful superconducting driver linac capable of accelerating any stable ion from protons to uranium and designed to allow simultaneous acceleration of multiple charge states of a given ion to attain unprecedented power for the heaviest ions. • A very efficient post-acceleration scheme based on a superconducting linac that delivers high quality radioactive beams starting from singly charged ions of mass up to 240 amu from ion source energy. • First rate experimental equipment to maximize the scientific output of the facility. 2.TECHNOLOGY DEVELOPMENT FOR AN ADVANCED EXOTIC BEAM FACILITY At this time concepts for a RIA-lite facility are being worked out at Argonne National Laboratory and at Michigan State University. A summary of recent plans was presented at the Erice summer school by Sherrill [5]. A simplified schematic layout of a RIA-lite facility on the Argonne site is shown in Fig. 1. It is referred to as the Advanced Exotic Beam Laboratory (AEBL). The driver linac option shown in the figure is the two-stripper version. However, there is also a one-stripper option being considered, corresponding to the beam list given in Table I. The main functional components and selected results from the R&D programs are described in the following sub-sections. 2.1. Heavy Ion Driver Accelerator The driver linac produces the primary high-power stable ion beam required to produce the radioisotopes. As in the baseline RIA concept, the facility will use a number of production mechanisms spanning the range from spallation on thick targets which requires high energy light ion beams (e.g. p, d, 3He), fragmentation which requires heavy-ion beams at high-intensity and energies in the 100’s of MeV per nucleon range for midmass nuclei, to in-flight fission which requires high intensity uranium beams at similar velocities. To be able to fully exploit the various production mechanisms it is, therefore, imperative that the driver linac be capable of accelerating essentially all stable nuclei to sufficiently high energies with high efficiency. The broad mass-to-charge ratio of the species to be accelerated together with the higher energy for lighter ions and high current
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Figure 2. Design drawing of a superconducting, triple-spoke resonator that is used at the high-energy end of the AEBL driver linac.
requirements dictate the use of a superconducting linac whose short independently phased cavities enable efficient acceleration over a wide velocity profile. The proposal for RIA-lite is to reduce the beam energies from the heavy ion driver linac by a factor of two relative to RIA while keeping the beam power at 400 kW by doubling the corresponding beam currents. A list of beam energies and intensities for selected ions from the driver is given in Table I. The rare isotope yields enabled by such a driver and the performance relative to the baseline RIA facility are given in section 3. The baseline RIA Driver Linac design and initial component prototyping were discussed in reference [6]. The baseline design comprises: (1) an advanced ECR ion source [7], (2) a 2-charge-state low-energy beam transport system and pre-buncher [8], (3) a low-frequency CW RFQ [9], (4) six classes of low to medium velocity superconducting resonators [6,10], (5) three classes of high-velocity compressed elliptical resonators [11, 12], or alternatively, two classes of triple-spoke resonators [13, 14], (6) two high-intensity charge-state strippers and associated multiple-charge-state beam transport sections [15, 16, 17, 18], and a (7) multiple-charge-state RF switcher and beam transport system for beam sharing [17, 19]. The design of the driver accelerator took a number of bold steps to meet the requirements of the RIA facility. A vigorous R&D program led to the development of the multiple charge state acceleration concept [20], its experimental demonstration at the ATLAS facility at ANL [21], the development of the spoke-cavity superconducting structures [13, 14] to fill the gap in the velocity regime between the low-β ATLAS-type cavities and the CEBAF-type velocity-of-light structures, and development of the SNS-type high-β cavities [11, 12] that will be used for the higher energy section of the linac. Successes with the R&D program for the baseline RIA Driver Linac as discussed above support the technological approach for the RIA-lite driver at one-half the beam energy and twice the beam current. For example, the high currents of medium charge-state bismuth beams achieved recently with the LBNL VENUS ECR ion source operating at 28 GHz [7] combined with the detailed studies of the multiple-charge-state operation of the driver with very low beam losses [20, 22] supports the goal of achieving 400-kW uranium beams at 200 MeV per nucleon with 8 particle microamperes of beam on target. The excellent
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Figure 3. A photograph of a liquid lithium film being developed for the stripper foil in the AEBL driver linac. The film velocity is 50 m/s and the thickness is 15 microns. The width is 6 mm.
beam dynamics associated with the very large longitudinal acceptances of the 345 MHz triple-spoke resonators and the successful operation of the prototypes of these resonators support a robust design of the high power driver. (Fig. 2 shows a CAD model of the beta=0.63 triple-spoke resonator and shows the Q-curves of the prototype are reported in [14].) The high power uranium beams in the one-stripper option requires stripping at a uranium beam energy of 17 MeV/u at a beam intensity of 10 particle microamperes. The development of high-velocity, windowless, thin-film, liquid-lithium stripper foils [16] is an ongoing project at Argonne [23]. Progress with this effort is indicated by the photograph of a 50 m/s liquid lithium film shown in Fig. 3. 2.2. Isotope Production complex The production of the radioisotopes will occur by interaction of the high-power primary beams on targets located in an enclosed production building. Because of the large inventories of radioactive materials that will be present remote handling capabilities and the required radiation control mechanisms will be in this building. The production of radioactive isotopes in this complex can occur through 4 main mechanisms, all of which are enabled by the versatile driver described above. The production area will house standard ISOL, two-step fission (actinide) targets, high power fragmentation targets such as liquid lithium, a large acceptance fragment separator and its associated high power beam dumps, and the gas catcher. The issues to be faced in the design of this area will be very similar to those that were required for the full RIA complex. Development of concepts for the RIA production areas has been the subject of a multi-laboratory collaboration under the auspices of the DOE Office of Nuclear Physics R&D program for the past three years. Some of the radiological and ISOL target design issues were addressed in the recent International Conference on Accelerator Applications 2005 [24, 25]. The special issues of contamination control associated with the use of actinide targets are common to all present and future ISOL-type radioactive beam facilities. A workshop to bring together the interested parties for discussion of these common challenges was hosted by TRIUMF recently [26]. The targets for in-flight fragmentation and fission of intense very heavy beams up to uranium present a major challenge. For the very high power densities produced by the
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Figure 4. CAD model and photograph of an upgraded prototype gas cell that has been equipped with RF electrodes on the body as well as on the extraction cone to increase the space charge limit of the system.
intense uranium beams with small beam spot diameters, a windowless liquid lithium concept has been developed and demonstrated in a prototype with a high power electron beam that simulates the power density encountered at RIA and RIA-lite [27, 28]. One of the features that makes RIA-lite globally competitive is its unique combination of high power uranium and other very heavy ion beams, large acceptance fragment separators, gas catcher technology, and a flexible and efficient post accelerator. Radioactive isotopes are produced by fragmentation (or in-flight fission) of fast heavy-ion beams on a ”thick” thin target. A large acceptance fragment separator is configured to collect, separate, and compensate for the fragment energy spread so that the rare isotopes are slowed down and stopped in a gas catcher system [29, 30], where they are thermalized but remain singly or doubly charged and can be extracted by a combination of gas flow, DC, and RF fields to be delivered to the stopped beam area or to the post accelerator. This results in beams of quality as good or better than those obtained by ISOL techniques, without the chemical limitations encountered by the ISOL technique in the diffusion and release out of thick, solid targets. A vigorous R&D program is ongoing to demonstrate and further improve gas catcher technology. A full-scale RIA prototype gas cell was constructed and evaluated at the Argonne ATLAS accelerator and then put on-line a successfully demonstrated with full energy secondary beams at the GSI fragment separator. Further development and characterization of gas cells at high intensities is underway at Argonne. Fig. 4 shows a CAD model and a photograph of the internal components of an upgraded small gas cell that is now in use at the ANL Canadian Penning Trap facility. To increase the space charge limit, this cell now has RF electrodes along the whole body rather than just in the extraction cone [31]. This cell was also adjusted to increase the efficiency for light isotopes such as 14 O. An alternative to the linear gas stopper geometry discussed above is an inverse gas-filled cyclotron device that is being pursued at RIKEN and MSU [32]. 2.3. Post-Accelerator The most efficient production mechanisms for slow radioactive ions yield these ions in the 1+ or 2+ charge states. The post-accelerator must therefore be able to accept such low charge-to-mass ratio ions from ion source energy. One approach to this problem is to
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Figure 5. Plot of the ratios of yields predicted for mass separated 1+ beams from the AEBL facility relative to those predicted for the baseline RIA facility. The ratios are generally between 10% and 100%. [42]
increase the charge state of the ions before acceleration via a charge booster stage, be it an ECR-type breeder [33] or an EBIS-based system [34]. In general, the breeders are more competitive with stripper methods for heavier isotopes. To ensure maximum efficiency in the post-acceleration process, especially for light isotopes, RIA-lite can use a very efficient low-charge-to-mass ratio injector based on a superconducting linac accelerator coupled to low frequency CW RFQs [35]. The singly charged radioactive ions extracted from the ion source are first mass separated by a large acceptance, high-resolution (m/Δm = 20,000) isobar separator [36]. The selected isotopes are then bunched by a multi-harmonic buncher and accelerated through a series of 3 low frequency RFQs, the first two located on an adjustable high-voltage platform. The first RFQ is a CW split-coax RFQ structure [37] recently developed at Argonne and capable of accelerating ions with mass-to-charge ratio as high as 240. The second and third RFQs use a novel hybrid structure [38] that mixes RFQ and IH-type sections to obtain higher accelerating gradient with still enough focusing force. The following low-beta superconducting linac section can accept ions of = 0.011 with q/m 1/66 so that ions with mass larger than 66 must be stripped. This is done in helium gas to achieve a high efficiency with essentially no emittance growth [39]. The stripped ions are then accelerated by low-beta inter-digital superconducting cavities up to an energy of at least 600 keV/u after which they are further stripped to q/m 0.15 for further acceleration in ATLAS type superconducting cavities. The beam is, therefore, available at all energies from ion source energy up to energies above 10 MeV/u for midmass ions. The acceptance of all parts of the linac being very large compared to typical emittance from ISOL sources, the transmission is also very high, limited by the bunching efficiency (about 85%) and the stripping efficiencies for the heavier beams. Multiple-
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Fr
AEBL 200 MeV/u 400 kW
80
Yields of mass separated + 1 ions for re-acceleration T-gas cell: 20 ms Proton Number Z
60 159
80
Nd Stable isotopes
Zr
>10 132,138
40
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6
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Li
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40
60
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Figure 6. Plot of the predicted yields for mass separated 1+ beams from the AEBL facility relative using the optimal production mechanism for each isotope [42].
charge-state acceleration can be used following the second stripper to further increase efficiency for experiments that are not sensitive to the resultant emittance growth [40]. The layout shown in Fig. 1 has both a low-charge-to-mass ratio injector and a charge breeder branch. The final configuration will depend on the cost constraints and relative priorities that are set for various facility capabilities. The ultimate combination is to use both types of injectors so that it is possible to simultaneously deliver light beams to the Astrophysics area and heavy beams to the Coulomb Barrier instruments. 2.4. Experimental Areas As was planned for RIA, RIA-lite will host four experimental areas for users, corresponding to the different energy regimes that are required to carry out the science program that is to be addressed at this facility. The four areas are indicated in Fig. 1, the Trap area, the Nuclear Astrophysics area, the Coulomb Barrier area, and the full-energy, in-flight area. Due to the lower target cost for RIA-lite, these areas, at least initially will be less extensive and less well equipped than was planned for the full RIA facility. Over the past few years there have been several workshop held for potential RIA users to participate in the planning of the layouts and instruments for the RIA experimental facilities. The most recent of these, jointly sponsored by the MSU/NSCL and the ANL Physics Division, was held in East Lansing, MI in March, 2004 [41]. As the plans for RIA-lite evolve, additional user workshops will be required to set priorities for equipping the experimental areas. 3. EXPECTED PERFORMANCE The proposed RIA-lite facility will provide yields of short-lived isotopes atunprecedented intensities. This is due to the high primary beam power, the flexibility in the
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primary beam selection which allows the best production mechanism to be used, new target technologies that allow the normal limitations of ISOL type systems to be overcome, and finally the use of a very efficient post-acceleration scheme. For the baseline RIA facility yields were calculated for the different reaction mechanisms and are available on the web [42]. For AEBL the predicted yields for beams of mass separated 1+ rare isotopes are in the range of approximately 10% to 100% of the full RIA yields because the yields are more dependent on the total beam power than on the beam energy. Below 200 MeV/u the yields fall rapidly due to decreasing effective target thicknesses, more charge state fractionation, and poorer kinematics for the collection of in-flight-produced fission products. The predicted ratios of AEBL to RIA yields are shown in Fig. 5 and the absolute yields for AEBL are shown in Fig. 6 [43]. Acknowledgements The RIA and RIA-lite concepts have evolved from ideas generated within the nuclear and accelerator physics communities worldwide. This work was supported by the US Department of Energy, Division of Nuclear Physics, under contract number W-31-109-ENG-38. REFERENCES 1. Opportunities in Nuclear Science, A Long-Range Plan for the Next Decade, Ed. J. Symons, et al., April 2002. 2. B.M. Sherrill, Nucl. Instr. Meth. in Phys. Res. B204 (2003) 765-770. 3. G. Savard, ibid., p. 771-779. 4. J.A. Nolen, Nucl. Phys. A734 (2004) 661-668. 5. B.M. Sherrill, INTERNATIONAL SCHOOL OF NUCLEAR PHYSICS, 28th COURSE, Radioactive Beams, Nuclear Dynamics and Astrophysics, ERICE-SICILY: 16 - 24 SEPTEMBER 2006. 6. K.W. Shepard, Proc. XXI Inter. Linear Accelerator Conference, Gyeongju, Korea, August 19-23, 2002, p. 598-601. 7. D. Leitner, C. M. Lyneis, T. Loew, D. S. Todd, and S. Virostek, Rev. Sci. Instrum. 77 (2006) 03A302,1-6. 8. P.N. Ostroumov, K.W. Shepard, V.N. Aseev, and A.A. Kolomiets, Proc. 20th LINAC Conference, August 21-25, 2000, Monterey, California, p. 202. 9. P.N. Ostroumov, et al., Phys. Rev. STAB 5, 060101 (2002), see also papers TU466 and TU467 at LINAC2002. 10. K.W. Shepard, M.P. Kelly, and J.D. Fuerst, Proc. XXI Inter. Linear Accelerator Conference, Gyeongju, Korea, August 19-23, 2002, p. 478-480. 11. I.E. Campisi, et al., Proc. XXI Inter. Linear Accelerator Conference, Gyeongju, Korea, August 19-23, 2002, p. 496-498. 12. W. HARTUNG, et al., Proc. 2003 Particle Accelerator Conference, Portland, OR, May 12-16, 2003, paper TPAB070. 13. K.W. Shepard, P. N. Ostroumov, and J. R. Delayen, Phys. Rev. ST Accel. Beams 6 (2003) 080101. 14. M.P. Kelly, Proceedings of the 39th ICFA Advanced Beam Dynamics Workshop ”High Intensity High Brightness Hadron Beams,” Tsukuba City, Japan, May 29-June 2, 2006.
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15. M. Portillo, et al., Proc. 2001 Particle Accelerator Conf., June 18-22, Chicago, IL, 2001, p. 3012. 16. X. Wu, et al., Proc. EPAC 2002, Paris, France, p. 1211. 17. J.A. Nolen, Proc. 2nd RIA Driver Workshop, Argonne National Laboratory, ArgDOE Office of Science Financial Assistance Program Notice LAB 05-22 entitled ”Research and Development for a Rare Isotope Accelerator”, May 22-24, 2002, copies available from the ANL Physics Division. 18. X. Wu, ”High symmetry stripping sections”, ibid. 19. X. Wu, ”Beam switch-yard”, ibid. 20. P.N. Ostroumov and K.W. Shepard, Phys. Rev. ST Accel. Beams 3 (2000) 030101. 21. P.N. Ostroumov et al., Phys. Rev. Lett. 86 (2001) 2798. 22. P.N. Ostroumov, Proc. Linac 2004, paper TH204. 23. C.B. Reed, 2006 R&D under the DOE Office of Science Financial Assistance Program Notice LAB 05-22. 24. I. Remec, et. al., Nucl. Instr. and Meth. in Phys. Res. A562 (2006) 896-899. 25. G. Bollen, et al., Nucl. Instr. and Meth. in Phys. Res. A562 (2006) 915-920. 26. First Workshop on Actinide Target Development, TRIUMF, Vancouver, B.C., Canada, April 27-29, 2006. 27. J.A. Nolen, et al., Nucl. Instr. Meth. in Phys. Res. B204 (2003) 293-297. 28. J.A. Nolen, et al., Rev. Sci. Instr. 76 (2005) 073501, p 1-6. 29. G. Savard, et al., Nucl. Instr. Meth. in Phys. Res. B204 (2003) 582-586. 30. G. Savard, Eur. Phys. J. A 25, supplement 1 (2005) 713-718 31. G. Savard, Workshop on the SPIRAL2 Super Separator Spectrograph, Paris, June 27-28, 2006, 32. G. Bollen, D.J. Morrissey, and S. Schwarz, Nucl. Instr. and Meth. in Physics A550 (2005) 27-38. 33. T. Lamy, R. Geller, P. Sortais, and T. Thuillier, Rev. Sci. Instrum. 77, 03B101 (2006). 34. F. Wenander, P. Delahaye, R. Scrivens, and R. Savreux, Rev. Sci. Instrum. 77, 03B104 (2006). 35. P.N. Ostroumov et al., Proc. 2001 Particle Accelerator Conf., June 18-22, Chicago, IL, 2001, p. 4080. 36. M. Portillo et al., Proc. 2001 Particle Accelerator Conf., June 18-22, Chicago, IL, 2001, p. 3015. 37. M.P. Kelly, et al. Proc. 2001 Particle Accelerator Conf., June 18-22, Chicago, IL, 2001, p. 506. 38. P.N. Ostroumov, et al., Proc. 2001 Particle Accelerator Conf., June 18-22, Chicago, IL, 2001, p. 4077. 39. P. Decrock et al., Rev. Sci. Instr. 68 (1997) 2322. 40. P.N. Ostroumov, et al., Nucl. Instr. Meth. in Phys. Res. B204 (2003) 433-439. 41. RIA Facility Workshop, Organized jointly by the MSU/NSCL and the ANL Physics Division, East Lansing, MI, March 9-13, 2004. 42. C.-L. Jiang, et al., Nucl. Intr. Meth. A492 (2002) 57-73. Graphical presentations of predicted yields are at: http://www.phy.anl.gov/ria/ria yields/yields home.html 43. B.B. Back and C.-L. Jiang, ”Beam intensity expectations for a 200 MeV/u 400 kW radioactive beam driver accelerator,” ANL Physics Division Internal Report (2006).
Plans for an Advanced Exotic Beam facility in the U.S. J.A. Nolena a
Physics Division, Argonne National Laboratory, 9700 South Cass Av., Argonne, IL 60439, USA The Office of Science of the Department of Energy is currently considering options for an advanced radioactive beam facility in the U.S. A committee, the Rare Isotope Science Assessment Committee, has been formed under the auspices of the Board of Physics and Astronomy of the National Academies of Science and Engineering with the charge of assessing the importance of and prospects for future research in this field world-wide. There is interest in identifying opportunities for a facility in the U.S. that will complement the capabilities that will exist elsewhere. There is an on-going R&D program sponsored by the DOE to develop the technologies that are essential to both the production of and use of beams of intense exotic beams at next-generation facilities. This paper presents the status of planning and options for such a facility, as well as, progress in the R&D. This program is developing concepts and prototypes for high power superconducting heavy ion linacs for rare isotope production and high power targets, ion sources, fragment separators, gas catchers, and reacceleration technologies that support the delivery of high quality, intense beams of exotic isotopes for research. 1. INTRODUCTION TO A ”RIA-LITE” CONCEPT The study of the properties of and reactions induced by short-lived isotopes is a key to our understanding of fundamental questions in nuclear physics, nuclear astrophysics and fundamental interactions at low-energy. This has long been recognized by the international nuclear physics community and is reflected by the investments in that field that have been or are being made in North America, Europe, and Asia. The US nuclear physics community addressed the importance of such studies in its 2002 Long Range Plan [1] where a Rare Isotope Accelerator Facility (RIA) was identified as its highest priority for new construction. The powerful combination of technologies that comprised the RIA concept were subsequently described in several papers [2,3,4] and much of the R&D towards realizing the facility was described in the references included in those publications. However, the estimated total project cost of the proposed RIA facility was $1.1B escalated to the planned years of construction, and this led to consideration of alternatives for how best to pursue science with radioactive beams. A committee, the Rare Isotope Science Assessment Committee, was formed under the auspices of the Board of Physics and Astronomy of the National Academies of Science and Engineering with the charge of assessing the importance of and prospects for future research in this field world-wide. The report of this committee is scheduled to be published in late October 2006. 0375-9474/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.nuclphysa.2006.12.018
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Figure 1. A simplified schematic layout of AEBL, a RIA-lite facility at Argonne.
Recently, the Department of Energy and the National Science Foundation have also charged the Nuclear Science Advisory Committee of the United States to carry out a study of the scientific capabilities and the technological feasibility of an advanced radioactive beam facility that could be constructed for approximately one half of the cost of the baseline RIA concept. The charge involves consideration of ”a modified RIA that focuses on capabilities which would make it unique in the world and would complement the rare isotope capabilities elsewhere.” The present plan is for completion of this study in early 2007 so that the recommendations can be provided as input to the upcoming long-range plan for nuclear physics in the United States which is due to be completed by the end of 2007.
Table 1 Beam list for selected ions from the AEBL Driver Linac with the onestripper option. The output currents indicated correspond to 400kW beam power for all beams. ECR Strip Output Beam Z A Source Qinj Energy Qstrip Frac Iout Energy pmicroA MeV/A pmicroA MeV/A 1 1 880 1 95.2 704.1 568 2 3 390 2 70.7 312.4 427 1 2 728 1 57.1 582.0 344 8 18 101 6 41.8 8.0 0.90 72.9 305 18 40 47 8 27.6 18.0 0.90 33.7 297 36 86 24 14 23.0 35.0 0.90 17.5 266 54 136 17 18 18.9 50.7 0.90 12.4 237 92 238 6+6 33+34 16.5 79.0 0.83 8.4 200
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Contingent upon the recommendations of these two committees and the United States Long-Range Plan for Nuclear Physics the tentative plan for a ”RIA-lite” facility could proceed with project engineering and design beginning in fiscal year 2011 with construction beginning soon after that. This schedule requires continued R&D on the technical components of the facility, a site selection, and the completion of a Conceptual Design Report in the coming years, prior to 2011. The following sections of this paper will address a possible technological approach and the associated R&D for the ”RIA-lite” facility. The goal is to design an advanced facility that in conjunction with other modern facilities worldwide will enable great advancements in this science. The resulting design will retain as many of the features of the baseline RIA concept as possible within the budget constraints, such as: • Use of a variety of production techniques for short-lived isotopes including new approaches based on gas catcher technology that remove the main existing limitations of standard ISOL techniques. • A powerful superconducting driver linac capable of accelerating any stable ion from protons to uranium and designed to allow simultaneous acceleration of multiple charge states of a given ion to attain unprecedented power for the heaviest ions. • A very efficient post-acceleration scheme based on a superconducting linac that delivers high quality radioactive beams starting from singly charged ions of mass up to 240 amu from ion source energy. • First rate experimental equipment to maximize the scientific output of the facility. 2.TECHNOLOGY DEVELOPMENT FOR AN ADVANCED EXOTIC BEAM FACILITY At this time concepts for a RIA-lite facility are being worked out at Argonne National Laboratory and at Michigan State University. A summary of recent plans was presented at the Erice summer school by Sherrill [5]. A simplified schematic layout of a RIA-lite facility on the Argonne site is shown in Fig. 1. It is referred to as the Advanced Exotic Beam Laboratory (AEBL). The driver linac option shown in the figure is the two-stripper version. However, there is also a one-stripper option being considered, corresponding to the beam list given in Table I. The main functional components and selected results from the R&D programs are described in the following sub-sections. 2.1. Heavy Ion Driver Accelerator The driver linac produces the primary high-power stable ion beam required to produce the radioisotopes. As in the baseline RIA concept, the facility will use a number of production mechanisms spanning the range from spallation on thick targets which requires high energy light ion beams (e.g. p, d, 3He), fragmentation which requires heavy-ion beams at high-intensity and energies in the 100’s of MeV per nucleon range for midmass nuclei, to in-flight fission which requires high intensity uranium beams at similar velocities. To be able to fully exploit the various production mechanisms it is, therefore, imperative that the driver linac be capable of accelerating essentially all stable nuclei to sufficiently high energies with high efficiency. The broad mass-to-charge ratio of the species to be accelerated together with the higher energy for lighter ions and high current
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Figure 2. Design drawing of a superconducting, triple-spoke resonator that is used at the high-energy end of the AEBL driver linac.
requirements dictate the use of a superconducting linac whose short independently phased cavities enable efficient acceleration over a wide velocity profile. The proposal for RIA-lite is to reduce the beam energies from the heavy ion driver linac by a factor of two relative to RIA while keeping the beam power at 400 kW by doubling the corresponding beam currents. A list of beam energies and intensities for selected ions from the driver is given in Table I. The rare isotope yields enabled by such a driver and the performance relative to the baseline RIA facility are given in section 3. The baseline RIA Driver Linac design and initial component prototyping were discussed in reference [6]. The baseline design comprises: (1) an advanced ECR ion source [7], (2) a 2-charge-state low-energy beam transport system and pre-buncher [8], (3) a low-frequency CW RFQ [9], (4) six classes of low to medium velocity superconducting resonators [6,10], (5) three classes of high-velocity compressed elliptical resonators [11, 12], or alternatively, two classes of triple-spoke resonators [13, 14], (6) two high-intensity charge-state strippers and associated multiple-charge-state beam transport sections [15, 16, 17, 18], and a (7) multiple-charge-state RF switcher and beam transport system for beam sharing [17, 19]. The design of the driver accelerator took a number of bold steps to meet the requirements of the RIA facility. A vigorous R&D program led to the development of the multiple charge state acceleration concept [20], its experimental demonstration at the ATLAS facility at ANL [21], the development of the spoke-cavity superconducting structures [13, 14] to fill the gap in the velocity regime between the low-β ATLAS-type cavities and the CEBAF-type velocity-of-light structures, and development of the SNS-type high-β cavities [11, 12] that will be used for the higher energy section of the linac. Successes with the R&D program for the baseline RIA Driver Linac as discussed above support the technological approach for the RIA-lite driver at one-half the beam energy and twice the beam current. For example, the high currents of medium charge-state bismuth beams achieved recently with the LBNL VENUS ECR ion source operating at 28 GHz [7] combined with the detailed studies of the multiple-charge-state operation of the driver with very low beam losses [20, 22] supports the goal of achieving 400-kW uranium beams at 200 MeV per nucleon with 8 particle microamperes of beam on target. The excellent
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Figure 3. A photograph of a liquid lithium film being developed for the stripper foil in the AEBL driver linac. The film velocity is 50 m/s and the thickness is 15 microns. The width is 6 mm.
beam dynamics associated with the very large longitudinal acceptances of the 345 MHz triple-spoke resonators and the successful operation of the prototypes of these resonators support a robust design of the high power driver. (Fig. 2 shows a CAD model of the beta=0.63 triple-spoke resonator and shows the Q-curves of the prototype are reported in [14].) The high power uranium beams in the one-stripper option requires stripping at a uranium beam energy of 17 MeV/u at a beam intensity of 10 particle microamperes. The development of high-velocity, windowless, thin-film, liquid-lithium stripper foils [16] is an ongoing project at Argonne [23]. Progress with this effort is indicated by the photograph of a 50 m/s liquid lithium film shown in Fig. 3. 2.2. Isotope Production complex The production of the radioisotopes will occur by interaction of the high-power primary beams on targets located in an enclosed production building. Because of the large inventories of radioactive materials that will be present remote handling capabilities and the required radiation control mechanisms will be in this building. The production of radioactive isotopes in this complex can occur through 4 main mechanisms, all of which are enabled by the versatile driver described above. The production area will house standard ISOL, two-step fission (actinide) targets, high power fragmentation targets such as liquid lithium, a large acceptance fragment separator and its associated high power beam dumps, and the gas catcher. The issues to be faced in the design of this area will be very similar to those that were required for the full RIA complex. Development of concepts for the RIA production areas has been the subject of a multi-laboratory collaboration under the auspices of the DOE Office of Nuclear Physics R&D program for the past three years. Some of the radiological and ISOL target design issues were addressed in the recent International Conference on Accelerator Applications 2005 [24, 25]. The special issues of contamination control associated with the use of actinide targets are common to all present and future ISOL-type radioactive beam facilities. A workshop to bring together the interested parties for discussion of these common challenges was hosted by TRIUMF recently [26]. The targets for in-flight fragmentation and fission of intense very heavy beams up to uranium present a major challenge. For the very high power densities produced by the
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Figure 4. CAD model and photograph of an upgraded prototype gas cell that has been equipped with RF electrodes on the body as well as on the extraction cone to increase the space charge limit of the system.
intense uranium beams with small beam spot diameters, a windowless liquid lithium concept has been developed and demonstrated in a prototype with a high power electron beam that simulates the power density encountered at RIA and RIA-lite [27, 28]. One of the features that makes RIA-lite globally competitive is its unique combination of high power uranium and other very heavy ion beams, large acceptance fragment separators, gas catcher technology, and a flexible and efficient post accelerator. Radioactive isotopes are produced by fragmentation (or in-flight fission) of fast heavy-ion beams on a ”thick” thin target. A large acceptance fragment separator is configured to collect, separate, and compensate for the fragment energy spread so that the rare isotopes are slowed down and stopped in a gas catcher system [29, 30], where they are thermalized but remain singly or doubly charged and can be extracted by a combination of gas flow, DC, and RF fields to be delivered to the stopped beam area or to the post accelerator. This results in beams of quality as good or better than those obtained by ISOL techniques, without the chemical limitations encountered by the ISOL technique in the diffusion and release out of thick, solid targets. A vigorous R&D program is ongoing to demonstrate and further improve gas catcher technology. A full-scale RIA prototype gas cell was constructed and evaluated at the Argonne ATLAS accelerator and then put on-line a successfully demonstrated with full energy secondary beams at the GSI fragment separator. Further development and characterization of gas cells at high intensities is underway at Argonne. Fig. 4 shows a CAD model and a photograph of the internal components of an upgraded small gas cell that is now in use at the ANL Canadian Penning Trap facility. To increase the space charge limit, this cell now has RF electrodes along the whole body rather than just in the extraction cone [31]. This cell was also adjusted to increase the efficiency for light isotopes such as 14 O. An alternative to the linear gas stopper geometry discussed above is an inverse gas-filled cyclotron device that is being pursued at RIKEN and MSU [32]. 2.3. Post-Accelerator The most efficient production mechanisms for slow radioactive ions yield these ions in the 1+ or 2+ charge states. The post-accelerator must therefore be able to accept such low charge-to-mass ratio ions from ion source energy. One approach to this problem is to
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Figure 5. Plot of the ratios of yields predicted for mass separated 1+ beams from the AEBL facility relative to those predicted for the baseline RIA facility. The ratios are generally between 10% and 100%. [42]
increase the charge state of the ions before acceleration via a charge booster stage, be it an ECR-type breeder [33] or an EBIS-based system [34]. In general, the breeders are more competitive with stripper methods for heavier isotopes. To ensure maximum efficiency in the post-acceleration process, especially for light isotopes, RIA-lite can use a very efficient low-charge-to-mass ratio injector based on a superconducting linac accelerator coupled to low frequency CW RFQs [35]. The singly charged radioactive ions extracted from the ion source are first mass separated by a large acceptance, high-resolution (m/Δm = 20,000) isobar separator [36]. The selected isotopes are then bunched by a multi-harmonic buncher and accelerated through a series of 3 low frequency RFQs, the first two located on an adjustable high-voltage platform. The first RFQ is a CW split-coax RFQ structure [37] recently developed at Argonne and capable of accelerating ions with mass-to-charge ratio as high as 240. The second and third RFQs use a novel hybrid structure [38] that mixes RFQ and IH-type sections to obtain higher accelerating gradient with still enough focusing force. The following low-beta superconducting linac section can accept ions of = 0.011 with q/m 1/66 so that ions with mass larger than 66 must be stripped. This is done in helium gas to achieve a high efficiency with essentially no emittance growth [39]. The stripped ions are then accelerated by low-beta inter-digital superconducting cavities up to an energy of at least 600 keV/u after which they are further stripped to q/m 0.15 for further acceleration in ATLAS type superconducting cavities. The beam is, therefore, available at all energies from ion source energy up to energies above 10 MeV/u for midmass ions. The acceptance of all parts of the linac being very large compared to typical emittance from ISOL sources, the transmission is also very high, limited by the bunching efficiency (about 85%) and the stripping efficiencies for the heavier beams. Multiple-
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Fr
AEBL 200 MeV/u 400 kW
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Yields of mass separated + 1 ions for re-acceleration T-gas cell: 20 ms Proton Number Z
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Figure 6. Plot of the predicted yields for mass separated 1+ beams from the AEBL facility relative using the optimal production mechanism for each isotope [42].
charge-state acceleration can be used following the second stripper to further increase efficiency for experiments that are not sensitive to the resultant emittance growth [40]. The layout shown in Fig. 1 has both a low-charge-to-mass ratio injector and a charge breeder branch. The final configuration will depend on the cost constraints and relative priorities that are set for various facility capabilities. The ultimate combination is to use both types of injectors so that it is possible to simultaneously deliver light beams to the Astrophysics area and heavy beams to the Coulomb Barrier instruments. 2.4. Experimental Areas As was planned for RIA, RIA-lite will host four experimental areas for users, corresponding to the different energy regimes that are required to carry out the science program that is to be addressed at this facility. The four areas are indicated in Fig. 1, the Trap area, the Nuclear Astrophysics area, the Coulomb Barrier area, and the full-energy, in-flight area. Due to the lower target cost for RIA-lite, these areas, at least initially will be less extensive and less well equipped than was planned for the full RIA facility. Over the past few years there have been several workshop held for potential RIA users to participate in the planning of the layouts and instruments for the RIA experimental facilities. The most recent of these, jointly sponsored by the MSU/NSCL and the ANL Physics Division, was held in East Lansing, MI in March, 2004 [41]. As the plans for RIA-lite evolve, additional user workshops will be required to set priorities for equipping the experimental areas. 3. EXPECTED PERFORMANCE The proposed RIA-lite facility will provide yields of short-lived isotopes atunprecedented intensities. This is due to the high primary beam power, the flexibility in the
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primary beam selection which allows the best production mechanism to be used, new target technologies that allow the normal limitations of ISOL type systems to be overcome, and finally the use of a very efficient post-acceleration scheme. For the baseline RIA facility yields were calculated for the different reaction mechanisms and are available on the web [42]. For AEBL the predicted yields for beams of mass separated 1+ rare isotopes are in the range of approximately 10% to 100% of the full RIA yields because the yields are more dependent on the total beam power than on the beam energy. Below 200 MeV/u the yields fall rapidly due to decreasing effective target thicknesses, more charge state fractionation, and poorer kinematics for the collection of in-flight-produced fission products. The predicted ratios of AEBL to RIA yields are shown in Fig. 5 and the absolute yields for AEBL are shown in Fig. 6 [43]. Acknowledgements The RIA and RIA-lite concepts have evolved from ideas generated within the nuclear and accelerator physics communities worldwide. This work was supported by the US Department of Energy, Division of Nuclear Physics, under contract number W-31-109-ENG-38. REFERENCES 1. Opportunities in Nuclear Science, A Long-Range Plan for the Next Decade, Ed. J. Symons, et al., April 2002. 2. B.M. Sherrill, Nucl. Instr. Meth. in Phys. Res. B204 (2003) 765-770. 3. G. Savard, ibid., p. 771-779. 4. J.A. Nolen, Nucl. Phys. A734 (2004) 661-668. 5. B.M. Sherrill, INTERNATIONAL SCHOOL OF NUCLEAR PHYSICS, 28th COURSE, Radioactive Beams, Nuclear Dynamics and Astrophysics, ERICE-SICILY: 16 - 24 SEPTEMBER 2006. 6. K.W. Shepard, Proc. XXI Inter. Linear Accelerator Conference, Gyeongju, Korea, August 19-23, 2002, p. 598-601. 7. D. Leitner, C. M. Lyneis, T. Loew, D. S. Todd, and S. Virostek, Rev. Sci. Instrum. 77 (2006) 03A302,1-6. 8. P.N. Ostroumov, K.W. Shepard, V.N. Aseev, and A.A. Kolomiets, Proc. 20th LINAC Conference, August 21-25, 2000, Monterey, California, p. 202. 9. P.N. Ostroumov, et al., Phys. Rev. STAB 5, 060101 (2002), see also papers TU466 and TU467 at LINAC2002. 10. K.W. Shepard, M.P. Kelly, and J.D. Fuerst, Proc. XXI Inter. Linear Accelerator Conference, Gyeongju, Korea, August 19-23, 2002, p. 478-480. 11. I.E. Campisi, et al., Proc. XXI Inter. Linear Accelerator Conference, Gyeongju, Korea, August 19-23, 2002, p. 496-498. 12. W. HARTUNG, et al., Proc. 2003 Particle Accelerator Conference, Portland, OR, May 12-16, 2003, paper TPAB070. 13. K.W. Shepard, P. N. Ostroumov, and J. R. Delayen, Phys. Rev. ST Accel. Beams 6 (2003) 080101. 14. M.P. Kelly, Proceedings of the 39th ICFA Advanced Beam Dynamics Workshop ”High Intensity High Brightness Hadron Beams,” Tsukuba City, Japan, May 29-June 2, 2006.
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15. M. Portillo, et al., Proc. 2001 Particle Accelerator Conf., June 18-22, Chicago, IL, 2001, p. 3012. 16. X. Wu, et al., Proc. EPAC 2002, Paris, France, p. 1211. 17. J.A. Nolen, Proc. 2nd RIA Driver Workshop, Argonne National Laboratory, ArgDOE Office of Science Financial Assistance Program Notice LAB 05-22 entitled ”Research and Development for a Rare Isotope Accelerator”, May 22-24, 2002, copies available from the ANL Physics Division. 18. X. Wu, ”High symmetry stripping sections”, ibid. 19. X. Wu, ”Beam switch-yard”, ibid. 20. P.N. Ostroumov and K.W. Shepard, Phys. Rev. ST Accel. Beams 3 (2000) 030101. 21. P.N. Ostroumov et al., Phys. Rev. Lett. 86 (2001) 2798. 22. P.N. Ostroumov, Proc. Linac 2004, paper TH204. 23. C.B. Reed, 2006 R&D under the DOE Office of Science Financial Assistance Program Notice LAB 05-22. 24. I. Remec, et. al., Nucl. Instr. and Meth. in Phys. Res. A562 (2006) 896-899. 25. G. Bollen, et al., Nucl. Instr. and Meth. in Phys. Res. A562 (2006) 915-920. 26. First Workshop on Actinide Target Development, TRIUMF, Vancouver, B.C., Canada, April 27-29, 2006. 27. J.A. Nolen, et al., Nucl. Instr. Meth. in Phys. Res. B204 (2003) 293-297. 28. J.A. Nolen, et al., Rev. Sci. Instr. 76 (2005) 073501, p 1-6. 29. G. Savard, et al., Nucl. Instr. Meth. in Phys. Res. B204 (2003) 582-586. 30. G. Savard, Eur. Phys. J. A 25, supplement 1 (2005) 713-718 31. G. Savard, Workshop on the SPIRAL2 Super Separator Spectrograph, Paris, June 27-28, 2006, 32. G. Bollen, D.J. Morrissey, and S. Schwarz, Nucl. Instr. and Meth. in Physics A550 (2005) 27-38. 33. T. Lamy, R. Geller, P. Sortais, and T. Thuillier, Rev. Sci. Instrum. 77, 03B101 (2006). 34. F. Wenander, P. Delahaye, R. Scrivens, and R. Savreux, Rev. Sci. Instrum. 77, 03B104 (2006). 35. P.N. Ostroumov et al., Proc. 2001 Particle Accelerator Conf., June 18-22, Chicago, IL, 2001, p. 4080. 36. M. Portillo et al., Proc. 2001 Particle Accelerator Conf., June 18-22, Chicago, IL, 2001, p. 3015. 37. M.P. Kelly, et al. Proc. 2001 Particle Accelerator Conf., June 18-22, Chicago, IL, 2001, p. 506. 38. P.N. Ostroumov, et al., Proc. 2001 Particle Accelerator Conf., June 18-22, Chicago, IL, 2001, p. 4077. 39. P. Decrock et al., Rev. Sci. Instr. 68 (1997) 2322. 40. P.N. Ostroumov, et al., Nucl. Instr. Meth. in Phys. Res. B204 (2003) 433-439. 41. RIA Facility Workshop, Organized jointly by the MSU/NSCL and the ANL Physics Division, East Lansing, MI, March 9-13, 2004. 42. C.-L. Jiang, et al., Nucl. Intr. Meth. A492 (2002) 57-73. Graphical presentations of predicted yields are at: http://www.phy.anl.gov/ria/ria yields/yields home.html 43. B.B. Back and C.-L. Jiang, ”Beam intensity expectations for a 200 MeV/u 400 kW radioactive beam driver accelerator,” ANL Physics Division Internal Report (2006).