- Email: [email protected]
Upgrade status and plans at the Holifield Radioactive Ion Beam Facility
Nuclear Instruments and Methods in Physics Research B 241 (2005) 926–930 www.elsevier.com/locate/nimb
Upgrade status and plans at the Holifield Radioactive Ion Beam Facility Brian Alan Tatum
*
Physics Division, Oak Ridge National Laboratory,1 Building 6000, MS-6368, Oak Ridge, TN 37831 06368, USA Available online 21 September 2005
Abstract The Holifield Radioactive Ion Beam Facility (HRIBF) is a national user facility dedicated to nuclear structure and astrophysics research with Radioactive Ion Beams (RIBs) using the Isotope Separator On-Line (ISOL) technique. HRIBF also maintains a vibrant development program for ISOL targets, ion sources and diagnostics. As a bridge to RIA, HRIBF continues to expand its technology. Presently, a $4.75M High Power Target Laboratory (HPTL) is being constructed to provide a facility for testing new targets, target geometries, ion sources and beam preparation techniques. HPTL will ultimately be co-located with a second RIB production system (IRIS2). An external axial injection system for the driver cyclotron, ORIC, is planned to provide higher beam intensities, reduce machine activation and eliminate cathode lifetime limitations. A multi-channel residual gas BPM is under development for measuring intensity and position of the driver beam. RIB production via electron-induced photofission is also being explored to attain higher intensities. Ó 2005 Elsevier B.V. All rights reserved. PACS: 21.10. k; 24.10. i; 26.30.+k Keywords: HRIBF; RIB; ISOL; ORNL; Accelerator
1. Introduction The Holifield Radioactive Ion Beam Facility (HRIBF) at Oak Ridge National Laboratory 1 Managed by UT-Battelle, LLC, for the US Department of Energy under contract DE-AC05-00OR22725. * Tel.: +1 865 574 4759; fax: +1 865 574 1268. E-mail address: [email protected]
(ORNL) is the only US national user facility dedicated to the production of high-quality post-accelerated Radioactive Ion Beams (RIBs) for nuclear structure and reactions, and nuclear astrophysics research. RIBs on both the proton- and neutronrich side of stability are produced by the Isotope Separator On-Line (ISOL) technique, and HRIBF is the only facility in the world capable of delivering beams of medium-mass neutron-rich RIBs at
0168-583X/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.07.171
B.A. Tatum / Nucl. Instr. and Meth. in Phys. Res. B 241 (2005) 926–930
energies above the Coulomb barrier. Additionally, HRIBF is making critical contributions to RIB production technology and producing high-impact scientific results in areas well-aligned with nuclear science, DOE and office of science strategic plans. HRIBF has a users group consisting of 423 experimentalists and theorists, and 80–100 users take part in experiments each year. Science with radioactive beams is a key component of the future of nuclear science in the US, demonstrated by the high priority given to the next-generation radioactive beam facility, the Rare Isotope Accelerator (RIA), in the DOE 20-year plan for facility construction. HRIBF facility and research staff will play important roles in the development of RIA and carry on research programs at RIA. HRIBF is playing a key role in supporting RIB research and development and nurturing the low-energy RIB user community, thus bridging the gap until RIA is on-line. The DOE Office of Nuclear Physics (DOEONP), following the final report of the 2001 NSAC Subcommittee on Low Energy Nuclear Physics, instructed HRIBF staff to develop a strategy for upgrading the facility at modest cost (<$20M) and with minimal down-time. The resultant plan addresses performance, reliability and operational efficiency.
2. Existing systems The HRIBF accelerator system has three main components: the Oak Ridge Isochronous Cyclotron (ORIC) [1], an ISOL production/beam preparation system with two stages of mass separation (Injector for Radioactive Ion Species #1, or IRIS1) [2], and a 25-MV tandem electrostatic accelerator post-accelerator [2,3]. ORIC is the driver accelerator for RIB production, presently providing up to 20 lA of proton, deuteron or alpha particle beams to IRIS1. The RIB production platform of IRIS1 operates up to 300 kV; a target/ion source (TIS) assembly mounted on the platform operates up to ±60 kV from platform potential. Radioactive atoms produced in the target are transported thermally by diffusion and effusion to the ionization region
927
Fig. 1. Schematic overview of principal components and operation of the HRIBF.
where they are ionized and formed into a beam. Target geometrical structure and temperature dramatically influence the speed at which atoms are transported for ionization. Speed is critical since the radioactive species we accelerate exist, by definition, for a finite time [4]. Several varieties of ion sources are employed. RIBs extracted from an ion source pass through a first-stage mass separator with resolving power (m/Dm) of approximately 1000 to select the mass number of the RIB of interest. Negative-ion beams, required for acceleration in the tandem, are accelerated directly off the platform while positive-ion beams must first be converted to negative-ions by charge exchange. After acceleration off the production platform, the beam is passed through a second-stage mass, or isobar separator, with resolving power (m/Dm) up to 20,000 in order to separate species with the same mass number (isobars). The desired RIB is postaccelerated by the 25-MV tandem electrostatic accelerator and then delivered to one of the experimental end stations, see Fig. 1.
3. HPTL The most critical need identified for performance improvement at HRIBF, based on programmatic demands, was enhanced capability for on-line high-power research on ISOL targets and ion sources. A proposal was submitted to the DOE Office of Nuclear Physics (DOE-ONP) in
928
B.A. Tatum / Nucl. Instr. and Meth. in Phys. Res. B 241 (2005) 926–930
January 2003 for an upgrade project to address this priority. The High Power Target Laboratory (HPTL), approved with initial funds provided late in FY03, will enable us to carry out an aggressive program of R&D at full driver-beam power, without impacting our single on-line RIB production station. In addition to high-power release studies, new target geometries/formats, ion sources and beam preparation techniques will be tested, as well as the effects of rastering the driver beam over a nominal 5 cm target to reduce the density of beam power deposited in the production target. The HPTL will be solely an R&D facility while IRIS1 will be used primarily for production. This, IRIS1 will not be impacted by the higher levels of activation, experimental target failures and associated delays between TIS target changes. Anticipated results are higher RIB intensities, higher purities and more available RIB species. The substantially larger size of the HPTL compared to IRIS1 will permit development of such technologies as laser and ECR ion sources and ion beam cooling. The scope of the HPTL project is divided into two subsystems, facility modifications and technical equipment. Facility modifications consists of two primary tasks: (1) construction of a heavilyshielded room where target bombardment and ion source testing will be carried out, and (2) construction of a moderately-shielded room to house peripheral equipment. Technical equipment consists of three primary tasks: (1) a light-ion beam transport line from ORIC to the target room, (2) a platform system that includes a high voltage structure with associated instrumentation and utilities and (3) a target station consisting of a source support structure, ion source assembly, target holder, remote decoupling mechanism and a RIB analysis system. Also under development is a
non-destructive beam imaging and intensity monitoring system, the residual gas beam profile monitor [5]. Fig. 2 shows the HRIBF with the HPTL included. The $4.75M project is scheduled for completion by the end of FY05.
4. IRIS2 The second most critical upgrade need for HRIBF was determined to be redundancy in the harsh production system environment. A large percentage of facility down-time is associated with the high radiation, high voltage and high temperature components of IRIS1 because of the obvious maintenance difficulties. Redundancy, crucial to all ISOL facilities and recommended for RIA, will result in an increase in HRIBF operational hours, improve overall reliability, decrease transition time between experimental campaigns, and allow new ion sources and techniques developed at HPTL to be used in production for the experimental program. To accomplish these objectives, a proposal for a second Injector for Radioactive Ion Species (IRIS2) was recently submitted to DOE-ONP. Although it would be desirable to build IRIS2 as a third high-power bombardment area and maintain HPTL as an R&D station independent of the production stations, it is not cost-effective to do so. Therefore, IRIS2 will be co-located with HPTL, a possibility considered in the original HPTL design and will duplicate all capabilities of the existing IRIS1 while providing scope for implementation of new features unavailable at IRIS1. In addition to improving efficiency, we believe that IRIS2 will result in roughly a 50% increase in RIB hours for experiments. We also expect a general increase of a factor of about 5 in currently
Fig. 2. The HRIBF with HPTL included.
B.A. Tatum / Nucl. Instr. and Meth. in Phys. Res. B 241 (2005) 926–930
929
Fig. 3. IRIS2 plan view.
available RIBs within a year of IRIS2 commissioning. A laser lab is included in the project scope: laser ion source developments should produce a significant impact on beam purity. IRIS2 requires little civil construction, so the primary effort is associated with technical equipment that includes: (1) a 225 kV high voltage platform system, (2) an injector beamline, (3) a transport beamline and (4) remote handling and localized shielding. Similar to IRIS1, a high voltage platform system will provide the radioactive ions with the necessary energy for injection into the tandem. Multiple platform structures are required to accommodate beamline components, instrumentation and power generation equipment. The injector beamline on the high voltage platform will include a first stage mass analysis system, electrostatic ion beam optics, RIB diagnostics and charge exchange capabilities. A transport line will deliver the RIB from the injector beamline to an existing beamline that contains the second stage isobar separator. Activated TIS assemblies associ-
ated with the production environment must be remotely removed, installed and stored via an expanded remotely-operable handling system. Localized shielding will be needed in the immediate vicinity of the TIS and at other locations within the room. Fig. 3 depicts the HRIBF in the IRIS2 configuration. The $4.7M IRIS2 project is proposed to begin in FY06 with completion in FY08. A scientific review has already been conducted by DOE-ONP and the response was quite positive.
5. ORIC axial injection The performance of the driver accelerator, ORIC, is limited by aspects of the PIG-type internal ion source and the multi-stage extraction system. Internal ion source limitations are short cathode lifetime when used with helium and poor beam quality. Electrostatic deflector septum extraction has low efficiency and limits the intensity of
930
B.A. Tatum / Nucl. Instr. and Meth. in Phys. Res. B 241 (2005) 926–930
production beams. It also results in high machine activation that impedes maintenance and poses risks to internal machine components. ORICÕs limitations can be addressed by a proposed axial injection system to replace the internal ion source [6]. This multi-source system would improve beam quality and power for both RIB production and the study of high power ISOL targets at the HPTL, would improve reliability and would make machine maintenance more efficient. An axial injection system would consist of a 40 kV platform that accommodates multiple ion sources, including a multi-cusp source for production of high intensity H and D beams and an ECR source for He2+. Magnetic beam transport components would be used to address space-charge effects and injection through the ORIC magnet yoke into the central region would be accomplished by a multi-harmonic (MH) beam pulsing system and a spiral inflector. Combined with a newly designed foil stripping extraction system, extracted light-ion beams with intensities up to 100 lA are attainable. Higher driver beam intensities combined with beam rastering will yield higher intensity RIBS.
6. Neutron-rich RIB production from high power electron beams Alternative driver accelerator options are also being explored. One attractive possibility involves the use of high power electrons to produce neutron-rich RIBs by photofission of actinide nuclei. A 50 kW, 100 MeV electron beam would generate a total uranium fission rate 25 times greater than a 20 lA 50 MeV proton beam, while depositing about the same power density in the target. In
addition, the photofission yield of neutron-rich species is shifted farther from stability than for proton induced fission: the yield of 132Sn, for example, is estimated to be about 500 times larger for 50 kW of electrons versus 20 lA 50 MeV protons. The fission yield per unit electron beam power is independent of electron energy above 30 MeV. Studies are underway with the Oak Ridge Electron Linear Accelerator (ORELA) to further explore the feasibility of this concept, and to experimentally determine RIB yields [7].
7. Summary HRIBF will indeed serve as a bridge to the RIA era. The systematic, modest-cost upgrades underway and planned will further enhance the HRIBFÕs role as a forefront laboratory for research and development with post-accelerated RIBS, and as a leader in ISOL science and technology. References [1] B.A. Tatum, D.T. Dowling, J.R. Beene, in: Proceedings of the 16th International Conference on Cyclotrons and their Applications, East Lansing, MI USA, May 2001. [2] G.D. Alton, J.R. Beene, Y. Liu, Nucl. Instr. and Meth. A 438 (1999) 190. [3] M.J. Meigs et al., in: Proceedings of Symposium of North Eastern Accelerator Personnel, Lund, Sweden, October 2002. [4] D.W. Stracener, Nucl. Instrum. and Meth. B 204 (2002) 42. [5] D. Shapira, 2003 Workshop for RIA R&D. [6] G.D. Alton, Y. Zhang, B.A. Tatum, in: Proceedings of the 2003 Particle Accelerator Conference, May 12–16, Portland, Oregon, USA, 2003 995. [7] William T. Diamond, Nucl. Instr. and Meth A 432 (1999) 471.
Upgrade status and plans at the Holifield Radioactive Ion Beam Facility Brian Alan Tatum
*
Physics Division, Oak Ridge National Laboratory,1 Building 6000, MS-6368, Oak Ridge, TN 37831 06368, USA Available online 21 September 2005
Abstract The Holifield Radioactive Ion Beam Facility (HRIBF) is a national user facility dedicated to nuclear structure and astrophysics research with Radioactive Ion Beams (RIBs) using the Isotope Separator On-Line (ISOL) technique. HRIBF also maintains a vibrant development program for ISOL targets, ion sources and diagnostics. As a bridge to RIA, HRIBF continues to expand its technology. Presently, a $4.75M High Power Target Laboratory (HPTL) is being constructed to provide a facility for testing new targets, target geometries, ion sources and beam preparation techniques. HPTL will ultimately be co-located with a second RIB production system (IRIS2). An external axial injection system for the driver cyclotron, ORIC, is planned to provide higher beam intensities, reduce machine activation and eliminate cathode lifetime limitations. A multi-channel residual gas BPM is under development for measuring intensity and position of the driver beam. RIB production via electron-induced photofission is also being explored to attain higher intensities. Ó 2005 Elsevier B.V. All rights reserved. PACS: 21.10. k; 24.10. i; 26.30.+k Keywords: HRIBF; RIB; ISOL; ORNL; Accelerator
1. Introduction The Holifield Radioactive Ion Beam Facility (HRIBF) at Oak Ridge National Laboratory 1 Managed by UT-Battelle, LLC, for the US Department of Energy under contract DE-AC05-00OR22725. * Tel.: +1 865 574 4759; fax: +1 865 574 1268. E-mail address: [email protected]
(ORNL) is the only US national user facility dedicated to the production of high-quality post-accelerated Radioactive Ion Beams (RIBs) for nuclear structure and reactions, and nuclear astrophysics research. RIBs on both the proton- and neutronrich side of stability are produced by the Isotope Separator On-Line (ISOL) technique, and HRIBF is the only facility in the world capable of delivering beams of medium-mass neutron-rich RIBs at
0168-583X/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.07.171
B.A. Tatum / Nucl. Instr. and Meth. in Phys. Res. B 241 (2005) 926–930
energies above the Coulomb barrier. Additionally, HRIBF is making critical contributions to RIB production technology and producing high-impact scientific results in areas well-aligned with nuclear science, DOE and office of science strategic plans. HRIBF has a users group consisting of 423 experimentalists and theorists, and 80–100 users take part in experiments each year. Science with radioactive beams is a key component of the future of nuclear science in the US, demonstrated by the high priority given to the next-generation radioactive beam facility, the Rare Isotope Accelerator (RIA), in the DOE 20-year plan for facility construction. HRIBF facility and research staff will play important roles in the development of RIA and carry on research programs at RIA. HRIBF is playing a key role in supporting RIB research and development and nurturing the low-energy RIB user community, thus bridging the gap until RIA is on-line. The DOE Office of Nuclear Physics (DOEONP), following the final report of the 2001 NSAC Subcommittee on Low Energy Nuclear Physics, instructed HRIBF staff to develop a strategy for upgrading the facility at modest cost (<$20M) and with minimal down-time. The resultant plan addresses performance, reliability and operational efficiency.
2. Existing systems The HRIBF accelerator system has three main components: the Oak Ridge Isochronous Cyclotron (ORIC) [1], an ISOL production/beam preparation system with two stages of mass separation (Injector for Radioactive Ion Species #1, or IRIS1) [2], and a 25-MV tandem electrostatic accelerator post-accelerator [2,3]. ORIC is the driver accelerator for RIB production, presently providing up to 20 lA of proton, deuteron or alpha particle beams to IRIS1. The RIB production platform of IRIS1 operates up to 300 kV; a target/ion source (TIS) assembly mounted on the platform operates up to ±60 kV from platform potential. Radioactive atoms produced in the target are transported thermally by diffusion and effusion to the ionization region
927
Fig. 1. Schematic overview of principal components and operation of the HRIBF.
where they are ionized and formed into a beam. Target geometrical structure and temperature dramatically influence the speed at which atoms are transported for ionization. Speed is critical since the radioactive species we accelerate exist, by definition, for a finite time [4]. Several varieties of ion sources are employed. RIBs extracted from an ion source pass through a first-stage mass separator with resolving power (m/Dm) of approximately 1000 to select the mass number of the RIB of interest. Negative-ion beams, required for acceleration in the tandem, are accelerated directly off the platform while positive-ion beams must first be converted to negative-ions by charge exchange. After acceleration off the production platform, the beam is passed through a second-stage mass, or isobar separator, with resolving power (m/Dm) up to 20,000 in order to separate species with the same mass number (isobars). The desired RIB is postaccelerated by the 25-MV tandem electrostatic accelerator and then delivered to one of the experimental end stations, see Fig. 1.
3. HPTL The most critical need identified for performance improvement at HRIBF, based on programmatic demands, was enhanced capability for on-line high-power research on ISOL targets and ion sources. A proposal was submitted to the DOE Office of Nuclear Physics (DOE-ONP) in
928
B.A. Tatum / Nucl. Instr. and Meth. in Phys. Res. B 241 (2005) 926–930
January 2003 for an upgrade project to address this priority. The High Power Target Laboratory (HPTL), approved with initial funds provided late in FY03, will enable us to carry out an aggressive program of R&D at full driver-beam power, without impacting our single on-line RIB production station. In addition to high-power release studies, new target geometries/formats, ion sources and beam preparation techniques will be tested, as well as the effects of rastering the driver beam over a nominal 5 cm target to reduce the density of beam power deposited in the production target. The HPTL will be solely an R&D facility while IRIS1 will be used primarily for production. This, IRIS1 will not be impacted by the higher levels of activation, experimental target failures and associated delays between TIS target changes. Anticipated results are higher RIB intensities, higher purities and more available RIB species. The substantially larger size of the HPTL compared to IRIS1 will permit development of such technologies as laser and ECR ion sources and ion beam cooling. The scope of the HPTL project is divided into two subsystems, facility modifications and technical equipment. Facility modifications consists of two primary tasks: (1) construction of a heavilyshielded room where target bombardment and ion source testing will be carried out, and (2) construction of a moderately-shielded room to house peripheral equipment. Technical equipment consists of three primary tasks: (1) a light-ion beam transport line from ORIC to the target room, (2) a platform system that includes a high voltage structure with associated instrumentation and utilities and (3) a target station consisting of a source support structure, ion source assembly, target holder, remote decoupling mechanism and a RIB analysis system. Also under development is a
non-destructive beam imaging and intensity monitoring system, the residual gas beam profile monitor [5]. Fig. 2 shows the HRIBF with the HPTL included. The $4.75M project is scheduled for completion by the end of FY05.
4. IRIS2 The second most critical upgrade need for HRIBF was determined to be redundancy in the harsh production system environment. A large percentage of facility down-time is associated with the high radiation, high voltage and high temperature components of IRIS1 because of the obvious maintenance difficulties. Redundancy, crucial to all ISOL facilities and recommended for RIA, will result in an increase in HRIBF operational hours, improve overall reliability, decrease transition time between experimental campaigns, and allow new ion sources and techniques developed at HPTL to be used in production for the experimental program. To accomplish these objectives, a proposal for a second Injector for Radioactive Ion Species (IRIS2) was recently submitted to DOE-ONP. Although it would be desirable to build IRIS2 as a third high-power bombardment area and maintain HPTL as an R&D station independent of the production stations, it is not cost-effective to do so. Therefore, IRIS2 will be co-located with HPTL, a possibility considered in the original HPTL design and will duplicate all capabilities of the existing IRIS1 while providing scope for implementation of new features unavailable at IRIS1. In addition to improving efficiency, we believe that IRIS2 will result in roughly a 50% increase in RIB hours for experiments. We also expect a general increase of a factor of about 5 in currently
Fig. 2. The HRIBF with HPTL included.
B.A. Tatum / Nucl. Instr. and Meth. in Phys. Res. B 241 (2005) 926–930
929
Fig. 3. IRIS2 plan view.
available RIBs within a year of IRIS2 commissioning. A laser lab is included in the project scope: laser ion source developments should produce a significant impact on beam purity. IRIS2 requires little civil construction, so the primary effort is associated with technical equipment that includes: (1) a 225 kV high voltage platform system, (2) an injector beamline, (3) a transport beamline and (4) remote handling and localized shielding. Similar to IRIS1, a high voltage platform system will provide the radioactive ions with the necessary energy for injection into the tandem. Multiple platform structures are required to accommodate beamline components, instrumentation and power generation equipment. The injector beamline on the high voltage platform will include a first stage mass analysis system, electrostatic ion beam optics, RIB diagnostics and charge exchange capabilities. A transport line will deliver the RIB from the injector beamline to an existing beamline that contains the second stage isobar separator. Activated TIS assemblies associ-
ated with the production environment must be remotely removed, installed and stored via an expanded remotely-operable handling system. Localized shielding will be needed in the immediate vicinity of the TIS and at other locations within the room. Fig. 3 depicts the HRIBF in the IRIS2 configuration. The $4.7M IRIS2 project is proposed to begin in FY06 with completion in FY08. A scientific review has already been conducted by DOE-ONP and the response was quite positive.
5. ORIC axial injection The performance of the driver accelerator, ORIC, is limited by aspects of the PIG-type internal ion source and the multi-stage extraction system. Internal ion source limitations are short cathode lifetime when used with helium and poor beam quality. Electrostatic deflector septum extraction has low efficiency and limits the intensity of
930
B.A. Tatum / Nucl. Instr. and Meth. in Phys. Res. B 241 (2005) 926–930
production beams. It also results in high machine activation that impedes maintenance and poses risks to internal machine components. ORICÕs limitations can be addressed by a proposed axial injection system to replace the internal ion source [6]. This multi-source system would improve beam quality and power for both RIB production and the study of high power ISOL targets at the HPTL, would improve reliability and would make machine maintenance more efficient. An axial injection system would consist of a 40 kV platform that accommodates multiple ion sources, including a multi-cusp source for production of high intensity H and D beams and an ECR source for He2+. Magnetic beam transport components would be used to address space-charge effects and injection through the ORIC magnet yoke into the central region would be accomplished by a multi-harmonic (MH) beam pulsing system and a spiral inflector. Combined with a newly designed foil stripping extraction system, extracted light-ion beams with intensities up to 100 lA are attainable. Higher driver beam intensities combined with beam rastering will yield higher intensity RIBS.
6. Neutron-rich RIB production from high power electron beams Alternative driver accelerator options are also being explored. One attractive possibility involves the use of high power electrons to produce neutron-rich RIBs by photofission of actinide nuclei. A 50 kW, 100 MeV electron beam would generate a total uranium fission rate 25 times greater than a 20 lA 50 MeV proton beam, while depositing about the same power density in the target. In
addition, the photofission yield of neutron-rich species is shifted farther from stability than for proton induced fission: the yield of 132Sn, for example, is estimated to be about 500 times larger for 50 kW of electrons versus 20 lA 50 MeV protons. The fission yield per unit electron beam power is independent of electron energy above 30 MeV. Studies are underway with the Oak Ridge Electron Linear Accelerator (ORELA) to further explore the feasibility of this concept, and to experimentally determine RIB yields [7].
7. Summary HRIBF will indeed serve as a bridge to the RIA era. The systematic, modest-cost upgrades underway and planned will further enhance the HRIBFÕs role as a forefront laboratory for research and development with post-accelerated RIBS, and as a leader in ISOL science and technology. References [1] B.A. Tatum, D.T. Dowling, J.R. Beene, in: Proceedings of the 16th International Conference on Cyclotrons and their Applications, East Lansing, MI USA, May 2001. [2] G.D. Alton, J.R. Beene, Y. Liu, Nucl. Instr. and Meth. A 438 (1999) 190. [3] M.J. Meigs et al., in: Proceedings of Symposium of North Eastern Accelerator Personnel, Lund, Sweden, October 2002. [4] D.W. Stracener, Nucl. Instrum. and Meth. B 204 (2002) 42. [5] D. Shapira, 2003 Workshop for RIA R&D. [6] G.D. Alton, Y. Zhang, B.A. Tatum, in: Proceedings of the 2003 Particle Accelerator Conference, May 12–16, Portland, Oregon, USA, 2003 995. [7] William T. Diamond, Nucl. Instr. and Meth A 432 (1999) 471.