G. D. Mills

Oak Ridge National Laboratory, Oak Ridge, Florida, United States

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Publications (37)22.74 Total impact

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    ABSTRACT: A renewed interest in Accelerator Mass Spectrometry (AMS) from nuclear physics laboratories is emerging in connection with Radioactive Ion Beams (RIBs). At the Holifield Radioactive Ion Beam Facility (HRIBF) at Oak Ridge National Laboratory (ORNL) we are exploring the AMS capabilities of the 25-MV tandem accelerator. Behind this effort is the realization that two fields of research – AMS and RIBs – complement each other in techniques. Development of effective and efficient beam purification techniques is of common interest to both AMS and the RIB program. Two main characteristics of the 25-MV tandem provide unique opportunities for performing the highest sensitivity measurements of AMS; namely (i) the highest operating voltage in the world, and (ii) a folded geometry which involves a 180° magnet in the terminal. For the RIB program, we have used AMS techniques to improve the sensitivity of detection of some radioactive species in the measurement of unknown masses of n-rich nuclei. For AMS, we have concentrated in exploring two important isotopes, 14C and 36Cl, for applications that require the highest sensitivity. We have successfully measured 36Cl/Cl ratios as low as a few times 10−16 in seawater samples demonstrating that our setup has the highest sensitivity for this isotope and proving that 36Cl can be measured at the levels required for a tracer in oceanography.
    Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms 06/2007; 259(1):123–130. · 1.27 Impact Factor
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    ABSTRACT: An all-permanent-magnet, 6-GHz "volume-type" ECR ion source has been constructed and evaluated. This source employs a novel magnetic field configuration with an extended central flat region to form a large, on-axis, ECR volume. It can also be converted to a traditional minimum-5 source where the ECR zones are surfaces. Comparisons are made of the performance of the source when operated in both "volume" and "surface" modes. According to the preliminary results, the "volume" mode is superior in terms of ion beam intensities and charge-state distributions.
    Particle Accelerator Conference, 2003. PAC 2003. Proceedings of the; 06/2003
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    ABSTRACT: Direct extraction, negative-surface ionization, and sputter-type sources have been designed to efficiently ionize specific members of the group VII A elements (F, Cl, Br, I, and At) for use in the nuclear structure physics and nuclear astrophysics research programs at the Holifield Radioactive Ion Beam Facility (HRIBF). A negative surface ionization source that utilizes a solid, spherical geometry, LaB6 ionizer (ϕ≅2.7 eV) for negatively ionizing Cl, Br, l, and At. During off-line evaluation, the ionization efficiency for Cl− generation ranged between 15% and 20% and for Br− generation, between 15% and 25%. Chemically active elements, such as fluorine, are often released from refractory host materials in a variety of molecular forms. Consequently, the LaB6 surface ionization source, described above, is ineffective for simultaneously dissociating and negatively ionizing this element. To overcome this problem, a new concept source, referred to as the kinetic ejection negative ion source (KENIS), was developed. The source utilizes the Cs-sputter technique to effectively dissociate molecular carriers and ionize F at efficiencies in excess of 7%. The source has been used on-line to inject >3×107 17F−/s into the tandem accelerator and deliver ∼3×106 17F9+/s (fully stripped) to the research station for completion of important nuclear-astrophysics experiments. In this article, emphasis will be placed on the design details, materials of construction, ion optics, operational parameters, thermal transport properties, emittances, and measured ionization efficiencies for these sources.
    Review of Scientific Instruments 02/2002; 73(2). · 1.60 Impact Factor
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    ABSTRACT: We are currently investigating the feasibility of using the 25 MV Tandem from the HRIBF for Accelerator Mass Spectrometry (AMS). The HRIBF Tandem accelerator is the highest operating voltage electrostatic accelerator in the world. There are a number of isotopes of interest for AMS where the high energies achieved for ions could represent a very important advantage for their detection. HRIBF has a variety of equipment for beam transport and analysis and for rejection of unwanted species, including a 180 degree magnet in the terminal. We are focusing on areas where the high voltage of the tandem plus the specialized instrumentation can play a unique role in AMS. Very recently, an AMS test run was performed at HRIBF to detect ^36Cl in sea water samples. With the tandem set at 22.5MV ^36Cl ions were stripped first to the 7+ charge state using N2 gas. By using a thin C foil after acceleration they were fully stripped (17+) with good efficiency, practically eliminating background events originating from ^36S. These initial results prove that the 25MV-tandem offers a very powerful tool for AMS.
    04/2001;
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    ABSTRACT: A simple and efficient negative surface ionization source has been designed, fabricated and initially tested for on-line generation of radioactive ion beams of the halogens (Cl, Br, I, and At) for use in the nuclear-structure and nuclear-astrophysics research programs at the Holifield Radioactive Ion Beam Facility. The source utilizes a solid, spherical geometry LaBâ surface ionizer for forming highly electronegative atoms and molecules. Despite its widely publicized propensity for being easily poisoned, no evidences of this effect were experienced during testing of the source. Nominal efficiencies of 15% for Br⁻ beam generation were obtained during off-line evaluation of the source with AlBr3 feed material when account is taken of the fractional dissociation of the molecule. Principles of operation, design features, operational parameter data, initial performance results, and beam quality data (emittance) are presented in this article.
    01/2001;
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    ABSTRACT: Negative radioactive-ion beams of isotopes with lifetimes in excess of a few hours can be efficiently generated in batch-mode. In this scenario, species of interest are sequentially produced by irradiating target materials with light-ion beams for a time period comparable with their half-lives followed by transfer of the target to a Cs-sputter ion source where the irradiated target material is sputtered by a 1-5 keV Cs^+ beam for negative-ion generation. We have developed a batch-mode Cs-sputter negative-ion source for generation of radioactive-ion beams of long-lived species for use in nuclear physics and nuclear astrophysics research programs at the Holifield Radioactive Ion Beam Facility. The source features a water-cooled target wheel that holds eight separate targets; each individual target can be remotely rotated from the primary-ion-beam production position to the Cs-sputter position. The target wheel is designed so that a production target can be exposed to the primary ion beam while generating negative-ion beams from a previously irradiated target. The source has been characterized for the future generation of several long-lived species such as ^56Ni and ^18F during off-line tests with stable beams. The design features, operational parameters, emittance and measured efficiencies of the source will be presented in this report.
    10/2000;
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    ABSTRACT: INTRODUCTION The design features of the source are described in reference 1. Plasma sputter negative ion sources of this type have been utilized by several groups for a number of applications including accelerator based atomic and nuclear physics, material processing and isotope separation (see e.g. the references contained in ref. 1). In addition to the sputter mode of operation, both positive and negative ions can be extracted directly from the RF plasma when operated in the volume mode. The capability to operate in * Managed by Martin Marietta Energy systems, Inc., under contract No. DE-AC05-84OR21400 with the U.S. Department of Energy. "The submitted manuscript has been authored by a contractor of the U.S. Government under contract No. DE-AC05-84OR21400. Accordingly, the U.S. Government retains a nonexclusive, royalty-free license to pulish or reproduce the published form of this contribution, or allow others to do so, for U.S. Government purposes." any of
    04/1998;
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    ABSTRACT: A compact, all-permanent-magnet, single-frequency electron cyclotron resonance (ECR) ion source with a large uniformly distributed ECR plasma volume has been designed and is presently under construction at the Oak Ridge National Laboratory. The central region of the field is designed to achieve a flat field (constant mod-B) which extends over the length of the central field region along the axis of symmetry and radially outward to form a uniformly distributed ECR plasma “volume.” The magnetic field design strongly contrasts with those used in conventional ECR ion sources where the central field regions are approximately parabolic and the resulting ECR zones are “surfaces.” The plasma confinement magnetic field mirror has a mirror ratio Bmax/BECR of slightly greater than 2. The source is designed to operate at a nominal rf frequency of 6 GHz. The central flat magnetic field region can be easily adjusted by mechanical means to tune the source to the resonant conditions within the limits of 5.5–6.8 GHz. The rf injection system is broadband to ensure excitation of transverse electric modes so that the rf power is largely concentrated in the resonant plasma volume which lies along and surrounds the axis of symmetry of the source. Because of the much larger ECR zone, the probability for absorption of microwave power is dramatically increased, thereby increasing the probability for acceleration of electrons, the electron temperature of the plasma, and, consequently, the “hot” electron population within the plasma volume of the source. The creation of an ECR “volume” rather than a “surface” is commensurate with higher charge states and higher beam intensities within a particular charge state. The source has also been designed so that it can be easily converted into a conventional magnetic field geometry source so that comparisons of the performances of the “volume” and “surface” forms of the source can be easily made. The design features of the source and rf injection system will be described in detail in this article. © 1998 American Institute of Physics.
    Review of Scientific Instruments 02/1998; 69(3):1311-1315. · 1.60 Impact Factor
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    ABSTRACT: A versatile, new concept, spherical-geometry, positive (negative) surface-ionization source has been designed, fabricated, and initial tests completed which can operate in either positive- or negative-ion beam generation modes without mechanical changes to the source. The highly permeable, composite Ir/C has an intrinsic work function of φ=5.29 eV and can be used directly for the generation of positive-ion beams of highly electropositive elements. For negative-surface ionization, the work function is lowered by dynamic flow of a highly electropositive adsorbate such as Cs through the ionizer matrix. The results of initial testing indicate that the source is reliable, stable and easy to operate, with efficiencies for Cs+ conservatively estimated to exceed 60% and as high as ∼50% for F− generation. However, the overall ionization efficiency for F− formation and extraction is ∼1% due to the low efficiency for thermal dissociation of the carrier molecules (assumed to be CsF and AlF). The design features, operational principles, and initial performance of the source for generating Cs+ and F−, when operated with Cs, are discussed in this article.
    Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms 01/1998; 142(4):578-591. · 1.27 Impact Factor
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    ABSTRACT: The Holifield Radioactive Ion Beam Facility (HRIBF) construction project has been completed and the first radioactive ion beam has been successfully accelerated. The project, which began in 1992, has involved numerous facility modifications. The Oak Ridge Isochronous Cyclotron has been converted from an energy booster for heavy ion beams to a light ion accelerator with internal ion source. A target-ion source and mass analysis system have been commissioned as key components of the facility's radioactive ion beam injector to the 25MV tandem electrostatic accelerator. Beam transport lines have been completed, and new diagnostics for very low intensity beams have been developed. Work continues on a unified control system. Development of research quality radioactive beams for the nuclear structure and nuclear astrophysics communities continues. The HRIBF was formally dedicated on December 12, 1996, and approved for high intensity operation as a National User Facility, the first of its kind in North America. This paper describes facility development to date.
    05/1997;
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    ABSTRACT: Over the last three years, the Holifield accelerator system has been reconfigured into a first-generation radioactive ion beam facility, the HRIBF, a national user facility for RIB research. The construction and reconfiguration have been completed and the equipment commissioning and beam development phases have started. The progress to date, the present status, and future plans will be given. The special problems connected with the production and acceleration of RIBs will be discussed.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 11/1996; 382(s 1–2):197–206. · 1.14 Impact Factor
  • G. D. Alton, G. D. Mills
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    ABSTRACT: A versatile, new concept, spherical‐geometry, positive (negative) surface‐ionization source has been designed and fabricated which will have the capability of generating both positive‐ and negative‐ion beams without mechanical changes to the source. The source utilizes a highly permeable, high‐work‐function Ir ionizer (ϕ≡5.29 eV) for ionizing highly electropositive atoms/molecules; while for negative‐surface ionization, the work function is lowered to ϕ≡1.43 eV by continually feeding cesium vapor through the ionizer matrix. The use of this technique for negative ion beam generation has the potential of overcoming the chronic poisoning effects experienced with LaB 6 while enhancing considerably the efficiency for negative surface ionization of atoms and molecules with intermediate electron affinities. The flexibility of operation in either mode makes it especially attractive for radioactive ion beam applications and, therefore, the source will be used as a complementary replacement for the high‐temperature electron impact ionization sources presently in use at the Holifield radioactive beam facility. The design features and operational principles of the source will be described in this report. © 1996 American Institute of Physics.
    Review of Scientific Instruments 05/1996; · 1.60 Impact Factor
  • G. D. Alton, G. D. Mills
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    ABSTRACT: A versatile, new concept, spherical-geometry, positive (negative) surface-ionization source has been designed and fabricated which will have the capability of generating both positive- and negative-ion beams without mechanical changes to the source. The source utilizes a highly permeable, high-work-function Ir ionizer (Ïâ¡5.29 eV) for ionizing highly electropositive atoms/molecules; while for negative-surface ionization, the work function is lowered to Ïâ¡1.43 eV by continually feeding cesium vapor through the ionizer matrix. The use of this technique for negative ion beam generation has the potential of overcoming the chronic poisoning effects experienced with LaBâ while enhancing considerably the efficiency for negative surface ionization of atoms and molecules with intermediate electron affinities. The flexibility of operation in either mode makes it especially attractive for radioactive ion beam applications and, therefore, the source will be used as a complementary replacement for the high-temperature electron impact ionization sources presently in use at the Holifield radioactive beam facility. The design features and operational principles of the source will be described in this report. {copyright} {ital 1996 American Institute of Physics.}
    Review of Scientific Instruments 01/1996; 67(3):1189-1633. · 1.60 Impact Factor
  • G. D. Alton, G. D. Mills
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    ABSTRACT: A novel, versatile, new concept, spherical-geometry, positive (negative) surface-ionization source has been designed and fabricated which will have the capability of generating both positive- and negative-ion beams without mechanical changes to the source. The source utilizes a highly permeable, high-work-function Ir ionizer (phi ≌ 5.29 eV) for ionizing highly electropositive atoms/molecules; while for negative-surface ionization, the work function is lowered by continually feeding a highly electropositive vapor through the ionizer matrix. The use of this technique to effect low work function surfaces for negative ion beam generation has the potential of overcoming the chronic poisoning effects experienced with LaB6 while enhancing the probability for negative ion formation of atomic and molecular species with low to intermediate electron affinities. The flexibility of operation in either mode makes it especially attractive for radioactive ion beam (RIB) applications and, therefore, the source will be used as a complementary replacement for the high-temperature electron impact ionization sources presently in the use at the Holifield Radioactive Ion Beam Facility (HRIBF). The design features and operational principles of the source will be described in this report.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 01/1996; 382:232-236. · 1.14 Impact Factor
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    ABSTRACT: The Holifield Radioactive Ion Beam Facility (HRIBF) is a first generation radioactive ion beam (RIB) facility. Project construction commenced in FY'93 with the initial emphasis placed on conversion of a heavily shielded room from an experiment area to an area suitable for housing the RIB Injector. The RIB Injector is the central component of the RIB project. The Injector consists of two connected high voltage platforms which are operate at -300 kilovolts and which are separated by a shield wall. One platform houses controls, instrumentation, and power supplies. The second platform houses an ISOLDE type target/ion source (TIS) which will be bombarded with light-ion beams from the Oak Ridge Isochronous Cyclotron (ORIC). Additionally, this platform houses the first stage mass separator system which is designed for 1 part in 1000 mass resolution, electrostatic quadrupole lenses for beam transport, and a cesium charge exchange cell for conversion of positive ions to negative ions for injection into the Tandem Accelerator. This paper details the design and beam development aspects of the RIB Injector
    Particle Accelerator Conference, 1995., Proceedings of the 1995; 06/1995
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    ABSTRACT: A versatile, high brightness, volume type, RF source, capable of producing positive ion beams with intensities as high as 1 mA from gaseous feed materials and microamperes of negative ion beams has been characterized. The source can also be operated as a plasma sputter negative ion source to generate up to 1 mA of a selected species. The performance of the source in the positive and negative volume modes of operation can be greatly enhanced by addition of a removable, water cooled filament assembly in place of the negative sputter probe. For example, the material utilization efficiencies of gaseous feed species can be more than doubled, total current intensities increased up to 40%, N<sub>2</sub> molecular dissociation fractions increased by 20% and minimum operating pressures reduced by a factor of four when operated in the volume mode. These added electrons also favorably effect, as a consequence of lower pressures, the emittance apparently through a reduction of scattering in the beam through the transport system. A brief description of the source and performance data for the positive volume mode of operation will be presented
    Particle Accelerator Conference, 1995., Proceedings of the 1995; 06/1995
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    ABSTRACT: The Holifield Radioactive Ion Beam Facility now under construction at the Oak Ridge National Laboratory will use the 25 MV tandem accelerator for the acceleration of radioactive ion beams to energies appropriate for research in nuclear physics; negative ion beams are, therefore, required for injection into the tandem accelerator. Because charge exchange is an efficient means for converting initially positive ion beams to negative ion beams, both positive and negative ion sources are viable options for use at the facility. The choice of the type of ion source will depend on the overall efficiency for generating the radioactive species of interest. Although direct‐extraction negative ion sources are clearly desirable, the ion formation efficiencies are often too low for practical consideration; for this situation, positive ion sources, in combination with charge exchange, are the logical choice. The high‐temperature version of the CERN‐ISOLDE positive ion source has been selected and a modified version of the source designed and fabricated for initial use at the facility because of its low emittance, relatively high ionization efficiencies, and species versatility, and because it has been engineered for remote installation, removal, and servicing as required for safe handling in a high‐radiation‐level ISOL facility. The source will be primarily used to generate ion beams from elements with intermediate to low electron affinities. Prototype plasma‐sputter negative ion sources and negative surface‐ionization sources are under design consideration for generating radioactive ion beams from high‐electron‐affinity elements. The design features of these sources and expected efficiencies and beam qualities (emittances) will be described in this report.  
    Review of Scientific Instruments 07/1994; · 1.60 Impact Factor
  • G. D. Alton, G. D. Mills, J. Dellwo
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    ABSTRACT: A versatile, high‐intensity, negative ion source has been designed and is now under construction which can be operated in either the cesium‐sputter or plasma‐sputter mode. The cesium‐sputter mode can be effected by installation of a newly designed conical‐geometry cesium‐surface ionizer; for operation in the plasma‐sputter mode, the surface ionizer is removed and either a hot filament or rf antenna plasma‐discharge igniter is installed. A multicusp magnetic field is specifically provided confining the plasma in the radial direction when the plasma‐sputter mode is selected. This arrangement allows comparison of the two modes of operation. Brief descriptions of the design features, ion optics, and anticipated performances of the two source geometries will be presented in this report.
    Review of Scientific Instruments 07/1994; · 1.60 Impact Factor
  • G. D. Alton, G. D. Mills, J. Dellwo
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    ABSTRACT: A versatile, high-intensity, negative ion source has been designed and is now under construction which can be operated in either the cesium-sputter or plasma-sputter mode. The cesium-sputter mode can be effected by installation of a newly designed conical-geometry cesium-surface ionizer; for operation in the plasma-sputter mode, the surface ionizer is removed and either a hot-filament or rf antenna plasma-discharge igniter is installed. A multicusp magnetic field is specifically provided for confining the plasma in the radial direction when the plasma-sputter mode is selected. This arrangement allows comparison of the two modes of operation. Brief descriptions of the design features, ion optics, and anticipated performances of the two source geometries will be presented in this report.
    Review of Scientific Instruments 06/1994; 65(6). · 1.60 Impact Factor
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    ABSTRACT: The Holifield Radioactive Ion Beam Facility now under construction at the Oak Ridge National Laboratory will use the 25‐MV tandem accelerator for the acceleration of radioactive ion beams to energies appropriate for research in nuclear physics; negative ion beams are, therefore, required for injection into the tandem accelerator. Because charge exchange is an efficient means for converting initially positive ion beams to negative ion beams, both positive and negative ion sources are viable options for use at the facility. The choice of the type of ion source will depend on the overall efficiency for generating the radioactive species of interest. Although direct‐extraction negative ion sources are clearly desirable, the ion formation efficiencies are often too low for practical consideration; for this situation, positive ion sources, in combination with charge exchange, are the logical choice. The high‐temperature version of the CERN‐ISOLDE positive ion source has been selected and a modified version of the source designed and fabricated for initial use at the facility because of its low emittance, relatively high ionization efficiencies and species versatility, and because it has been engineered for remote installation, removal and servicing as required for safe handling in a high‐radiation‐level ISOL facility. The source will be primarily used to generate ion beams from elements with intermediate to low ionization potentials. Prototype plasma‐sputter negative ion sources and negative surface‐ionization sources are under design consideration for generating radioactive ion beams from high‐electron‐affinity elements. The design features of these sources and expected efficiencies and beam qualities (emittances) will be described in this report.
    Review of Scientific Instruments 05/1994; · 1.60 Impact Factor