Abstract:
A method for performing accelerator mass spectrometry, includes producing a beam of positive ions having different multiple charges from a multicharged ion source; selecting positive ions having a charge state of from +2 to +4 to define a portion of the beam of positive ions; and scattering at least a portion of the portion of the beam of positive ions off a surface of a target to directly convert a portion of the positive ions in the portion of the beam of positive ions to negative ions.

Description:
STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT 
     This invention was made with Government support under contract No. DE-AC05-960R22464 awarded by the United States Department of Energy to Lockheed Martin Energy Research Corporation, and the Government has certain rights in this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates generally to the field of accelerator mass spectrometry. More particularly, the invention relates to accelerator mass spectrometers employing a multicharged ion source. 
     2. Discussion of the Related Art 
     The main difficulty in single atom detection of C-14 arises from the isobaric interferences due to N-14 atomic ions and  12 CH 2  and  13 CH molecular ions. In conventional accelerator mass spectrometry (AMS) the approach consists of using a negative ion source to eliminate the  14 N contamination, since it does not support a stable negative ion, accelerating the negative ion beam in a tandem accelerator to high energy (few MeV), and then dissociating molecular ions isobaric with  14 C −  also present in the ion beam either in a foil or gas target. Subsequent stages of electrostatic and magnetic analysis are then used to isolate the  14 C ions prior to their detection. Conventional AMS requires large, nuclear physics scale facilities, with correspondingly high cost, which are usually not dedicated to a single task, and entails time consuming sample preparation prior to the actual measurements, and so is not suited to quasi-real time monitoring Of C-14 levels. 
     SUMMARY OF THE INVENTION 
     The invention includes an apparatus and method for the detection of carbon-14 and other rare isotopes where molecular ion isobaric interferences are a problem, and where interfering atomic isobars do not form stable negative ions. In this invention, large nuclear physics scale facilities such as used in conventional accelerator mass spectrometry (AMS), for example, are not needed. 
     These, and other, goals and embodiments of the invention will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following description, while indicating preferred embodiments of the invention and numerous specific details thereof, is given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the invention without departing from the spirit thereof, and the invention includes all such modifications. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A clear conception of the advantages and features constituting the invention, and of the components and operation of model systems provided with the invention, will become more readily apparent by referring to the exemplary, and therefore nonlimiting, embodiment illustrated in the drawing accompanying and forming a part of this specification. 
     FIG. 1 a  illustrates a high level schematic view of an accelerator mass spectrometry apparatus, representing an embodiment of the invention. 
     FIG. 1 b  illustrates a high level schemative view of another accelerator mass spectrometry apparatus, representing an embodiment of the invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The invention and the various features and advantageous details thereof are explained more fully with reference to the nonlimiting embodiments that are illustrated in the accompanying drawings and detailed in the following description of preferred embodiments. Descriptions of well known components and processing techniques are omitted so as not to unnecessarily obscure the invention in detail. 
     Within this application several publications are referenced by Arabic numerals within parentheses. Full citations for these, and other, publications may be found at the end of the specification immediately preceding the claims after the section heading References. The disclosures of all these publications in their entireties are hereby expressly incorporated by reference into the present application for the purpose of indicating the background of the invention and illustrating the state of the art. 
     The below-referenced U.S. Patents disclose embodiments that were satisfactory for the purposes for which they were intended. The entire contents of U.S. Pat. Nos. 4,489,237; 5,661,299; 5,621,209; and 4,830,010 are hereby expressly incorporated by reference into the present application as if fully set forth herein. 
     FIG. 1 a  illustrates an embodiment of an apparatus  10  according to the invention. The apparatus  10  includes multicharged ion source  12  for the production of a multicharged ion beam  14 . Suitable multicharged ion sources include, but are not limited to, an electron cyclotron resonance (ECR) ion source. An ECR ion source  12  contains a high-temperature electron plasma which is very efficient in removing electrons from source atoms to form multiply charged positive ions. If mass-14 ions are extracted from the ECR plasma in sufficiently high charge states, one is assured that there will be no molecular species in the extracted multicharged ion beam  14 , since the loosely bound electrons forming the relevant chemical bonds of the molecule will have been removed, leading to immediate breakup of the molecule into smaller mass fragments. During operation of the apparatus  10 , the ECR preferably produces a multicharged ion beam  14  with a charge state which is high enough to eliminate molecular isobar interference. The charge state is preferably at least +2, and more preferably at least +3. 
     In addition to facilitating removal of one of the isobaric interferences, a further advantage of using an multicharged ion source such as an ECR source  12  is very high ionization efficiency. Stable C beams with ionization efficiencies as high as 24% are achievable for formation of +4 ions. For low abundance isotopic species, this value is reduced about a factor of two due to adsorption on the source chamber wall. However, the use of a hot liner can reduce wall adsorption, and result in values very close to that obtained for the corresponding stable isotope. 
     After the multicharged ion beam  14  is extracted from the multicharged ion source  12  and accelerated to energies in the keV range, the beam  14  passes to an analyzing magnet  15  where the ions are separated into different regions according to their mass-14 charge state. The ions in different regions can be processed independently of one another. For instance, the ions in one region can be received by a detector  16  for monitoring the C-12 intensity for reference purposes and the ions in another region can be selected for further processing by the apparatus. The charge state for the selected ions is preferably at least +2, more preferably +3, and most preferably +4 to maximize ionization efficiency. The beam  14  from the analyzing magnet  15  will be completely free of any interfering molecular isobars, but will still contain a strongly dominant  14 N component of the same charge state. 
     The beam  14  from the analyzing magnet  15  enters a UHV chamber  17  where the beam  14  is incident upon a target surface  18  at a grazing angle of incidence for the formation of negative ions. The angle of incidence is preferably at most approximately 5° (e.g. from approximately 1° to approximately 5.0°), but depends on the energy of the multicharged ion beam. Suitable target surfaces  18  include, but are not limited to, a metal or insulator high quality single crystal. With a LiF (100) target, very high efficiency for converting incident multicharged O and F projectiles into scattered negative ions can be obtained, that is essentially independent of incident charge state. Maximum efficiencies for converting incident C 4+  to C −  are estimated to be in the 50% range. Operating the insulator target surface at high temperature where the ionic conductivity will be sufficiently high will ameliorate sample-charging effects due to impact of the high intensity ion beam  14 . Alternatively a single crystal metal target can be used, with a concomitant decrease of negative ion yield of about an order of magnitude, but having the same feature of the negative ion yield being independent of incident charge state. This feature is a key one, in that it permits the choice of charge state to be determined solely on the basis of maximum ionization efficiency. Since specular reflection conditions apply, the scattered beam  14  will still have low divergence, small size, and very close to its original energy. 
     The scattered beam  14  passes from the target  18  to a first (primary) electrostatic analyzer  20  to disperse the different scattered charge states. The different scattered charge states can be dispersed into different regions  21 . The different regions  21  can be discrete or can overlap. Suitable first electrostatic analyzers  20  include, but are not limited to, low resolution electrostatic analyzers and low resolution deflection plates. 
     The ions in the zone  21  receiving the charge state of interest pass to a second (secondary) electrostatic analyzer  22  which further spatially separates the desired  14 C −  ions from other scattered charge states. The secondary analyzer can provide high resolution. For instance, the negative ion component of the beam  14  can be further separated from the other scattered charge states to further reduce background and discriminate against other negative ions of different energy (e.g.  28 Si −  from  28 Si 8+  having the same mass to charge ratio as the  14 C 4+  ions extracted from the ECR source  12 ). The negative ion component of the beam  14  will not exhibit interference from  14 N due to the instability of  14 N as a negative ion. The second electrostatic analyzer  22  preferably has a higher resolution than the first electrostatic analyzer  20 . Suitable second electrostatic analyzers  22  include, but are not limited to cylindrical or hemispherical analyzers. The beam  14  from the second electrostatic analyzer  22  is received by a particle detector  24  such as a channel electron multiplier or multichannel plate, which may be position sensitive. 
     FIG. 1 b  illustrates an alternative embodiment of the apparatus  10  according to the invention. This embodiment of the apparatus adds an electrostatic analysis apparatus  13  prior to the surface scattering stage to remove possible contamination due to charge exchange of the extracted beams with residual gas prior to magnetic analysis. As depicted, the electrostatic analysis apparatus involves turning an additional turn of the beam  14 . 
     In place of the beam/solid target negative ion formation process, a gas cell could be introduced in which multiple electron capture could occur to form the fast neutrals, followed by a second gas cell for negative ion formation. If a suitable gas could be found, the two steps could be performed in a single gas cell. Any approach involving gas phase collisions for the neutralization of the multicharged ions and negative ion formation will have much lower efficiency than the ion-target surface interaction process. 
     In place of the ECR ion source  12 , other ion sources of low energy multicharged ions could, in principle, be employed. But at present only the ECR source  12  combines the high ionization efficiency and beam intensity characteristics required for this apparatus  10 . 
     This invention can in principle be used for detection of other rare isotopes where molecular ion isobaric interferences are a problem, and where interfering atomic isobars do not form stable negative ions, provided the specie of interest can be formed in a charge state sufficiently high that the interfering molecular ion is no longer stable. 
     The apparatus  10  has value within the technological arts. As medical diagnostic, for measurements of in vivo  14 C uptake related to detection of cancer or other pathologies, for biomedical research into oral availability of drugs or transport across cell membranes, for radiocarbon dating applications in the areas of paleoclimatology and archaeology, for tracer studies of atmospheric chemistry and transport, ocean mixing, erosional processes and glacial recession, diffusion through soils, as diagnostic in studies of diesel exhaust pollution, lubricant consumption and degradation, wear analyses of graphite composite materials, and of various petroleum industry problems (see Ref. 10). There are virtually innumerable uses for the invention, all of which need not be detailed here. 
     The ion-target surface interaction process described above (see also Ref. 8 and 9) essentially combines two steps: neutralization of the multicharged C ions and negative ion formation. This results in simplicity, compactness of design and low cost. Additionally, the apparatus  10  requires voltages in the range 5-20 kV in contrast to the conventional approaches (Ref. 1 and 3) which can require at least a factor of 100 higher voltages. The reduced voltage requirements can also translate into increased simplicity, compactness and reduced cost. Additionally, the compact ECR source  12  (Ref. 7) combined with the highly efficient process for converting multicharged positive ions to negative ions (Ref. 7 and 8) provides an increased efficiency and throughput than those obtained with existing approaches. 
     The difficulty of sample preparation is substantially reduced in the invention as compared to conventional accelerator mass spectrometry (AMS) hardware. In previous approaches the samples had to be converted off line to solid pellets (Ref. 1) that could be inserted into a negative ion sputter source. The present scheme can use samples in gaseous (see Ref. 5) form directly (Ref. 6 and 7). Together with the highly efficient compact ECR source  12  and method for converting multicharged positive ions to negative ions, this makes possible much faster processing times, and opens the possibility of quasi real-time monitoring. 
     The apparatus  10  can also provide an increased sensitivity above what can be achieved with conventional biomedical tracer measurement methods. This increased sensitivity permits usage of lower radioactive tracer levels, with corresponding positive environmental, health and safety, and financial impacts. 
     Because of the above advantages, this apparatus  10  should find great utility in quasi-real time monitoring of C-14 based chemical tracer uptake in biological systems for the purposes of atmospheric pollution studies, cancer research, medical diagnostics, or other biomedical studies. 
     The term “approximately”, as used herein, is defined as at least close to a given value (e.g., preferably within 10% of, more preferably within 1% of, and most preferably within 0.1% of). The term “substantially”, as used herein, is defined as at least approaching a given state (e.g., preferably witin 10%, more preferably within 1% of, and most preferably within 0.1% of). The term “coupled”, as used herein, is defined as connected, although not necessarily directly, and not necessarily mechanically. 
     All the disclosed embodiments of the invention described herein can be realized and practiced without undue experimentation. Although the best mode of carrying out the invention contemplated by the inventors is disclosed above, practice of the invention is not limited thereto. Accordingly, it will be appreciated by those skilled in the art that the invention may be practiced otherwise than as specifically described herein. 
     For example, the individual components need not be formed in the disclosed shapes, or assembled in the disclosed configuration, but could be provided in virtually any shape, and assembled in virtually any configuration. Further, the individual components need not be fabricated from the disclosed materials, but could be fabricated from virtually any suitable materials. Further, although the components of the apparatus described herein can be constructed from physically separate modules, it will be manifest that any two or more of the components may be integrated into a single modules. Furthermore, all the disclosed elements and features of each disclosed embodiment can be combined with, or substituted for, the disclosed elements and features of every other disclosed embodiment except where such elements or features are mutually exclusive. 
     It will be manifest that various additions, modifications and rearrangements of the features of the invention may be made without deviating from the spirit and scope of the underlying inventive concept. It is intended that the scope of the invention as defined by the appended claims and their equivalents cover all such additions, modifications, and rearrangements. The appended claims are not to be interpreted as including means-plus-function limitations, unless such a limitation is explicitly recited in a given claim using the phrase “means-for.” Expedient embodiments of the invention are differentiated by the appended subclaims. 
     REFERENCES 
     1. D. Elmore and F. M. Phillips, Science 236, 543 (1987). 
     2. U.S. Pat. No. 4,489,237: Method of broad band mass spectrometry and apparatus therefor. 
     3. U.S. Pat. No. 5,661,299: Miniature AMS detector for ultrasensitive detection of individual carbon-14 and tritium atoms. 
     4. U.S. Pat. No. 5,621,209: Attomole detector. 
     5. U.S. Pat. No. 4,830,010: Methods for the diagnosis of gastrointestinal disorders. 
     6. R. Geller and B. Jacquot, Physica Scripta T3 (1983); R. Geller, IEEE Trans. Nucl. Sci. NS-26, 2120 (1979). 
     7. L. Maunoury et al., Proc. 13th Int. Workshop on ECR Ion Sources, D. May, ed., Texas A&amp;M, 26-28 February 1997. 
     8. L. Folkerts, S. Schippers, D. M. Zehner, and F. W. Meyer, Phys. Rev. Lett. 74, 2204 (1995). FIG.  3 . 
     9. F. W. Meyer, Q. Yan, P. Zeijlmans van Emmichoven, I. G. Hughes, and G. Spierings, NIMB 125, 138 (1997). FIG.  12 . 
     10. J. C. Davis, NIMB 92, 1 (1994).