Abstract:
A negative ion source placed inside a negatively-charged high voltage electrode emits a beam which is accelerated to moderate energy, approximately 35,000 electron volts, and filtered by a momentum analyzer i.e. an analyzing bending magnet, to remove unwanted ions. Reference ions such as carbon-12 are deflected and measured in an off-axis Faraday cup. Ions of interest, such as carbon ions of mass 14, are accelerated through 300 kV to ground potential and passed through a gas stripper where the ions undergo charge exchange and molecular destruction. The desired isotope, carbon-14 along with fragments of the interfering molecular ions, emerge from the stripper into a momentum analyzer which removes undesirable isotope ions. The ions are further filtered by passing through an electrostatic spherical analyzer to remove ions which have undergone charge exchange. The ions remaining after the spherical analyzer are transmitted to a detector and counted.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     Not applicable. 
     STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     The invention relates to electrostatic accelerators in general and to the use of electrostaic accelerators to perform accelerator mass spectrometry in particular. 
     Since the late 1970&#39;s techniques have been developed for using tandem electrostatic accelerators to develop extremely sensitive mass spectrometers able to distinguish the presence of atomic isotopic ratios as small as 10 −15 , for example between carbon-12 and carbon-14. The detection of very small quantities of isotopes from samples of less than 1 mg has revolutionized the process of carbon dating. The ability to uniquely detect the presence of atomic isotopes finds many uses, for example, carbon dating, or using atomic isotopes as chemical labels. The use of long-lived radioactive compounds as labels forms an important subset of the possible uses to which accelerator mass spectrometry (AMS) can be employed. Radioactive isotopes with long half-lives are difficult to measure by detection of radioactive decay if the sample size is small and the half-life of the radioactive isotope is large. For radioactive carbon-14, with a half-life of 5,730 years, a sample size of one gram is generally considered necessary for radioactive carbon dating. A one-gram sample of modern carbon contains approximately 10 −12  grams  14 C or approximately 5×10 10  atoms of  14 C and produces only 14 disintegrations per minute. Using an accelerator mass spectrometer (AMS) as much as 10 percent of the atoms of  14 C present in a sample can be directly detected. The result is that the concentration of carbon-14 can be measured with a precision of better than one percent in a modern sample, using a sample size of less than one mg in only a few minutes. 
     Mass spectrometry uses the principal that a charged particle is deflected more or less by a magnetic or static electric field depending on the velocity and mass of the particle. By the proper combination of magnetic and/or electrostatic analyzers it is possible to separate particles by mass and velocity and thus to detect the mass and energy of individual particles. The detection of a particular atomic isotope, however, requires for unique detection that all molecular isobars be eliminated. For example, in the case of carbon-14 molecular isobars of  13 CH and  12 CH 2  are perhaps one million times more prevalent than the carbon-14 to be measured. To detect carbon-14, negatively charged particles of mass 14 are accelerated in the tandem accelerator through a potential of about one-half million volts to several million volts. The negatively charged particles of mass 14 are passed through a stripping column of rarefied gas in the high voltage positively charged electrode. The stripping column causes the particles to lose electrons and in the process breaks up any molecular isobars into their constituent parts. The positively charged ions are accelerated away from the positively charged high voltage electrode to ground and the particles of mass 14 are separated and counted. 
     Although very successful accelerator mass spectrometers (AMS) are relatively expensive and of large size, and have certain operation requirements such as the handling of sulfur hexafluoride insulating gas which contribute to the expensive operation. A smaller and simpler design for an accelerator mass spectrometer (AMS) is needed to facilitate the continued growth of AMS applications. 
     SUMMARY OF THE INVENTION 
     The accelerator mass spectrometer of this invention utilizes a single stage air insulated accelerator (SSAMS). A negative carbon ion source is placed inside a negatively-charged high voltage terminal. The ion beam emerges from the ion source and is accelerated to moderate energy, approximately 35,000 electron volts, and is filtered by a momentum analyzer, i.e., an analyzing bending magnet, to remove unwanted ions. Reference ions such as carbon-12 are deflected and measured in an off-axis Faraday cup. Ions of mass 14 are accelerated to ground potential and passed through a gas stripper where the ions undergo charge exchange and molecular destruction. The desired isotope, carbon-14 along with fragments of the interfering molecular ions emerge from a stripper into a momentum analyzer (analyzing bending magnet) which removes all but the desired isotope ions from the beam. The ions in emerging from the analyzing magnet are further filtered by passing through an electrostatic spherical analyzer to remove ions which have undergone charge exchange while passing through the analyzing magnet. The ions remaining after the spherical analyzer are transmitted to a detector and counted. 
     It is an object of the present invention to provide an accelerator mass spectrometer of lower-cost, simpler operation and smaller size. 
     It is a further object of the present invention to provide an accelerator mass spectrometer for detecting carbon-12 to carbon-14 ratios. 
     It is another object of the present invention to provide an accelerator mass spectrometer utilizing an air insulated high voltage electrode. 
     Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a somewhat schematic top plan view of the accelerator mass spectrometer of this invention. 
     FIG. 2 is somewhat schematic side elevational view of the accelerator mass spectrometer of FIG.  1 . 
     FIG. 3 is a schematic view of the beam profile in the x-axis and y-axis of the beam as it moves through the accelerator of FIG.  1 . 
     FIG. 4 is a somewhat schematic top plan view of an alternative embodiment of the accelerator mass spectrometer of this invention. 
     FIG. 5 is a somewhat schematic side elevational view of the accelerator mass spectrometer of FIG.  4 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring more particularly to FIGS. 1-3, wherein like numbers refer to similar parts, a Single Stage Accelerator Mass Spectrometer (SSAMS)  20  is shown in FIG.  1  and FIG.  2 . The SSAMS  20  has an air insulated high voltage electrode  22  which is isolated from ground  24  by conventional high voltage ceramic insulators  26 . A solid-state high voltage power supply  28  is positioned between ground  24  and the high voltage electrode  22  and raises the potential of the high voltage electrode to 300,000 volts. The high voltage electrode  22  is constructed of a steel frame  30  which supports an equipment deck  32 . The equipment deck  32  is enclosed by removable metal panels (not shown) creating a Faraday cage within the high voltage electrode. 
     Mounted on the equipment deck  32  are a multi-sample carbon negative ion source  34 , which produces 1 − carbon ions with an energy of about six keV, followed by a beam extractor  36  with an extracting acceleration of about twelve KV which is followed by an Einzel lens  38  followed by a preacceleration tube  40  producing an additional acceleration of about twenty-two KV. The carbon ion beam  41  thus produced has an energy of about 35 keV. An electrostatic quadruple singlet  42  focuses the beam  41  into an analyzer  44  consisting of a 90-degree permanent magnet of 10 inch radius. The analyzer magnet  44  separates the negative ions contained in the beam by mass, lighter weight ions being caused to bend more than heavier ions. The dominant ions present consist of carbon-12, carbon-13, carbon-14 and various molecular isobars such as  13 CH,  13 CH 2 ,  12 CH 2 , and  12 CH. The analyzing magnet  44  bends the molecular weight 12 particles so they are captured in a Faraday cup  46  positioned for that purpose. The Faraday cup  46  thus produces a current which is a direct measurement of the rate of molecular weight 12 particles produced by the ion source and transmitted through the analyzer. The molecular weight 12 particles are substantially all carbon-12 atoms and thus the outlet of the Faraday cup  46  corresponds to carbon-12 contained in the particle beam  41 . 
     Molecular weight 14 particles consisting of carbon-14,  13 CH, and  12 CH 2 , are passed through a second electrostatic quadruple singlet lens  48  followed by a resolving aperture  50  followed by a second Einzel lens  52  and are injected into a 300 kV acceleration tube  54  which extends between the high voltage electrode  22  and ground  24 . A grounded cage or preferably room  56  surrounds the high voltage electrode  22  and the acceleration tube  54 . The room  56  isolates the high voltage components of the SSAMS  20  from the human operator of the SSAMS for safety reasons, and allows the high voltage electrode  22  to be surrounded by air which has been conditioned to remove moisture and dust particles by an air supply unit  58 . The air supply unit  58  creates a slight positive pressure within the room  56  preventing the inflow of unconditioned air into the room. By controlling moisture the breakdown resistance of the air is controlled, and by removing particles, the precipitation of dust onto the high voltage electrode  22  is prevented. 
     Immediately following the acceleration tube  54  the ion beam  41  passes through a gas stripper column  60  of argon gas having a density of two micrograms per square cm, along the axis of the beam  41 . The stripper column causes the mass 14 ions to collide with argon atoms which breaks up the molecular isobars  13 CH, and  12 CH 2  so that the only remaining mass 14 ions are carbon-14 ions in the +1, +2, or +3 state. The gas stripper  60  necessarily results in gas leaking into the evacuated beam transport pipe  61 . Where stripping occurs at the high voltage electrode, such as typically done in the tandem accelerator, removal of gas is complicated by the necessity of locating the pumping equipment within the high voltage electrode. In the SSAMS  20  of this invention the stripping column  60  is located at ground potential allowing vacuum pumps  62  located on either side of the stripping column  60  to easily remove the gas injected into the beam transport  61 . 
     A second analyzer  64  receives the beam  41  as it leaves the gas stripping column  60  and is composed of an electromagnetic bending magnet  66  and an electrostatic spherical analyzer  68  separated by a resolving aperture  69 . The bending magnet  66  alone is not sufficient to separate the carbon-14 atoms from the other atomic species because lighter weight ions can be neutralized by charge exchange just as they reach the same amount of deflection as the carbon-14 atoms experiences and thus these neutral particles follow the same trajectory as the carbon-14 atoms and, in the absence of an additional analyzing component, strike the detector. Utilizing an electrostatic spherical analyzer  68  which is of the same radius as the electromagnetic bending magnet  66  produces an achromatic lens system which reduces the dispersion caused by the variation in particle energy produced by energy loss in the stripping column  60 . 
     Following the spherical analyzer, the beam passes through a final resolving aperture  70  into a silicon surface barrier detector  72  which counts individual carbon-14 ions. Typically the bending magnet  66  and the electrostatic spherical analyzer  68  are adjusted so that carbon-14 +1  ions impact the detector  72 . Carbon-14 +1  ions predominate because of the relatively low beam energy, approximately 335 keV, making up about 50 percent of the carbon-14 ions present in the stripped beam. 
     An important feature of the SSAMS  20  is the multi-sample carbon source  34 . Such multi-sample sources are well known in the prior art, and may be based on solid or gaseous samples as taught in U.S. Pat. No. 5,644,130 to James E. Raatz which is incorporated herein by reference. The multi-sample carbon source  34  when combined with the beam extractor  36  forms a multiply selectable negative carbon ion source. A multiple cathode ion source in a 40 or a 134-sample configuration is available from National Electrostatic Corporation of Middleton, Wis. The multi-sample carbon source  34  allows unknown samples to be compared against known samples. The known samples of particular use are carbon derived from modern biological materials, and old carbon samples derived from geologically old carbon sources, such as coal which contains essentially no carbon-14. The old carbon allows calibrations of the SSAMS  20  to be sure that the stripper is adequately breaking down molecular isobars and that the second analyzer is removing all non carbon-14 particles. On the other hand, modem carbon has a known ratio between carbon-12 and carbon-14 which can be used to calibrate the relationship between the current produced by the carbon-12 beam in the Faraday cup  46 , and the carbon-14 as detected by the silicon surface barrier detector  72 . Thus the errors due to a certain amount of the carbon-12 which forms hydrogen compounds not reaching the Faraday cup  46 , or losses of carbon-14 atoms due to the fact the stripping process produces only about 50 percent carbon-14 +1  ions, can be substantially eliminated. By repeatedly analyzing the known samples between unknown samples the SSAMS  20  has produced sample measurement precision of better than one percent with a background of better than 40,000 years. 
     It will be understood by those skilled in the art of electrostatic accelerator and beam optic design that it will be useful or desirable to place additional Faraday cup and beam monitors along the beam path through the evacuated beam transport pipe  61 . In particular, an adjustable Faraday cup and beam monitor may be placed between the electromagnetic bending magnet  66  and the electrostatic spherical analyzer  68 . Similarly, a beam monitor and Faraday cup may be placed after the pre-acceleration tube  40 , and at other places as those skilled in the art may find useful, in setting up and calibrating the SSAMS  20 . In addition, vacuum pumps will be placed within the high voltage electrode  22  and in the evacuated beam transport pipe  61 . 
     The use of an air insulated high voltage electrode  22  allows ready access to the multi-sample carbon ion source  34 . The high voltage electrode  22  is grounded, and a door  74  connected to a safety interlock  76  which also grounds the electrode  22 , allows access to the high voltage electrode  22 . The multi-sample carbon ion source  34  contained within the electrode  22  is accessed by removing metal panels (not shown) which cover the vertical faces of the high voltage electrode  22 . In a typical accelerator mass spectrometer, beam currents are substancially higher than in the SSAMS  20  due to the practice of continuously accelerating carbon-13 ions and periodically accelerating carbon-12 ions. The SSAMS  20 , by accelerating only mass-14 ions, reduces beam current and the undesirable production of x-rays which can result from higher beam currents. The relatively large easily accessible high voltage electrode allows the positioning of electronic controllers (not shown) within equipment boxes  78 , within the high voltage electrode  22 . The electronic control box  80  which controls and supplies voltage to the ion source  34  may be held at about 35 kV voltage above that of the high voltage electrode. 
     Electrical power to operate the various pieces of equipment located within the high voltage electrode are supplied by a pair of isolation transformers (not shown) connected in series which supply conventional wall plug power to the electronic controllers and equipment located on the equipment deck  32 . Control commands are communicated by means of optical fiber. 
     The SSAMS  20  of this invention may be used for the detection of other atomic isotopes. The applicability of the SSAMS  20  design to other isotopes depends on the particular isotope being considered. For many isotopes such as chlorine, very high beam energies are required so the isotope of interest can be distinguished from isotopes having the same mass but different atomic numbers. However, for some isotopes such as tritium a relatively low acceleration voltage such as supplied by the air-insulated accelerator of this invention can be effective. Of course, for various other ions the individual beam handling components such as the beam optics, including the first beam bending magnet, will need to be configured to the particular isotope of interest. 
     The essential components for any SSAMS include a high voltage air insulated electrode having a potential of less than 500 kilovolts, preferably less than 300 kilovolts, and located at the high voltage electrode an ion source which may be remotely controlled or automatically controlled to produce ions from multiple samples sequentially in time. Also located at the high voltage electrode is a mass spectrometer consisting of an analyzer which breaks ions produced by the ion source into at least two species on the basis of mass. One of the two species of ions is directed into the Faraday cup to produce a reference current proportional to the rate of collection of the one ion. The mass spectrometer injecting the second of the two ion species into an acceleration column. A gas stripper will preferably be used, because its mass density can be readily adjusted, although thin foil stripping could be used. The stripper is followed by an analyzer and finally a particle detector. 
     Preferably the high voltage electrode SSAMS will be located within a safety cage or room which is supplied with conditioned air, the entrance of the room being connected with a safety interlock to ground the high voltage electrode before or as the door is opened. Preferably wall socket power will be transmitted to the high voltage electrode deck through one or more isolation transformers arranged in series, and the high voltage electrode deck will be supplied with a solid-state high voltage source. 
     It should be understood that although air insulated electrodes of more than one million volts are known, because of their size, space and cost limitations, it is desirable that the high voltage electrode be as low voltage as possible, and that high voltage electrodes above about 500 kilovolts will not be economically desirable. 
     It should be understood that where the invention is defined with respect to ground, ground potential would not necessarily be equivalent to an earth ground, but may vary by such small potential as does not interfere with the practicality and simplicity of the accelerator described herein. 
     It should be understood that the term “single stage electrostatic accelerator” means that the ion beam used in the mass spectrometer passes only once between the high-voltage electrode and ground. 
     It should be understood that the location of the SSAMS components could be reversed so that the ion source  34  within a separate lower voltage electrode  82 , the pre-acceleration tube  40 , and the permanent magnet  44 , together with the Faraday cup  46 , could all be located at ground, and the gas stripping column  60 , having analyzing magnet  66 , electrostatic spherical analyzer  68  and the silicon surface barrier detector  72 , could all be located within the high-voltage electrode as shown in FIGS. 4 and 5, wherein like reference numbers refer to like parts. The ion source when positioned at ground must still be raised to approximately 35,000 volts requiring a voltage isolation chamber  84 , and the additional power and control which would be necessary at the high-voltage electrode, to handle the electromagnet and data collection at the detector. However the invention is not intended to be limited to the particular configuration shown and described but only by the claims. 
     It should also be understood that the description of the ion source, the ion filter, and the ion accelerator, as being within the high voltage electrode, is defined to include positioning of these component parts such that they are substantially included within the Faraday shield defining the high voltage electrode, or are positioned within a Faraday cage of a second higher voltage electrode mounted on the high voltage electrode. 
     It is understood that the invention is not limited to the particular construction and arrangement of parts herein illustrated and described, but embraces all such modified forms thereof as come within the scope of the following claims.