Patent Publication Number: US-6218672-B1

Title: Ion source

Description:
This application claims the benefit of U.S. Provisional Application Ser. No. 60/093,971 Filed Jul. 24, 1998. 
    
    
     This invention was made with U.S. government support under contract number S-97526-Z with the National Aeronautics and Space Administration. The U.S. government has certain rights in this invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention relations to an ion source, and more particularly to a device which generates ions using a beam of generated electrons. 
     BACKGROUND 
     An ion source utilizes an electron gun to generate electrons, which ionize neutral atoms and molecules. There are various apparatus, such as a mass spectrometer, which require an ion source. In a mass spectrometer, the ion source is used to ionize a gas which consists of neutral particles so that the ions of the gas can be analyzed. The electron-ionization source which has been generally used for mass spectrometers is the Nier source. Referring to FIGS. 1 and 2 there is shown diagrams of two views of a typical Nier source  10 . The Nier source  10  comprises a chamber  12  having a front wall  14  and a back wall  16 . A gas inlet passage  18  extends through the back wall  16  to allow a gas to be ionized to enter the chamber  12 . An exit slit  20  is in the front wall  14  of the chamber  12 . An electron source filament  22  is in the chamber adjacent the side wall  24  of the chamber  12 . The filament  20  is connected to a source of electricity (not shown) for heating the filament  20 . An accelerator plate  26  is adjacent but spaced from the outer surface of the front wall  14  of the chamber  12 . The accelerator plate  26  has a slit  28  therethrough which is aligned with the exit slit  20  in the front wall  14  of the chamber  12 . 
     In the operation of the Nier ion source  10 , a gas of the sample material passes through the gas inlet passage  18  into the chamber  12 . The filament  20  is heated so as to emit electrons. The electrons from the filament  20  pass transversely across the chamber  12  and across the flow of the gas. The electrons bombard the gas so that ions are generated by the electron impact. These ions are drawn through the exit slit  20  in the front wall  14  of the chamber  12  by the accelerator plate  26 . The ions then pass through the slit  28  in the accelerator plate  26  and flow into a mass spectrometer (not shown). In the Nier ion source  10 , ionization takes place only in the area of the gas which is traversed by the electron beam so that the amount of ions generated is relatively small. Also, the ions are generated only over an area defined by the length and width of the exit slit  20 . These ions must then be focused onto the entrance of a quadrupole mass spectrometer, which is typically a small circular aperture. Therefore, additional focusing plates are required to ensure that all of the ions generated enter the mass spectrometer. 
     SUMMARY OF THE INVENTION 
     An ion source includes an electron generating chamber and a ionization chamber adjacent the electron generating chamber. A gas inlet opens into the ionization chamber and an opening extends between the ionization chamber and the electron generating chamber. The electron generating chamber has an outlet opening therein. The gas inlet, opening between the ionization chamber and electron generating chamber and the outlet opening are all along a common axis. The electron generating chamber has therein means for generating electrons completely around the axis and means for directing the electrons radially toward the axis. Also in the electron generating chamber is means for deflecting the radial flow of electrons longitudinally along the axis and into the ionization chamber. At the outlet opening is means for attracting the ions toward and through the outlet opening. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an end view of a diagram of a prior art Nier ion source; 
     FIG. 2 is a side view of the Nier ion source shown in FIG. 1; 
     FIG. 3 is a schematic view of an ion source of the present invention; 
     FIG. 4 is a schematic view of the ion generating chamber of the ion source of the present invention showing the trajectory of the electrons; 
     FIG. 5 is a schematic view showing the electron trajectories in the ionization chamber of the ion source of the present invention; 
     FIG. 6 is a schematic view showing the ion trajectories in the ionization chamber of the ion source of the present invention; and 
     FIG. 7 is a schematic view showing the ion trajectories in the electron generating chamber of the ion source of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring initially to FIG. 3, there is schematically shown a ion source  30  in accordance with the present invention. Ion source  30  comprises a housing  31  having therein an ionization chamber  32  and an electron generating chamber  34  adjacent the ionization chamber  32 . A gas inlet tube  36  extends into the ionization chamber  32  at its side opposite the electron generating chamber  34 . The ionization chamber  32  has an opening  38  in its wall  40  adjacent the electron generating chamber  34 . 
     In the electron generating chamber  34  is a cathode filament  42  which extends completely around the electron generating chamber  34  so that the filament  42  extends completely around an axis  44  which extends longitudinally along the center of the opening  38 . Electrodes  46  are in the electron generating chamber  34  between the filament  42  and the axis  44  to guide electrons from the filament  42  across the electron generating chamber  34  transversely to the axis  44 . A deflection electrode  48  is within the electron generating chamber, within the electrodes  46 , and around the axis  44 . The deflection electrode  48  has a conical deflection plate  50  facing the opening  38  in the wall  40  of the ionization chamber  32 . An accelerator electrode  52  is on the side of the electron generating chamber  34  opposite the ionization chamber  32 . The accelerator electrode  52  has a tubular portion  54  which extends within the deflection electrode  48  and around the axis  44 . 
     In the operation of the ion source  30 , a current is provided through the cathode filament  42  to generate electrons. The electrodes  46  generate an electric field which causes the electrons to flow as a beam  56  radially toward the axis  44  of the electron generating chamber. The deflection electrode  48  generates an electric field which causes the beam  56  to be deflected along the axis  44  into the ionization chamber  32 . Thus, there is provided an electron beam  58  which extends longitudinally along the axis  44  and into the flow of gas entering the ionization chamber  32  through the inlet tube  36 . The electrons impinge on the gas causing the generation of ions of the material forming the gas. The ions are drawn along the axis  44  into the electron generating chamber  34  and then through to cylindrical portion  54  of the accelerator plate  50  into a device (not shown) for using or analyzing the ions. The charge of the electrons helps draw the ions from the ionization chamber  32  into the electron generating chamber  34  and the charge on the accelerator plate  50  draws the flow of ions the rest of the way. 
     The transverse beam of electrons  56  and the longitudinal beam of electrons  58  is achieved by the size of the electrodes  46  and  50  and the voltage applied thereto. FIG. 4 is a sectional view of the electron generating chamber  32  showing typical arrangement and size of the electrodes  46  and  50  as well as typical voltages applied to the electrodes to achieve the desired beam of the electrons. Although the dimensions shown in FIG. 4 can be of any dimension, they would typically be in mils. Also, it is desirable that the longitudinal beam of electrons  58  be as collimated as possible. FIG. 5 is a diagram of a typical ionization chamber  32  which can achieve improved collimation of the beam  58 . The ionization chamber  32  is formed of three cylindrical sections  60 ,  62  and  64  of different diameters. The section  60  is of the smallest diameter and is adjacent the electron generating chamber  34 . The section  62  is the next largest section and the section  64  is the largest section. The outer wall of each of the sections are at the same voltage whereas the radial walls connecting the sections are at a lower voltage. The dimensions of the ionization chamber  32  and the voltages applied to the various portions of the ionization chamber  32  shown in FIG. 5 will collimate the electron beam  58 . 
     Referring to FIG. 6 there is shown trajectories of the beam of ions  66  generated in the ionization chamber  32  of the structure shown in FIG.  5 . This generated beam of ions  66  is drawn out of the ionization chamber  32  into the electron generating chamber  34  as previously described. Referring to FIG. 7 there is shown the trajectories of the beam of ions  66  through the electron generating chamber  34 . This FIG. also shows a typical voltage which is applied to the cylindrical portion  54  of the accelerator electrode  52  to achieve a flow of the ions out of the electron generating chamber  34  and into the adjacent device. This voltage also forms the beam of ions  66  into a cylindrical beam of relatively small diameter. 
     In the ion source  30  of the present invention, the generated electrons in the electron generating chamber  34  do not just flow transversely across the flow of the gas, but is deflected to flow as a beam longitudinally along the axis along which the gas flows. This results in a much longer contact between the electrons and the gas so that a larger number of ions is generated. Thus there is provided a larger signal to noise ratio. In addition, the generated ions are formed into a cylindrical beam of relatively small diameter so that it can more easily pass through a cylindrical opening into the device which is to either use or analyze the ions. Thus there is a smaller loss of ions from the ion source into the device which is to use or analyze the ions. Thus, there is provided by the present invention an ion source which provides a larger number of generated ions and which provides a cylindrical beam of the generated ions for a small loss of ions from the ion source to the device which either uses or analyzes the ions.