Patent Publication Number: US-7217918-B1

Title: Apparatus and method for hydrogen and oxygen mass spectrometry of the terrestrial magnetosphere

Description:
STATEMENT REGARDING FEDERAL RIGHTS 
   This invention was made with government support under Contract No. W-7405-ENG-36 awarded by the U.S. Department of Energy. The government has certain rights in the invention. 

   FIELD OF THE INVENTION 
   The present invention relates generally to spectrometry, and, more particularly, to a spectrometer that uses an electrostatic energy-per-charge analyzer to select an energy passband of incident ions. 
   BACKGROUND OF THE INVENTION 
   Driven by disturbances in the solar wind, geomagnetic storms and substorms energize the near-Earth space environment, causing numerous deleterious effects such as damage to spacecraft materials, communications problems, and spacecraft charging. While hydrogen ions (H + ) are ubiquitous in the Earth&#39;s magnetosphere, oxygen ions (O + ) at keV energies form a substantial but variable component of the Earth&#39;s plasma environment. Geostationary Operational Environmental Satellites (GOES) measurements indicate that in the geosynchronous region the concentration of 1–15 keV O +  is highly dependent on geomagnetic activity, local time, and distance from Earth. Additionally, at magnetic shells L˜7–8 (which are defined as the extension of the Earth&#39;s dipole magnetic field lines from their radial distance L (in units of earth radii) from the Earth at the Earth&#39;s magnetic equator), the most abundant ion during quiet time and moderately disturbed conditions is H + , while during strongly disturbed conditions the O +  density and energy density can be comparable to that of H +  on the dayside magnetosphere. 
   A summary of results from measurements of the space environment for ions averaged over 0.1–17 keV/e and pitch angles in the range 45°–135° at L˜5 is [W. Lennartsson and R. D. Sharp, “A comparison of 0.1–17 keV/e ion composition in the near equatorial magnetosphere between quiet and disturbed conditions,” J. Geophys. Res. 87 (1982) 6109]:
     (1) O +  is typically comparable to H +  in density and is often the dominant species, particularly during quiet times.   (2) The density ratio n(O + )/n(H + ) peaks at the lowest magnetic L-shell values and are, on average, higher during quiet times than during the early main phase of major geomagnetic storms.   (3) H +  and O +  have comparable mean energies (usually 2–7 keV) within the measured energy window, and the energies are highest during geomagnetically disturbed times.
 
Not only is O +  a major, but variable, constituent of the terrestrial magnetosphere, O +  can also damage spacecraft materials through a different process than H + . For ion energies less than approximately 50 keV, H +  loses most of its energy (e.g., 94% for 10 keV H +  incident on silicon [H. O. Funsten, S. M. Ritzau, R. W. Harper, J. E. Borovsky, and R. E. Johnson, Energy Loss by keV Ions in Silicon,  Phys. Rev. Lett.,  92 (2004) 212301–212304.] to excitations and ionizations of electrons in the target material. While this energy loss process cannot damage conductors or semiconductors, in dielectric material this can result in charging (and therefore damaging electrostatic discharges), chemical modification of the material, and degradation of electronics and electrical components. Over the same energy range, O +  loses most of its energy (e.g., 66% for 10 keV O +  incident on silicon [H. O. Funsten, S. M. Ritzau, R. W. Harper, J. E. Borovsky, and R. E. Johnson, Energy Loss by keV Ions in Silicon,  Phys. Rev. Lett.,  92 (2004) 212301–212304] to Coulombic interactions with nuclei in the target, causing atomic displacement and rearrangement along the ion track in both dielectric and conductive material. This can result in chemical modification, physical modification of the material structure, sputtering, and degradation of electronics and electrical components. Due to the large difference in the types of damage induced by H +  and O + , and substantial variation in abundances of these species, their measurement is critical for understanding the plasma environment and effects on spacecraft materials.
   

   Mass spectrometers have been flown in the Earth&#39;s magnetosphere to study the composition of the terrestrial magnetosphere in order to better understand its structure, dynamics, and coupling to the ionosphere and solar wind. These instruments typically utilize a foil-based time-of-flight (TOF) technique in which an ion of known energy transits a thin foil, where it emits secondary electrons that are detected and used to start a timer, and then continues through a drift section and is detected, stopping the timer. The time-of-flight of an ion across a known distance of travel allows the ion&#39;s mass to be determined if its energy is known. These TOF instruments, which have a field-free drift section, have a mass resolution m/Δm typically in the range of 7–10. However, these instruments require fast timing circuits, long drift lengths, and, often, multiple detectors, resulting in large mass, volume, and power requirements. 
   The present invention addresses the negative aspects of prior art instruments by exhibiting simplicity, lower mass, lower power, lower volume, and lower cost, which results from the absence of a traditional drift region that uses considerable volume and the fast timing circuitry of the TOF system. 
   Various objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims 
   SUMMARY OF THE INVENTION 
   In accordance with the purposes of the present invention, as embodied and broadly described herein, the present invention, a detector element for mass spectrometry of a flux of heavy and light ions, includes: a first detector to detect light ions that transit through a foil operatively placed in front of the first detector, and a second detector that detects the flux of heavy and light ions. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and form a part of the specification, illustrate the embodiments of the present invention and, together with the description, serve to explain the principles of the invention. In the drawings: 
       FIG. 1   a  shows a schematic of one embodiment of a detector element. 
       FIG. 1   b  shows a schematic of another embodiment of the invention where a thin foil is placed over the open aperture. 
       FIG. 1   c  shows a schematic of another embodiment of the present invention where a top-hat electrostatic energy analyzer is shown with a circular array of channel electron multiplier detector elements located at the exit of the energy-per-charge analyzer that are alternately covered with a foil or are foil-less. 
       FIG. 1   d  shows a schematic of another embodiment of the present invention where an electrostatic energy analyzer is followed by a single foil and a detector. 
       FIG. 2  graphically shows the range of H +  and O +  in carbon and is shown as a function of incident ion energy E 0 . 
       FIG. 3  graphically shows the measured ratio of counts (Equation 3) for beams of O +  and H +  incident on a 6 μg cm −2  carbon foil as a function of the incident ion energy. The solid lines are analytic fits to the data. 
       FIG. 4  graphically shows the measured ratio of counts (Equation 3) for beams of O +  and H +  incident on a 12 μg cm −2  carbon foil as a function of the incident ion energy. The solid lines are analytic fits to the data. 
   

   DETAILED DESCRIPTION 
   The present invention comprises a low resource thin-foil technique to distinguish a light ion, such as hydrogen ions (H + ), from heavy ions, such as oxygen ions (O + ), in magnetospheric plasmas, where light and heavy ions are defined hereinafter that, over a specified energy range, most or all light ions can transit a foil of a specified composition and thickness, whereas most or all heavy ions cannot transit the same foil. Thus, measurements of H+ from O+ flux distributions enables monitoring and assessment of the plasma environment of a spacecraft and the activity level of the magnetosphere. 
   Advantages of the invention relative to current space-based mass spectrometer techniques include simplicity, lower mass, lower power, lower volume, and lower cost. This primarily results from the absence of a traditional drift region, which uses considerable volume and the fast timing circuitry of the time-of-flight (TOF) system. 
   Measurement of O +  in the Earth&#39;s magnetosphere is important for understanding the initiation and evolution of geomagnetic activity. Furthermore, since ambient O +  and H +  fluxes from the ubiquitous plasma in the Earth&#39;s magnetosphere damage exposed spacecraft materials through different processes, measurement of the O +  and H +  fluxes is important for understanding cumulative damage effects to these materials due to the ambient plasma environment. 
   Referring now to  FIG. 1   a  showing a detector element, electrostatic energy-per-charge analyzer (ESA)  20  is used to select an energy passband of incident ions  10 . Ions  10  subsequently travel through either foil-less aperture  50  or aperture  30  covered by foil  40  of sufficient thickness so that most or all H +  ions of the selected energy can transit foil  40 , however, most or all O +  ions of the selected energy are stopped in foil  40 . Ions  10  that transit foil  40  are detected using first electron multiplier detector  70 , such as a microchannel plate detector or channel electron multiplier. Ions that transit foil-less aperture  50  are detected using second electron multiplier detector  80  (also a microchannel plate detector or channel electron multiplier). 
   Comparison of the counts obtained for ions transiting foil-less aperture  50  and detected by second detector  80  and counts obtained for ions transiting aperture  30  with foil  40  and detected by first detector  70  allow measurement of the incident H +  flux and the incident O +  flux. In practice, the count rate of H +  that transits foil  40  and is measured by first detector  70  and the count rate of H +  and O +  that transit foil-less aperture  50  and are measured by second detector  80  are functions of the detection probabilities of H +  and O + , the transmission probability of H +  and O +  through foil  40  at the selected energy, directional asymmetries in the incident plasma flux, and the transmission of a grid, if used, to support foil  40 . 
   A similar embodiment is shown in  FIG. 1   b , the difference being the use of thin foil  45  covering aperture  50  where thin foil  45  is sufficiently thin so that most or all H +  ions and most or all O +  ions incident on thin foil  45  transit thin foil  45 . The advantage of using thin foil  45  is that the detection efficiency of ions transiting thin foil  45  and detected by detector  80  is similar to the detection efficiency of ions transiting foil  40  and detected by detector  70 . In both cases, an ion transiting the foil can be detected by detecting the ion itself, detecting secondary electrons generated by the ion at the back surface of the foil, or a combination of the two. 
   Referring to  FIG. 1   b , ions  10  transiting electrostatic energy-per-charge analyzer  20  subsequently travel through either aperture  50  covered by thin foil  45  or aperture  30  covered by foil  40  of sufficient thickness so that most or all H +  ions of the selected energy can transit foil  40 , however, most or all O +  ions of the selected energy are stopped in foil  40 . Ions  10  that transit foil  40  are detected using first electron multiplier detector  70 , such as a microchannel plate detector or channel electron multiplier. Ions that transit thin foil  45  are detected using second electron multiplier detector  80  (also a microchannel plate detector or channel electron multiplier). Foil  40  and thin foil  45  may include a grid for structural support of the foil. The count rate of H+that transits foil  40  and is measured by first detector  70  and the count rate of H +  and O +  that transit thin foil  45  by second detector  80  are functions of the detection probabilities of H +  and O + , the transmission probability of H +  and O +  through foil  40  and thin foil  45  at the selected energy, directional asymmetries in the incident plasma flux, and the transmission of grids, if used, to support foil  40  and thin foil  45 . 
   Calculation 
   The count rate C 1  resulting from ions that are incident on first detector  70  is
 
C 1 =AφT G ε 1 T F   (1)
 
where A is the aperture area, φ is the ion flux incident on foil  40 , T G  is the grid transmission if a grid is used (otherwise T G =1), and T F  is the probability of ion transmission through foil  40 . The detection efficiency ε 1  includes the detection efficiency of ions  10  that are transmitted through foil  40  and, assuming that foil  40  is biased negative with respect to the front of first detector  70 , secondary electrons  60  generated by ions  10  at the back surface of foil  40 .
 
   The count rate C 2  resulting from ions  10  detected by second detector  80 , which lies behind foil-less aperture  50 , is
 
C 2 =AφT G ε 2   (2)
 
where ε 2  is the detection efficiency of ions that are incident on second detector  80 . The grid transmission (T G ), aperture area (A), and ion beam flux (φ) for each ion species are assumed to be the same as in Equation 1. Referring again to  FIG. 1   a , conductive grid  41  may be attached spanning foil-less aperture  50  to generate a uniform, planar electric field in front of second detector  80  that helps to ensure consistent detector performance. Conductive grid  41  also blocks the same fraction of ions as are incident on foil  40  so that the count rates C 1  and C 2  can be directly compared to derive the fluxes of light and heavy ions.
 
   The ratio R j  of count rates measured by both detectors  70  and  80 , respectively, for a particular ion species j (e.g., j equals H +  or O + ) is 
                   R   j     =         C   1       C   2       =         ɛ   1       ɛ   2       ⁢       T   F     .                 (   3   )               
The ratio R j , which depends on the energy and species of incident ions  10  and the thickness and composition of foil  40 , is related to the probability T F  that an ion is transmitted through foil  40  by the ratio of the energy-dependent detection efficiencies ε 1  and ε 2 .
 
   When using the present invention to measure combined fluxes of H +  and O + , as would be the case for measuring the plasma environment of a spacecraft, the mean count rates in first detector  70 , i.e., C 1 =C 1 (H + )+C 1 (O + ), and second detector  80 , C 2 =C 2 (H + )+C 2 (O + ), are measured. Thus, using Eqs. 1–3, the absolute H +  and O +  fluxes at the exit of the ESA  20  are: 
   
     
       
         
           
             
               
                 
                   ϕ 
                   
                     H 
                     + 
                   
                 
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                     1 
                     
                       
                         AT 
                         G 
                       
                       ⁢ 
                       
                         ɛ 
                         
                           2 
                           , 
                           
                             H 
                             + 
                           
                         
                       
                     
                   
                   ⁢ 
                   
                     
                       
                         C 
                         1 
                       
                       - 
                       
                         
                           R 
                           
                             0 
                             + 
                           
                         
                         ⁢ 
                         
                           C 
                           2 
                         
                       
                     
                     
                       
                         R 
                         
                           H 
                           + 
                         
                       
                       - 
                       
                         R 
                         
                           0 
                           + 
                         
                       
                     
                   
                 
               
             
             
               
                 ( 
                 4 
                 ) 
               
             
           
         
       
     
   
                   ϕ     O   +       =       1       AT   G     ⁢     ɛ     2   ,     O   +             ⁢           R     H   +       ⁢     C   2       -     C   1           R     H   +       -     R     0   +                     (   5   )               
The absolute H +  and O +  fluxes incident on the instrument are derived using Equations 4 and 5 combined with the energy response, angular response, and entrance aperture area of the electrostatic energy analyzer. The detection efficiencies ε 2,H+  and ε 2,O+  and count ratios R H+  and R O+  can be determined by laboratory calibration.
 
   Of particular interest is measuring the abundance of O +  relative to H + , which is simply the ratio of Equations 4 and 5: 
                     ϕ     O   +         ϕ     H   +         =         ɛ     2   ,     H   +           ɛ     2   ,     O   +           ⁢           R     H   +       ⁢     C   2       -     C   1           C   1     -       R     0   +       ⁢     C   2                     (   6   )               
While the measurement accuracy of the relative fluxes φ O+ /φ H+  depends on the total accumulated counts in detectors  70  and  80  during the time interval of the measurement, the accuracy is maximized when the thickness of foil  40  results in a transmission of H +  that is much greater than that of O + , for example when R H+ &gt;0.75 and R O+ &lt;0.25.
 
   Referring to  FIG. 1   c , typical ion energy-per-charge (E/q) spectrometer  90  consists of ESA  20  in a “top hat” configuration or a spherical section geometry. ESA  20  is normally followed by an array of detectors alternately with a foil (detector elements consisting of combined first detector  70  and foil  40  in  FIG. 1   a  are noted as even-numbered elements  2 ,  4 ,  6 , and  8  with corresponding foils  42 ,  44 ,  46 , and  48  in  FIG. 1   c ) and without a foil (detector elements consisting of second detector  80  in  FIG. 1   a  are noted as odd-numbered elements  1 ,  3 ,  5 , and  7  in  FIG. 1   c ) is located at the circular exit of ESA  20 . Each detector element views a different azimuthal angle, providing angle-resolved measurements over a wide (360°) azimuthal field-of-view (FOV). An advantage of top hat ESA  20  is that ion  10  throughput from the entrance of ESA  20  to the exit of ESA  20  is the same for all detector elements  1 ,  2 ,  3 ,  4 ,  5 ,  6 ,  7  and  8 . 
   The ion fluxes in space plasmas can be anisotropic, having a cylindrical symmetry about the local magnetic field direction. An asymmetric ion distribution results in an ion flux  10  incident on ESA  20  that is a smoothly-varying function of the azimuthal angle and of the orientation of the circular entrance aperture of ESA  20  relative to the direction of the magnetic field. To compensate for the difference in the incident ion flux on the detector elements  1 ,  2 ,  3 ,  4 ,  5 ,  6 ,  7  and  8 , a simple derivation of the fluxes φ H+  and φ O+  in Equations 4 and 5 at one azimuthal angle uses the counts measured by the detector element located at that azimuthal angle counts and the average of the counts measured by the two adjacent detector elements. For example, the fluxes φ H+  and φ O+  in Equations 4 and 5 at detector  2  in  FIG. 1   c  are derived using the measured count rate C 1  in detector  2  and the count rate C 2 , which equals the average of the measured count rate in detector  1  and the measured count rate in detector  3 . 
   In another example, the fluxes φ H+  and φ O+  in Equations 4 and 5 at detector  3  in  FIG. 1   c  are derived using the measured count rate C 2  in detector  3  and count rate C 1 , which is the average of the measured counts rate in detector  2  and the measured count rate in detector  4 . When the spacecraft spin axis is parallel to the plane of the field-of-view of the ESA, the full  4 π steradian distribution of ions, including a directional anisotropy if present, can be measured in one-half spacecraft spin. 
   A more complex analysis of the H +  and O +  fluxes can be performed by fitting a smoothly-varying count rate function C 1 (θ) that depends on incident azimuthal angle θ of ions  10  using the measured count rates in detector elements  2 ,  4 ,  6 , and  8  having a foil and another smoothly varying function C 2 (θ) that depends on incident azimuthal angle  0  using the measured count rates in detector elements  1 ,  3 ,  5 , and  7 . The azimuthal-dependent fluxes φ H+ (θ) and φ O+ (θ) are then derived using C 1 (θ) for C 1  in Equations 4 and 5 and using C 2 (θ) for C 2  in Equations 4 and 5. 
   Therefore, the count rates in the array of alternating detectors  1 ,  3 ,  5 , and  7  having no foils and the count rates in the array of alternating detectors  2 ,  4 ,  6 , and  8  having foils can be used to monitor and interpolate anisotropies in the incident ion fluxes that might falsely appear as a variation in O +  abundance relative to H +  if the relative count rate between a single pair of adjacent foil and foil-less channels is compared. 
   Another embodiment of the present invention shown in  FIG. 1   d  utilizes electrostatic energy analyzer  110  to select an energy passband of incident ions  100 . Ions  100 , falling within this energy passband and transiting the electrostatic energy analyzer, subsequently are incident on foil  120  of sufficient thickness so that most or all H +  atoms of the selected energy transit foil  120 , but most or all O +  of the selected energy are stopped in foil  120 . Note that support grid  122  may be used as structural support to foil  120 . 
   First electron multiplier detector  160  detects ions  100  that transit foil  120 . First electron multiplier detector  160  can be placed to measure the ions that transit foil  120 , secondary electrons  140  emitted from the rear of foil  120  that indicate that an ion has transited the foil, or both the ions that transit foil  120  and secondary electrons  140  emitted from the back of foil  120 . 
   Ions  100  incident on foil  120  generate secondary electrons  130  at the entrance surface of foil  120 . Secondary electrons  130  are detected by second electron multiplier detector  150  placed in a location to detect secondary electrons emitted off of the front of foil  120 . Detection by second detector  150  indicates that an ion is incident on foil  120 . Comparison of the counts measured in first detector  160  and counts measured in second detector  150  allow determination of the incident H +  flux and the incident O +  flux. 
   In practice, the count rates in detectors  160  and  150  resulting from H +  and O +  are a function of factors including the detection probabilities of H +  and O + , the yields by H +  and O + of secondary electrons  130  and  140  from the front and rear surfaces of foil  120 , the detection efficiencies of secondary electrons  130  and  140 , the transmission probability of H +  and O +  through foil  120  at the selected energy of the electrostatic energy analyzer  110 , and the transmission of support grid  122 , if used, for structural support of foil  120 . 
   At energies for which O +  cannot transit the foil, a coincidence of detected events between first detector  160  and second detector  150  indicates that the incident ion was H + . This coincidence measurement can be important for separating H +  from background counts at times in which penetrating radiation, for example during a geomagnetic storm or when the spacecraft is in the terrestrial radiation belts, stimulates detectors  150  and  160 . 
   For the embodiment in  FIG. 1   d , the count rate of first detector  160  located behind foil  120  follows from, and is identical to, that of first detector  70  in the embodiment shown in  FIG. 1   a:  
 
C 1 =AφT G ε 1 T F   (7)
 
The count rate C 3  resulting from ions impacting foil  120 , generation of secondary electrons  130  at the front surface of foil  120 , and detecting secondary electrons  130  in second detector  150  is
 
C 3 =Aφε 3   (8)
 
where ε 3  is the combination of the probability that an incident ion generates secondary electrons  130  at the front surface of foil  120  and the probability that secondary electrons  130  register a pulse in second detector  150 . The area (A) and incident flux (φ) of the ion beam incident on foil  120  for each ion species are identical to those described in Equation 7.
 
   From Equations 7 and 8 the ratio R j  of count rates of detectors  160  and  150  for a particular ion species j at incident energy E 0  is 
                   R   j     =         C   1       C   3       =         ɛ   1       ɛ   3       ⁢     T   G     ⁢       T   F     .                 (   9   )               
The value of R j  for H +  and O +  as a function of incident energy can be derived in the laboratory by directing an ion beam of H +  or O +  of known energy onto the apparatus and recording the count rates C 1  and C 3 .
 
   When using the apparatus to measure combined fluxes of H +  and O +  as would be the case for measuring the plasma environment of a spacecraft, the mean count rates in first detector  160 , i.e., C 1 =C 1 (H + )+C 1 (O + ), and second detector  150 , C 3 =C 3 (H + )+C 3 (O + ), are measured. Using Equations 7–9, the absolute H +  and O +  fluxes at the exit of the ESA  110  are: 
   
     
       
         
           
             
               
                 
                   ϕ 
                   
                     H 
                     + 
                   
                 
                 = 
                 
                   
                     1 
                     
                       A 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         ɛ 
                         
                           3 
                           , 
                           
                             H 
                             + 
                           
                         
                       
                     
                   
                   ⁢ 
                   
                     
                       
                         C 
                         1 
                       
                       - 
                       
                         
                           R 
                           
                             0 
                             + 
                           
                         
                         ⁢ 
                         
                           C 
                           3 
                         
                       
                     
                     
                       
                         R 
                         
                           H 
                           + 
                         
                       
                       - 
                       
                         R 
                         
                           0 
                           + 
                         
                       
                     
                   
                 
               
             
             
               
                 ( 
                 10 
                 ) 
               
             
           
         
       
     
   
                   ϕ     O   +       =       1     A   ⁢           ⁢     ɛ     3   ,     O   +             ⁢           R     H   +       ⁢     C   3       -     C   1           R     H   +       -     R     0   +                     (   11   )               
At energies for which no or few O +  are detected (i.e., R O+ ≈0) so that only or mostly H +  is detected, coincidence measurements between detectors  150  and  160  enable derivation of the absolute detection efficiency ε 3 [H. O. Funsten, R. W. Harper, and D. J. McComas, Absolute Detection Efficiency of Space-Based Ion Mass Spectrometers and Neutral Atom Imagers,  Review of Scientific Instruments,  76 (2005)053301.]. The probability of a coincidence between detectors  150  and  160  is simply the product of their detection efficiencies, so the coincidence count rate is
   C   1+3   =AφT   G ε 1   T   F ε 3   (12) 
The ratio C 1+3 /C 1  of coincident counts to counts in first detector  160  yields
 
                     C     1   +   3         C   1       =     ɛ   3             (   13   )               
Therefore, the absolute detection efficiency ε 3  of second detector  150  for incident H +  can be measured while the instrument is in space without absolute knowledge of the incident H +  flux. Once ε 3  is measured, the absolute incident H +  flux can be determined using Equation 10.
 
   By combining Equations 10 and 11, the abundance of O +  relative to the abundance of H +  is: 
                     ϕ     O   +         ϕ     H   +         =         ɛ     3   ,     H   +           ɛ     3   ,     O   +           ⁢             R     H   +       ⁢     C   3       -     C   1           C   1     -       R     0   +       ⁢     C   3           .               (   14   )               
The statistical error associated with the ratio φ O+ /φ H+  is minimized when the value R H+  approaches 1 and the value R O+  approaches 0. While the measurement accuracy of the relative fluxes of H +  and O +  depends on the total accumulated counts in detectors  150  and  160 , the accuracy is maximized when the thickness of foil  120  results in a transmission of H +  that is much greater than that of O + , for example when R H+ &gt;0.75 and R O+ &lt;0.25.
 
Transmission of O +  and H +  Through Thin Foils
 
   Thin foils can be fabricated of any material, although the preferred embodiment is carbon as the foil material because it is easily fabricated, the thickness can be controlled with reasonable accuracy (typically ±0.5 μg cm −2  as cited by the manufacturer ACF Metals, Inc.), and it has been successfully used on more than 45 space-based instruments [D. J. McComas, F. Allegrini, C. J. Pollock, H. O. Funsten, S. Ritzau, G. Gloeckler, Ultra-thin (˜10 nm) Carbon Foils in Space Instrumentation, Rev. Sci. Instrum., 75 (2004) 4863–4870]. Often, it is preferred that the foil be affixed to a support grid to maintain structural integrity of the foil.  FIG. 2  shows the mean projected range of H +  and O +  incident on a carbon target derived using the Stopping and Range of Ions in Matter code [J. F. Ziegler, J. P. Biersack, and U. Littmark, Stopping and Range of Ions in Solids, Pergamon, New York, 1985, Vol. 1.]. The range of H +  is approximately 3.5 times the range of O + , which results in a wide energy range over which H +  will transit a foil and O +  will not transit the foil. An optimal foil thickness for mass discrimination between H +  and O +  is selected based on the energy range whose measurement will yield key information for assessing the activity level of the magnetosphere or the potential damage to spacecraft materials due to O +  and H + . 
   Measurement Methodology 
   To demonstrate the invention shown in  FIG. 1   a  and described in Eqs. 1–6, carbon foils of thicknesses 6 and 12 μg cm −2  were procured from ACF Metals, Inc. The foils were subsequently mounted on a 333 line-per-inch (lpi) electroformed grid that was affixed to an aperture frame having a 5.9-mm-diameter aperture. The grid transmission τ G  was measured to be 83%. 
   A 2.7-mm-diameter, magnetically mass-resolved beam of H +  or O +  ions of known energy E 0  and constant flux φ was first directed toward an aperture frame with a grid only and no foil. Then the output count rate C 2  of a microchannel plate (MCP) detector, due to detection of ions that passed through the foil-less grid, was recorded. Then, the foil-less, gridded aperture was replaced by an aperture frame with a foil on a support grid, and the detector count rate C 1  due to ions transmitted through the foil and support grid was recorded with the same MCP detector. 
   Both the foil-less gridded aperture frame and the aperture frame with grid and foil were located 6.5 mm from the MCP detector and were biased to −100 V relative to the front surface of the MCP detector. This bias maximized the detection efficiency of ions for two reasons. First, secondary electrons created by ions that struck the web region of the front MCP were electrostatically suppressed back toward the MCP, and could thus initiate an electron avalanche in the MCP so that the ion is detected. Second, secondary electrons generated at the exit surface of the foil by ions that transit the foil were accelerated toward the MCP and could also initiate an avalanche, thereby increasing the ion detection efficiency when ions transited the foil. 
   Foil Transmission Measurement Results 
     FIG. 3  shows the measured ratios R H+  and R O+  as a function of incident ion energy E 0  as defined by Equation 3 for a carbon foil of thickness 6 μg cm −2 , and  FIG. 4  shows the same ratios (R H+  and R O+ ) for a carbon foil of thickness 12 μg cm −2 . Qualitatively, several expected features are apparent for a particular combination of ion species and foil thickness. First, no ions are observed to transit a foil and be detected at low energies (e.g., in  FIG. 4  below 1 keV for H +  and below 10 keV for O + ) because they lose all or most of their energy in the foil and are not detected. Second, the energy at which ions begin to transit a foil and be detected is higher for a thicker foil. Third, O +  begins to transit a foil of a particular thickness at an energy that is substantially higher than for H +  due to the large energy loss of O +  in the foil relative to H + . Fourth, at higher energies the ratio R increases toward a maximum of R≈1 as more ions successfully transit a foil and are detected (although R H+  for H +  reaches a value slightly higher than unity and subsequently decreases toward R H+ =1). 
   The presence of an overshoot for H +  in which R H+ &gt;1 is associated with the detection efficiencies ε 1  and ε 2 . While ε 2  describes the detection efficiency of ions transiting the foil-less aperture and striking the detector, ε 1  includes the combined detection efficiencies of both an incident ion that transits the foil and the secondary electrons emitted from the foil&#39;s rear surface by the ion, yielding ε 2 &gt;ε 1  for energies at which most H +  transits the foil (i.e., T F ≈1). Therefore, when ε 2 &gt;ε 1  and T F ≈1, Equation 6 yields R H+ &gt;1. This is the case for E 0 &gt;4 keV in  FIG. 3  and E 0 &gt;10.5 keV in  FIG. 4 . 
   In  FIGS. 1   a ,  1   b , and  1   d , a negative bias can be applied to the foil to accelerate low energy H +  to an energy that is sufficient for its transmission through the foil and subsequent detection by the MCP detector. The magnitude of the applied bias is selected based on the minimum energy H +  needed for detection to characterize the ambient space environment. For example, in  FIG. 3   a  foil bias of −3 kV yields a ratio R H+ &gt;90% of H +  at an incident energy of 1 eV for a 6 μg cm −2  carbon foil, resulting in the detection of 1 eV H + . As another example, in  FIG. 4   a  foil bias of −6 kV yields a ratio R H+ &gt;80% of H +  at an incident energy of 1 eV for a 12 μg cm −2  carbon foil, resulting in the detection of 1 eV H + . Furthermore, a single power supply can be used to bias both the foil and the front of the MCP detector so that the detector anode, from which the signal is extracted, is referenced to ground potential. An advantage of this configuration is that a high voltage coupling capacitor is not needed to extract the signal from the detector. 
   The energy range over which the present invention produces accurate measurements of the relative abundances of H +  and O +  depends on several factors. First, increasing the total counts accumulated in each detector over the period of measurement maximizes the measurement accuracy. Increasing the time over which measurements are accumulated and increasing the aperture area A both increase the total accumulated counts. Second, the abundance ratio is maximized by maximizing R H+  and minimizing R O+ . For example, an accurate determination of φ H+  and φ O+  results when, for example, R H+ ≧0.75 and R O+ ≦0.25. For the 6 μg cm −2  carbon foil whose data is shown in  FIG. 3 , this results in an energy range of approximately 2.5 keV to 13 keV. For the 12 μg cm −2  carbon foil whose data is shown in  FIG. 4 , this results in an energy range of approximately 5 keV to 28 keV. 
   The foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching. 
   The embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.