Abstract:
A system and method for suppressing secondary electron counts in systems that count particles. The secondary electrons are produced in a foil or other secondary electron emitting surface. A suppression grid is placed in front of the particle detector. The grid is held at an applied voltage that prevents some portion of the electrons from being transmitted through the grid and reaching the detector. The applied voltage may be adjusted and varied in a manner that provides a predicable electron count and an enhanced dynamic range of measurements at the detector.

Description:
TECHNICAL FIELD OF THE INVENTION 
     This invention relates to instrumentation that involves particle counting, and more particularly to a time of flight mass spectrometer capable of handling a large dynamic range of ion fluxes. 
     BACKGROUND OF THE INVENTION 
     Mass spectrometers use the difference in mass-to-charge ratio (m/e) of ionized atoms or molecules to separate them from each other. Mass spectrometry is therefore useful for quantization of atoms or molecules and also for determining chemical and structural information about molecules. Molecules have distinctive fragmentation patterns that provide structural information to identify structural components. 
     Neutral mass spectrometers must first create gas-phase ions, whereas ion mass spectrometers analyze pre-existing ions. In either case, the ions are then separated in space or time based on their mass-to-charge ratio. Next, the quantity of ions of each mass-to-charge ratio is measured. 
     In general a mass spectrometer consists of an ionizer (neutral mass spectrometers only), a mass-selective analyzer, and an ion detector. The magnetic-sector, quadrupole, and time-of-flight designs also require extraction and acceleration ion optics to transfer ions from the source region into the mass analyzer. 
     A time-of-flight (TOF) mass spectrometer uses the differences in transit time through a drift region to separate ions of different masses. Some operate in a pulsed mode so ions must be produced or extracted in pulses, whereas other TOF mass spectrometers measure the times of single ions. An electric field accelerates all ions into a field-free drift region with a kinetic energy of qV, where q is the ion charge and V is the applied voltage. Lighter ions have a higher velocity than heavier ions and reach the detector at the end of the drift region sooner. 
     SUMMARY OF THE INVENTION 
     One aspect of the invention is a particle detection unit that detects secondary electrons produced in a foil or other emission surface. A detector, such as a microchannel plate detector is used to detect the electrons. A suppression grid is placed in the electron flight path in front of the detector. The grid is made from a conductive material and receives an applied voltage. The applied voltage is set to value that results in a known percentage of the secondary electrons being transmitted through the grid to be detected by the detector. 
     An advantage of the invention is that it may be used to increase the dynamic range of many types of particle counting instrumentation. More specifically, instruments whose maximum counting rates are limited may be equipped with a particle suppression grid in accordance with the invention, then used for particle fluxes that would otherwise exceed the maximum counting rate. 
     The invention is especially useful for space applications of time-of-flight mass spectrometers, but may be used for ground spectrometers and other particle counting instrumentation. 
     In addition to increasing dynamic range, the invention provides a means for maintaining calibration of the counting rate. The same source particles that are analyzed may be used for the calibration, that is, there is no need for any sort of external stimulus or calibration equipment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a time-of-flight mass spectrometer having an electron suppression unit in accordance with the invention. 
     FIG. 2 illustrates the energy distribution of the electrons emitted by the foil of FIG.  1 . 
     FIG. 3 illustrates how the grid of FIG. 1 may be used to suppress electrons that would otherwise be incident on the start detector. 
     FIG. 4 illustrates how the voltage applied to the suppression grid may be varied to control electron suppression. 
     FIG. 5 illustrates the efficiency of the electron suppression as a function of the voltage applied to the suppression grid. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a time of flight mass spectrometer  10  in accordance with the invention. For purposes of example, spectrometer  10  receives pre-existing ions, such as is the case for space applications. However, the invention is also applicable to time of flight mass spectrometers that use an ionizer. In general, spectrometer  10  is just one example of instrumentation that counts particles, and is thus one example of instrumentation to which the invention described herein may be applied. 
     Spectrometer  10  is a “single coincidence” spectrometer, meaning that a single start and a single stop per particle define an event. Primary ions enter the spectrometer  10  and pass through an ultra-thin foil  11 , such as a carbon foil. The interaction of the ions and the foil  11  produces secondary electrons, which are emitted from the exit locations of the ions. The foil may be more generally referred to as a type of “secondary electron emission surface.” These secondary electrons are electrically directed to a start detector  13  to provide a start signal. An example of a suitable start detector  13  is a microchannel plate. 
     A suppression grid  12  is placed in front of the start detector  13 . As explained below, grid  12  permits only a fraction of the electrons to pass through and impinge on detector  13 . Although typically, each primary particle that enters the spectrometer  10  provides only at most a few electrons, the number of primary particles may be quite high. The suppression of electrons by grid  12  prevents spectrometer  10  from being saturated in the case of high primary particle rates. Grid  12  is made from a highly transmissive conductive material, examples being nickel or gold. 
     The ions pass through the foil  11 , traverse a drift space and impinge upon a stop detector  14 . A microchannel plate may also be used for detector  14 . Although some ions may become neutralized at foil  11 , because the drift space has a small electric field, the times of flight are not much different for the ions and the neutrals. 
     The time lapse between the “start” pulse from the electron detector and the “stop” pulses from the stop detector  14  represents the time-of-flight of the respective ion. This time-of-flight is proportional to the square root of the ratio of the ion&#39;s mass over its charge. 
     If desired, the biasing of foil  11  may be used to reduce false start electrons. As shown in FIG. 1, foil  11  is held at a voltage, V foil , which is more negative than any part of spectrometer  10  other than suppression grid  12 . With sufficient biasing, the result is suppression of electrons arriving from anywhere other than from foil  11 . 
     Electron suppression at grid  12  is achieved by applying a voltage, V cutoff , to grid  12 , where V cutoff  is more negative than V foil . Electron suppression by grid  12  is based on the fact that the electrons are emitted from foil  11  with a very low but highly repeatable energy distribution. As explained below, V cutoff  may be adjusted so that only a known fraction of the secondary electrons that would otherwise reach detector  13  are transmitted through grid  12 . 
     A control unit  15  may be used to provide appropriate voltage for V cutoff  as well as V foil , with appropriate control electronics for grid  12  and foil  11 . Processing unit  16  receives the output of detectors  13  and  14  and may be programmed to analyze the output data and to implement various calibration techniques discussed below. 
     FIG. 2 illustrates the energy distribution of the electrons emitted by foil  11 . As illustrated, this energy peaks at only a few electron volts (eV). This secondary electron spectrum is independent of the energy of the primary ions. 
     FIG. 3 illustrates how grid  12  is used to suppress the count rate of start electrons that reach detector  13 . A voltage, V cutoff , is applied to grid  12 , such that only the fraction of electrons that have sufficiently high energy, E&gt;V cutoff , pass through grid  12 . 
     FIG. 4 illustrates how V cutoff  may be varied to control the fraction of start electrons that pass through grid  12 . The number of electrons with sufficiently high energy to pass through grid  12  is the area of the curve to the right of V cutoff . As V cutoff  is increased, the fraction of start electrons that reach detector  13  is reduced. In effect, grid  12  acts as a variable “electrostatic choke” on the count rate of start electrons. 
     FIG. 5 illustrates the resulting efficiency of this throttling as a function of V cutoff . As can be seen, grid  12  provides a controllable variable count efficiency. Like the curve of FIG. 4, the curve of FIG. 5 is predictable and particle independent. 
     The secondary electron suppression provided by grid  12  can be introduced anywhere along the electron flight path. Grid  12  may be placed immediately after foil  11  or just in front of detector  13 . Also, additional grids could be used for additional throttling. 
     An alternative embodiment of spectroscope  10  could be equipped with a stop foil and stop detector for electrons produced on stop foil (not shown). This would permit secondary electrons to be produced and collected, to produce stop electrons and a stop signal, in a manner similar to the production of start electrons. This alternative embodiment could be further equipped with a suppression grid associated with the stop detector, which could be used to throttle the stop electrons in a manner similar to the above-described throttling of start electrons. 
     Using the above-described electron suppression method, it is expected that, if desired, more than 99% of the secondary electrons from foil  11  may be suppressed. Because of the nature of the electron emission curve of FIG. 5, it can be determined with accuracy, what percent of electrons are being detected at detector  13 . Specifically, a particular value of V cutoff  can be expected to suppress a known percent of electrons at grid  12 . 
     The above-described method of electron suppression may also be used for purposes of calibrating the spectrometer  10 . The same ions being analyzed may be used as the calibration source. Measurements that vary from the curves illustrated in FIGS. 4 and 5 indicate that the applied voltage, V cutoff , may require calibration. 
     One approach to calibration is to scan the suppression voltage, V cutoff , while ions are received at various constant fluxes. The secondary electron counting rates may be measured as a function of the suppression voltage. 
     Calibration may be also performed as a function of energy, to remove energy dependent effects. Or, calibration may be performed as a function of ion species, to remove species dependent effects. For ions of mixed species, calibration using the actual ions to be detected guarantees that the calibration is appropriate for the particular mixture of ions observed. 
     Because of the ease of calibration, routine calibrations may be incorporated into the normal data collection cycle of spectrometer  10 . At any point in time, or at periodic intervals, the curves of FIG. 4 or  5  may be run to determine what counts were detected for the V cutoff  that was set. 
     Measuring calibration factors as a function of energy may be used to increase the accuracy of the absolute count rate measurement. This is because the secondary electron emission curve, illustrated in FIG. 2, is extremely reproducible. This means that a fit to a known curve shape is used rather than simple count ratios. Reproducible discrepancies from this curve can provide an onboard internal measure of problems with the measurements, internal to spectrometer  10 , such as errors in the applied voltage, or with the detector or counter. 
     Calibration is especially effective when combined with the use of separate stop and start channels. This approach is useful for space-based spectrometers, which often use a single stop channel in association with multiple start channels. The different start channels are used for effecting different viewing directions simultaneously. Comparison of start and stop rates provides improved knowledge of absolute calibration because the different channels provide independent measurements of the same incident ions. 
     As stated above, the particle suppression concepts described herein may be applied to any particle counting instrumentation. In fact, detector  13  and suppression grid  12  could be manufactured as a unit to be installed in such instruments. Appropriate voltage controls could be implemented. Two such units could be used in a time of flight spectrometer for counting both start and stop electrons. 
     Other Embodiments 
     Although the present invention has been described in detail, it should be understood that various changes, substitutions, and alterations can be made hereto without departing from the spirit and scope of the invention as defined by the appended claims.