Abstract:
In a conventional a photomultiplier tube, the present method provides for shorting the last stage or stages of dynodes to the anode, thereby causing photoelectrons therein to impact a smaller number of dynodes effectively reducing the transit time of electrons through the photomultiplier.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    Conventional time-of-flight mass spectrometry (TOFMS) is a technique that uses electron impact (EI) ionization. EI ionization involves irradiating a gas phase molecule of the unknown composition with an electron beam, which displaces outer orbital electrons, thereby producing a net positive charge on the newly formed ion.  
           [0002]    TOFMS has seen a resurgence due to the commercial development of two new ionization methods: electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI). The availability of low cost pulsed extraction electronics, high speed digital oscilloscopes and ultra-high speed microchannel plate detectors have improved the mass resolution capability of the traditional TOFMS technique.  
           [0003]    Mass spectrometers include three major components: (1) an ionization source; (2) a mass filter; and (3) a detector. The ionization source ionizes an unknown composition. The mass filter temporally separates the resultant ions so that lighter ions reach the detector before the heavier ions. The detector converts the ions into a charge pulse. The detector ascertains the arrival times of the charge pulses, which correspond to the masses of the ions. Identifying the masses of the ions enables identification of the unknown composition.  
           [0004]    Typically, a TOF mass spectrometer also has a digitizer connected to the detector to process the signals.  
           [0005]    In the MALDI technique, the analyte of interest is usually mixed in solution with a large excess of light absorbing matrix material. The sample mixture is placed on a mass spectrometer sample plate and illuminated with a pulse of light from a pulsed laser. The matrix material absorbs the laser light, the analyte molecules are desorbed from the sample surface and ionized by one of a number of ionization mechanisms.  
           [0006]    In ESI, the analyte of interest is normally dissolved in an acidified solution. This solution is pumped out the end of a metallic capillary tube held at a high potential. This potential causes the evaporation of extremely small droplets that acquire a high positive charge. Through one of a number of mechanisms, these small droplets continue to evaporate until individual molecular ions are evaporated from the droplet surface into the gas phase. These ions then are extracted through a series of ion optics into the source region of the TOFMS.  
           [0007]    The mass filter temporally separates ions by accelerating the ions with a bias voltage ranging up to ±30 kV. Since like charges repel, negative ions, for example, experience repulsive forces, thus tend to accelerate from, a negative potential toward a positive or less negative potential. A higher bias voltage will generate stronger repelling forces, thus greater ion acceleration. The repelling force accelerates lighter particles faster than heavier particles. Although smaller voltages foster better temporal separation, larger voltages allow for greater detection efficiency.  
           [0008]    Detectors typically convert an ion into many electrons, forming an electron cloud which is more readily discernable. Three conventional types of detectors, or electron multipliers, generally have been used. The first type of electron multiplier is a single channel electron multiplier (SCEM). SCEMs typically are not used in modem TOFMS instruments because SCEMs provide limited dynamic range and temporal resolution, in the order of 20-30 nanoseconds to full width at half maximum (ns FWHM).  
           [0009]    The second type of electron multiplier is a discrete dynode electron multiplier (DDEM). DDEMs exhibit good dynamic range, and are used in moderate and low resolution applications because of relatively poor pulse widths, in the order of 6-10 ns FWHM.  
           [0010]    The third type of electron multiplier is a microchannel plate (MCP) electron multiplier. MCPs typically have limited dynamic range, in the order of 20 mHz/cm2 of active area. However, MCPs provide the highest temporal resolution, in the order of 650 ps FWHM.  
           [0011]    Although the invention is not limited to use with an MCP-type electron multiplier, for ease of understanding, the following reviews the general operating characteristics of an MCP.  
           [0012]    [0012]FIG. 1 shows an MCP  10 . MCP  10  typically is constructed from a fused array of drawn glass tubes filled with a solid, acid-etchable core. Each tube is drawn according to conventional fiber-optic techniques to form single fibers called mono-fibers. A number of these mono-fibers then are stacked in a hexagonal array called a multi. The entire assembly is drawn again to form multi-fibers. The multi-fibers then are stacked to form a boule or billet which is fused together at high temperature. The fused billet is sliced on a wafer saw to the required bias angle, edged to size, then ground and polished to an optical finish, defining a glass wafer  15 . Glass wafer  15  is chemically processed to remove the solid core material, leaving a honeycomb structure of millions of pores, also known as holes or channels,  20 , which extend at an angle  25  relative to the normal flight trajectory of an ion between the surfaces  30  and  32  of MCP  10 .  
           [0013]    Referring also to FIG. 2, subsequent processing of the interior surface  35  of each channel  20  produces conductive and secondary electron emissive properties. These secondary electron emissive properties cause channel  20  to produce one or more electrons upon absorption or conversion of a particle, such as an ion, impacting surface  35 . As a result, each channel  20  functions like an SCEM, having a continuous dynode source which operates relatively independently of surrounding channels  20 .  
           [0014]    Finally, a thin metal electrode  40 , typically constructed from Inconel or Nichrome, is vacuum deposited on the surfaces  30  and  32  of wafer  15 , electrically connecting all channels  20  in parallel. Electrodes  40  permit application of a voltage  45  across MCP  10 .  
           [0015]    MCP  10  receives ions  50  accelerated thereto by an ion-separating voltage  55 . Ion  50  enters an input end  60  of channel  20  and strikes interior surface  35  at a point  62 . The impact on surface  35  causes the emission of at least one secondary electron  65 . Each secondary electron  65  is accelerated by the electrostatic field created by voltage  45  across channel  20  until electron  65  strikes another point (not shown) on interior surface  35 . Assuming secondary electrons  65  have accumulated enough energy from the electrostatic field, each impact releases more secondary electrons  70 . This process typically occurs ten to twenty times in channel  20 , depending upon the design and use thereof, resulting in a significant signal gain or cascade of output electrons  80 . For example, channel  20  may generate 50-500 electrons for each ion.  
           [0016]    Gain impacts the sensitivity, or ability to detect an ion, of a spectrometer. A spectrometer with a high gain produces many electrons in an electron cloud corresponding to an ion, thus providing a larger target to detect.  
           [0017]    Some TOF mass spectrometers drive electron clouds produced at the channel output toward an anode or charge collector, such as a Faraday cup (not shown). The charge collector sums or integrates the electron charges into a charge pulse, which is analyzed by a digitizer. Because lighter ions accelerate faster than the heavier ions, the voltage pulses correspond to the masses of the respective ions. The aggregate of arrival times of the voltage pulses corresponds to the mass spectrum of the ions. The mass spectrum of the ions aids in discerning the composition of the unknown composition.  
           [0018]    Detecting the masses of very massive ions requires a high “post acceleration” potential between the ionization source and the MCP. A high post acceleration potential permits sufficient high mass ion conversion efficiency to enable detection of massive ions. However, MCPs cannot withstand excessive voltages thereacross without risk of significant degradation. Accordingly, some MCP-based spectrometers “float” or electronically isolate the anode from the charge collectror. To this end, the MCP output voltage is dropped to ground through a voltage divider. Unfortunately, this creates great potential for arcing or short circuiting between the output and the anode, the energy from which could damage or destroy sensitive and expensive spectrometry equipment. Thus, attaining superior temporal range with an MCP-based spectrometer which also has superior dynamic capabilities, or high sensitivity, may come at significant, unpredictable cost.  
           [0019]    [0019]FIG. 3 shows a modular detector assembly  100  assembled with a modified vacuum flange  200  of a TOF spectrometer (not shown) described in U.S. patent application Ser. No. 09/809,090. Detector assembly  100  includes a detector cartridge  300 , a scintillator  400  and a charge collector  500 . Detector cartridge  300  receives the ions which enter an input end  105  from an ionization source (not shown) and produces electrons at intervals that correspond to the respective masses of the ions, as described above. Scintillator  400  receives output electrons from detector cartridge  300  and produces approximately 400 output photons for every electron absorbed. Collector  500  receives and converts the output photons into up to 5×10 6  electrons and sums the electrons into a charge pulse. As discussed above, the timing of the pulses correspond to the masses of the ions, thereby aiding identification of an unknown composition.  
           [0020]    Collector  500  includes a photomultiplier  505  which, responsive to the output photons of scintillator  400 , generates on the order of 5×10 6  electrons for every photon that strikes photomultiplier  505 . Collector  500  also includes a socket  510  into which photomultiplier is received. Photomultiplier  505  and socket  510  are electrically connected with pins (not shown) extending from photomultiplier  505  and received in electrical contacts (not shown) in socket  510  in a known manner.  
           [0021]    Referring to FIGS. 4 and 5, in operation, detector assembly  100  may be used to detect, for example, large negative ions. An ionization source S has multiple plates (not shown) across which a voltage repels only negative ions −i into a field free drift tube. A net +10 kV voltage exists across the gap between ionization source S and detector cartridge  300 , which may contain, for example, an MCP, between ionization source output S 0 , which is at ground, and MCP input voltage P mi . Ions −i are attracted to detector cartridge  300  by the net positive voltage bias with respect to detector cartridge  300 . The voltage between ionization source S and detector cartridge  300  temporally separates negative ions −i by mass. Ions −i may be post-accelerated with a high voltage to increase overall ion detection efficiency.  
           [0022]    A net positive potential, such as +1 kV, across detector cartridge  300 , i.e. between detector cartridge input (P mi =+10 kV) and detector cartridge output (P mo =+11 kV), accelerates electrons −e, converted from ions −i, as discussed above, through detector cartridge  300 . A net positive voltage, such as +2 kV, between detector cartridge  300  and scintillator  400 , i.e. between detector cartridge output (P mo =+11 kV) and scintillator input (P si =+13 kV), accelerates electrons −e from detector cartridge  300  toward scintillator  400 .  
           [0023]    Scintillator  400  converts electrons −e into photons P. Photons P are insensitive to electrical fields, therefore the voltage across scintillator  400  may drop to ground. Photons P strike collector  500 .  
           [0024]    The photomultiplier (not shown) of collector  900  converts photons P into electrons (not shown). A net positive voltage across collector  500 , such as +600 kV, from collector input (P co =−600 kV) to the grounded output, urges electrons through collector  500 . The electrons are summed into a charge pulse at the output C.  
           [0025]    As shown in FIG. 5, detector assembly  100  is bi-polar in that detector assembly  100  may be operated to detect large positive ions as well as negative ions. Similar to the above, ionization source S directs only positive ions +i toward detector cartridge  300 . A net −10 kV voltage between ionization source S and detector cartridge  300 , i.e. between ionization source output S 0  and detector cartridge input voltage P mi . Ions +i are attracted to detector cartridge  300  by the net negative voltage bias with respect to detector cartridge  300 .  
           [0026]    A net positive potential, such as +1 kV, across detector cartridge  300 , between detector cartridge input voltage P mi  (e.g. −10 kV) and detector cartridge output voltage P mo  (e.g. −9 kV), likewise accelerates electrons −e through detector cartridge  300 .  
           [0027]    Electrons −e from detector cartridge  300  travel toward scintillator  400 , driven by a net positive voltage, such as +3 kV, between detector cartridge  300  and scintillator  400 , i.e. between detector cartridge output (P mo =9 kV) and scintillator input (P si =6 kV).  
           [0028]    Scintillator  400  converts electrons −e into photons P. The output of scintillator  400  is grounded.  
           [0029]    Photomultiplier (not shown) in collector  500  converts photons P into electrons (not shown), which are urged therethrough with a net +600 kV voltage and summed into a charge pulse at output C.  
           [0030]    [0030]FIG. 6 schematically shows photomultiplier  505  of FIG. 3. Photomultiplier  505  includes an evacuated envelope or vessel  515  which has a cylindrical wall  520  and a faceplate  525 . A photocathode  530  is formed on an interior surface of faceplate  525  and on the interior surface of a portion of cylindrical wall  520 . Light incident on faceplate  525  enters the envelope  535 . Photocathode  530  converts the incident light into a plurality of first photoelectrons e 1 . First photoelectrons e 1  impact a first dynode DY 1 . First dynode DY 1  absorbs and generates for each first photoelectron e 1  a plurality of second photoelectrons e 2  which impact a second dynode DY 2 . Second dynode DY 2  absorbs and generates for each second photoelectron e 2  a plurality of third photoelectrons e 3  which impact a third dynode DY 3 . Successive dynode absorption and generation by downstream dynodes DY 3 -DY 12 , similar to that described with respect to dynodes DY 1  and DY 2 , substantially increases the gain of the energy of each photon converted by photocathode  530  into a first photoelectron e 1 .  
           [0031]    Photomultiplier  505  includes an anode A which receives photoelectrons e 13  generated by dynode DY 12 . Anode A effectively sums the charges of photoelectrons e 13  into a pulse. Typically, the pulse is carried from anode A to digitizing equipment for quantizing and sequencing successive pulses. Anode A is surrounded by a shield S which has the same potential as final dynode DY 12  to prevent noise from developing at anode A.  
           [0032]    As shown in FIG. 7, resistors R 1 -R 11  are interposed among dynodes DY 1 -DY 12  to equally divide voltage applied between terminals T 1  and T 2 , between first dynode DY 1  and final dynode DY 12 . Initial, intermediate and final terminals T 1 -T 3  have voltages that establish a net positive field to urge electrons toward anode A. Thus, terminal T 1  may have a negative voltage, T 2  may be grounded and T 3  may have a positive voltage. A resistor R 12  between dynode DY 12  and terminal T 2  provides a potential with which to detect a signal.  
           [0033]    An exemplary photomultiplier  505  is a Hamamatsu RU7400 photomultiplier tube, regarded as a “fast” photomultiplier. “Fast” refers to the reaction time from when a photon strikes a dynode to when a resultant electron strikes an anode of the photomultiplier. For example, the RU7400 has a reaction time of approximately 3.2 ns FWHM. Faster reaction times improve the dynamic range of a detector because the detector may identify individual ions, rather than groups of ions. Faster reaction times may be possible by connecting one or more downstream dynodes with the anode.  
           [0034]    Although photomultiplier tubes, such as the Hamamatsu RU7400, generally are fairly fast, the photomultiplier tube stage represents the bottleneck that slows the overall responsiveness of a TOFMS. What is needed, and what is not taught or suggested by known prior art, is a method for enhancing photomultiplier tube speed by shorting out the last few stages of a photomultiplier tube.  
         SUMMARY OF THE INVENTION  
         [0035]    The invention provides a method for enhancing photomultiplier tube speed by shorting out the last few stages of a photomultiplier tube and/or employing a dynode closer than the anode to the cathode as the anode.  
           [0036]    Accordingly, the invention is a method for enhancing the speed of a photomultiplier tube, including a plurality of discrete dynodes in multiple stages, each dynode facing an adjacent dynode within the plurality of dynodes, and an anode, involving electrically connecting a selected dynode with the anode.  
           [0037]    The invention provides improved elements and arrangements thereof, for the purposes described, which are inexpensive, dependable and effective in accomplishing intended purposes of the invention.  
           [0038]    Other features and advantages of the present invention will become apparent from the following description of the preferred embodiments which refers to the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0039]    The invention is described in detail below with reference to the following figures, throughout which similar reference characters denote corresponding features consistently, wherein:  
         [0040]    [0040]FIG. 1 is a perspective view, partially in section, of a multichannel plate;  
         [0041]    [0041]FIG. 2 is a schematic view of a single channel of the multichannel plate of FIG. 1;  
         [0042]    [0042]FIG. 3 is a cross-sectional view of a detector assembly;  
         [0043]    [0043]FIGS. 4 and 5 are schematic views of alternative voltages across a mass spectrometer incorporating the detector assembly of FIG. 3.  
         [0044]    [0044]FIG. 6 is a cross-sectional view of a photomultiplier tube;  
         [0045]    [0045]FIG. 7 is a schematic view of the electronic circuitry of the photomultiplier tube of FIG. 6; and  
         [0046]    [0046]FIG. 8 is a schematic view of electronic circuitry for the photomultiplier tube of FIG. 6 according to the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0047]    The invention is a method for enhancing photomultiplier tube speed by shorting out one or more final stages of a photomultiplier tube.  
         [0048]    [0048]FIG. 8 shows a basic electronic circuitry schematic for a photomultiplier comparable to photomultiplier  505  of FIGS. 3 and 6, which is described above. As with photomultiplier  505 , the photomultiplier with the circuitry of FIG. 8 also encounters and promotes photoelectrons that successively impact cascaded dynodes, each dynode absorbing and generating for photoelectron a plurality of resultant photoelectrons. Resistors R 1 -R 3 , RW, RX and RY are interposed among dynodes DY 1 -DY 4 , DYX, DYY and DYZ to equally divide voltage applied between terminals T 1  and T 2 , between first dynode DY 1  and final dynode DYZ. Initial, intermediate and final terminals T 1 -T 3  have voltages that establish a net positive field to urge electrons toward anode A. Thus, terminal T 1  may have a negative voltage, T 2  may be grounded and T 3  may have a positive voltage. A resistor RZ between dynode DYZ and terminal T 2  provides a potential with which to detect a signal.  
         [0049]    The present method provides for shorting a selected dynode, such as dynode DY 4 , to anode A. Thus, the photoelectrons impact a smaller number of dynodes within the associated PMT (not shown). Dynodes DYX-DYZ also are shorted to anode A to prevent unwanted charge buildup on dynodes DYX-DYZ.  
         [0050]    The method also connects an intermediate terminal T 2 , between the selected dynode (DY 4 ) and an adjacent dynode between initial terminal T 1  and the selected dynode. As shown, resister R 3  is disposed between dynode DY 3  and terminal T 2 , to provide a potential with which to detect a signal.  
         [0051]    Although FIG. 8 shows dynodes DY 4  and DYX-DYZ shorted to anode A, the invention is not limited to such configuration. Rather, the invention extends to all applications in which the final dynode or dynodes of a PMT are shorted to the anode thereof. Also, as with photomultiplier  505  of FIGS. 3 and 6, anode A is surrounded by a shield S which has the same potential as the final dynode, DY 4  in FIG. 8, to prevent noise from developing at anode A.  
         [0052]    Shorting the last dynode(s) to the anode effectively reduces the transit time of electrons through a photomultiplier. Such configuration may sacrifice some of the gain of the photomultiplier. However, in most applications, the gain sacrificed is not required.  
         [0053]    The amount of transit time reduced due to the foregoing technique is difficult to calculate. First, even though dynodes DY 1 -DYZ all may have substantially the same geometry, be equally spaced and have equal voltages applied thereacross, transit time from the photocathode is likely to differ between stages. Second, transit time for an electron between two electrodes is a function of electric field strength therebetween, therefore a function of voltage and the shape and spacing of the electrodes, which may differ ever so slightly. Third, dynodes are not intended to serve as anodes for fast timing applications, thus may introduce inefficiencies at the collection point.  
         [0054]    Following is a formula for calculating electron transit time. The formula is predicated on the existence of a uniform electric field between dynode stages.  
       T   =       L        [     2     v        (     q   m     )         ]         1   /   2                             
 
         [0055]    where q/m=1.76×10 11  C/kg, v=voltage and L=length.  
         [0056]    Alternatively, the transit time may be derived from measuring peak pulse current against average pulse current to calculate pulse width. Fifty percent of the pulse width is calculated by:  
         Δ t   s =τ f 1 n (2)* t   p +τ r 1 n ( I− 0.5( I   p   /I   o )) 
         [0057]    where τ f  is pulse fall time; t p  is time to peak; τ r  is pulse rise time; I p  is peak pulse current; and I o  is average current.  
         [0058]    Due to the unpredictability of the reduction in electron transit time, the speed of a photomultiplier configured in accordance with the invention is not necessarily in proportion to the number of dynodes shorted out. However, the the foregoing dynode-shorting technique allows for operating a photomultiplier with a gain of less than 10×10 4 , rather than the more typically imposed gain of 10×10 10 .  
         [0059]    Although the invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. The invention is not limited by the specific disclosure herein, but only by the appended claims.