Abstract:
A detector assembly for a time-of-flight (TOF) mass spectrometer has a modular microchannel plate (MCP)-based detector and a replaceable MCP cartridge containing at least one MCP secured within the detector. The detector includes an integrated support for a voltage divider and high voltage connections. The detector is also adapted to be secured to a modified vacuum flange for the instrument.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to detectors for analytical instruments such as time-of-flight (TOF) mass spectrometers. In particular, the invention pertains to a modular microchannel plate (MCP)-based detector of reduced size for a TOF mass spectrometer. 
     2. Description of the Prior Art 
     A microchannel plate (MCP)-based detector is the device of choice in many analytical instrumentation applications. The compact nature, high-speed response and position sensing capability of the detector makes it ideal for applications in, for example, scanning electron microscopes (SEM), miniature residual gas analyzers, magnetic sector mass spectrometers, and time-of-flight mass (TOF) spectrometers. However, conventional detector arrangements are fragile, complex and relatively large, and do not allow for field replacement of the MCP. The detector must be returned to the factory for refurbishment. This is undesirable in terms of cost and out-of-service time for the instrument. In addition, the total number of parts required to manufacture a conventional detector is relatively high, thus adding to the cost. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to prove a modular MCP-based detector for an analytical instrument such as a TOF mass spectrometer having a replaceable MCP cartridge. 
     It is a further object of the invention to provide an MCP-based detector for a TOF mass spectrometer having an integrated voltage dividing means and high voltage connection unit of simplified design and reduced size. 
     The foregoing objects are achieved according to an exemplary embodiment of a detector assembly for a time-of-flight (TOF) mass spectrometer comprising a modular microchannel plate (MCP)-based detector having an input end for receiving charged and neutral particles, and a replaceable cartridge containing at least one MCP adapted to be secured within the detector. The detector includes an integrated voltage dividing means and high voltage connection means. The detector is also adapted to be secured to a modified vacuum flange of the spectrometer. 
     The invention is also directed to various constructions of the replaceable MCP cartridge. For example, the cartridge may include a plurality of stacked MCPs, a grid plate and spacers. 
     Other features and advantages of the invention will become apparent upon reference to the following description and drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevation of a modular detector secured in a modified vacuum flange for use in a TOF mass spectrometer according to an exemplary embodiment of the invention; 
     FIG. 2 is a top plan view of the arrangement shown in FIG. 1; 
     FIG. 3 is an exploded side elevation of the arrangement shown in FIG. 1; 
     FIG. 4 is a side sectional view of a modular MCP-based detector according to an exemplary embodiment of the invention; 
     FIG. 5 is a to plan view of the detector shown in FIG. 4; 
     FIG. 6 is an exploded side sectional view of the detector shown in FIG. 4; 
     FIG. 7 is a cross-sectional view of a high voltage contact; and 
     FIG. 8 is a side sectional view of a replaceable MCP cartridge according to an exemplary embodiment of the invention; 
     FIG. 9 is a top plan view of the cartridge shown in FIG. 8; 
     FIG. 10 is an exploded side sectional view of the cartridge shown in FIG. 8; 
     FIG. 11 is a schematic diagram of the electrical circuit of the detector shown in FIG. 1. 
    
    
     DESCRIPTION OF THE INVENTION 
     FIGS. 1-4 illustrate a modular detector assembly 10 according to an exemplary embodiment of the invention, including a modular detector 12 secured in a modified vacuum flange 14 for use in a time-of-flight spectrometer 15 (shown as a fragment) (not shown). The detector 12 has an input end 16 for receiving charged or neutral particles 17, for example, electrons, ions and photons. The vacuum flange 14 has a vacuum side 18 and an atmosphere side 20. The detector 12 is adapted to be secured in a first recess 22 formed in the vacuum side 18 of the flange 14. A plurality of inboard screws 24 passing through the vacuum flange 14 secure the detector 12 within the recess 22, as shown. 
     The atmosphere side 20 of the vacuum flange 14 has a second recess 26 for receiving a secondary flange 28 which is secured to the vacuum flange 14 by a plurality of outboard screws 30. An O-ring 32 is located between respective confronting surfaces 34 and 35 of the secondary recess 26 and second flange 28. The O-ring 32 is sized so that it surrounds the inboard screws 24 and is inboard of the outboard screws 30 to secure the vacuum when assembled. An anode connector 40 is secured in vacuum tight relation in an opening 42 of the second flange 28, as shown. The anode connector 40 has a pin 44 which is adapted to engage an anode connector (not shown in FIG. 3, but hereinafter shown and described). 
     A high voltage connector 46 is secured in vacuum tight relation in an opening 48 of the vacuum flange 14, as shown. A conductive pin 50 extends through the vacuum side 18 of the flange 14. A flexible connector 52 is electrically connected to the detector 12, as shown, and has a free connector end 54 which is adapted to engage the pin 50 of the high voltage connector 46. A distal end 56 of the connector 52 is secured in a modular high voltage connector terminal 58. 
     A electrically conductive shield 60 is secured to the vacuum side 18 of the flange 14 by means of screws 62. In an exemplary embodiment, the screws 62 are of a so-called vented type which are known to those skilled in the art. The shield 60 is located in spaced relation with the vacuum flange and shades the flexible connector 52, connector end 54 and the pin 50 to shield the input end 16 of the detector 12 from stray electric fields produced by the high voltage connector. 
     The modular detector 12 is illustrated in further detail in FIGS. 4-6 and includes base 70, a cap 72 and a removable MCP cartridge 74 which is captured between base 70 and the cap 72. The base 70 further includes a 50 Ω coaxial conical anode 76 secured in a stepped and tapered central opening 78 of the base 70. The cap 74 has an opening 80 formed at the input end 16 of the detector 12, for allowing the particles to enter the cartridge 74 at the input face 82. The anode 76 has a signal receiving face 84 in confronting relation with the output face 86 of the cartridge 74. The anode 76 tapers down the signal receiving surface 84 to a cylindrical tail piece 88. The anode 76 is held in spaced relationship with the base member 70 by first and second insulated washers 90 and 92 which engage opposite sides of a stepped wall 94. A threaded nut 96 and a threaded end 97 of the tail piece 88 secure the anode 76 in position, as shown. The tail piece 88 has a central opening 98 for receiving the anode pin 44, shown in FIG. 3, to thereby make electrical contact for carrying the output signal from the detector 12. The opening 78 has a tapered wall 98 in spaced relation with the tapered wall of the anode 76 and with the insulating dielectric washers 90 and 92, and the tail piece 88 establishes an impedance matched anode structure. 
     The detector 12 further includes an integrated high voltage connector unit 100 (FIG. 10) in the form of an insulated housing 102 having a spring loaded electrical connector 104 secured therein. The spring loaded connector 104 has a high voltage connector end opening 106 which is adapted to receive the distal end 56 of flexible connector 52 (FIG. 3). A screw 108 secures the high voltage connector end 56 in position, as shown. Set screw 109 secures the spring loaded connector 104 in position. Screws 110 secure the connector unit 100 to the base portion 70 through openings 111. 
     A spring loaded resistor 120 is located in an opening 122 having a through hole 124 in the lower side, as illustrated. The resistor 120 has a lead 126 which passes through the opening 124 and which is bent over at 127 (FIG. 4) to hold the resistor 120 in position. A spring 128 is captured between the bottom of the opening 122 in the resistor 120 for urging the resistor upwardly. A contact side 130 of the resistor engages an output electrode secured in the cartridge 74 which is hereinafter described. The resistor forms a voltage divider to drop the output electrode voltage. 
     Overall, the detector 12 has a relatively low profile about five times lower than a conventional device. In the exemplary embodiment shown, the detector has a height to width ratio near about one. 
     The cartridge 74 as illustrated in FIGS. 8-10 in respective top plan, side sectional and exploded views. The cartridge 74 includes an insulated cartridge body 110 having an interior chamber 112 for receiving the cartridge elements, including a conductive output plate 114 having a central opening, an insulating centering ring 116 having a central opening 118, a pair of opposed MCP elements 120 and 122 having a contact ring 123 disposed between the confronting surfaces of the MCP, as shown; a conductive input plate assembly 124, an insulated spacer 126, a conductive grid 128 and an insulating ring retainer 130. The ring retainer 130 is secured in the cartridge body 110 by means of mating threads 132. The centering ring 118 receives the MCPs 120,122 and contact ring 123 as a stacked array in the opening 118 which is sized for receiving the MCPs therein. The centering ring locates the MCPs 120, 122 and contact ring 123 and spaces the respective input and output electrodes 124 and 114 thereby preventing shorting. 
     The output plate 114 is generally circular and has an edge portion 134 removed for engaging a corresponding seat 136 in the lower portion of the cartridge body 110. A first contact opening 138 is formed in a lower portion of the cartridge body 110 and is in registration with the contact surface 130 of the resistor 120 (FIG. 6) which makes contact a corresponding portion 131 of one side of the output plate 114, as shown in the assembly drawing of FIG. 9. The housing 110 has a second contact opening 140. The input plate assembly 124 has a contact member 142 engaging the opening 140 as shown in the assembly drawing of FIG. 9. The contact member 142 engages the spring contact 104 of the high voltage connector 100 (FIG. 4). Thus, in accordance with the arrangement illustrated, a high potential difference may be established between the input plate 124 and the output plate 114. The dropping resistor or voltage divider resistor 120 establishes a relatively low negative bias on the output plate 114. 
     As can be seen in FIGS. 5, 6 and 8, an upper side 170 of the base 70 has upstanding registration pins 150 which mate with corresponding apertures 152 in the body 110 for locating the high and low voltage connectors as previously described. The components thus described are arranged in a stack within the body, as illustrated in FIG. 9, to thereby form a removable and replaceable cartridge assembly 74. This can be readily and easily replaced by removing the flange assembly 10 from the instrument and removing the cap 72 from detector 12 and exchanging the cartridge 74. 
     FIG. 11 illustrates an electrical schematic of the detector assembly 10. In the arrangement, input plate 124 has a bias of about -2,000 V, output plate 114 has a potential of about -100 V and voltage divider 120 drops the voltage to ground. The housing base 70 is at ground and the anode is electrically separated from the housing at virtual ground to form an input to a detector amplifier 154. 
     While there has been described what is considered to be the exemplary embodiment of the invention, it will be apparent to those skilled in the art the various changes and modifications that may be made therein without departing from the invention, and it is intended in the appended claims to cover such changes and modifications as fall within the true spirit and scope of the invention.