Abstract:
A photomultiplier tube having an ion suppression electrode positioned between a photocathode and an electron multiplying device in the photomultiplier tube is disclosed. The ion suppression electrode includes a grid that is configured to provide sufficient rigidity to avoid deformation during operation of the photomultiplier tube. The photomultiplier tube also includes a source of electric potential connected to the electron multiplying device and to the ion suppression electrode to provide a first voltage to the second electrode and a second voltage to the suppression grid electrode wherein the second voltage has a magnitude equal to or greater than the magnitude of the first voltage. A method of making the photomultiplier and a method of using it are also disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/831,808, filed Jun. 6, 2013, the entirety of which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates to photomultiplier tubes and in particular, to a microchannel plate photomultiplier tube that provides suppression of ions generated throughout the microchannel plate when the photomultiplier tube is in operation. 
         [0004]    2. Description of the Related Art 
         [0005]    During operation of a transmission-mode microchannel plate photomultiplier tube (MCP-PMT) positive ions are generated along the length of the MCP pores and are accelerated directly towards the photocathode, where they impact with significant energy. This phenomenon is termed “ion feedback” and is responsible to a significant degree for degradation of photocathode sensitivity and adversely affects the expected lifetime of the device. There are known techniques directed at reducing or eliminating the ion feedback effect that generally involve reducing the number of ions through the use of sophisticated materials engineering and/or vacuum processing. Alternatively, physical ion barriers formed in the MCP geometry and/or ion barrier films deposited on an external surface of the MCP have been used. 
         [0006]    In a transmission-mode MCP-PMT, photons are detected by their absorption and the subsequent ejection of photoelectrons from a semi-transparent photocathode deposited on the vacuum side of a window. The photoelectrons are amplified by a factor of at least 10 3  by means of a secondary-electron cascade in one or more MCP&#39;s. The electrons emitted by the MCP are collected as charge pulses on a single or multi-segment anode. The operational principle of a PMT having a single MCP is illustrated in  FIG. 1 . An MCP-based image intensifier tube operates according to the same principle as the MCP-PMT, but the charge collecting anode is replaced by an imaging system. 
         [0007]    MCP&#39;s are wafers containing millions of high aspect-ratio hollow channels, the walls of which have been treated to provide a desired electrical conductivity and a high probability of releasing secondary electrons. Generally, MCP&#39;s are made using leaded-glass, although the use of conformal thin-film coatings has more recently enabled MCP&#39;s to be fabricated using other substrate materials. 
         [0008]    When an energetic primary particle such as a photoelectron strikes the wall of an MCP pore channel, it can release one or more secondary electrons. In MCP-PMTs this initial event is facilitated by (i) accelerating the photoelectron across a potential difference of at least 100 V and (ii) orienting the MCP pores at an angle relative to the wafer normal direction. The secondary electrons are accelerated down the length of the pore channel by a large electric field (˜10 6  V/m) until they strike the channel wall and liberate additional secondary electrons. This cascade process is repeated numerous times as illustrated in  FIG. 2  and results in a pulse comprising at least 1000 electrons leaving the output side of the MCP. The output electrons are then accelerated to the charge collecting anode. 
         [0009]    Throughout the amplification process positive ions are also generated by electron-molecule collisions. Given the ultrahigh vacuum (UHV) conditions inside the MCP-PMT, direct ionization of residual gases is relatively unimportant and the ion generation occurs predominately by electron stimulated desorption (ESD) from the surfaces of the MCP pore channels. Inside the MCP pores the electric field is axial, so the ions generated can be accelerated out of the MCP back toward and into the photocathode where they adversely affect the lifetime of the device. For a typical MCP the ion yield increases exponentially along the length of the MCP pores in direct correlation with the electron density and as a result, there is an increasing distribution of higher energy ions originating nearer the output side of the MCP as illustrated in  FIG. 3 . If one neglects the relatively small internal energies from the ESD process, the high-energy cutoff of this distribution occurs at the full potential energy difference between the MCP output and the photocathode which is typically greater than 1000 eV. 
         [0010]    A common method of minimizing ion feedback is to treat the MCP surfaces such that fewer ions are created during the multiplication process. At a minimum this is done through the use of UHV techniques involving extreme cleanliness in the handling and processing environments and extended bake-outs of the MCP at elevated temperature. Additionally, extensive operation of MCP&#39;s under UHV conditions before their assembly into the PMT allows the ESD process to “scrub” the MCP surfaces which also decreases the ion feedback rate. In addition, techniques that involve either conformally depositing on the MCP a film with desirable properties to minimize damaging ion feedback or functionalizing the MCP entirely through the use of conformal coatings of desired materials have been demonstrated in the art. 
         [0011]    Complementing the ion-minimizing methods, one solution is to physically interrupt the ions while they are in transit towards the photocathode. Certain devices such as Gen III image intensifiers make use of a thin barrier film deposited over the input of the MCP that can ensure that energetic ions cannot reach the photocathode. However, that technique is not without drawbacks in complexity and in certain aspects of performance. Another physical-barrier technique is to arrange multiple MCPs in series with their pore channel directions staggered, such that the majority of ions are guaranteed to collide with the MCP channel surfaces. The most common configurations are termed “chevron” and “Z-stack” when using two or three plates, respectively. A chevron arrangement of MCPs is shown in  FIG. 4A  and a Z-stack configuration is shown in  FIG. 4B . In these staggered configurations the majority of ions generated deep in the MCP pores are forced to strike the upper plate where the channel wall changes their direction and the number of ions reaching the photocathode is greatly reduced although not entirely eliminated. 
         [0012]    The PLANACON photon detector is a square-shaped, multi-anode MCP-PMT that is manufactured and sold by PHOTONIS USA Pennsylvania Inc., of Lancaster, Pa. The PLANACON photon detector is used for many photon detection applications where large detection areas are required. The unique format of the PLANACON detector makes it the largest detector areally of its type on the market and allows for many PLANACON detector units to be tiled together in order to form a larger image. 
       SUMMARY OF THE INVENTION 
       [0013]    The problems associated with ion feedback in an MCP-PMT are solved to a large degree by a photomultiplier tube in accordance with the present invention. In accordance with one aspect of the present invention there is provided a photomultiplier tube that includes a photocathode having a first surface for receiving light and a second surface opposite the first surface from which electrons are emitted in response to light that is incident on the first surface. The photomultiplier also includes an electron multiplying device positioned in spaced relation to the photocathode. The electron multiplying device has an electron receiving side that faces the second surface of the photocathode and an electron emission side opposite the electron receiving side. The electron multiplying device is positioned such that the electron receiving side is located at a preselected distance from the second surface of the photocathode. A first electrode is operatively connected to the electron receiving side of the electron multiplying device. A second electrode is operatively connected to the electron emission side of the electron multiplying device. An ion suppression electrode is positioned between the photocathode and the electron multiplying device and spaced therefrom. The ion suppression electrode preferably includes a conductive grid. The photomultiplier according to the present invention further includes a source of electric potential connected to the second electrode and to the ion suppression electrode. The electric potential source is configured and adapted to provide a first voltage to the second electrode and a second voltage to the suppression grid electrode wherein the second voltage has a magnitude equal to or greater than the magnitude of the first voltage. 
         [0014]    In accordance with another aspect of the present invention there is described a method of making a photomultiplier that provides suppression of ions. The method includes the steps of providing a photocathode having a first surface for receiving light and a second surface opposite the first surface from which electrons are emitted in response to light that is incident on the first surface and providing an electron multiplying device in spaced relation from the photocathode, wherein the electron multiplying device has an electron receiving side that faces the second surface of the photocathode and an electron emission side opposing the electron receiving side. The electron multiplying device is positioned such that the electron receiving side is located at a preselected distance from the second surface of said photocathode. The method according to this invention also includes the steps of providing an ion suppression electrode between the photocathode and the electron multiplying device. Preferably, the ion suppression electrode is formed as a grid. Further steps of the method include energizing the electron receiving surface of the electron multiplying device with a first voltage, energizing the electron emission surface of the electron multiplying device with a second voltage that is greater in magnitude than the first voltage, and energizing the suppression electrode with a third voltage having a magnitude that is equal to or greater than the magnitude of the second voltage. 
         [0015]    In accordance with a further aspect of the present invention, there is disclosed a method of suppressing feedback ions in the photomultiplier described above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The foregoing summary as well as the following detailed description will be better understood when read with reference to the several views of the drawing, wherein: 
           [0017]      FIG. 1  is a schematic diagram showing the operation of a known photomultiplier tube; 
           [0018]      FIG. 2  is a schematic diagram of a known microchannel plate and its principle of operation; 
           [0019]      FIG. 3  is a graph of ion yield as a function of energy as formed along the length of a pore channel in a known microchannel plate; 
           [0020]      FIG. 4A  is a schematic view of two microchannel plates in the known chevron configuration; 
           [0021]      FIG. 4B  is a schematic view of three microchannel plates in the known Z-stack configuration; 
           [0022]      FIG. 5  is a schematic diagram showing the operation of a photomultiplier tube in accordance with the present invention; 
           [0023]      FIG. 6  is a perspective view of a photomultiplier in accordance with the present invention; 
           [0024]      FIG. 7  is cross-sectional view of the photomultiplier of  FIG. 6 ; 
           [0025]      FIG. 8  is a plan view of a first embodiment of an ion suppression grid used in the photomultiplier of  FIGS. 6 and 7 ; 
           [0026]      FIG. 9  is a plan view of a second embodiment of an ion suppression grid used in the photomultiplier of  FIGS. 6 and 7 ; 
           [0027]      FIG. 10  is a plan view of a third embodiment of an ion suppression grid used in the photomultiplier of  FIGS. 6 and 7 ; 
           [0028]      FIG. 11  is a plan view of a fourth embodiment of an ion suppression grid used in the photomultiplier of  FIGS. 6 and 7 ; 
           [0029]      FIG. 12  is a schematic diagram of a first embodiment of an electric potential source used with the photomultiplier according to the present invention; 
           [0030]      FIG. 13  is a schematic diagram of a second embodiment of the electric potential source used with the photomultiplier according to the present invention; and 
           [0031]      FIG. 14  is a schematic diagram of a third embodiment of the electric potential source used with the photomultiplier according to the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0032]    Referring now to the drawings and in particular to  FIGS. 6 and 7 , there is shown a photomultiplier tube in accordance with the present invention. The photomultiplier tube  10  includes a housing in which the internal components of the device are sealed so that a vacuum can be maintained inside the photomultiplier tube  10 . The photomultiplier tube  10  preferably has a high useful area ratio (open area ratio) and a footprint having one or more flat sides so that the photomultiplier tube can be butted up against one or more similar units. Such an arrangement provides a wide imaging area and permits tiling of multiple units to provide a wide variety of imaging areas and geometries. 
         [0033]    Referring now to  FIG. 7 , the photomultiplier tube  10  includes an input window  12  for receiving light. The window  12  is formed of a light transmitting material such as a glass or transparent crystal. Preferred materials for the window of a photomultiplier tube are known to those skilled in the art. A photocathode  14  is positioned internally to the photomultiplier tube  10  adjacent the window  12 . Preferably the photocathode is formed as a thin layer on the inside surface of the window. An electron multiplying device is positioned inside the photomultiplier tube  10  in spaced relation to the photocathode  14 . In the embodiment shown in  FIG. 7 , the electron multiplying device includes a first microchannel plate  17  and a second microchannel plate  18 . The first and second microchannel plates  17  and  18  are stacked on each other such that their respective pore channels are oriented at an angle to each other so as to provide the known chevron configuration. In a different embodiment there may be three or more microchannel plates stacked vertically with their respective pore channels oriented at angles to each other so as to provide the known z-stack configuration. It is also contemplated that the electron multiplying device may consist of a single microchannel plate. 
         [0034]    A first contact or electrode  20  is connected to the input surface of first microchannel plate  17 . A second contact or electrode  22  is connected to the output surface of second microchannel plate  18 . Suitable leads or other terminals are connected to the first and second electrodes so that the electrodes can be connected to a source of electric voltage. A charge collecting anode  24  is positioned between the microchannel plate  18  and the base of the photomultiplier tube  10 . The anode  24  may consist of a single electrode or multiple electrodes depending on the application in which the photomultiplier will be used. A suitable lead or leads are connected to the anode so that it can be connected to a signal analyzing instrument that converts the collected charges into signal that can be used to generate and/or display useful information. 
         [0035]    In addition to the foregoing features, the photomultiplier tube  10  has an ion suppression electrode  16  that is positioned between the photocathode  14  and the first microchannel plate  17 . The ion suppression electrode  16  includes a grid that is preferably formed of a material and in a configuration that results in sufficient rigidity that the electrode  16  maintains a substantially planar form. The ability to maintain a planar form is important because of the relatively wide viewing/imaging area that the electrode  16  covers. Too much sagging of the electrode  16  will adversely affect performance of the device and in extreme cases could result in a catastrophic short circuit when the device is in operation. 
         [0036]    Referring now to  FIG. 8 , there is shown a first embodiment of the grid for ion suppression electrode  16  according to the present invention. The electrode  16  preferably includes a grid formed of metallic elements  26  that are spaced from each other to provide small openings  28  that are dimensioned to permit electrons to pass. Moreover, each opening  28  is dimensioned to be small enough to minimize or substantially eliminate a potential (voltage) gradient between the metallic elements that define the opening. In a preferred embodiment, the opening is dimensioned to be not greater than about one-tenth of the distance between the photocathode and the input side of the electron multiplying device. 
         [0037]    In the embodiment of  FIG. 8 , the metallic elements  26  are realized as fine wires that are equi-spaced and aligned in parallel. The openings  28  have an elongated geometry. In the embodiment shown in  FIG. 9 , the grid has a first set of metallic elements  26  arranged as in  FIG. 8  and a second set of metallic elements  26 ′ that are equi-spaced and oriented transversely to the first set of metallic elements  26 . In the embodiment shown in  FIG. 9 , the openings  28  have a square geometry. In  FIG. 10 , the electrode  16  has a grid that includes a plurality of metallic elements  26  that are constructed and arranged with hexagonal geometries.  FIG. 11  shows an electrode grid  16  that is formed from thin plate or foil which functions as the metallic elements. The openings  28  are typically formed in the thin plate or foil using photochemical etching or any other known microfabrication technique. 
         [0038]    Referring to  FIG. 12 , there is shown a first embodiment of an electric potential source  30  to which the photomultiplier tube of this invention is connected for operation. The electric potential source  30  includes a first terminal  32  that is connected to the output terminal of a dc voltage supply  34 . A second terminal  36  is connected to ground potential or to a reference terminal of the dc voltage supply. The electric potential source  30  includes a voltage divider network  37  having a first terminal  38  that is connected to the photocathode  14  for applying a first electric potential to the photocathode. The electric potential source  30  has second terminal  40  that is connected to the ion suppression electrode  16  for applying a second electric potential thereto. Potential source  30  further includes third and fourth terminals  42 ,  44  that are connected respectively to the input and output electrodes  20 ,  22  of the electron multiplying device for applying third and fourth electric potentials thereto. In the embodiment shown in  FIG. 12 , the voltage divider network  37  is constructed and arranged such that when it is energized by the dc voltage supply  34 , the electric potential provided at the second terminal  40  has a magnitude that is equal to the electric potential provided at the fourth terminal  44  in order to suppress positive ion feedback from the electron multiplier. In the embodiment shown in  FIG. 13 , the voltage divider network  37  is constructed and arranged such that when it is energized by the dc voltage supply  34 , the electric potential provided at the second terminal  40  has a magnitude that is greater than the electric potential provided at the fourth terminal  44  in order to suppress positive ion feedback from the electron multiplier to a greater degree than with the embodiment of  FIG. 12 . 
         [0039]    It is also contemplated that the electric potential source  30  may include means for varying the magnitude of the voltage applied to the suppression electrode. Referring to  FIG. 14  there is shown a further embodiment of electric potential source  30  that provides such functionality. As shown in  FIG. 14 , the voltage divider network includes a variable resistor  46  connected between the first terminal  32  and the second terminal  40 . By adjusting variable resistor  46 , the electric potential at second terminal  40  is varied. Since the ion suppression electrode is connected to second terminal  40 , the potential of the ion suppression electrode is also varied. In this manner, the degree of ion suppression can be adjusted depending upon the application in which the photomultiplier tube is used. 
         [0040]    The operation of a photomultiplier tube with a properly biased, ion suppression grid electrode located between the photocathode and input of the MCP in accordance with the present invention can effectively prevent positive ions from reaching the photocathode. The reduction of positive ion impingement on the photocathode effectively improves (increases) the life cycle of the photocathode. As illustrated in  FIG. 5 , when the ion suppression grid voltage exceeds the MCP output voltage substantially all positive ions are returned to the MCP where they are neutralized. If the voltage is maintained below that cutoff value, only those ions originating from the corresponding shallower (nearer to the input) regions of the MCP pores will be suppressed. The inventive concept can be extended to other variations, for example, an MCP-PMT that has a chevron MCP assembly or a Z-stack MCP assembly, so long as the suppression grid bias voltage can be energized above the maximum possible value for complete cutoff 
       Working Example 
       [0041]    In order to demonstrate the effectiveness of the photomultiplier (PMT) according to the present invention in suppressing ion feedback, a prototype device was constructed and tested as described below. The prototype device was constructed in accordance with the description presented in this specification and as shown in  FIG. 7 . The device included a bialkali photocathode deposited on a quartz window. A pair of microchannel plates with 25 micron diameter pores was arranged in a chevron configuration. A metallic anode was positioned adjacent the output surface of the microchannel plate stack and a conductive ion-suppression grid was located between the photocathode and the input surface of the microchannel plate stack. Testing was performed as follows to determine the operational effectiveness of the ion-suppression grid. 
         [0042]    The window of the PMT was illuminated with a  35 -picosecond width laser pulse that was filtered to single photoelectron intensity. The corresponding charge pulses were measured using a high-speed digitizing oscilloscope connected to the anode. On the occasion when a positive ion from the MCP stack was accelerated to the photocathode, electrons would be released from the photocathode resulting in an after-pulse that followed the primary photoelectron pulse in time. The total after-pulse occurrence rates were measured with the ion suppression grid energized at each of six different electric potentials starting at the same potential as the input of the MCP stack and increased in five increments up to the potential of the output surface of the MCP stack. Additionally, the late arrival time region containing large ion masses (i.e., ions having mass/charge&gt;100 AMU) was separately analyzed and tabulated as such ions are presumed to be more damaging to the photocathode. 
         [0043]    The results of the testing are shown in the table below including the electric potential of the ion suppression grid as a percentage of the electric potential at the Chevron MCP interface, the total raw after-pulsing rate in % per photoelectron, the total after-pulse rate normalized relative to the unsuppressed rate, the raw high mass after-pulsing rate in % per photoelectron, and the normalized high mass after-pulse rate. The Chevron MCP interface is defined as the plane where the upper and lower MCP&#39;s meet in the stacked arrangement. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
               
               
                 Suppression Grid 
                   
                   
                 High Mass 
                   
               
               
                 Potential (% of 
                 Total Afterpulsing 
                   
                 Afterpulsing 
                 Normalized High 
               
               
                 Chevron Interface 
                 Rate (% per 
                 Normalized Total 
                 Rate (% per 
                 Mass After-pulse 
               
               
                 Potential) 
                 photoelectron) 
                 After-pulse Rate 
                 photoelectron) 
                 Rate 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 0 
                 0.105 
                 1.00 
                 0.020 
                 1.00 
               
               
                 40 
                 0.025 
                 0.24 
                 0.0096 
                 0.47 
               
               
                 80 
                 0.017 
                 0.16 
                 0.0045 
                 0.22 
               
               
                 120 
                 0.017 
                 0.16 
                 0.0037 
                 0.18 
               
               
                 160 
                 0.018 
                 0.17 
                 0.0040 
                 0.20 
               
               
                 200 
                 0.018 
                 0.17 
                 0.0045 
                 0.22 
               
               
                   
               
             
          
         
       
     
         [0044]    The results reported in the table show a clear effect of the ion suppression grid in significantly reducing the rate of positive ions reaching the photocathode. The data show that ion suppression appears to level off when the suppression grid potential is about 80% or more of the Chevron MCP interface potential which verifies that ions are in fact originating deep in the MCP pores. The data represent a minimum expectation for ion feedback suppression because some of the after-pulses can be attributed to suppressed ions directly generating electrons by impinging on the input ends of the MCP pores. Another possible contribution of after-pulses may result from energetic neutral atoms or molecules that would not be affected by the suppression grid. 
         [0045]    It will be recognized by those skilled in the art that changes or modifications may be made to the above-described embodiments without departing from the broad inventive concepts of the invention. It is understood, therefore, that the invention is not limited to the particular embodiments which are described, but is intended to cover all modifications and changes within the scope and spirit of the invention as described above and set forth in the appended claims.