Abstract:
A detector for an electron multiplier comprising: a substrate comprising a dielectric material, the substrate having a first face and an opposing second face; a charge collector provided adjacent the first face of the substrate; an anode within the substrate, the anode spaced from first face, such that the anode is capacitively coupled to the charge collector, so that charge incident on the charge collector generates an image charge on the anode; and a conduit contact, coupled to the anode and passing through the substrate to the second face of the substrate layer.

Description:
RELATED APPLICATION INFORMATION 
       [0001]    This application is based on and claims the benefit of priority from UK Patent Application No. GB 1510859.0, filed on Jun. 19, 2015, the content of which is incorporated herein by reference. 
       BACKGROUND 
       [0002]    A detector, namely a detector for an electron multiplier and an electron multiplier comprising the electron detector, is disclosed herein. 
         [0003]    Electron multipliers can be used to detect the presence of single electrons or other quanta or particles.  FIG. 2  shows a cut-through view of an electron multiplier  200  according to the prior art. 
         [0004]    Generally, an electron multiplier  200  comprises a chamber  202  defined by side walls  204 , a front cover  206  and a rear cover  208 . The front cover  206  is transparent to the quanta or particle that the multiplier  200  is designed to detect. 
         [0005]    The chamber  202  is held under a vacuum, and contains an electron avalanche mechanism  210 . A single electron  212  incident on the front face of the avalanche mechanism  210  is absorbed and causes an electron avalanche  214  to be emitted from the rear face. 
         [0006]    Electron multipliers  200  can be used to directly detect incident electrons, quanta or other particles. The avalanche mechanism  210  may initiate an electron avalanche in response to the absorption of an electron, quanta or other particle. For example, an electron avalanche may be initiated in response to absorption of electrons, positively or negatively charged ions or photons. 
         [0007]    Alternatively, as shown in  FIG. 2 , the electron multiplier  200  may comprise a converter  216 . The converter  216  absorbs an incident quanta or particle  218  (such as a photon or ion) on its front face, and emits a single electron  212  from its rear face. In this case, the electron multiplier  200  detects an electron which is representative of the incident quanta or particle. 
         [0008]    In the example shown in  FIG. 2 , a single photon  218  is absorbed by the converter  216 , which emits a single electron  212 . The single electron  212  is then absorbed by the avalanche mechanism  210 , which emits an electron avalanche  214 . 
         [0009]    The electron avalanche  214  is detected by a sensor (not shown in  FIG. 2 ), provided after the avalanche mechanism  210 . It is known, for example, to provide the sensor within the chamber  202 , having electrical connects provided though the side walls  204  or rear cover  208 . This can be used to detect the present of quanta or particles and, for example, count the number of quanta and particles. 
         [0010]    The electron multiplier  200  may be provided on its own, such that the outside of the side walls  204 , front cover  206  and rear cover  208  is open to atmospheric conditions. 
         [0011]    The electron multiplier  200  could also be coupled to a larger vacuum system. In examples where the electron multiplier is coupled to a larger vacuum system, the electron multiplier may be held within the larger vacuum or adjacent a housing forming the larger vacuum (the housing defining the larger vacuum should include a window transparent to the quanta or particle to be detected aligned with the front cover  206  of the electron multiplier). 
         [0012]    In another example where the electron multiplier  200  is coupled to a larger vacuum, the chamber  202  may be formed as part of the larger vacuum, with the front cover  206  omitted, and the sidewalls  204  joining directly to the housing defining the larger vacuum. 
         [0013]    Any suitable technique can be used to couple the electron multiplier to the larger vacuum. 
         [0014]    Avalanche mechanisms  210 , converters  216  and electron multipliers  200  of this type are generally well known in the art. However, the electrical connections through the side walls  204  make the sensor difficult to implement. Also, in some applications, it may be important to know the position of detected quanta or particles. 
         [0015]    To detect the position of the quanta or particle  218 , the electron multiplier  200  should preserve position throughout the multiplier  200 . Therefore, at the avalanche mechanism  210  the electron avalanche  214  is emitted from the same (two dimensional) position as where the incident electron  212  is absorbed. Similarly, at the converter  216 , the single electron  212  should be emitted from the same (two-dimensional) position as where the incident quanta or particle  218  is absorbed. 
         [0016]    Because the avalanche mechanism  210  (and converter) preserve the position of the incident quanta or particle  218 , it is possible to determine the position at which the incident quanta or particle  218  over the face of the electron multiplier  200  by determining the position of the electron avalanche  214 . 
         [0017]    The electron avalanche  214  is first collected for a short time inside the vacuum via a high-resistance collector. The electron avalanche is then read out capacitively as an image charge, through the rear cover  208 . This is done via a low-resistance anode layer, outside the chamber  202 . 
         [0018]    The anode layer comprises a plurality of contact regions. The image charge spreads as it capacitively couples through the counter-substrate, so the image charge is incident on more than one of the contact regions. The position of the incident electron can be determined by processing the charges induced on the different contact regions (also known as “centroiding”). 
         [0019]    U.S. Pat. No. 5,686,721 is one example of a sensor that can be used for determining the position of an incident electron. 
         [0020]    Two events (detection of quanta  218 ) will be considered simultaneous, or near-simultaneous, if they result in image charges incident on one or more of the contact regions at the same time. If simultaneous or near-simultaneous events induce image charges in one or more of the same contact regions, centroiding is not able to determine the two separate events. Instead, the two induced charges are interpreted as a single event, with a position derived from treating the two induced image charges as a single image charge. Therefore, centroiding is limited to serial events (i.e. one after another) for events in close proximity and cannot be used to detect simultaneous or near-simultaneous events. 
         [0021]    According to a first aspect of the disclosure, there is provided a detector for an electron multiplier, the detector comprising: a substrate comprising a dielectric material, the substrate having a first face and an opposing second face; a charge collector provided adjacent the first face of the substrate; an anode within the substrate, the anode spaced from first face, such that the anode is capacitively coupled to the charge collector, so that charge incident on the charge collector generates an image charge on the anode; and a conduit contact, coupled to the anode and passing through the substrate to the second face of the substrate layer. 
         [0022]    Burying the anode within the substrate means that the image charge spreads less before being detected, without having to provide electrical connections all the way through the rear wall. As such, there is less chance of two simultaneous or near-simultaneous events occurring in close proximity to one another inducing image charges on a common contact of the anode, improving the resolution of the detector. 
         [0023]    The substrate may comprise a dielectric layer forming the first face. The anode may be provided within the dielectric layer or at the edge of the dielectric layer. 
         [0024]    The substrate may comprise a ceramic layer forming the second face. The anode may be provided at the interface between the ceramic layer and the dielectric layer. 
         [0025]    The spacing between the first face of the substrate and the anode may be at most 1 mm. 
         [0026]    The conduit contact may comprise a via portion passing through the substrate and a planar portion, on the second face of the substrate. 
         [0027]    The detector may further comprise circuitry, coupled to the conduit contact. The circuitry may be arranged to read-out the image charge induced in the anode and may comprise a processor arranged to determine the position of an electron avalanche incident on the charge collector, based on the detected charge. 
         [0028]    The anode may comprise a plurality of electrically isolated anode plates. Each anode plate may be coupled to a separate conduit contact and the circuitry may be arranged to separately readout the image charge from each anode plate. 
         [0029]    The circuitry may be arranged to readout the charge induced in at least two anode plates simultaneously. 
         [0030]    The processor may be arranged to compare the image charge simultaneously induced in neighboring anode plates and to determine the position of an electron avalanche incident on the charge collector based on the comparison. 
         [0031]    The charge collector may be a resistive layer. The resistive layer may have a sheet resistance of at least 250 kohm/square. The resistive layer may have a sheet resistance of 500 kohm/square; 750 kohm/square or 1 Mohm/square. 
         [0032]    According to a second aspect of the disclosure, there is provided an electron multiplier comprising means for initiating an electron avalanche in response to a single detected electron, the electron avalanche emitted in the same location as the single electron is detected; and a detector, according to the first aspect, for detecting the electron avalanche and determining the position of the single electron from the detected position of the electron avalanche. 
         [0033]    The electron multiplier may comprise a measurement chamber holding the means of initiating an electron avalanche in a vacuum. The chamber may be defined by a housing, the substrate forming at least part of the housing, such that the charge collector is held in the vacuum and the second face of the substrate is held outside the vacuum. 
         [0034]    The electron multiplier may further comprise an electron converter having a first face and an opposing second face, the electron converter, arranged to receive a photon or other quanta at a first location on the first face, and, in response to a received photon or quanta, emit an electron from the first location on the second face, the electron emitted towards the means for initiating an electron avalanche. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0035]      FIG. 1  is a schematic representation a detector for an electron multiplier as described herein. 
           [0036]      FIG. 2  is a schematic representation of an electron multiplier according to the prior art. 
       
    
    
     DETAILED DESCRIPTION 
       [0037]      FIG. 1  shows a detector  100  for use in an electron multiplier  200  in accordance with one embodiment. Elements of the detector  100  are provided within the body of the rear cover  208  of the chamber  202 . 
         [0038]    In the current embodiment, the detector  100  comprises a substrate layer  102 . A first side  110  of the substrate layer  102  is arranged to be provided within the chamber  202  and hence within the vacuum. The opposite side  108  is arranged outside the chamber  202 . 
         [0039]    A resistive layer  122  is provided over the vacuum side  110  of the substrate layer  102 . The resistive layer  122  functions as a charge collector. An electron avalanche  214  incident on the resistive layer  122  causes a charge build up in the insulting layer  122 . 
         [0040]    Anode plates  104   a - d  are embedded within the substrate layer  102 , to form an anode  104  for the detector  100 . The anode  104  should be spaced from the resistive layer  122  such that the charge collected as a result of the electron avalanche  214  capacitively induces an image charge on the anode plates  104 . 
         [0041]    Conduit contacts  106   a - d  passing through the substrate layer  102  electrically connect the anode plates  104  to rear contacts  112   a - d  provided on the non-vacuum side  108  of the substrate  102 . Each anode plate  104   a - d  and its associated conduit contact  106   a - d  and rear contact  108   a - d  are electrically isolated from the other anode plates  104   a - d  and contacts  106   a - d,    108   a - d.  Therefore, the image charge induced on each anode plate  104  can be read out from the respective rear face contact  112 . 
         [0042]    The substrate layer  102  and resistive layer  122  (alone or in combination) are used to provide the rear cover  208  of the vacuum chamber  202 . In this way, the capacitive coupling can be optimized, without having to provide complicated contacts through the walls of the chamber  202 . 
         [0043]    Read-out circuitry  114  is electrically coupled to the rear face contacts  112 . The read-out circuitry  114  includes read-out contacts  116   a - d  and a processor (not shown). Each read-out contact  116  is arranged to detect the charge from a single rear face contact  112  and forward the charge to the processor. The processor analyses the measured charges to determine the location of the quanta  218  or particle that initiated the electron avalanche  214 . 
         [0044]    The processor is able to differentiate between charges provided by the different anode plates  104 . The processor includes information correlating each anode plate  104  to its location. When a quanta or particle  218  is incident directly over the center of an anode plate  104 , the spreading of the charge from the electron avalanche  214  is such that the image charge is only induced in a single anode plate  104 . When the quanta or particle is incident over the periphery of an anode plate  104 , or between anode plates  104 , the image charge will be induced in neighboring anode plates  104 . The processor is able to compare the charge induced in neighboring anode plates  104  to determine the location of the quanta or particle  218 . 
         [0045]    In this way, the detector  100  is able to detect and locate quanta or particle  218  strike events that are simultaneous or near simultaneous, and located in close proximity, provided the quanta or particles  218  do not induce an image charge on one or more common anode plates  104 . 
         [0046]    In the current embodiment, the substrate layer  102  is formed by a layered structure. A ceramic layer  118  is provided at the non-vacuum side  108  of the substrate  102 . The anode plates  104  are provided such that they are partially embedded in the ceramic layer  118  and partially project from the ceramic layer  118 . A layer of dielectric material  120  is provided over the anode plates  104 . The resistive layer  122  is then provided over the dielectric. 
         [0047]    To achieve the desired charge collecting performance, the resistive layer  122  should have a minimum sheet resistance of 250 kohm/square. In one example, the resistive layer has a sheet resistance of 500 kohm/square. In another example, the resistive layer has a sheet resistance of 750 kohm/square or 1 Mohm/square. 
         [0048]    It will be appreciated that although  FIG. 1  only shows a single row of anode plates, the anode plates  104  can be of any suitable size. There may be any number of anode plates  104  (one or more), and the anode plates  104  may also be of any shape and arranged in any suitable pattern, with any suitable spacing between anode plates  104 . For example, the anode plates may be square or circular and arranged in a square grid or in concentric circles. In general, the smaller the anode plates  104  and the closer the spacing, the better the spatial resolution of the detector  100 . 
         [0049]    In some embodiments, the size of the anode plates  104  may be such that an image charge is induced on multiple plates  104 , no matter where the electron avalanche  214  is absorbed by the resistive layer  122 . In such embodiments, the signals from neighboring plates  104  are processed accordingly. 
         [0050]    The ceramic layer  118 , dielectric layer  120  and resistive layer  122  can be made from any suitable materials. 
         [0051]    The minimum spacing of the anode plates  104  from the resistive layer  122  is dependent on the resistive material  122 , dielectric material  120  and size of the anode plates  104 . In one example, the anode plates  104  may be 0.5 mm from the resistive layer  122 . This may be provided by a 0.5 mm thick dielectric layer. 
         [0052]    The anode plates  104  may be buried at any suitable position within the layered structure of the substrate  102 . For example, they may be encased within the dielectric layer  120 . 
         [0053]    In other embodiment, the substrate  102  may comprise a single material that achieves the desired capacitive performance and is able to form the rear cover  208  of the chamber  202 . 
         [0054]    It will be appreciated that no matter what the structure of the substrate layer  102 , the overall thickness of the substrate layer  102  can be any suitable value to obtain the desired vacuum in the chamber  202 . In one example, the substrate  102  may be 2 mm thick. 
         [0055]    It will be appreciated that, although the embodiment shown in  FIG. 1  shows rear contacts  112  provide on the non-vacuum face  108  of the substrate layer, the contacts  112  may be provided any suitable way. For example, they may be partially or wholly recessed into the non-vacuum face  108  of the substrate  102 . In some embodiments, the rear face contacts may be omitted altogether and the read-out contacts  116  may couple directly to the conduit contacts  106 . 
         [0056]    It will also be appreciated that any suitable read-out circuitry  114  may be used to detect and process the charges on the anode plates  104 . 
         [0057]    The detector  100  described above may be used in any suitable type of electron multiplier  200 . It will also be appreciated that the same principle can be applied to detect any other form of charge. 
         [0058]    It will further be appreciated that features which are described in different embodiments may be combined in a single embodiment. Similarly, where several features are described in combination in a single embodiment, such features may also be provided separately or in suitable sub-combinations.