Patent Publication Number: US-9420677-B2

Title: X-ray tube electron sources

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a national stage application of PCT Application Number PCT/GB2010/050125, which was filed on Jan. 27, 2010, and relies on Great Britain Patent Application No. 0901338.4 filed on Jan. 28, 2009. 
     FIELD OF THE INVENTION 
     The present invention relates to X-ray tubes, to electron sources for X-ray tubes, and to X-ray imaging systems. 
     BACKGROUND OF THE INVENTION 
     X-ray tubes include an electron source, which can be a thermionic emitter or a cold cathode source, some form of extraction device, such as a grid, which is arranged to control the extraction of electrons from the emitter, and an anode which produces the X-rays when impacted by the electrons. Examples of such systems are disclosed in U.S. Pat. No. 4,274,005 and U.S. Pat. No. 5,259,014. 
     With the increasing use of X-ray scanners, for example for medical and security purposes, it is becoming increasingly desirable to produce X-ray tubes which are relatively inexpensive and which have a long lifetime. 
     SUMMARY OF THE INVENTION 
     Accordingly the present invention provides an electron source for an X-ray scanner comprising an emitter support block. An electron-emitting region may be formed on the support block and arranged to emit electrons. An electrical connector may be arranged to connect a source of electric current to the electron-emitting region. Heating means may be arranged to heat the support block. 
     The present invention further provides a control system for an X-ray scanner. The system may comprise an input arranged to receive an input signal identifying which of a plurality of electron emitters is to be active. The system may be arranged to produce a plurality of outputs each arranged to control operation of one of the emitters. In some embodiments each of the outputs can be in a first state arranged to activate its respective emitter, a second state arranged to de-activate said emitter, or a third state arranged to put said emitter into a floating state. 
     The present invention further provides a control system for an X-ray scanner, the system comprising an input arranged to receive an input signal identifying which of a plurality of electron emitters is to be active, and to produce a plurality of outputs each arranged to control operation of one of the emitters. The system may further comprise output monitoring means arranged to monitor each of the outputs, and the monitoring means may be arranged to generate a feedback signal indicating if any of the outputs exceeds a predetermined threshold. 
     The present invention further provides a control system for an X-ray scanner, the system comprising an input arranged to receive an input signal identifying which of a plurality of electron emitters is to be active, and to produce a plurality of outputs each arranged to control operation of one of the emitters, wherein each of the outputs can be in a first state arranged to activate its respective emitter, and a second state arranged to de-activate said emitter. The system may further comprise blanking means arranged to fix all of the outputs in the second state irrespective of which state the input signal indicates they should nominally be in. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings in which: 
         FIG. 1  is schematic view of an X-ray scanner according to an embodiment of the invention; 
         FIG. 2 a    is top perspective view of an emitter element of the scanner of  FIG. 1 ; 
         FIG. 2 b    is a bottom perspective view of the emitter element of  FIG. 2   a;    
         FIG. 3  is a transverse section through an X-ray emitter unit of the system of  FIG. 1 ; 
         FIG. 4  is a plan view of the emitter of  FIG. 3 ; 
         FIG. 5  is a diagram of an output stage forming part of a control device of the emitter unit of  FIG. 3 ; 
         FIG. 6  is a circuit diagram of a control device of the emitter of  FIG. 3 ; 
         FIGS. 7, 8 and 9  are timing diagrams showing operation of the control device of  FIG. 6  in three different operating modes; 
         FIG. 10  is a diagram of an output stage forming part of a further embodiment of the invention; 
         FIGS. 11 a  and 11 b    are top and bottom perspective views of an emitter element according to a further embodiment of the invention; and 
         FIG. 12  is a transverse section through an electron source unit including the emitter element of  FIGS. 11 a    and  11   b.    
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , an X-ray scanner  50  comprises an array of X-ray emitter units  25  arranged in an arc around a central scanner Z axis, and orientated so as to emit X-rays towards the scanner Z axis. A ring of sensors  52  is placed inside the emitters, directed inwards towards the scanner Z axis. The sensors  52  and emitter units  25  are offset from each other along the Z axis so that X-rays emitted from the emitter units pass by the sensors nearest to them, through the Z axis, and are detected by the sensors furthest from them. 
     Referring to  FIGS. 2 a  and 2 b    each of the emitter units  25  includes an electron emitter element  116  which comprises an aluminium nitride (AlN) emitter support block  117  with low work function emitters  118  on its top surface  120  and platinum (Pt) heater element  122 , on its bottom surface  121 . The emitters  118  are formed from platinum-based ink coated with a highly emitting coating, and the heater element is also formed from Pt-based ink. The emitters cover discrete spaced apart areas on the surface  120  of the block  117 , spaced along its length, and a connecting strip  123  of electrically conducting material extends from each of the emitters  118  around the side of the block  117  to its under side  121 , where they form connector pads  124 . The connecting strips are also spaced apart from each other, so that each emitter  118  is electrically isolated from the other emitters  118 . Aluminium nitride (AlN) is a high thermal conductivity, strong, ceramic material and the thermal expansion coefficient of AlN is closely matched to that of platinum (Pt). Alumina (Al 2 O 3 ) can also be used for the substrate as it has similar properties. These properties lead to the design of an integrated heater-electron emitter element suitable for use in X-ray tube applications. 
     AlN is a wide bandgap semiconductor material and a semiconductor injecting contact is formed between Pt and AlN. To reduce injected current that can occur at high operating temperatures, it is advantageous to replace the injecting contact with a blocking contact. This may be achieved, for example, by growing an aluminium oxide layer on the surface of the AlN substrate  120  prior to fabrication of the Pt metallisation. The provision of an oxide layer between the AlN and the Pt emitter forms a suitable blocking contact. 
     Alternatively, a number of other materials may be used in place of Pt, such as tungsten or nickel. Typically, such metals may be sintered into the ceramic during its firing process to give a robust hybrid device. 
     In some cases, it is advantageous to coat the metal on the AlN substrate with a second metal such as Ni. This can help to extend lifetime of the oxide emitter or control the resistance of the heater, for example. 
     To form the heater element  122  of this embodiment the Pt metal is formed into a track of 1-3 mm wide with a thickness of 10-200 microns to give a track resistance at room temperature in the range 5 to 200 ohms. It is advantageous to limit the heater voltage to below 100V to avoid electrical cross talk to the emitter pads  118  on the upper surface  120  of the substrate. By passing an electrical current through the track, the track will start to heat up and this thermal energy is dissipated directly into the AlN substrate. Due to the excellent thermal conductivity of AlN, the heating of the AlN is very uniform across the substrate, typically to within 10 to 20 degrees. Depending on the current flow and the ambient environment, stable substrate temperatures in excess of 1100 C can be achieved. Since both AlN and Pt are resistant to attack by oxygen, such temperatures can be achieved with the substrate in air. However, for X-ray tube applications, the substrate is typically heated in vacuum. 
     The emitter pads  118 , heater element  122 , and connecting strips  123 , are applied to the surface of the substrate block  117  in the required pattern by printing. The connector pads  124  are formed by applying several layers of ink by means of multiple printing so that they are thicker than the connecting strips  123 . The connectors at the ends of the heater element  122  are built up in the same way. The substrate block  117  is then heated to around 1100 C to sinter the ink into the surface of the substrate block  117 . The emitter pads  118  are then coated with a Ba:Sr:Ca carbonate material in the form of an emulsion with an organic binder. This coating can be applied using electrophoretic deposition or silk screen printing. When the emitter element  116  is installed, before it is used, the heater element  122  is used to heat the substrate block  117  to over 700 C, which causes the carbonate material firstly to eject the organic binder material, and then to convert from the carbonate to the oxide form. This process is known as activation. The most active material remaining in the emitter pad coating is then barium oxide, and electron emission densities in excess of 1 mA/mm 2  can be achieved at operating temperatures of around 850-950 C. 
     Referring to  FIG. 3 , each emitter unit  25  comprises an emitter element  116 , a circuit board  310  that provides the electrical control of the emitter element  116 , a grid  312  arranged to control extraction of electrons from each of the emitter pads  118 , and focusing elements  314  arranged to focus the beam of extracted electrons towards a target area on an anode  311 . Typically, the underlying circuit board  310  will provide vacuum feedthrus for the control/power signals that are individually controlled on an emitter-by-emitter basis. The circuit board is best made of a material with low outgassing properties such as alumina ceramic. 
     The emitter element  116  is connected to the circuit board  310  by means of sprung connection elements  316 . These provide physical support of the emitter element over the circuit board  310 , and also each connection element  316  provides electrical connection between a respective one of the connector pads  124  on the emitter element  116  and a respective connector on the circuit board  310 . Each connection element  316  comprises an upper tube  318  connected at its upper end to the emitter element so that it is in electrical contact with one of the connector pads  124 , and a lower tube  320  of smaller diameter, mounted on the circuit board  310  with its lower end in electrical contact with the relevant contact on the circuit board  310 . The upper end of the lower tube  320  is slidingly received within the lower end of the upper tube  318 , and a coil spring  322  acts between the two tubes to locate them resiliently relative to each other, and therefore to locate the emitter element  116  resiliently relative to the circuit board  310 . 
     The connector elements  316  provide electrical connection to the connector pads  124 , and hence to the emitter pads  118 , and mechanical connection to, and support of, the AlN substrate. Preferably the springs  322  will be made of tungsten although molybdenum or other materials may be used. These springs  322  flex according to the thermal expansion of the electron emitter assembly  116 , providing a reliable interconnect method. The grid  312  and focussing elements  324  are less affected by thermal expansion and therefore provide a fixed location. The top of the emitter element  116  is kept at a fixed distance from the grid  312  by spacers in the form of sapphire spheres  317 . Hence the emitter pads  118  are held stationary by being clamped against the grid  312 , via the sapphire spacers  317 , during any thermal expansion or contraction of the emitter assembly  116 . The potential of each of the emitter pads  118  can therefore be switched between an emitting potential, which is lower than that of the grid  312  such that electrons will be extracted from the emitter  118  towards the grid  312 , and a blocking, or non-emitting, potential, which is higher than that of the grid, so that electrons will tend not to leave the surface of the emitter  118 , or if they do, will be attracted back towards the emitter. 
     Referring also to  FIG. 4  the grid  312  is formed from a thin foil of tungsten. It extends over the upper surface  120  of the emitter element  116 , and down past the sides of the emitter element  116 , through the plane containing the lower surface  121  of the emitter block. It also extends down as far as the circuit board  310 , passing through the plane including the front face of the circuit board, and the plane containing the rear face of the circuit board. The grid  312  includes a number of extraction areas  313  each of which extends over a respective one of the emitter pads  118  and has a series of narrow apertures  315  formed in it. The apertures  315  make up at least 50% of the area of the extraction areas. Each extraction area  313  covers an area approximately equal to the area of, and located directly above, the emitter pad  118 . The areas of the grid  312  between the extraction areas are solid. The grid  312  therefore helps to focus the extracted electron beams from the individual emitter pads  118 . The apertures  315  are formed using chemical etching of the tungsten foil. The grid  312  therefore forms an almost continuous layer over the emitter element  116 , apart from the apertures  315 . The sapphire spacers  317  maintain a fixed spatial relationship between the top surface  120  of the emitter element and the upper portion of the grid  312 , and the side portions of the grid  312  are spaced from the emitter element  116  and the circuit board  310 . The grid  312  therefore forms an effective heat shield enclosing the emitter element  116  on both sides and over its top surface. This partial enclosure reduces the radiation of heat from the emitter element. Other materials such as molybdenum can also be used for the grid  312 . The grid  312  is connected to an electrical connector on the circuit board  310  so that its electrical potential can be controlled. The grid  312  is supported close to the emitter pads  118 , with a gap between them of the order of 1 mm. This enables the extraction voltage (i.e. the difference in voltage between an active emitter pad  118  and the grid) to be kept low, for example below 200V while achieving beam currents in excess of 1 mA/mm 2 . 
     The focusing elements  314  extend one along each side of the emitter element  116 . Each focusing element  314  is mounted on isolating mountings  323  so as to be electrically isolated from the grid  312  and the emitter element  116 . It includes a flat lower portion  324  that extends parallel to, and spaced from, the side portions of the grid  312 , and a curved portion  326  that extends upwards from the lower portion  324  beyond the grid upper portion, over in a curved cross section, and back towards the grid  312 , with its inner edge  328  extending along the length of the emitter, spaced from the grid  312  and approximately level, in the lateral direction, with the edge of the emitter pads  118 . This leaves a gap between the two focusing elements  314  that is approximately equal in width to the emitter pads  118  and the apertured areas of the grid  312 . The focusing elements  314  are both held at an electric potential that is negative with respect to the grid  312 , and this causes an electric field that focuses in the lateral direction the electrons extracted from the emitters. The focusing elements form a further, outer heat shield, spaced from the grid  312 , which further reduces the radiation of heat away from the emitter elements  116 . 
     Referring to  FIG. 3 , a heat shield or reflector  330  is located between the emitter element  116  and the circuit board  310 . In this embodiment, the heat shield  330  is formed from a mica sheet coated in a thin layer of gold. The addition of a titanium layer between the gold and the mica improves adhesion of the gold. The heat reflector  330  is supported on the sprung connection elements  316 , which extend through holes in the reflector  330 . Its coated upper surface is located close to, but spaced from, and facing, the lower heated side  121  of the AlN substrate. This reflects heat from the emitter element back towards it, and thereby improves the heater efficiency, reducing the loss of heat through radiative heat transfer. With the grid  312  enclosing the top and sides of the emitter element, and the shield  330  enclosing its under-side, the emitter element is surrounded on all four sides by heat shielding. Heat shielding can also be provided at the ends of the emitter element  116 , but this is less important as the emitter elements are placed end-to-end in close proximity to each other. Silica can be used as an alternative substrate for the reflector, and other reflective materials such as Ti or multi-layer IR mirrors can also be used. Further similar reflectors can also be provided between the emitter element  116  and the grid  312 . 
     Referring back to  FIG. 1 , the scanner is controlled by a control system which performs a number of functions represented by functional blocks in  FIG. 1 . A system control block  54  controls, and receives data from, an image display unit  56 , an X-ray tube control block  58  and an image reconstruction block  60 . The X-ray tube control block  58  controls a focus control block  62  which controls the potentials of the focus elements  314  in each of the emitter units  25 , an emitter control block  64  which controls the potential of the individual emitter pads  118  in each emitter unit  25 , and a high voltage supply  68  which provides the power to the anode  311  of each of the emitter blocks and the power to the emitter elements  118 . The image reconstruction block  60  controls and receives data from a sensor control block  70  which in turn controls and receives data from the sensors  52 . 
     Referring to  FIG. 5 , the circuit board  310  includes a high voltage push-pull output stage  500  for each of the emitter pads  118 , arranged to provide a high voltage signal to it to control the emission of electrons from it. Each output stage  500  comprises a pair of transistors, in this case FETs,  502 ,  504  connected in series between the supply  506  and ground  508 . The HV output  510  is connected between the two FETs. A drive input  512  is connected directly to the second FET  504  and via an XOR gate  514  and an inverter to the first FET  502 . The XOR gate  514  has a second input en (the drive input being the first). This input en is usually low, so that the output of the XOR gate  514  matches the input, but can be used to provide further control as will be described further below with reference to  FIG. 6 . When the input signal goes low, the first FET  502  is switched on and connects the output  510  to the supply voltage and the second FET  504  is switched off and isolates the output from ground. The output voltage therefore rises quickly to the supply voltage. When the input goes low, the first FET is switched off and isolates the output  510  from the supply voltage and the second FET  504  is switched on and connects it to ground, so that the output voltage falls rapidly to zero. Therefore this output stage  500  allows the emitter to be switched on and off rapidly in a well controlled manner, so that the position of the source of the X-ray beam can be accurately controlled. If the input is at an intermediate level that is not high enough to turn on the second FET  504  or low enough to turn on the first FET  502 , then both of the FETs are turned off and the output is in a floating tri-state condition, in which it is disconnected from the fixed potentials of the HV supply and ground and is free to fluctuate. This puts the emitter pad  118  into an electrically isolated state which inhibits electron extraction from the emitter pad  118 . 
     Referring to  FIG. 6 , the emitter control block  64  of the control system provides a digital emitter control signal which is input to a number of emitter control devices  600 , each of which is arranged to control the operation of 32 electron emitter pads in one of the X-ray emitter units  25 . Each control device  600  receives as an input signal a serial digital signal Din which includes data indicating which of the emitters should be turned on and which turned off, and which should be in the floating state. This input signal is fed to a processor  601 , and a number of shift registers  602 ,  604 ,  608 ,  614  which control and monitor the output of the output stages  500 , of which there is one for each of the 32 controlled emitters. 
     The processor  601  is arranged to receive a control signal CTRL as well as the data signal Din and a clock signal SCLK, and to output a number of signals that control operation of the shift registers, and other functions of the device  600 . 
     One of the registers is a data register  602 , in the form of 32 bit serial-in-parallel-out (SIPO) shift register, and is arranged to receive the serial input signal Din, which includes data indicating the required state of each of the emitters  118  for a particular cycle, to load that data under control of a signal ld_dat from the processor  601  and a clock signal SCLK. It is arranged to output the 32 required states to the inputs of a parallel-in-parallel-out data register  604 , which loads them under control of a clock signal XCLK. The data register  604  presents the loaded data at its parallel outputs to one of three inputs to respective NAND gates  606 . Assuming for now that the other inputs to the NAND gates  606  are all high, the outputs of each NAND gate  606  will be low if its respective emitter  118  is to be active, and high if it is to be inactive. The output from each NAND gate  606  is fed to one input of an exclusive-OR (EOR) gate  609 , the other input of which is arranged to receive a polarity signal POL. The output of each EOR gate  609  is input to a respective output stage  500 , each of which is as shown in  FIG. 5 , which therefore provides the controlled HV output HVout to the emitter. The polarity signal POL allows the polarity of the system to be reversed. For example when the grid is fixed at −HV (or ground) potential then a positive voltage is needed on the emitter to turn the beam off. If, however, the emitter potential at −HV (or ground) then a negative potential is needed to turn the beam off. The XOR gate and POL input allows the circuit to be used in either configuration. 
     A tri-state register  608 , in the form of a second 32 bit SIPO register, is arranged to receive the serial input signal Din which also includes data indicating which of the outputs should be set to the tri-state (or floating state) condition. This data is read from the input signal and loaded into the tri-state register  608  under the control of the signal ld_en and the clock signal SCLK. This data is then output in parallel to the respective output stages  500 , with the output en being high if the output stage  500  is to be switched to the tri-state condition, and low if the output stage is to be set to the high or low level as determined by the output from the respective NAND gate  606 . Referring back to  FIG. 3 , when the enable signal en is high, the emitter will be in the floating condition when the input signal to the output stage is low. Data from the serial input signal Din can therefore be used to set any one or more of the emitters  118  to the tri-state condition. This is useful, for example, during initial activation of the emitters  118 , when all of the emitters are set to the tri-state condition, or if short circuits occurred affecting one or more emitters, for example connecting it to the grid, in which case setting the affected emitters to the floating state would allow the short circuit to be mitigated. 
     Each NAND gate  606  also has one input connected to a blanking signal BLA. Therefore if the blanking signal BLA is high, the output of the NAND gate  606  will be low regardless of the output from the data register  602 . The blanking signal can therefore be used to set the outputs of any of the NAND gates to a blanked state, in which they are constant or at least independent of the input data, or an active state, in which they are controlled by the input data. A further chip select input CS is provided to all of the NAND gates and can be used to activate or de-activate the whole control chip  600 . 
     Each HV output HVout is input to a respective comparator  612  which is arranged to compare it to a threshold signal VREF, and produce a feedback output indicative of whether the output drive signal is above or below the threshold. This feedback data, for all 32 output signals, is input to a parallel-in-serial-out feedback register  614 , under control of a signal rd_fb from the processor  601 , and the feedback register  614  converts it to a serial feedback output  616 . This output  616  therefore indicates if any of the outputs is supplying excessive current, which can be used as an indication of, for example, a short circuit problem. The level of the reference signal VREF is set by the processor  601 . 
     A serial output  618  is also provided from the data register  602  which is indicative of whether each of the output signals is nominally at the high or low level. These two serial outputs are multiplexed by a multiplexer  620 , under the control of a multiplexing control signal mux from the processor  601 , to produce a single serial digital output signal Dout. This allows the expected output values to be checked from  618 , for example to check the programming of the device, and the actual values to be checked from  616  to check that the correct outputs have actually been achieved. 
     The control device  600  is arranged to operate in three different modes: a sequential access mode, a random access mode, and a non-scanning or reset mode. In the sequential access mode, the X-ray beam is scanned around the X-ray sources sequentially. Therefore in each full scan of all emitters, each control device will be active for a single period within the scan, and during that period, will activate each of the emitters it controls in sequence for respective activation periods. In the random access mode, the X-ray source is moved around the X-ray source array in a pseudo-random manner. Therefore, in each scan of all emitters, each control device will activate one of its emitters for one activation period, and then will be inactive for a number of activation periods while emitters controlled by other devices  600  are active, and will then be active again for a further activation period when another of its emitters is active. Some of the control inputs for the sequential access mode are shown in  FIG. 7 , and for the random access mode in  FIG. 8 . The signal marked ‘LOAD’ is not a specific signal, but is indicative of the times at which the device  600  is actively providing power to one of the emitters. As can be seen, in the sequential access mode, the blanking signal BLA is held high for a period covering successive activation periods while the emitters are activated in turn. Therefore the outputs track the data loaded in the data register  604 . In the random access mode, the blanking signal is high only during the activation periods in which one of the emitters controlled by the device  600  is active. Therefore, for those activation periods, the outputs track the data in the data register  604 . For intervening activation periods, the blanking signal is low, so the outputs are all turned off. This ensures that whatever processing is being carried out by the device  600  during those intervening periods does not affect the device&#39;s outputs and therefore does not affect the control of the emitters. This mode is suitable for scanning methods such as the random-access method described above, in which the emitter unit is active, in the sense of emitting electrons and hence X-rays, for a number of activation periods during a scan, but inactive for a number of intervening periods during which other emitter units are active. 
     Referring to  FIG. 9 , in the non-scanning mode, which is the default mode, the blanking signal is kept low, so the outputs are all maintained in the off condition and all of the emitters are inactive. This mode is used, for example, to enable calibration of the data acquisition system. 
     The format of the data input signal Din is a 5-byte programming pattern having the following format: 
                                                MSB   Control word               Status/data word 0 (MSB)               Status/data word 1               Status/data word 2           LSB   Status/data word 3 (LSB)                        
The control word has a bit configuration such as:
 
     
       
         
           
               
               
               
               
             
               
                   
               
             
            
               
                 MSB 
                 7 
                 1 = load data register 
                 0 = no action 
               
               
                   
                 6 
                 1 = read status register 
                 0 = read data register 
               
               
                   
                 5 
                 1 = set tri-state register 
                 0 = no action 
               
               
                   
                 4 
                 1 = set BLA hi (Mode 1) 
                 0 = normal action (Mode 2) 
               
               
                   
                 3 
                 1 = set BLA lo (Mode 3) 
                 0 = normal action (Mode 2) 
               
               
                   
                 2 
                 Don&#39;t care 
               
               
                   
                 1 
                 Don&#39;t care 
               
               
                 LSB 
                 0 
                 Don&#39;t care 
               
               
                   
               
            
           
         
       
     
     Therefore one 5-byte input signal is required for each emitter activation period, and the signal indicates by means of the four data/status bytes which emitter is to be active, and by means of the control byte which mode the system is in. The emitter control block  64  sends a serial data input signal to a control device  600  for each of the emitter units  25  of the scanner so as to coordinate operation of all of the emitters in the scanner. 
     In this embodiment as shown in  FIG. 6 , the threshold voltage is generated by an on-chip DAC programmed using the lower 3 bits of the control word. The same threshold programming voltage is applied to all 32 output channels. In this case, the control word is assigned the following bit pattern: 
     
       
         
           
               
               
               
               
             
               
                   
               
             
            
               
                 MSB 
                 7 
                 1 = load data register 
                 0 = no action 
               
               
                   
                 6 
                 1 = read status register 
                 0 = read data register 
               
               
                   
                 5 
                 1 = set tri-state register 
                 0 = no action 
               
               
                   
                 4 
                 1 = set BLA hi (Mode 1) 
                 0 = normal action (Mode 2) 
               
               
                   
                 3 
                 1 = set BLA lo (Mode 3) 
                 0 = normal action (Mode 2) 
               
               
                   
                 2 
                 1 = set threshold voltage 
                 0 = no action 
               
            
           
           
               
               
               
            
               
                   
                 1 
                 Threshold voltage DAC bit 1 (MSB) 
               
               
                 LSB 
                 0 
                 Threshold voltage DAC bit 0 (LSB) 
               
               
                   
               
            
           
         
       
     
     In operation, an object to be scanned is passed along the Z axis, and the X-ray beam is generated by controlling the emitter pad potentials so that electrons from each of the emitter pads  118  in turn are directed at respective target positions on the anode  311  in turn, and the X-rays passing through the object from each X-ray source position in each unit detected by the sensors  52 . As described above, for some applications the beam is arranged to scan along the emitter in discrete steps, and for some it is arranged to switch between the emitter pads  118  in a pseudo-random manner to spread the thermal load on the emitter. Data from the sensors  52  for each X-ray source point in the scan is recorded as a respective data set. The data set from each scan of the X-ray source position can be analysed to produce an image of a plane through the object. The beam is scanned repeatedly as the object passes along the Z axis so as to build up a three dimensional tomographic image of the entire object. 
     In an alternative embodiment the connector elements  316  of  FIG. 3  are inverted. However, the advantage of the spring  322  being near to the circuit board and further from the emitter element  118  is that the upper tube  318  runs at high temperature and the spring  322  at low temperature. This affords a greater choice of spring materials since creeping of the spring is lower at lower temperatures. 
     As an alternative to the wraparound interconnects  124  of the embodiment of  FIGS. 2 a  and 2 b   , through-hole Pt interconnects can be used which extend through holes in the AlN substrate  120  to connect the emitter pads  118  to connectors on the underside of the emitter element  116 . In a further modification, a clip arrangement may be used to connect the electrical power source to the top surface of the AlN substrate. 
     It will be appreciated that alternative assembly methods can be used including welded assemblies, high temperature soldered assemblies and other mechanical connections such as press-studs and loop springs. 
     Referring to  FIG. 10 , in a further embodiment, the output stages of  FIG. 5  are each replaced by an alternative output stage  700 . In this embodiment, the output  710  is connected to the supply  706  via a resistance  702 , and to ground  708  via an FET  704  which is switched on and off by the input signal on the input  712 . (The XOR gate is omitted from the drawing for simplicity). While the input signal is high, the FET  704  is switched on and the output  710  is connected to ground. When the input signal goes low, the FET  704  is switched off, and the output is connected via the resistor  702  to the supply  706 , switching the source on slowly. When the input signal goes high again, the FET  704  again connects the output  710  to ground, switching the source off more rapidly. 
     Referring to  FIGS. 11 a  and 11 b   , in a further embodiment each emitter element  810  is formed from a ceramic substrate  812 , in this case alumina (Al 2 O 3 ) although AlN can again also be used, with individual spaced apart metal emitter pads  814  formed on its upper surface  816  by sputter coating. The emitter pads can be formed of any suitable metal, such as Ni, Pt or W, and they are covered with an active oxide layer to enhance electron emission as in the embodiment of  FIGS. 2 a  and 2 b   . Patterning of the individual emitter pads  814  is achieved using shadow masks during sputter coating, although photolithographic methods can also be used. 
     On the opposite side  818  of the substrate from the emitter pads, a heating element in the form of a continuous conductive film  820  is applied, which in this case covers the whole of the rear side  818  of the emitter element  810 . The heating element is also formed by means of sputter coating, and at each end of the emitter element, the conductive film is made thicker, by further sputter coating, to form contact areas  822 ,  824 . Clearly since the substrate is electrically non-conducting, the heating element  820  is electrically isolated from the emitter pads, which in turn are electrically isolated from each other. 
     Referring to  FIG. 12 , each emitter element  810  is supported in a supporting heat shield structure  830 , which is formed from two side elements  832 ,  834  and two cross members  836 ,  838  which extend between the side elements  832 ,  834  and hold them parallel to, and spaced apart from, each other. The emitter element  810  is supported between the upper (as shown in  FIG. 9 ) edges of the side elements, parallel to the cross members  836 ,  838 , with the emitter pads  814  facing outwards, i.e. upwards as shown, and a circuit card  840  is supported between the lower edges of the side elements  832 ,  834 . 
     The side elements  832 ,  834  and the cross members  836 ,  838  are formed from silica plates, which are formed into interlocking shapes by laser cutting. These plates therefore interlock to form a stable mechanical structure. The silica material is coated on one side, the side facing the emitter element  810 , with a high reflectance low emissivity material, such as Au or Ti. Alternatively the silica may be coated with a multi-layer infra-red mirror. 
     A series of connecting wires  842  each have one end connected to a respective one of the emitter pads  814 , and extend around the outside of the heat shield structure  830 , having their other ends connected to respective connectors on the circuit card  840 . The interconnecting circuit card  840  is used to transfer signals from outside the vacuum envelope of the scanner, either directly through a hermetic seal or indirectly through a metal contact which engages with a hermetic electrical feedthru. 
     A grid  844 , similar to that of  FIG. 3 , extends over the top of the emitter element  810  and down the sides of the heat shield structure  830 , being spaced from the side elements  832 ,  834  to leave an insulating gap, through which the connecting wires  842  extend. The top part of the grid  844  is parallel to the upper surface  816  of the emitter element and spaced from it by a small distance of, in this case, about 1 mm. Focusing elements  846  are arranged on either side of the heat shield  830  and emitter  810 . Each one extends along parallel to the side elements  832 ,  834 , outside the grid  844  and spaced from it by a further insulating gap, and up over the side of the emitter element  810 . Each focusing element has its upper edge extending part way over the emitter element  810 , so that a focusing gap is left, between the focusing elements  846 , which extends along the emitter over the emitter pads  814 . 
     As with the embodiment of  FIG. 3 , the grid  844  and focusing elements  846  are connected to appropriate electrical potentials and serve, as well as their primary functions, as additional heat shields to reduce the radiation of heat away from the emitter element  810 .