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The Effect of Electron Optics on the Properties of the X-Ray Image Intensifier
ADVANCES IN ELECTRONICS AND ELECTRON PHYSICS. VOL. 640
The Effect of Electron Optics on the Properties of the X-Ray Image Intensifier
v. JARES TESLA-Vucuurn Technics, Prugue, Czechoslovakia
The determination of the imaging properties of electron optical systems, ensuring the transfer of an electron image from the photocathode to the luminescent screen of an image intensifier, represents a complex problem.l.*Problems peculiar to the electron optics of X-ray image intensifiers are the large entrance diameter, dictated by the human anatomy, and the strong demagnification, necessitated by the brightness requirements of the subsequent systems, such as vidicons and film cameras. Like any imaging devices, X-ray image intensifiers suffer from image distortions such as spherical and chromatic aberrations, coma, astigmatism, and geometric distortion. Besides the well-known types of aberrations, generally based on the assumption of cylindrical symmetry, another kind of image deterioration which results from the deviations from cylindrical symmetry claims particular attention. Classical methods of electron optics are cumbersome and do not yield results of the desired accuracy. It is furthermore possible to obtain the required values experimentally, from measurements on complete experimental samples of tubes; their preparation, however, is laborious and expensive. Two methods used for determining the imaging properties of the electronoptical systems of X-ray image intenders will be briefly discussed: (1) an experimental method based on the use of a demountable and permanently pumped model of the investigated tube enabling easy alterations in the geometry of the electronoptical systems and (2) a computer method utilizing preset programs for determining the imaging properties of the investigated electronoptical systems. The techniques described were used in the design of electronoptical systems for second- and third-generation X-ray image intensifiers with constant and variable image magnification. The imaging performance of experimental tubes with 190and 270-mm useful input fields3 will be described and some parameters determining the image quality will be compared. 549 Copyright €3 1985 by Academic Press, Inc. (London) Lid. All rights of reproduction in any form reserved. ISBN 0-12-014724-6
550
v. JARES A DEMOUNTABLE MODEL
A diagrammatic section of the demountable system of the X-ray image intensifier is shown in Fig. 1. The input section, primary screen, and photocathode were replaced by a concave aluminum disk with apertures. Tungsten cathodes were placed behind the individual apertures, the apertures were covered by metal grids with a variable pitch. The ceramic supports with the thermionic cathodes were inserted in metal cylinders which were provided with additional metal grids (see Fig. 1). The metal cylinders with the grids were mounted under the metal disk. The fine metal grids with variable pitch which form a suitable test pattern were produced electrolytically from a glass matrix. The grids were secured in
FIG. 1. Section of the demountable model of the X-ray image intensifier with variable demagnification.M, microscope; S, luminescent screen; A, anode; A,, zooming anode; F, focusing electrode; D, aluminum disk with thermionic cathode; M I , metal grid with variable pitch; M2,metal grid; K, tungsten filament; R, rubber seal.
ELECTRON OPTICS OF THE
X-RAYIMAGE
INTENSIFIER
551
the plane of the apertures above the tungsten cathodes by means of a second metal disk made of aluminum foil. The focusing electrode of the intensifier which covers the wall of the glass bulb is also made of aluminum foil. The anode, together with the output screen, are movably mounted in the upper narrowed section of the glass bulb. To investigate the imaging properties of the experimental model of the intensifier for various geometric configurations and electrode spacings, the demagnified electron images of the metal grids are observed on the output screen with the help of a microscope. The resolution of the electronoptical system of the intensifier can be determined from the number of grid apertures discernible in the individual images on the output screen. The image distortion is apparent from the geometry and the position of the electron images of the grids. The curvature of the image plane can be estimated from the magnitude of the focusing voltages required for focusing the images corresponding to the central and peripheral areas of the cathode. Similarly, the influence of both the cathode curvature and the variations of the form of the anode on the sharpness and geometry of the image can be deduced. COMPUTATION METHOD
When designing an electronoptical system economic considerations often necessitate a theoretical analysis of some of its parameters. By utilizing an established numerical method for calculating electrostatic fields and electron trajectories in designing a system with wide electron beams, a safe analysis of the influence of geometrical variations upon the electron trajectories can be obtained. It is then possible to determine, at certain stages of the design, the efficiency of certain modifications of the system geometry and thus to improve upon the quality and speed of the final design. For analyzing the imaging properties of electronoptical systems by means of a computer, a program consisting of subprograms for solving the distribution of an electrostatic field and the path of electron trajectories in an axially symmetric electrostatic field was prepared. The programs involve the automatic adjustment of the focusing voltage and a recalculation of the electrostatic field, until the central trajectory with the chosen initial values crosses the chosen plane (e.g., the plane of the luminescent screen) within a circle of a predetermined diameter. When this condition is satisfied, or when the focusing voltage lies outside the preselected range, it is possible to calculate any number of trajectories with predetermined initial conditions. The input data and programs are fed into the computer by means of an eight-track tape or by means of cards. The program was set up in the FORTRAN language for computation on a Hewlett-Packard 2100 or EC 1030. The prepared program repre-
552
v. JARES
sents an aid for calculating the electrostatic field and the selected trajectories in a focused system consisting of given electrodes. One of these electrodes serves for focusing the system which is achieved by varying the electrode voltage automatically. The calculated results may be obtained from the computer in a graphical form (distribution of electrostatic field and electron trajectories in this field) and in the form of tables serving for further evaluations of the electronoptical system. The method described was used to design the electronoptical system of an X-ray image intensifier with a spherically, hyperbolically, and ovally shaped photocathode.
MEASUREMENT RESULTS
A demountable vacuum test tube was used in combination with digital computer techniques for determining the electrostatic field distribution and the variation of electron trajectories in designing the electronoptical system of X-ray image intensifiers having input fields of 190 and 270 mm with variable image demagnification. Attention was paid to the effect on imaging properties of the electrode shape as well as deformations occurring during assembly. In the following, first the design of the electronoptical system for the 190-mrn X-ray image intensifiers will be described; further, results obtained in designing the 270-mm multiple field intensifier will be given. A 190-mm X-RAYIMAGEINTENSIFIER In designing the electronoptical system, particular attention was aimed at attaining the required center and edge resolution, demagnification, image geometry, and tolerable outer tube dimensions. Original configurations were based on imaging systems employing a spherically curved cathode meniscus, a focusing electrode deposited on the inner surface of the glass tube, and a cylindrical anode containing the output phosphor screen. These electrode systems met the requirements of first-generation image intensifiers; they were, however, unable to yield higher image resolution, particularly in peripheral regions. Hence, computer techniques and demountable test tubes were employed to investigate the effect of the cathode curvature, the height of the peripheral ring electrode, and the shape and position of the anode on the center and edge resolution, and the geometry and the demagnification of the image. As to cathode shapes, best results were obtained with a rotational hyperboloid approximated from center to edge by six spherical arcs. Experimental X-ray image intensifier tubes employing the new cathode meniscus and modified anode
ELECTRON OPTICS OF THE X-RAY IMAGE INTENSIFIER
553
system (tube type 012QA41) yielded a center resolution of 4 Ip mm-I and 1.6 to 1.8 lp mm-I edge resolution, respectively. To meet the requirements of third-generation X-ray image intensifiers, particularly increased edge resolution, the effect of the cathode shape on the image area curvature in the plane of the output screen was computed. Upon optimization of the imaging properties of the electrode systems comprising the new cathode shape, best results were obtained with an oval cathode shape approximated by two circles of different diameter. Use was made of the demountable tube to test several configurations differing in the mutual arrangement of their electrodes. The results obtained were plotted in the diagram shown in Fig. 2. The diagrams may be used to determine the optimal adjustment of the chosen electrodes while maintaining the edge conditions. A triode type electronoptical configuration was deduced from these measurements and tested in a number of experimental tubes. In some of these tubes, limitation phenomena were observed at the image edges. It is evident from Fig. 2b that the region of optimum image adjustment is very narrow calling for high precision in assembling the electrode system. The differing focusing voltages indicate deviations in the location of the system within the tube invelope. The image limitation in the peripheral region was eliminated by modifying the front part of the anode. The 014QA41 X-ray image intensifier whose triode mode electronoptical system employs an oval shaped photocathode is fitted with a cesium iodide input screen and an output phosphor based on zinc-cadmium-sulfide. The brightness intensification, as compared to direct X-ray screens, reaches values up to 10,000; the conversion factor is equivalent to 382 cd m-* pC-I kg-' sec-I. Image resolution is 4.5 Ip mm-' in the central region, edge resolution (0.9R) is 3.5 lp mm-I, and distortion is less than 15%. This type of X-ray image intensifier is finding applications in light mobile equipment as well as diagnostic units. A 270-mm X-RAYIMAGE INTENSIFIER WITH DEMAGNIFICATION
VARIABLE
For electronoptical systems with variable demagnification, greatest efforts have been concentrated on attaining the highest possible center and edge resolution. Emphasis was on maintaining the outer dimensions of the standard 270-mm tubes as well as their all-glass design, keeping production costs within reasonable limits. In designing new configurations of the tetrode systems, computer techniques were used to study the effect of the geometric electrode arrangement on the image area curvature in the plane of the phosphor screen. The results obtained were checked in a demount-
v. JARES
554
mm mm mm
a
Lo= 266 mm Lo=268 mm LO=270 mm Lo=271 mm
350; 300
3
250
I
1Region of impaired edge resolution
1Region of peripheral
electron beam limitation
150
L
-
- b
555
ELECTRON OPTICS OF THE X-RAY IMAQE INTENSIFIER
able vacuum test tube. Primary attention was paid to the effect of inaccuracies arising during tube assembly, such as sealing processes, and, further, to ovalities and nonconcentric mounting of the electrodes. The properties of the resulting electrode arrangement are shown in Fig. 3 which gives the dependences of the electrode voltages on the size of the output image. The figure shows the dependence of the second anode
4 1
41
100
I
I
I
I
I
I
I
I
I
150
I
200
I
l
l
I
I
250
Focusing voltage, UF ( V )
FIG.3, The imaging and focusing properties of a tetrode electrode system with variable demagnification of the image. For explanation see text.
556
V. JARES
(zooming) voltage UT on the focusing voltage UFfor the focused image center (curve 1). It also shows the changes in demagnification in the central image region (curve 2, diameter 45 mm) and the peripheral region (curve 3, diameter 225 mm) as a function of the focusing voltage UF. An image reduction of 10 (output image 27 mm) is achieved according to curve 3 for a focusing voltage UF= 128 V; this corresponds to a zoom anode voltage of UT = 8.5 kV (curve 1). Curves 2 and 3 may be employed for determining the linear image distortion. The 01 1QA41 type image intensifier comprising the described tetrode electrode system with spherical photocathode is fitted with a cesium iodide input screen and an output screen based on a zinc-cadmium-sulfide phosphor. The conversion factor is about 343.6 cd m-2 pC-' kg-I sec-I for the input field of 270 mm and 114.5 cd m-2 pC-I kg-I sec-I for the 175-mm field. The central and peripheral resolutions are 2.5 and 1.5 Ip mm-I, respectively, for the large field, and 3 and 2.5 Ip mm-I, respectively for the reduced field. Image distortion is less than 15%. This tube type finds applications in diagnostic X-ray units for routine as well as special procedures. The tetrode electrode system has been further investigated with the aim of increasing the resolution in the peripheral area. The outer tube envelope and the anode system remain constant and further progress was attempted by varying the shape of the cathode meniscus. Various shapes of the cathode (hyperbolic, oval) were tried without success. Further designs represented pentode systems achieved by separating some of the electrodes of the tetrode system. For instance, the effect of separating the 160 4 mm
FE
\
A
0s 0 1530476685
0
,
20
'
,
40
'
129 1
60
'
22 k V
179206252 305366 4 3 9 5 2 8 6 3 9 7 8 0 9 6 2 V 1 2 1 5 2 1 9 6 2 5 9 3 5 1 4 9 6 6 5 8 9 1 3 2 1 9 6 2 1 1 2 1 9 1
80
'
1
'
1
'
1
'
1
100 120 140 160
~
1
'
1
'
180 200 220
1
~
1
'
1
'
'
~
1
'
1
'
240 260 280 300 320 340
1
'
1
'
1
'
1
360 380 4Wmm
FIG.4. Distribution of calculated equipotential lines with the shape and location of the zooming electrode. CM, Cathode meniscus; FE, focusing electrode;AS, zooming electrode; A, anode; OS, output screen.
'
ELECTRON OPTICS OF THE X-RAY IMAGE INTENSIFIER
557
558
v. JARES
focusing electrode on the inner wall into two parts was investigated; further, the auxiliary anode used for image zoom was made of two separate parts and the effect of separating the output screen from the anode was studied. Neither of these modifications brought the desired improvement in peripheral resolution. Results obtained with triode electrode systems led to the idea of constructing a 270-mm tube containing a triode system with a 27-mm output screen. These tubes yielded the required edge resolution of 3 lp mm-'. Subsequently, electrode systems for image zoom were considered to give the required performance without disturbing the electrostatic field distribution. Employing the EC 1030 computer, a suitable spherical equipotential area in the anode region was selected from the electrostatic field distribution in order to determine the proper shape of the zoom electrode (Fig. 4). The shape of the electrode appears as a dashed line in the computed field and consists of a section of a spherical cap with a circular aperture. The imaging properties of this tetrode system were investigated by means of the computer techniques and a demountable test tube. Particular attention was given to the optimal position of the zoom electrode for center and edge resolution, and the required geometry and demagnification of the image. The calculated values of the electrostatic field and the variation of the electron trajectories from the chosen points on the photocathode for the required demagnifications 270/27 and 17927 mm were in full agreement with measurements on the experimental model. The new electrode design (Fig. 5 ) was used in an experimental 270-mm X-ray image intensifier with zoom. For a 270-mm input field, a central resolution of 4.5 to 5 lp mm-I and edge resolution of 2.8 to 3.15 lp mm-l, respectively, was achieved; for the 175-mm input field, the central resolution was 5 lp mm-' and the edge resolution 1.8 to 2.2 lp mm-'. Further development work is taking place to improve the edge resolution for the 175-mm input image. CONCLUSION The use of demountable vacuum test tubes and computation in designing new electronoptical systems for X-ray image intensifiers helped to reduce the effect of geometric aberrations and image distortion. The demountable model yields data on the exit and incidence of the electron, while computation enables the electron path to be traced through the electronoptical system. Increased resolution in the central and peripheral regions in combination with new high-efficiency output screens ensure high image quality in X-ray image intensifiers and represent a valuable contribution to X-ray diagnostic equipment.
ELECTRON OPTICS OF THE X-RAY IMAGE INTENSIFIER
REFERENCES 1. JareS, V. and Novotny, B., I n “Adv. E.E.P.” Vol. 28A, p. 523 (1%9). 2. JareS, V. and Novotny, B . , I n “Adv. E.E.P.” Vol. 40A, p. 473 (1976). 3. JareS, V., Boc. IMEKO Sec. H-1371, Budapest p. 91 (1982).
559
The Effect of Electron Optics on the Properties of the X-Ray Image Intensifier
v. JARES TESLA-Vucuurn Technics, Prugue, Czechoslovakia
The determination of the imaging properties of electron optical systems, ensuring the transfer of an electron image from the photocathode to the luminescent screen of an image intensifier, represents a complex problem.l.*Problems peculiar to the electron optics of X-ray image intensifiers are the large entrance diameter, dictated by the human anatomy, and the strong demagnification, necessitated by the brightness requirements of the subsequent systems, such as vidicons and film cameras. Like any imaging devices, X-ray image intensifiers suffer from image distortions such as spherical and chromatic aberrations, coma, astigmatism, and geometric distortion. Besides the well-known types of aberrations, generally based on the assumption of cylindrical symmetry, another kind of image deterioration which results from the deviations from cylindrical symmetry claims particular attention. Classical methods of electron optics are cumbersome and do not yield results of the desired accuracy. It is furthermore possible to obtain the required values experimentally, from measurements on complete experimental samples of tubes; their preparation, however, is laborious and expensive. Two methods used for determining the imaging properties of the electronoptical systems of X-ray image intenders will be briefly discussed: (1) an experimental method based on the use of a demountable and permanently pumped model of the investigated tube enabling easy alterations in the geometry of the electronoptical systems and (2) a computer method utilizing preset programs for determining the imaging properties of the investigated electronoptical systems. The techniques described were used in the design of electronoptical systems for second- and third-generation X-ray image intensifiers with constant and variable image magnification. The imaging performance of experimental tubes with 190and 270-mm useful input fields3 will be described and some parameters determining the image quality will be compared. 549 Copyright €3 1985 by Academic Press, Inc. (London) Lid. All rights of reproduction in any form reserved. ISBN 0-12-014724-6
550
v. JARES A DEMOUNTABLE MODEL
A diagrammatic section of the demountable system of the X-ray image intensifier is shown in Fig. 1. The input section, primary screen, and photocathode were replaced by a concave aluminum disk with apertures. Tungsten cathodes were placed behind the individual apertures, the apertures were covered by metal grids with a variable pitch. The ceramic supports with the thermionic cathodes were inserted in metal cylinders which were provided with additional metal grids (see Fig. 1). The metal cylinders with the grids were mounted under the metal disk. The fine metal grids with variable pitch which form a suitable test pattern were produced electrolytically from a glass matrix. The grids were secured in
FIG. 1. Section of the demountable model of the X-ray image intensifier with variable demagnification.M, microscope; S, luminescent screen; A, anode; A,, zooming anode; F, focusing electrode; D, aluminum disk with thermionic cathode; M I , metal grid with variable pitch; M2,metal grid; K, tungsten filament; R, rubber seal.
ELECTRON OPTICS OF THE
X-RAYIMAGE
INTENSIFIER
551
the plane of the apertures above the tungsten cathodes by means of a second metal disk made of aluminum foil. The focusing electrode of the intensifier which covers the wall of the glass bulb is also made of aluminum foil. The anode, together with the output screen, are movably mounted in the upper narrowed section of the glass bulb. To investigate the imaging properties of the experimental model of the intensifier for various geometric configurations and electrode spacings, the demagnified electron images of the metal grids are observed on the output screen with the help of a microscope. The resolution of the electronoptical system of the intensifier can be determined from the number of grid apertures discernible in the individual images on the output screen. The image distortion is apparent from the geometry and the position of the electron images of the grids. The curvature of the image plane can be estimated from the magnitude of the focusing voltages required for focusing the images corresponding to the central and peripheral areas of the cathode. Similarly, the influence of both the cathode curvature and the variations of the form of the anode on the sharpness and geometry of the image can be deduced. COMPUTATION METHOD
When designing an electronoptical system economic considerations often necessitate a theoretical analysis of some of its parameters. By utilizing an established numerical method for calculating electrostatic fields and electron trajectories in designing a system with wide electron beams, a safe analysis of the influence of geometrical variations upon the electron trajectories can be obtained. It is then possible to determine, at certain stages of the design, the efficiency of certain modifications of the system geometry and thus to improve upon the quality and speed of the final design. For analyzing the imaging properties of electronoptical systems by means of a computer, a program consisting of subprograms for solving the distribution of an electrostatic field and the path of electron trajectories in an axially symmetric electrostatic field was prepared. The programs involve the automatic adjustment of the focusing voltage and a recalculation of the electrostatic field, until the central trajectory with the chosen initial values crosses the chosen plane (e.g., the plane of the luminescent screen) within a circle of a predetermined diameter. When this condition is satisfied, or when the focusing voltage lies outside the preselected range, it is possible to calculate any number of trajectories with predetermined initial conditions. The input data and programs are fed into the computer by means of an eight-track tape or by means of cards. The program was set up in the FORTRAN language for computation on a Hewlett-Packard 2100 or EC 1030. The prepared program repre-
552
v. JARES
sents an aid for calculating the electrostatic field and the selected trajectories in a focused system consisting of given electrodes. One of these electrodes serves for focusing the system which is achieved by varying the electrode voltage automatically. The calculated results may be obtained from the computer in a graphical form (distribution of electrostatic field and electron trajectories in this field) and in the form of tables serving for further evaluations of the electronoptical system. The method described was used to design the electronoptical system of an X-ray image intensifier with a spherically, hyperbolically, and ovally shaped photocathode.
MEASUREMENT RESULTS
A demountable vacuum test tube was used in combination with digital computer techniques for determining the electrostatic field distribution and the variation of electron trajectories in designing the electronoptical system of X-ray image intensifiers having input fields of 190 and 270 mm with variable image demagnification. Attention was paid to the effect on imaging properties of the electrode shape as well as deformations occurring during assembly. In the following, first the design of the electronoptical system for the 190-mrn X-ray image intensifiers will be described; further, results obtained in designing the 270-mm multiple field intensifier will be given. A 190-mm X-RAYIMAGEINTENSIFIER In designing the electronoptical system, particular attention was aimed at attaining the required center and edge resolution, demagnification, image geometry, and tolerable outer tube dimensions. Original configurations were based on imaging systems employing a spherically curved cathode meniscus, a focusing electrode deposited on the inner surface of the glass tube, and a cylindrical anode containing the output phosphor screen. These electrode systems met the requirements of first-generation image intensifiers; they were, however, unable to yield higher image resolution, particularly in peripheral regions. Hence, computer techniques and demountable test tubes were employed to investigate the effect of the cathode curvature, the height of the peripheral ring electrode, and the shape and position of the anode on the center and edge resolution, and the geometry and the demagnification of the image. As to cathode shapes, best results were obtained with a rotational hyperboloid approximated from center to edge by six spherical arcs. Experimental X-ray image intensifier tubes employing the new cathode meniscus and modified anode
ELECTRON OPTICS OF THE X-RAY IMAGE INTENSIFIER
553
system (tube type 012QA41) yielded a center resolution of 4 Ip mm-I and 1.6 to 1.8 lp mm-I edge resolution, respectively. To meet the requirements of third-generation X-ray image intensifiers, particularly increased edge resolution, the effect of the cathode shape on the image area curvature in the plane of the output screen was computed. Upon optimization of the imaging properties of the electrode systems comprising the new cathode shape, best results were obtained with an oval cathode shape approximated by two circles of different diameter. Use was made of the demountable tube to test several configurations differing in the mutual arrangement of their electrodes. The results obtained were plotted in the diagram shown in Fig. 2. The diagrams may be used to determine the optimal adjustment of the chosen electrodes while maintaining the edge conditions. A triode type electronoptical configuration was deduced from these measurements and tested in a number of experimental tubes. In some of these tubes, limitation phenomena were observed at the image edges. It is evident from Fig. 2b that the region of optimum image adjustment is very narrow calling for high precision in assembling the electrode system. The differing focusing voltages indicate deviations in the location of the system within the tube invelope. The image limitation in the peripheral region was eliminated by modifying the front part of the anode. The 014QA41 X-ray image intensifier whose triode mode electronoptical system employs an oval shaped photocathode is fitted with a cesium iodide input screen and an output phosphor based on zinc-cadmium-sulfide. The brightness intensification, as compared to direct X-ray screens, reaches values up to 10,000; the conversion factor is equivalent to 382 cd m-* pC-I kg-' sec-I. Image resolution is 4.5 Ip mm-' in the central region, edge resolution (0.9R) is 3.5 lp mm-I, and distortion is less than 15%. This type of X-ray image intensifier is finding applications in light mobile equipment as well as diagnostic units. A 270-mm X-RAYIMAGE INTENSIFIER WITH DEMAGNIFICATION
VARIABLE
For electronoptical systems with variable demagnification, greatest efforts have been concentrated on attaining the highest possible center and edge resolution. Emphasis was on maintaining the outer dimensions of the standard 270-mm tubes as well as their all-glass design, keeping production costs within reasonable limits. In designing new configurations of the tetrode systems, computer techniques were used to study the effect of the geometric electrode arrangement on the image area curvature in the plane of the phosphor screen. The results obtained were checked in a demount-
v. JARES
554
mm mm mm
a
Lo= 266 mm Lo=268 mm LO=270 mm Lo=271 mm
350; 300
3
250
I
1Region of impaired edge resolution
1Region of peripheral
electron beam limitation
150
L
-
- b
555
ELECTRON OPTICS OF THE X-RAY IMAQE INTENSIFIER
able vacuum test tube. Primary attention was paid to the effect of inaccuracies arising during tube assembly, such as sealing processes, and, further, to ovalities and nonconcentric mounting of the electrodes. The properties of the resulting electrode arrangement are shown in Fig. 3 which gives the dependences of the electrode voltages on the size of the output image. The figure shows the dependence of the second anode
4 1
41
100
I
I
I
I
I
I
I
I
I
150
I
200
I
l
l
I
I
250
Focusing voltage, UF ( V )
FIG.3, The imaging and focusing properties of a tetrode electrode system with variable demagnification of the image. For explanation see text.
556
V. JARES
(zooming) voltage UT on the focusing voltage UFfor the focused image center (curve 1). It also shows the changes in demagnification in the central image region (curve 2, diameter 45 mm) and the peripheral region (curve 3, diameter 225 mm) as a function of the focusing voltage UF. An image reduction of 10 (output image 27 mm) is achieved according to curve 3 for a focusing voltage UF= 128 V; this corresponds to a zoom anode voltage of UT = 8.5 kV (curve 1). Curves 2 and 3 may be employed for determining the linear image distortion. The 01 1QA41 type image intensifier comprising the described tetrode electrode system with spherical photocathode is fitted with a cesium iodide input screen and an output screen based on a zinc-cadmium-sulfide phosphor. The conversion factor is about 343.6 cd m-2 pC-' kg-I sec-I for the input field of 270 mm and 114.5 cd m-2 pC-I kg-I sec-I for the 175-mm field. The central and peripheral resolutions are 2.5 and 1.5 Ip mm-I, respectively, for the large field, and 3 and 2.5 Ip mm-I, respectively for the reduced field. Image distortion is less than 15%. This tube type finds applications in diagnostic X-ray units for routine as well as special procedures. The tetrode electrode system has been further investigated with the aim of increasing the resolution in the peripheral area. The outer tube envelope and the anode system remain constant and further progress was attempted by varying the shape of the cathode meniscus. Various shapes of the cathode (hyperbolic, oval) were tried without success. Further designs represented pentode systems achieved by separating some of the electrodes of the tetrode system. For instance, the effect of separating the 160 4 mm
FE
\
A
0s 0 1530476685
0
,
20
'
,
40
'
129 1
60
'
22 k V
179206252 305366 4 3 9 5 2 8 6 3 9 7 8 0 9 6 2 V 1 2 1 5 2 1 9 6 2 5 9 3 5 1 4 9 6 6 5 8 9 1 3 2 1 9 6 2 1 1 2 1 9 1
80
'
1
'
1
'
1
'
1
100 120 140 160
~
1
'
1
'
180 200 220
1
~
1
'
1
'
'
~
1
'
1
'
240 260 280 300 320 340
1
'
1
'
1
'
1
360 380 4Wmm
FIG.4. Distribution of calculated equipotential lines with the shape and location of the zooming electrode. CM, Cathode meniscus; FE, focusing electrode;AS, zooming electrode; A, anode; OS, output screen.
'
ELECTRON OPTICS OF THE X-RAY IMAGE INTENSIFIER
557
558
v. JARES
focusing electrode on the inner wall into two parts was investigated; further, the auxiliary anode used for image zoom was made of two separate parts and the effect of separating the output screen from the anode was studied. Neither of these modifications brought the desired improvement in peripheral resolution. Results obtained with triode electrode systems led to the idea of constructing a 270-mm tube containing a triode system with a 27-mm output screen. These tubes yielded the required edge resolution of 3 lp mm-'. Subsequently, electrode systems for image zoom were considered to give the required performance without disturbing the electrostatic field distribution. Employing the EC 1030 computer, a suitable spherical equipotential area in the anode region was selected from the electrostatic field distribution in order to determine the proper shape of the zoom electrode (Fig. 4). The shape of the electrode appears as a dashed line in the computed field and consists of a section of a spherical cap with a circular aperture. The imaging properties of this tetrode system were investigated by means of the computer techniques and a demountable test tube. Particular attention was given to the optimal position of the zoom electrode for center and edge resolution, and the required geometry and demagnification of the image. The calculated values of the electrostatic field and the variation of the electron trajectories from the chosen points on the photocathode for the required demagnifications 270/27 and 17927 mm were in full agreement with measurements on the experimental model. The new electrode design (Fig. 5 ) was used in an experimental 270-mm X-ray image intensifier with zoom. For a 270-mm input field, a central resolution of 4.5 to 5 lp mm-I and edge resolution of 2.8 to 3.15 lp mm-l, respectively, was achieved; for the 175-mm input field, the central resolution was 5 lp mm-' and the edge resolution 1.8 to 2.2 lp mm-'. Further development work is taking place to improve the edge resolution for the 175-mm input image. CONCLUSION The use of demountable vacuum test tubes and computation in designing new electronoptical systems for X-ray image intensifiers helped to reduce the effect of geometric aberrations and image distortion. The demountable model yields data on the exit and incidence of the electron, while computation enables the electron path to be traced through the electronoptical system. Increased resolution in the central and peripheral regions in combination with new high-efficiency output screens ensure high image quality in X-ray image intensifiers and represent a valuable contribution to X-ray diagnostic equipment.
ELECTRON OPTICS OF THE X-RAY IMAGE INTENSIFIER
REFERENCES 1. JareS, V. and Novotny, B., I n “Adv. E.E.P.” Vol. 28A, p. 523 (1%9). 2. JareS, V. and Novotny, B . , I n “Adv. E.E.P.” Vol. 40A, p. 473 (1976). 3. JareS, V., Boc. IMEKO Sec. H-1371, Budapest p. 91 (1982).
559